Entries in Strength (14)


Core Stability: The Inner Unit

By: Paul Chek.
From: A new frontier in abdominal training: IAAF/NSA 4.99.
Site Link: Coachr.
Article Link: The Inner Unit.

ALSO SEE: Core Stability: The Outer Unit.

A new frontier in abdominal training


Paul Chek is an expert in the fields of corrective exercise and high performance conditioning and is the founder of the C.H.E.K Institute in San Diego, California. For over fifteen years he has traveled around the world lecturing, consulting and giving seminars. Paul Chek has been a consultant to the Los Angeles Chiropractic College, the Chicago Bulls, the Denver Nuggets, the US Army Boxing team, Australia's Canberra Raiders and the US Air Force Academy. 


The author states that abdominal exercises can be performed in various ways and asks if the exercises commonly practiced really improve the functionality of the abdominal muscles. From his own studies with patients and clients who performed a high volume of abdominal routines, he concludes that the usual theories of explanation and treatment for back pain are wrong. He recommends the concept of "The Inner Unit", which is a term describing the functional synergy between specific abdominal muscle groups. He describes ideas for Inner Unit conditioning and concludes that Inner Unit training provides the essential joint stiffness and stability needed to give the large prime movers of the body a working foundation.

How many ways can you do an abdominal exercise? Well, if you have been reading the muscle tabloids for the past 20 years you could probably come up with well over 100. Today we have classes devoted to nothing but TRASHING people's abdominal muscles, complete with every variation of crunch, jack knife, side bend and leg raise exercise known to man. Are these classes, or these exercises, really improving the way you look or function, or reducing your chances of back pain?

To find the answers to these questions, in 1992 I began investigating the correlation between abdominal exercises performed, exercise volume and the postural alignment, pain complaints and overall appearance of my clients. To ensure objective observations of postural alignment and responses to specific exercises, I designed and patented calibrated instruments to measure structural misalignment.
In the first year of recording such information as forward head posture, rib cage posture, pelvic tilt and overall postural alignment, it became evident that those performing high volume sit-up/crunch exercise programmes were not showing promising results (see Figure 1)! Those attending "Ab Blast" classes and/or performing high repetition/high volume abdominal routines were not only having a harder time recovering from back pain, they were also showing little or no improvement in their postural alignment.

While studying patients and clients who performed high volume abdominal routines, it became very evident that there was a common link. About 98% of those with back pain had weak lower abdominal and transversus abdominis muscles, while those with no history of back pain were frequently able to activate the transversus abdominis and scored better on lower abdominal strength and coordination tests. To alleviate back pain, I frequently had to suggest that clients stay completely away from any form of sit-up or crunch type exercises. When this advice was adhered to, and exercises for the lower abdominal and transversus abdominis were practiced regularly, back pain either decreased or was completely alleviated and posture routinely improved.

One can always find some "experts" in the health and fitness industries who state that "there is no such thing as lower abdominal muscles," while others suggest that the best treatment for back pain is to exercise on machines that isolate the lower back muscles. My clinical observations lead me to believe both theories are wrong.

In 1987, "Clinical Anatomy of the Lumbar Spine" by Nikolai Bogduk and Lance Twomey was published. This book is important because it was Bogduk who made the first clinical observations of how the abdominal and back muscles worked together as a functional unit. This occurs via the connection of the transversus abdominis and internal oblique muscles to the envelope of connective tissue (thoraco-Iumbar fascia) surrounding the back muscles (Figure 2).

A few years ago, Australian researchers Richardson, Jull, Hodges and Hides began making significant headway in understanding how the deep abdominal wall worked in concert with other muscles, creating what they would later call THE INNER UNIT.

The Inner Unit

The Inner Unit became accepted as a term describing the functional synergy between the transversus abdominis and posterior fibers of the obliquus intern us abdominis, pelvic floor muscles, multifidus and lumbar portions of the longisssimus and iliocostalis, as well as the diaphragm (Figure 3). Research showed that the inner unit was under separate neurological control from the other muscles of the core. This explained why exercises targeting muscles such as the rectus abdominis, obliquus extern us abdominis and psoas, (the same muscles exercised in traditional abdominal conditioning programmes common all over the world) were very ineffective at stabilising the spine and reducing chronic back pain.

Exercising the big muscles (prime movers) was not providing the correct strengthening for such essential small muscles as the multifidus, transversus abdominis and pelvic floor muscles. When working properly, these muscles provide the necessary increases in joint stiffness and stability to the spine, pelvis and rib cage to provide a stable platform for the big muscles. In a sense, as the big muscles (outer unit) become stronger and tighter, the delicate balance between the inner and outer units becomes disrupted. This concept is easier to understand using the pirate ship model (Figure 4).

The mast of the pirate ship is made of vertebra which are held together (stiffened) by the small guy wires running from vertebra to vertebra. just like the role of the multifidus (a member of the inner unit) in the human spinal column.

Although the big guy wires (representing the outer unit) are essential to hold up the mast of the pirate ship (our spine), they could never perform this function effectively if the small segmental stabilizers (inner unit) were to fail. By viewing the pirate ship's large guy wires, it becomes easy to see how developing too much tension from the overuse of exercises such as the crunch, could disrupt the posture of the mast, or spinal column in the case of a human.

To better apply the concept of the pirate ship, let's examine how the inner and outer units work in a common situation such as picking dumbbells up from the floor in the gym (Figure 5). Almost in synchrony with the thought, "Pick up the weights from the floor," the brain activates the inner unit, contracting the multifidus and drawing in the transversus abdominis. This tightens the thoraco-Iumbar fascia in a weight belt-like fashion (Figure 2). Just as this is happening, there is simultaneous activation of the diaphragm above and the pelvic floor below. The effect is to encapsulate the internal organs as they are compressed by the transversus abdominis. This process creates both stiffness of the trunk and stabilises the joints of the pelvis, spine and rib cage, allowing effective force transfer from the leg musculature, trunk and large prime movers of the back and arms to the dumbbells.

