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Original Research

The Effects of Training Volume and Competition on the Salivary Cortisol Concentrations of Olympic Weightlifters

Crewther, Blair T1; Heke, Taati2; Keogh, Justin W L2

Author Information
Journal of Strength and Conditioning Research: January 2011 - Volume 25 - Issue 1 - p 10-15
doi: 10.1519/JSC.0b013e3181fb47f5
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Abstract

Introduction

Cortisol (C) is an important steroid hormone mediating resistance training adaptations. Traditionally, C is regarded as the primary catabolic hormone, because it decreases protein synthesis and increases protein breakdown (3,40). Likewise, the antianabolic properties of C are linked to the attenuation of anabolic hormones such as testosterone (T) and growth hormone (3). However, changes in C secretion may be a prerequisite for the repartitioning of metabolic resources during exercise and training (40). It is important to note that the majority of C (95-99%) is bound to proteins in blood, with the remaining portion (1-5%) circulating freely, thereby representing the biologically active hormone (9).

Training volume is a key factor influencing endogenous hormones. For example, adjustments in resistance training volume can elevate or reduce C concentrations (13,14,18,20,21,23). Some training studies have also reported correlations between the individual changes in the T/C ratio and strength adaptations (13,22,23). In female weightlifters, the changes in the T/C ratio also negatively correlated to the changes in load volume and to improvements in force production (18). Thus, the physiological stressors imposed by different volumes of training may influence these hormones and subsequent performance. It is possible that these outcomes are largely driven by the stress hormone C.

Competition can also influence the hormonal and performance outcomes in weightlifters. For instance, the salivary C (Sal-C) concentrations of Olympic weightlifters were found to be more than twofold higher during actual competition, than a simulated event (32). The 1 repetition maximum (1RM) lifts were also greater during the actual competition. Similarly, a rise in C concentrations has been implicated in the performance and behavioral outcomes in other sports competitions (12,36-38). In this environment, the psychological stressors of competition may be the prime regulator of C and performance. To our knowledge, no research has examined the effects of both training volume and competition (simulated and actual) on the C concentrations of Olympic weightlifters.

Another limitation of the literature is the amount of time between pre and posttraining assessments, which can span from several weeks to months (21-23). Intuitively, monitoring weekly workouts would allow for a more thorough analysis of those factors mediating training adaptation and how adjustments in 1 variable (e.g., volume) might affect others (e.g., hormones and performance) (5,18). Most studies have also assessed the bound hormone in blood, rather than the biologically active free hormone. Saliva offers a stress-free method for monitoring blood free hormones and the bioavailable portion (i.e., free plus albumin bound) that is potentially available to target tissue (2,16).

This study sought to assess the effects of training volume and competition on the Sal-C concentrations of Olympic weightlifters. It was hypothesized that the weekly changes in Sal-C concentrations would positively mirror the weekly changes in training volume. It was also hypothesized that Sal-C concentrations and 1RM performance would be greater in an actual rather than a simulated competition.

Methods

Experimental Approach to the Problem

This study was designed to evaluate the effects of training volume and competition on the Sal-C concentrations of Olympic weightlifters. The basic design was based on published research (5,18,32). In the first part of this study, Sal-C concentrations were monitored across 5 workouts to assess the weekly effects of changes in training volume. In the second part, Sal-C concentrations and 1RM performance from 2 simulated and 2 actual competitions were compared. Cortisol was assessed in saliva, a surrogate marker of the blood-free hormone (2,16). The snatch and clean and jerk exercises were assessed during competition, these being the main exercises for Olympic weightlifters (28).

Subjects

Five male (age 21.2 ± 3.8 years, height 171.4 ± 4.7 cm, body mass 82.0 ± 22.8 kg) and 4 female (age 23.8 ± 4.6 years, height 161.1 ± 5.5 cm, body mass 68.6 ± 15.3 kg) weightlifters with an average training experience of 4.7 ± 3.1 and 4.6 ± 1.8 years, respectively, volunteered for this study. The best 1RM lifts for the snatch, clean and jerk, and the Olympic total were 119.4 ± 27.8, 150.8 ± 34.6, and 270.2 ± 62.4 kg for men and 66.6 ± 15.8, 83.6 ± 18.8, and 150.3 ± 34.6 kg for women. All subjects had competed in national and international weightlifting competitions before this study, and each had a National ranking in the top 5 for their weight class and age. Subjects were informed of the experimental risks and signed an informed consent form before the start of this investigation. The investigation was approved by an Institutional Review Board (Waikato Institute of Technology, Hamilton, New Zealand) for the use of human subjects.

