Dr. Brown, Chair of USATF's Sports Medicine and Sciences Committee, referred to this in a note to us as a "think piece." He calls for coaches and scientists to continue this kind of study. This is a well-documented article, which may lead the interested coach to investigate further. We welcome response to this important article.
Strength training to enhance sports performance and improve fitness is now a common means of exercise for women. It has progressed to the point that there is now a world championships in weightlifting for women.
For many years resistive exercises for women were shunned for fear of these athletes becoming "masculinized" through the use of heavy weights. However, early studies showed that women were able to exhibit considerable improvements in strength with only minimal degrees of muscle hypertrophy (2). These researchers pointed out that the likelihood of major muscle hypertrophy from resistance training was small in comparison to males, as women have blood levels of the anabolic hormone testosterone which are only 5-10 per cent of those of men.
Many subsequent studies have borne out these early findings. Further, resistance training itself does not appear to increase basal levels of testosterone in women, and strength gains are not correlated with blood testosterone levels (3-5).
The endocrine aspects of exercise science have increased greatly in recent years, especially in the areas associated with resistance training. Assessing the roles of the various hormones as to the cause-and- effect relationships in response to any exercise stimulus can be very complex.
Hormonal levels in blood and tissues are influenced by their production from the parent organ, clearance from the blood by the liver, kidneys, and other peripheral tissues, and their binding to specific receptor sites in target organs.
In addition, steroidal hormones such as androgens, adrenal hormones, and ovarian hormones circulate in the blood bound to specific carrier proteins, with only a tiny fraction in the "free" form which is available to tissues.
Evaluation of the numerous studies which have been carried out concerning the responses of the endocrine system are further complicated by the variety of test protocols which have been utilized. Aerobic vs. resistance loading produces different hormonal responses, and even seemingly similar studies may yield different results. The athlete's state of training and nutrition can also influence the metabolic and hormonal outcomes.
There are at least three anabolic hormones which are responsible for muscle hypertrophy: testosterone (and dihydrotestosterone), pituitary growth hormone (GH), and insulin-like growth factor I (IGF-I), formerly called somatomedin-C.
Initially, studies focused on the role of testosterone in response to an exercise stimulus, especially resistive loading. It soon became apparent that, in addition to different basal levels between men and women, the response to exercise is quite different. Following a bout of resistive exercise, the male's testosterone level rises considerably, while in women the values change little, if at all.
Further, the disposition of testosterone in the body differs between the sexes. In males, about 50 per cent of the testosterone is bound to receptors in muscle, while only about 10 percent is cleared in this manner in women. However, women do show a greater response of the weaker adrenal androgen, androstenedione.
Concerns that resistive training in women raises basal testosterone levels, or that higher basal testosterone levels are accompanied by greater strength gains, have not been borne out.
Growth hormone responds to both aerobic and resistive exercise. Growth hormone stimulates muscle growth by facilitating the transport of amino acids across cell membranes, activating DNA transcription in the muscle cell nucleus, thus increasing the amounts of RNA and protein synthesis.
INSULIN-LIKE GROWTH FACTOR
Insulin-like growth factor I (IGF- I) is a potent anabolic factor. It is believed that growth hormone's effects are mediated through IGF-I. IGF-I is stored in the liver and peripheral tissues. It is released slowly (16-28 hours) after growth hormone stimulation. In those situations in which it was measured, IGF-I levels have risen little or not at all after exercise bouts which have been sufficient to elevate growth hormone concentrations. Further, increases in IGF-I did not seem to correlate with the rises in GH. The reasons for this are not clear.
It would appear from the foregoing that the growth hormone IGF-I complex plays a significant role in the development of muscle hypertrophy and strength in women. Hence, strength programs for women should focus on maximizing growth hormone production.
ELEVATING GH LEVELS
In a series of elegant studies, W. Kraemer, et al. (9-12) examined the hormonal responses to a wide variety of resistive training protocols in both men and women. By varying the resistive load (5-RM vs. l0-RM) and the rest interval (1 minute vs. 3 minutes), they were able to demonstrate considerable differences in the response of several hormones.
In summary, the greatest rises in GH occurred with the protocol in which eight different exercises were used, with three sets of each exercise. The resistance was l0-RM, or approximately 70-75 percent of the l-RM, and the rest interval was one minute between each exercise and between each set.
THE EFFECT OF THE MENSTRUAL CYCLE
Similar responses were seen in both men and women, with the women having somewhat higher baseline GH levels, and slightly greater exercise responses. However, in all prior studies by these and other authors, women were studied only during the follicular phase (first half) of the menstrual cycle.
Only recently has the effect of the menstrual cycle on various hormonal responses to resistive training been assessed (6). RR Kraemer, et al. (7, 8) studied the changes in hormonal response which occurred during both the follicular and luteal phases of the cycle in the same group of subjects. These women were subjected to a moderate exercise regimen of three sets of 10 repetitions of four different exercises with a 2- minute rest interval. There was a significantly higher GH response during the luteal phase, as well as much higher estradiol levels. Other studies have suggested that the female hormone estradiol facilitates the release of growth hormone.
TAILORING PROGRAMS TO THE MENSTRUAL CYCLE
These studies suggest that strength training programs for women should be tailored to each athlete's menstrual cycle. Although there have been no studies to validate the effect of these cyclic hormonal variations on muscle growth and strength development, the research findings are strongly suggestive that such a study would be of considerable value.
Until such a study is done, however, imaginative strength coaches should consider devising strength development programs which take into account these hormonal fluctuations which occur during the menstrual cycle. Such considerations might be especially valuable during the basic "hypertrophy" mesocycle of a strength development program.
These programs should consider:
- During the luteal phase (second half) of the menstrual cycle strength training should consist of "moderate intensity" loading, using 3-4 sets of 8-10 repetitions at 65- 75% of the 1- RM, done three times a week. These exercises should involve the large muscle groups of the upper and lower extremities and trunk, i.e., bench press, squats, power cleans, leg press, sit-ups, dead lifts, etc.
- The rest interval between sets and exercises should be no more than two minutes, and preferably shorter.
- A similar routine also may be of value during the follicular phase (first half) of the cycle, although the estradiol and GH responses may be lower.
- f the athlete is using oral contraceptives, no phasic change in GH response can be expected (1) unless the oral contraceptive is of the "tri-phasic" variety. Several studies of athletes using oral contraceptives have been done. These have yielded conflicting results as to whether there is a greater-than-expected GH response, probably because of variations in hormonal strength and type, and in exercise protocol.
- During the "strength" phase of the training cycle (lower repetitions, higher loading), i.e., four sets of five repetitions at 80% of l-RM, a lesser response of estradiol and GH is to be expected, and the training program need not be adapted to the menstrual cycle.
It is hoped that this paper will stir some thought and even controversy in the strength-training community and will lead to further studies and some empirical trials by innovative coaches and scientists.