When the inner unit is functioning correctly, joint injury is infrequent, even under extreme loads such as pushing a car, tackling an opponent in football or lifting large weights in the gym. When it is not functioning correctly, activation of the large prime movers will be no different than a large wind hitting the sail of the pirate ship in the presence of loose guy wires running from vertebra to vertebra in the mast. Any system is only as strong as its weakest link!

Inner Unit Conditioning Tips

The first and most important step towards reducing back pain, improving posture and the general visual appearance, is to stop all crunch and/or sit-up type exercises until you become proficient at activating your inner unit! Although the assessment procedures for the inner unit are beyond the scope of this article, the interested reader may find detailed information in the video series "Scientific Core Conditioning". With inner unit dysfunction being extremely common in today's working and exercising population, it is safe to assume that everyone needs to start with novice exercises, even the most elite of athletes.

To begin conditioning the transversus abdominis, use the 4 Point Transversus Abdominis Trainer (Figure 6). For conditioning of the multifidus and related stabiliser and postural muscles, the Horse Stance exercises may be used (Figures 7-9).

 ALSO SEE: Core Stability: The Outer Unit.


Core Stability: The Outer Unit

By: Paul Chek.
From: IAAF/NSA 1-2.00.
Site Link: Coachr.
Article Link: The Outer Unit.


Paul Chek is an expert in the fields of corrective exercise and high performance conditioning and is the founder of the C.H.E.K Institute in San Diego, California. For over fifteen years he has traveled around the world lecturing, consulting and giving seminars. Paul Chek has been a consultant to the Los Angeles Chiropractic College, the Chicago Bulls, the Denver Nuggets, the US Army Boxing team, Australia's Canberra Raiders and the US Air Force Academy.


The author stated that abdominal exercises can be performed in various ways and asks if the common exercises really improve the functionality of the abdominal muscles. In this article the author explains first, the anatomy of the outer unit, second, he describes the function of the four sling systems of the outer unit and, finally, he demonstrates exercises targeting one or all of the sling systems in a methodical manner.

In the previous article titled The Inner Unit A New Frontier In Abdominal Training, we discussed the function of the transversus abdominis, multifidus, diaphragm and pelvic floor musculature with regard to their significant functions as stabilizers of both the spine and extremities. The main message of this article was that stabilization of the core via the inner unit must always precede force generation by the core or extremities.

The scope of this article will be, first, to explain the anatomy of the outer unit, second, to describe the function of the four sling systems of the outer unit and, finally, to demonstrate exercises targeting one or all of the sling systems in a methodical manner.   

Functional Anatomy of the outer unit

The outer unit consists primarily of phasic muscles (Table 1), although there are many muscles such as the oblique abdominals, quadratus lumborum, hamstrings and adductors which serve a dual role, acting in a tonic role as stabilizers and a phasic role as prime movers. To be technically correct, we may say that outer unit functions are predominantly phasic functions (geared toward movement).  

Superficial to the musculature of the inner unit are the outer unit systems, sometimes referred to as slings. The Deep Longitudinal System (DLS) is composed of the erector muscles of the spine and their investing fascia. The spinal erectors communicate with the biceps femoris through the sacrotuberous ligament of the pelvis and to the lower extremity via the peroneus longus muscle (Figure 1).

The Posterior Oblique System (PS) or sling consists primarily of the latissimus dorsi and the contralateral gluteus maxim us (Figure 2).    

The Anterior Oblique System (AS) consists of a working relationship between the oblique abdominal muscles and the contralateral adductor musculature and the intervening anterior abdominal fascia (Figure 3).   

The Lateral System (LS) (Figure 4) consists of a working relationship between the gluteus medius, gluteus minimus and ipsilateral adductors (1,3). Porterfield and DeRosa (3) indicate a working relationship between the gluteus medius and adductors of one leg with the opposite quadratus lumborum. The author's clinical experience strongly suggests that the oblique musculature is synergistic with the quadratus lumborum during lateral sling functions such as those seen in Figure 4.  



The deep longitudinal and posterior systems

 To better understand how the DLS and PS function, we will explore their actions in what is certainly one of our most primal movement patterns, gait (walking). While walking, there is a consistent low level activation of the inner unit muscles to provide the necessary joint stiffness to protect the joints and support the actions of the larger outer unit muscles. Recruitment of the inner unit muscles will fluctuate in intensity as needed to maintain adequate joint stiffness and support, as the inertial forces of limb movement, kinetic forces and intradiscal pressures increase.  

As we walk, we swing one leg and the opposite arm forward in what is termed counter rotation. Just prior to foot strike, the hamstrings become active . The DLS, uses the thoracolumbar fascia and paraspinal muscle system to transmit kinetic energy above the pelvis, while using the biceps femoris as a communicating link between the pelvis and lower extremity. For example, Vleeming shows that the biceps femoris communicates with the peroneus longus at the fibular head, transmitting approximately 18% of the contraction force of the biceps femoris through the fascial system into the peroneus longus.  

Interestingly, the anterior tibialis, like the peroneus longus, attaches to the plantar side of the proximal head of the first metatarsal. The significance of this relationship is appreciated when considering that there is recruitment of the biceps femoris and the anterior tibialis just prior to heel strike in concert with the peroneal muscles, which act as dynamic stabilizers of the lower leg and foot. Dorsiflexion of the foot and activation of the biceps femoris just prior to heel strike, therefore, serves to "wind up" the thoracolumbar fascia mechanism as a means of stabilizing the lower extremity and storing kinetic energy that will be released during the propulsive phase of gait (4). 

As you can see by observing Figure 2, just prior to heel strike the gluteus maximus reaches maximum stretch as the latissimus dorsi is being stretched by the forward swing of the opposite arm. Heel strike signifies transition into the propulsive phase of gait, at which time the gluteus maximus contraction is superimposed upon that of the hamstrings. Activation of the gluteus maximus occurs in concert with activation of the contralateral latissimus dorsi, which is now extending the arm in concert with the propelling leg. The synergistic contraction of the gluteus maximus and latissimus dorsi creates tension in the thoracolumbar fascia, which will be released in a pulse of energy that will assist the muscles of locomotion, reducing the metabolic cost of gait.

The anterior oblique system

The concept of the Anterior Oblique System (AS Figure 3) appears to have become popular very recently. A review of the literature shows that spiral concept of muscle-joint action was understood as integral to human movement and corrective exercise by Robert W. Lovett, M.D. and by anatomist Raymond A. Dart in the early 1900's.  