Training Procedures

The 2 study aims were addressed concurrently across a 5-week period during the competitive season. Subjects were training 1-2 times daily, 5 days a week from Monday to Friday. Each training workout involved 3 main exercises; snatch, clean and jerk, and the front squat. The order of exercises was consistent across each workout, as outlined above. The assessed workouts were performed on the first training day (Monday) of weeks 1-5. Weekly training volume was described as being either ‘high’ or ‘low,’ based on the total number of sets completed across the 3 main exercises (Table 1). The criterion for a high-volume week was the performance of 200 or more sets, with 100 or less sets performed across a low-volume week. Competition data (see below) were included in these calculations. Workout duration differed considerably, depending on whether a high- or low-volume session was undertaken, and generally ranged from 45 to 120 minutes. Despite the weekly differences in volume, a similar loading range or training intensity (% of personal best 1RM lifts) was employed by subjects across each week (Table 1).

Table 1
Table 1:
Training volume across the experimental period.*

Competition Procedures

The competition procedures were based on previous research (32). The simulated competitions were performed under normal training conditions (i.e., venue, time of day, exercises), but longer recovery periods (>5 minutes) were used with greater loading intensities and fewer repetitions to replicate an actual event. Single repetition lifts were performed with increasing loads until a successful 1RM lift was achieved. The simulated competitions were performed on the last training day (Friday) of weeks 1 and 4, with the actual competitions performed at the end (Saturday) of weeks 2 and 5. The snatch and clean and jerk exercises were assessed during the competition settings. The performance results for the Olympic total lift (i.e., sum of the snatch and clean and jerk) were also calculated. To account for gender differences in body size, relative 1RM (rel1RM) performance was calculated by dividing the measured 1RM by fat-free mass in kilograms. The pretraining values for fat-free mass were used to normalize the 1RM data from weeks 1 and 2, with the posttraining values applied to the 1RM data from weeks 4 and 5.

A standard warm-up was performed before training and competition comprising low-intensity aerobic exercise, light lifts (Olympic bar only) focusing on technique, and stretching of the major muscle groups. The assessment of Olympic lifts by trained weightlifters are highly reliable with coefficients of variation from 2.3 to 2.7% (28). Lifting platforms and standard gymnasium equipment were used during the training and competition procedures including squat racks, Olympic barbells and free weights (Eleiko Sports, Halmstad, Sweden). The training workouts and simulated competitions were conducted between 3 pm and 6 pm, with the actual competitions undertaken between 10 am and 4 pm. This difference was unavoidable because of the prior scheduling of training and the weightlifting events. However, the effect of diurnal variations on Sal-C secretion (26) and sporting performance (8) was minimized, to some extent, by ensuring that individuals were assessed at the same time of day during all procedures. There was further consistency during the actual competitions with subjects tested at the same time of day.

Each weightlifter was instructed to maintain their normal dietary intake across this experiment (5). Daily information on nutritional intake and training loads were noted in a diary and examined at the completion of the study. The food consumed by each subject was similar before each training and competition session, as were the frequency and timing of these meals. Thus, the study population exhibited consistent nutritional habits across the experimental period, and this helped to partially offset the nutritional effects on Sal-C (15), because each subject acted as their own control. Subjects also monitored their hydration levels each day by recording fluid intake.

Body Composition

Body composition was assessed on the first and last days of this study using a wall-mounted stadiometer, electronic scales (SECA 702, Birmingham, United Kingdom) and skinfold measurements taken by a qualified anthropometrist using body fat callipers (Holtain, Crymmych, United Kingdom). The sum of 4 skinfolds (i.e., subscapular, suprailliac, biceps, triceps) was converted to a body fat percentage using a standard formula (10). The same tester performed all of the body composition analyses.

Hormone Assessment

The saliva samples (1 ml) were collected before and after the assessed training workouts, simulated competitions, and actual competitions using standard procedures (5,6). Briefly, sugar-free gum (Extra-peppermint, Wrigley's, Auckland, New Zealand) was first used to increase saliva flow for a short period, after which a small saliva sample was collected in sterile containers and stored at −20°C. To prevent saliva contamination, no hot drinks or food were taken 1 hour before each session. Saliva was assayed in duplicate using a commercial enzyme-immunoassay kit (Salimetrics, State College, PA, USA) and the manufacturer's instructions. Cortisol assay sensitivity was 0.012 μg·dL−1 with interassay coefficients of variation ranging from 10 to 13%, based on high and low control samples in each assay. The Sal-C concentrations of men and women are similar both at rest and in response to exercise (11,25,27,39), so the hormonal data were subsequently pooled for analysis.

Statistical Analyses

Changes in body composition from pre to posttraining were assessed using paired t-tests. The effects of weekly training on Sal-C concentrations were examined using a 2-way (workout × sample) analysis of variance with repeated measures. The pooled differences in Sal-C concentrations and 1RM performance between the simulated and actual competitions were compared using paired t-tests. Power calculations revealed values of 0.95-0.97 and 0.39-0.46 for the Sal-C results during competition and training, respectively, based on an alpha level of 0.05 and a sample size of 9 subjects. Relationships between the Sal-C and 1RM competition data were examined using Pearson product moment correlation coefficients. Significance was set at an alpha level of p ≤ 0.05.