To clarify the point that movement originates in the spine (core), Gracovetsky describes torque generation by an S-shaped spinal column. He exemplifies the point that the legs are not responsible for gait, but merely instruments of expression, by showing that a man with no legs whatsoever can walk. In both the examples of what Gracovetsky calls the spinal engine, it is evident that the kinetic and potential energies of the oblique abdominal musculature, in concert with other core muscles, are primarily responsible for creating the torque that drives the spinal engine; the oblique abdominal being best situated to create rotary torque.  

The oblique abdominals, like the adductors, serve to provide stability and mobility in gait. When looking at the EMG recordings of the oblique abdominals during gait and superimposing them upon the cycle of adductor activity in gait demonstrated by Inman, it is clear that both sets of muscles contribute to stability at the initiation of the stance phase of gait, as well as to rotating the pelvis and pulling the leg through during the swing phase of gait. As the speed of walking progresses to running, activation of the anterior oblique system becomes more prominent.  

The AS is very important, particularly in sprinting, where the limbs and torso must be accelerated. The demands on the AS are great in multi-directional sports such as tennis, soccer, football, basketball and hockey. In such sporting environments the AS must not only contribute to accelerating the body, but also to changing direction and decelerating it. One need not see an EMG study to appreciate the strong contribution of the AS; just ask anyone that has experienced an abdominal strain! Accelerating, decelerating and changing directions are all activities that result in immediate pain in the presence of both abdominal and groin strains or tears.  

AS functions can be appreciated when running in sand. Because sand gives away during the initiation of the stance and propulsive phases of gait, the impulse timing of ground reaction forces is disrupted, resulting in poor use of the thoracolumbar fascia, or what Margaria calls the smart spring system. The result is that you must muscle your way through the sand. Many athletes having performed sand sprints, will note abdominal fatigue in the following day or two after the sand sprints. This is due to the increased activation of the AS to compensate for the lost kinetic, potential and muscular energy, which is usually stored and released in part by the thoracolumbar fascia system. Gracovetsky states that wearing soft soled sporting shoes, as athletes often do today, can easily disrupt the body's timing mechanism, which could very well result in increased work and may result in injury.  

During explosive activities, such as swinging a sledge hammer (Figure 5), the AS serves critical function, stabilizing as in gait, yet assisting in propelling the hammer. Trunk flexion and rotation, as a closed chain movement atop of the lead leg, is generated by the adductors as they assist in trunk flexion and internal rotation of the pelvis and assisted by gravity. Activation of the adductors occurs in concert with activation of the ipsilateal (stance leg side) internal oblique and contralateral (throwing arm side) external oblique, pulling the trunk in the necessary direction to propel the shoulder/arm complex. The forces of the shoulder/arm unit summate with those of the legs and trunk below to produce a powerful hammer swing. Here one can clearly see the phasic functions of the AS at work.

 The lateral system

Porterfield and De Rosa (3) suggest that functional anatomy dictates that the lateral system provide essential frontal plane stability. While walking, the LS will be active at heel strike (initiation of stance phase), providing frontal plane stability. This is accomplished by a force-couple action between the gluteus medius and minimus pulling the iliac crest toward the stable femur while the opposite quadratus lumborum and oblique abdominal musculature assist by elevating the ilium. This action is necessary to help create the freeway space needed to swing the leg in gait, particularly when you consider the terrain we ambulated across during developmental years. 

During functional activities such as participating in Step class (Figure 4) or simply walking up stairs (Figure 6), the LS plays a critical role, stabilizing the spine in the frontal plane. Stability in the frontal plane is very important to the longevity of the lumbar spine because frontal plane motions of the lumbar and thoracic spine are mechanically coupled with transverse plane motions; excessive amounts of either will quickly aggravate spinal joints.    

The LS provides stability that not only protects the working spinal and hip joints, but is a necessary contributor to overall stability of the pelvis and trunk. Should the trunk become unstable, the diminished stability will compromise ones ability to generate the forces necessary to move the swing leg quickly, as required by many work and sports environments. Attempts to move the swing leg, or generate force with the stance leg during gait and other functional activities, can easily disrupt the sacroiliac joints and pubic symphysis and cause kinetic dysfunction in joints throughout the entire kinetic chain.     

A classic example of distal expression of LS dysfunction was illustrated by Sahrmann. She described a lateral shift of an athlete's center of gravity over the subtalar joint while going through the stance phase of gait (Trendelenburg's Sign) resulting in an inversion ankle sprain. Since attending her course in 1992, the author has found gluteus medius weakness and contralateral low back pain due to quadratus lumborum overload common among athletes exhibiting recurring ankle sprain.  


Although the outer unit is thought of as a phasic system, (a system for moving the body) by most, it does provide crucial stabilizer functions. We must remember that the muscles of the inner unit are relatively small, with less potential to generate force than the large outer unit muscles.

The inner unit muscles are concerned with providing joint stiffness and segmental stability. They work for extended periods of time at low levels of maximal contraction. The outer unit muscles, while very well oriented for moving the body, are also very important to stability, often serving to protect the inner unit muscles, spinal ligaments and joints from damaging overload. For example, consider this common scenario:

The coach instructs two football players to engage in an oblique medicine ball toss drill. One player is much bigger and stronger than the other, as the other player finds out as he attempts to catch the 8 kg. (17.5 lbs.) medicine ball traveling at him at over 60 kph (40mph)! The smaller player does not have the strength in his outer unit to decelerate the ball and is forced into end-range trunk flexion and rotation, traumatizing his lower lumbar discs, ligaments and intrinsic spinal muscles (multifidus, rotatores, intertransversarii and interspinales).

Regardless of how well conditioned the inner unit of the smaller player may have been, lack of strength in his outer unit relative to his partner, or the demands of the task at hand resulted in inner unit overload and injury! With careful scrutiny of most activities in the work or sports environment, you will find that good eccentric strength in the outer unit systems is critical to protecting the inner unit from damage. Protection of the inner unit through proper conditioning of the outer unit is a worthy goal when one considers that optimal proprioception is dependant upon the health of the inner unit muscles and the joints they protect!   


Now that we have taken a detailed look at the anatomy and function of the outer unit, it should be clear that modern exercise technology has taken us a long way from conditioning the outer unit systems the way they were designed to work! For example, can you see any way the following exercises condition the outer unit systems in such a way that they could provide carryover to most functional work or sport activities?    

  • Crunch on Floor
  • Crunch Machines of all types
  • Sit-up
  • Hanging Leg Raises of All Types
  • Bench Press
  • Leg Press
  • Seated Row Machines?

I could go on, filling the page with exercises that do very little to enhance function. Many of you will no doubt recognize the above exercises as traditional bodybuilding exercises. What has happened? Only a few years back in the days of Bill Pearl, bodybuilders were building beautiful physiques with functional exercises like squats, lunges, barbell rows, cable rows, deadlifts and the like. Today, we are overrun by the machine era, the era of the aesthetic emotional hook so carefully used by the machine manufacturers to convince you that you will look better using their machines.    

Our bodies were not designed to exercise on machines, they were designed to function in the wild. We are designed for three-dimensional freedom, not two dimensional guided, unrealistic exercise that encourages muscle imbalance between those muscles used to stabilize and those used in a phasic manner for any given movement. The motor programs developed on machines are of little use to the body for anything other than pushing or pulling the levers of that very machine during that very exercise. This limits functional carryover to those that operate cranes, excavators, bulldozers, and buses for a living; they are about the only people that must apply force to levers in a seated, supported, two-dimensional environment.    


Using your new understanding of the outer unit systems, carefully analyze such functional pushing and pulling exercises as the single arm standing cable row (Figure 7) and standing single arm cable push (Figure 8). You will see all outer unit systems being conditioned simultaneously, jU5t as they are used in most of our work and sport environments. 



Medicine ball exercise, like free weight training, was much more popular in the 40s, 50s, 60s, and 70s than it is today. Great athletes of those decades used exercises such as the oblique medicine ball toss and push-pass, not to mention almost 100 other variations of medicine ball exercises.  

The Swiss Ball can be used to effectively condition the outer unit systems in three-dimensional movement while providing unloading opportunities for those recovering from injury. For example, analyze the Supine Lateral Ball Roll (Figure 9) and see if you can determine which outer unit systems are being used and categorize them in the order of demand during this exercise. This will be a great start toward a better understanding of functional exercise.  



The outer unit consists of four systems, the deep longitudinal, posterior oblique, anterior oblique and lateral. These systems are dependent upon the inner unit for the joint stiffness and stability necessary to create an effective force generation platform. Failure of the inner unit to work in the presence of outer unit demand often results in muscle imbalance, joint injury and poor performance. The outer unit cannot be effectively conditioned in patterns of movement that carryover to function when using modern bodybuilding machines. Effective conditioning of the outer unit should include exercises that require integrated function of the inner and outer units, using movement patterns common to any given client's work or sport environment.



Muscle Architecture, Mechanics and Specific Adaptation to Resistance Training

By: Thrasivoulos Paxinos, M.Sc., Ph.D. Athens College of Sport Sciences.
Site Link: Coachr.


Skeletal muscle represents the largest organ of the body. It makes up approximately 40% of the total body weight and it is organized into hundreds of separate entities, or body muscles, each of which has been assigned a specific task to enable the great variety of movements that are essential to normal life:
Each muscle is composed of a great number of subunits, muscle fibers, that are arranged in parallel and typically extend from one tendon to another. In order to understand the performance of muscle it is essential to know the properties of the individual fibers. With the laboratory techniques now available it is possible to study the contractile behavior of intact single fiber. The single fiber preparation offers the possibility to study the mechanical performance under strict control of sarcomere length. This is of particular importance as the sarcomere length reflects the state of overlap between the two sets of filaments that constitute the main functional elements of the contractile system.

Architecture of Muscle

The muscle fiber is composed of tightly packed subunits, myofibrils, that fill up most of the fiber volume. They contain the contractile element and are therefore the structures within the muscle that are responsible for force generation and active shortening. The basic functional unit of the myofibril is the sarcomere. Many sarcomeres packed together in series form the myofibril. The principal elements in the myofibrillar structure are two sets of filaments of different thickness that show a highly ordered, segmental arrangement that corresponds to the striated appearance of the myofibril.
1. The thicker filaments are made up of a fibrous protein, myosin (Hanson and Huxley, 1953; Hasselbach, 1953).
2. The second set of filaments are mainly built up of a globular protein, actin.

Interaction of these two filaments is the primary cause of muscle contraction and therefore force production. The degree of interaction is controlled by tropomyosin and troponin which are helicaly based on the actin filament.

Sliding Filaments Theory

Our knowledge about the structural organization of the contractile system in the form of two sets of filaments, stems from the pioneering work of H. B. Huxley and J. Hanson (Hanson and Huxley, 1953; Huxley, 1953; Huxley and Hanson, 1954). Their observation that the thick and thin filaments remain constant in length during muscle contraction, while the region of overlap between the two filaments changes with the fiber length, led these authors to suggest that muscle contraction is based on a sliding motion of the two sets of filaments. This idea has now gained general acceptance.
According to this view, the driving force for the sliding motion is generated by the myosin cross-bridges within the region where the thick and thin filaments overlap. The experimental evidence suggests that the myosin bridges make repeated contacts with adjacent thin filaments and that each such contact makes a contribution to the force developed during contraction. This occurs when the fiber is stimulated and calcium is released into the myoplasm from its storage site in the sarcoplasmic reticulum. Adenosine triphosphate offers the energy for the continuous action of the cross-bridges.

Motor Units

The smallest subunit that can be controlled is called a motor unit because it is separately innervated by a motor axon. Neurologically, the motor unit consists of:

  • A synaptic junction in the ventral root of the spinal cord
  • A motor axon, and
  • A motor end place in the muscle fibres. 

Under the control of the motor units are as few as three fibers or as many as 2000, depending on the fineness of the control required. Muscles of the fingers, face and eyes have a small number of shorter fibers in a motor unit, while the large muscles of the leg have a large number of long fibers in their motor units.

Each muscle has a finite number of motor units, each of which is controlled by a separate nerve ending. Excitation of each is an all-or- nothing event. The electrical indication is a motor unit action potential; the mechanical result is a twitch of tension. An increase in tension can therefore be accomplished in two ways:

1. By an increase in stimulation rate for that motor unit, or
2. By the excitation (recruitment) of an additional motor unit.

Recruitment of motor units follows the size principle, which states that the size of the newly recruited motor unit increases with the tension level at which it is recruited (Henneman, 1974). This means that the smallest unit is recruited first and the largest unit last. In this manner low tension movements can be achieved in finely graded steps. Conversely, those movements requiring high forces but not needing fine control are accomplished by recruiting the larger motor units. When maximum voluntary contraction is needed, all motor units will be firing at their maximum frequencies. Tension is reduced by the reverse process: successive reduction of firing rates and dropping out of the larger units first (Milner-Brown and Stein, 1975).

Two types of motor units are present in the muscle.

It must be added that there have been many criteria and varying terminologies associated with the types of motor units present in any muscle. Biochemists have used metabolic or staining measures to categorize the fiber types. Biomechanics researchers have used force (twitch) measures (Milner-Brown et, al., 1973), and electrophysiologists have used electromyographic indicators (Warmolts and Engel, 1973; Milner- Brown et, al., 1975).

Furthermore, a muscle with a higher percent- age of type II fibers reacts with shol1er electro- mechanical delay, time to peak tension and relaxation. Persons having this type of muscles, produce higher maximal speeds and a higher level of force for a cel1ain speed of contraction.

Muscle Cross-Sectional Area

Muscle force is defined as the maximum tension produced during one contraction. This is related to the number of myosin cross-bridges, in parallel formation, which are able to interact with actin and produce tension. Each cross-bridge is a separate factor of force production. When air is present and calcium passes into the fiber, the cross-bridges stal1 a cyclic procedure of attachment to actin, tension production and relaxation. This cyclic procedure is not the same for the different types of muscles (it depends on the type of the heavy meromyosin of the cross-bridges). Scientific results also show that different types of cross-bridges produce various levels of tension. Also, even during maximum contraction, only a pal1 of the whole of cross-bridges is active.
The maximum force that a muscle can generate is directly related to its cross-sectional area (Morris, 1949; Tricker and Tricker, 1967; Ikai and Fukunaga, 1970; Norman, 1977). Hypothesizing that the number of myofibrils of muscle fibers are not significantly different, cross-sectional area is an accurate way to foresee the maximum tension of the muscle.

Mechanical Model of the Muscle

Three elements compose the mechanical model of the muscle influencing its mechanical behavior and effecting contraction:

  1. The contractile element
  2. The series elastic element, and
  3. The parallel elastic element

This model could be useful in order to explain the dynamic properties of the muscle and to understand its mechanical behavior.

The contractile element represents the muscle fibers, which are the active part of the muscle and are competent to produce tension. The parallel elastic element represents the connective tissue surrounding each muscle fiber, groups of fibers and the whole of the muscle. Furthermore, the elastic element represents the elasticity of cross- bridges (Huxley, 1974). They lengthen and respond like a spring. The series elastic element refers mainly to the tendons of the muscle which are placed "in series" with the contractile and parallel elastic elements. Finally, friction is represented on the model by a viscous piston used to explain the passive viscoelastic characteristics of muscle influenced by intracellular and extracellular fluids of muscle fibers.

Types of Muscle Contraction

The term contraction can be thought of as the state of muscle when tension is generated across a number of actin and myosin filaments. Depending on the external load, its direction of action, and its magnitude, contraction has been given different names.

  1. Concentric Contraction refers to the situation in the muscle when the muscle shortens its length during contraction: at a joint the term describes the situation in which the net muscle movement is in the same direction as the change in joint angle. Utilizing concentric exercises, mechanical work is positive.

  2. In Eccentric Contraction muscle is lengthened while it is contracting. The net muscle movement is now in the opposite direction from the change in the joint angle. In eccentric exercises mechanical work is negative.

  3. Isometric Contraction refers to the condition where neither the muscle nor the joint angle changes. The corresponding mechanical work is zero.

For a muscle, eccentric contraction produces the highest tension while concentric the lowest with isometric in between.

Force-Length Relationship of the Muscle

This relationship refers to force production from the muscle depending on its initial length before contraction. According to this, the muscle produces the highest force when it starts contraction from its resting length and possibly with a small elongation. The key to the shape of the force-length curve is the changes of the structure of the myofibril at the sarcomere level (Gordon et. al., 1966). At resting length, there are a maximum number of cross-bridges between the filaments, and therefore, maximum tension is possible. As the muscle lengthens the filaments are pulled apart and the number of cross-bridges and the tension reduces to zero. As the muscle shortens to less than resting length there is an overlapping of the cross-bridges and an interference takes place. This results in a reduction of tension that continues until a full overlap occurs. The tension never drops to zero, but is drastically reduced by these interfering elements.

In the human body the starting length of the muscle is effected by the joint angles. When the joint is in full extension, the extensors are shortened while the flexors are extended. Intermediate joint angles produce different muscle lengths and different force production according to force- length relationship. This has an effect on resistance training programs especially when free weights are used.
The connective tissue that surrounds the contractile element also influences the force- length curve. It is called the parallel elastic component, and it acts much like an elastic band. When the muscle is at resting length or less, the parallel elastic component is in a slack state with no tension. As the muscle lengthens, the parallel element is no longer loose, so tension begins to build up, slowly at first, and then more rapidly. Unlike most springs, which have a linear force- length relationship, the parallel element is quite nonlinear. The passive force of the parallel element is always present, but the amount of active tension in the contractile element at any given length is under voluntary control. Thus the overall force-Iength characteristics is a function of percent of excitation. 

Series Elastic Element and Electromechanical Delay

The relative speed of elongation of the series elastic element seems to be the most important factor for the electromechanical delay observed in the muscle. It is defined as the time lag between the onset of electromyographic activity and tension in the muscle. Other factors associated with electromechanical delay are conduction of the action potential in the t-tubulus system, release of calcium from the sarcoplasmic reticu- lum and the subsequent formation of the cross- bridges between actin and myosin filaments. These events are likely to be short when com- pared to the rate of lengthening of the series elastic element, which might be the primary cause for the value of electromechanical delay in a given muscle.

In isometric contraction the force is generated through the action of contractile element on the series elastic element, which is stretched (Braun- wald et. al., 1967). Concentric contraction, where the load is attached to the end of the muscle, is always preceded by an isometric type of contraction with rearrangements of lengths of contractile and series elastic elements. The final movement begins when the pulling force of contractile element on the series elastic element equals, or slightly exceeds, that of the load.

Electromechanical delay is shorter during eccentric contraction in comparison to concentric (Komi, 1973; Komi and Cavanagh, 1977). This can partially be explained by the fact that during eccentric contraction the direction of lengthening of series elastic element is the same with the action of the contractile element. The reverse is the case for concentric contraction. This is also one of the factors for greater tension production with eccentric contraction.

Force-Velocity Relationship of the Muscle

This relationship refers to force production from the muscle according its speed of contraction. The tension in a muscle decreases as it shortens under load (concentric contraction) while the reverse is true in eccentric contraction (muscle lengthens under load).

During concentric contraction, the decrease of tension as the shortening velocity increases has been attributed to two main causes:

Such viscosity requires internal force to overcome and therefore results in a lower tendon force. There is relatively little knowledge about the details of the force-velocity curve as the muscle lengthens (eccentric contraction). Experimentally, it is somewhat more difficult to conduct experiments involving eccentric work because an external device must be available to do the work on the human muscle. The reasons given for the forces increasing as the velocity of lengthening increases are similar to those that account for the drop of tension during concentric contractions. Within the contractile element it is understood that the force required to break the cross-bridges protein links is greater than that required to hold it at its isometric length, and that this force in- creases as the rate of breaking increases. Furthermore, the viscous friction of shortening is still very much present. However, because the direction of shortening has reversed, the tendon force must now be higher in order to overcome the damping friction. 

Specific Adaptation to Resistance Training

Most of the studies exploring this area use isokinetic contraction. One of the unique features of isokinetic training is that the speed of movement may be controlled during the exercise. This is perhaps the most important feature of isokinetic training as related to sports training since, in most sports activities, muscular force is applied during movement at various speeds. The force-velocity relationship is shifted upward and to the right in athletes, particularly those whose muscles contain a high percentage of fast twitch fibers. In view of the fact that fiber type distribution cannot be changed through training, studies try to answer the question if the force-velocity curve can be shifted upward and to the right following isokinetic resistance training.

The results of these studies present the following conclusions:

Following the above conclusions, specific needs can be tackled accordingly. However, in order to shift the entire curve, fast-speed isokinetic training must be used.


Bergstrom, J. (1962) Muscle Electrolytes in Man, Scandinavian Journal of Clinical Laboratory Investigation, Suppl. 68

Braunwald, E., J. Ross and E. H. Sonnenblick (1967) Mechanisms of Contraction of Normal and Failing Heart, New England Journal of Medicine, 277: 853-863

Burke, R. E. and V. R. Edgerton (1975) Motor Unit Properties and Selective Involvement in Movement, Exer. Sports Science Review; 3: 31-81

Di Prampero, P .E. (1985) Metabolic and Circulatory Limitations to VO2max at the Whole Animal Level, Journal of Experimental Biology, 115, 319-332

Ehashi S. (1980) Regulation of Muscle Contraction, Procedures of the Royal Society B ., 207, 259-286

Edman, K.A.P. (1988) Double- Hyperbolic Force -Velocity Relation in Frog Muscle Fibers, Journal of Physiology, 404, 301-321

Frost, H.M. (1973) Orthopaedic Biomechanics, Charles C. Thomas, Springfield, IL

Gordon, A.M., A.F. Huxley and F. J. Julian (1966) The Variation in Isometric Tension with Sarcomere Length in Vertegrate Muscle Fibers, Journal of Physiology, 184: 170

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Hanson, J. and Huxley, H.E. (1953) The Structural Basis of the Cross-Striations in Muscle, Nature, 172, 530-2

Hasselback, W. (1953) Elektrommikroskopische Untersuchungen and Juske!flbrillen beitota!er und partieller Extraktion des L-Myosins, Zeitschrift fur Naturforschung, 8b, 449-54

Hill, A.V. (1938) The Heat of Shortening and the Dynamic Constants of Muscle, Proc. R. Sac. Land., 126B: 136-195

Huxley, A.F. (1953) Electron-microscope Studies of the Organization of the Filaments in Striated Muscle, Biochimica et Biophysica Acta, 12, 387

Huxley, H.E. and Hanson, J. (1954) Changes in the Cross-striations of Muscle During Contraction and Stretch and their Structural Interpretation, Nature, 173, 973-7

Huxley, A.F. (1974) Muscular Contraction, Journal of Physiology (London), 243: 1-43

Ikai, M. and Fukunaga T. (1968) Calculation of Muscle Strength Per Unit Cross-sectional Area of Human Muscle by Means of Ultrasonic Measurement, Int. Z. angew, Physiology, 26, 26-32

Ikai, M. and Fukunaga T. (1970) A Study of Training Effect on Strength Per Unit Cross-sectional of Muscle by Means of Ultrasonic Measurement, Int. Z. angew, Physiology, 28, 173-180

Komi, P. V. (1973) Measurement of the Force-Velocity Relationship in Human Muscle Under Concentric and Eccentric Contractions, Biomechanics III, Ed. by S. Cergniglini, Karger, Basel, 224-229

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Strength Assessment

By: Matt Brzycki.
From: Coachr.

Brzycki is the coordinator of health, fitness, strength and conditioning programs at Princeton University. He has authored three books, co-authored another and written more than 180 articles on strength and fitness for 33 different publications.

Strength tests are used for a variety of populations, from professional athletes to recreational fitness enthusiasts. The main reasons for performing strength tests are to evaluate initial strength levels and to assess changes in strength. Regardless of the reason for testing muscular strength, trainers and other staff need to perform testing in a safe, efficient manner. Examined here are some traditional forms of strength testing, as well as some alternate ways to test to ensure accuracy and the test's safety.

The history of strength assessments

Strength tests and measurements began in the U.S. around 1860. At that time, the major focus was on anthropometric measurements, such as size and symmetry. Around 1880, physical testing shifted from anatomical proportions to muscular strength. Then, in the 1920s, new and more scientific test methods were developed, and statistical techniques for data analysis became available. Through the years, a few specific types of strength tests have become the most popular. 

Traditional 1-RM testing requires skill, proper warm-up, instruction, supervision and practice.

Traditional test methods

Two basic types of strength tests have evolved: static and dynamic. In a static (or isometric) test, a muscle exerts tension against a fixed, nonmoving resistance. In a dynamic (or isotonic) test, one or more body parts moves against a resistance.

Strength testing has gradually become more sophisticated. Now tests can be conducted in a formal, scientific setting - such as a laboratory or sports medicine facility with equipment ranging from relatively simple dynamometers and tensiometers to more elaborate isokinetic and motor- driven testing devices. Some equipment can even provide both static and dynamic tests that measure strength at different joint angles over a full range of motion, and then plot a "strength curve" with an incredible degree of accuracy. Unfortunately, such scientific testing can be expensive and involves a considerable amount of time. In addition, sophisticated scientific tests are usually not practical to assess a large number of individuals.

Fortunately, there is a more convenient way to assess muscular strength without the drawbacks of elaborate scientific testing. Since these easier assessments are performed outside of a formal scientific setting, they are referred to as "field tests." Field tests represent simple, convenient, easy-to-administer methods of measurement that require a minimum amount of time, cost and equipment. For these reasons, many strength and fitness professionals rely on field tests to assess muscular strength.

The most popular (and traditional) way to assess dynamic strength is to determine how much weight an individual can lift for one repetition. A one-repetition maximum (1- RM) is usually performed using three or four exercises that are representative of the body's major muscle groups. For example, a bench press or an incline press is typically used to assess the strength of the chest, shoulders and triceps, while a squat or a leg press is often used to measure the strength of the hips and legs.

Traditional 1-RM testing

The traditional way to test strength using a 1-RM raises a number of concerns. One reservation is that performing a 1-RM is a highly specialized skill, requiring proper warm-up, instruction, supervision and practice.1 In addition, traditional1-RM testing can be time-consuming, due to the number of warm-up sets that are required to prepare for the maximal attempt. These problems are magnified when evaluating a large group of people. 1,2 

Another concern with traditional1-RM testing is an increased risk of musculoskeletal injury.l,2,3,4 Attempting a 1-RM with a maximal or near-maximal weight can place an inordinate amount of stress on muscles, bones and connective tissues. Injuries occur when the stress exceeds the tensile strength of these structural components. The concern for safety increases when testing certain populations, such as younger adolescents and older adults who are at greater risk for orthopedic injury.1

Fitness professionals must identify a means to assess the muscular strength of their clients that is safe and efficient, but also inexpensive, practical and reasonably accurate.

Strength and anaerobic endurance

To discuss alternate ways to test strength, it's necessary to distinguish between strength and anaerobic endurance. In basic terms, strength is the ability to exert force, and maximal strength is a measure of the ability to exert force during a single muscular contraction with a maximal load. In contrast, anaerobic endurance is the ability to exert force during successive muscular contractions with a submaximal load. It is important not to confuse anaerobic endurance with cardiovascular endurance. Anaerobic endurance is a short-term, high-intensity muscular effort---less than about two minutes; cardiovascular endurance involves muscular effort for a much longer duration. 

Strength and anaerobic endurance are highly related.4 A review of strength-training literature indicates that there is a direct relationship between reps-to-fatigue and the percentage of maximal load (or weight): As the percentage of maximal load increases, the number of repetitions decreases in an almost linear fashion.5

Data also suggests that 10 reps-to-fatigue could be performed with a weight equal to approximately 75 percent of a maximal load.5 For example, if your 1-RM is 200 pounds, then you should be able to perform 10 reps-to-fatigue with 150 pounds (75 percent of 200). Expressed in other terms, if your maximal strength is 200 pounds, then a measure of your anaerobic endurance is your ability to perform 10 repetitions with 150 pounds before experiencing muscular fatigue. This would also be known as your 10-repetition maximum (10-RM).

Unless you have an injury or other musculoskeletal disorder, the relationship between your muscular strength and your anaerobic endurance remains relatively constant.4 Therefore, regardless of whether your strength increases or decreases, you should always be able to perform the same number of repetitions with a given percentage of your 1-RM. This also suggests that if you improve your 1-RM by 20 percent, then your 10-RM should also improve by 20 percent. Conversely, if you increase your anaerobic endurance, then you also increase your muscular strength. So, if you improve your 10-RM by 20 percent, then your 1-RM should also improve by 20 percent. Keep in mind, however, that the actual improvement in a 1-RM may be less if you haven't practiced the requisite skill in performing a 1-RM.

Implications for testing

Since there is a direct relationship between anaerobic endurance and strength, you can determine anaerobic endurance by measuring strength, and determine strength by measuring anaerobic endurance. Though it doesn't directly measure pure maximal strength, testing anaerobic endurance is much safer than attempting a 1-RM because it involves submaximal loads.

A number of prediction equations have been developed and used to estimate a 1-RM based on the relationship between strength and anaerobic endurance. While some of the equations have proven to be reasonably accurate, one problem with them is that they do not take into consideration individual differences.2,3

Genetic influences on testing Each individual inherits a different potential for improving muscular size and strength, cardiovascular' endurance, anaerobic endurance and other physical attributes. Indeed, a person's physical profile is largely determined by several inherited characteristics, including the ratio of fast-twitch (FT) to slow-twitch (ST) muscle fibers, limb length and neurological ability.

Because of these genetic influences, especially muscle fibers, some people perform either less than or more than 10 reps-to-fatigue with 75 percent of their maximal strength. Westcott reported data on 141 subjects who did a test of anaerobic endurance with 75 percent of their 1-RM.6 (Remember, it has been suggested that an individual could do 10 reps-to-fatigue with this workload.) According to the data, the subjects completed an average of 10.5 repetitions. However, only 16 of the 141 subjects (11.35 percent) did exactly 10 reps-to-fatigue with 75 percent of their 1-RM. Many of the subjects were within a few repetitions of 10. In fact, 66 of the subjects (46.81 percent) were able to do between eight and 13 reps-to-fatigue. On the other hand, 75 of the subjects (53.19 percent) did either less than eight reps-to-fatigue or more than 13. At the extremes, two subjects did only five reps-to-fatigue and one managed 24.

If predicting a 1-RM from reps-to-fatigue is to be as accurate as possible, individual differences in anaerobic endurance must be considered. There are several ways to determine an individual-specific estimate of a 1-RM.

1-RM and anaerobic endurance tests.

One way to obtain an individual-specific estimate of a 1-RM is to perform actual tests of muscular strength and anaerobic endurance. To do this, first determine the maximal weight that you can lift for one repetition using good technique. Next, assess your anaerobic endurance by taking 75 percent of your 1-RM and performing as many repetitions as possible using good technique. For instance, if your 1-RM is 200 pounds, do a set with 150 pounds (75 percent of 200). Suppose that you are able to do eight reps-to-fatigue with 75 percent of your 1- RM (instead of 10 reps-to-fatigue as has been suggested). You have just established an individual-specific relationship between your strength and anaerobic endurance based upon your inherited characteristics. More specifically, you now know that you can do eight reps-to-fatigue with 75 percent of your 1-RM. In the future, you can estimate your 1-RM based upon your inherited characteristics by dividing the most weight you can lift for eight repetitions by 0.75.

A two-set prediction equation.

Another approach to attain an individual-specific estimate of a 1-RM is to use a prediction equation. The most frequently used prediction equations are based on the reps-to-fatigue done in one set.2,3 However, a test using one set does not account for individual differences in anaerobic endurance. A better way to assess muscular strength from anaerobic endurance is to use a prediction equation that is based on the reps-to-fatigue obtained in two sets. A two-set prediction equation is shown in Figure 1.

To illustrate the equation, guesstimate a weight that will allow you to reach muscular fatigue in approximately four or five repetitions. On a later date, guesstimate a weight that will allow you to reach muscular fatigue in approximately nine or 10 repetitions. It doesn't really matter how many repetitions you do in the two sets, as long as you do not exceed 10. Now, suppose that you did five reps with 165 pounds in the first set and 10 reps with 135 pounds in the subsequent set. Inserting these values into the equation yields an individual-specific predicted 1-RM of 189 pounds.

l-RM graphing method

A final way to make an individual-specific estimate of a 1-RM is to use a graph and plot the reps-to-fatigue obtained in two sets. On a sheet of graph paper, draw a vertical line down the left-hand side of the page. Starting at the bottom of this vertical line, draw a horizontal line across the page. Label the vertical line "weight" and mark off five- or 10-pound increments; label the horizontal line "reps-to-fatigue" and mark off 10 increments, numbering them from one to 10. The intervals between the numbers on both lines must be equidistant.

Once the graph is set up, perform two sets with the same guidelines as previously stated. On the graph, plot the weight that you used and the number of reps-to-fatigue that you did in both sets. Using a ruler, connect these two points and extend this line to the left until it intersects the vertical line that designates one repetition. This extrapolation is an individual-specific estimate of your 1-RM.

An application of the graphing method appears in Figure 2. In this instance, consider again that you did 5 reps with 165 pounds and 10 reps with 135 pounds. When these two points are plotted on the graph and the line is extrapolated to the left, it yields a predicted 1-RM of 189 pounds---the same maximum that was estimated by the two-set prediction equation.

Implications for training

There is not currently any consensus on the percentage of maximal weight that is necessary to stimulate optimal gains in strength. For the moment, however, imagine that it is 75 percent. According to the study by Westcott, this workload appears to allow an average of about 10 reps-to-fatigue.6 Recall, though, that his data also showed that many individuals can do either less than or more than 10 reps-to-fatigue. These individual differences in anaerobic endurance suggest the need to customize repetition ranges to maximize the response to strength training. For example, those who cannot do more than 10 reps-to-fatigue with 75 percent of their 1-RM have a relatively low level of anaerobic endurance (and likely a high percentage of fast-twitch muscle fibers). These individuals would benefit more by training with slightly lower repetition ranges. Conversely, those who can do more than 10 reps-to-fatigue with 75 percent of their 1-RM have a relatively high level of anaerobic endurance (and likely a high percentage of slow-twitch muscle fibers). These individuals would benefit more by training with slightly higher repetition ranges. This is not to say that 75 percent is the optimal workload for stimulating increases in strength. The use of 75 percent of a 1-RM is only to illustrate the point about the need for individualized repetition ranges. 

There are also implications for pre-planned or "periodized" workouts that demand specific numbers of repetitions with certain percentages of a 1-RM. For instance, a workout might requite individuals to perform 10 repetitions with 75 percent of their 1-RM. Because of wide variations in anaerobic endurance, however, such a prescription might be far too easy for some and literally impossible for others. Therefore, pre-planned workout schedules that stipulate the same number of repetitions with a specific percentage of maximal load for everyone may be minimally effective, except for the segment of the population who have a particular level of anaerobic endurance that corresponds exactly to the specifications and parameters of the training prescription. 


1. LeSuer, D., andJ. McCormick. An alternative to 1 RM strength testing. Unpublished paper, 1993. 

2. Ware, J ., et al. Muscular endurance repetitions to predict bench press and squat strength in college football players. Journal of Strength and Conditioning Research 9: 99-103, 1995.

3. Mayhew, J., J. Prinster, J. Ware, et al. Muscular endurance repeti- tions to predict bench press strength in men of different train- ing levels. Journal of Sports Medidne and Physical Fitness 35: 108-113, 1995. 

4. Carpinelli, R.N. How much can you bench press? High Intensity Training Newsletter 5: 2-3, 1994. 

5. Sale, D.G., and D. MacDougall. Specifidty in strength training: A review for the coach and athlete. Canadian Journal of Applied Sport Sdences 6: 87-92, 1981.

6. Westcott, W. Building Strength and Stamina. Human Kinetics: Champaign, Ill., 1996.




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