Results

There were no significant changes in body mass and fat-free mass across the training period (Table 2). Body fat percentage decreased slightly posttraining (p < 0.05). No significant main workout and sample effects and no interactions were found when examining the weekly changes in Sal-C concentrations with training (p > 0.05, Figure 1).

Table 2
Table 2:
Subject information pre and posttraining.*
Figure 1
Figure 1:
Weekly changes in training volume and salivary cortisol (Sal-C) concentrations (mean ± SD).

No significant main effects or interactions were identified for the Sal-C, and 1RM data across either the simulated or actual competitions (p > 0.05). Thus, data from each competition setting were pooled for analysis. Pre and post Sal-C values were higher during the actual (vs. simulated) competitions (p < 0.001, Figure 2), but no significant changes (from pre to post) in Sal-C occurred across either condition (p > 0.05).

Figure 2
Figure 2:
Pooled differences in salivary cortisol (Sal-C) concentrations between the simulated and actual competitions (mean ± SD). *Significantly different from the simulated competitions p < 0.001.

The pooled 1RM and rel1RM values for the clean and jerk, and the Olympic total lift, were both found to be greater in the actual (vs. simulated) competitions (p < 0.05, Table 3). The snatch 1RM and rel1RM lifts were not significantly different (p > 0.05).

Table 3
Table 3:
Pooled differences in 1RM performance between the simulated and actual competitions.*†

Significant correlations were found between individual Sal-C concentrations before the simulated competitions and the snatch, clean and jerk, and Olympic total 1RM lifts (r = 0.48-0.49, p < 0.05). No significant correlations were identified between these parameters during the actual competitions (r = 0.09-0.012, p > 0.05) or between Sal-C and the rel1RM lifts in either competition (r = 0.16-0.27, p > 0.05).

Discussion

This study on Olympic weightlifters revealed 3 main findings: First, the weekly changes in training volume had no apparent effect on Sal-C concentrations; second, actual competitions produced higher Sal-C concentrations and superior 1RM lifts (clean and jerk, Olympic total) than the simulated competitions; third, individual Sal-C concentrations before the simulated competitions correlated to all of the 1RM lifts.

Adjustments in training volume have been shown to alter C concentrations in male and female weightlifters (13,14,18,20,21,23). In general, the greater the training stress (i.e., higher training volume and intensity) imposed on the neuromuscular system, the greater the C changes occurring. The lack of any significant Sal-C responses over the 5 weeks of this study could be explained by the absolute volume of training, the time of exposure to different training volumes, and the competition phase, relative to previous research. Baseline C concentrations at the start of this study could also be important in this regard. Previous studies also suggest that C modifications may affect the T/C ratio and thus one's ability to express strength and recovery from training and competitions (13,18,22,23). This is difficult to confirm with only C monitored in this study and given the low statistical power of the training results.

The actual weightlifting competitions produced higher (128-130%) Sal-C concentrations than the simulated ones, which could be explained by the greater challenge, motivation, and stress presented by a real event (32,35). Highly trained athletes might also possess an adrenal system that responds more to competition (i.e., greater C response) than lesser trained individuals (32,37). Other competition factors influencing hormone secretion include physical fitness, physical effort or exertion, mood, motivation, coping styles, and player position (35). We recognize that the Sal-C results in this study might be affected by circadian variation (26), because the actual events began earlier in the day than the simulated ones. However, the differences in Sal-C concentrations between the respective competitions were still much greater than time-matched control data (26,39). Nutritional intake is another confounding variable when assessing Sal-C levels (15).

The 1RM lifts in the actual competitions were generally superior (1.9-2.6%) to that seen in the simulated events and this may be explained. Similarly, improvements in the 1RM performance of Olympic weightlifters were noted during actual (vs. simulated) competition when Sal-C also increased by more than twofold (32). A rise in C levels has been observed in many sporting competitions (11,12,25,31,36-38), and these hormonal changes often enhanced the performance and behavioral outcomes. For example, judoists displaying higher Sal-C levels also had higher motivation to perform and obtained the better outcome (36), and higher C levels were found in judo winners when compared with losers (38). and. These data confirm suggestions that C may be essential for working capacity and performance (40). Thus, acute elevations in C may actually benefit athlete performance during competition.

Positive correlations were demonstrated between individual Sal-C concentrations, and the 1RM lifts during the simulated competitions, similar to previous work (5,6,32) This work implies that higher Sal-C concentrations (on an individual level) may also benefit 1RM performance during weight training procedures. No significant relationships were observed during the actual competitions, which could be explained by individual variation in the adrenal responses to the competitive environment, especially since participant C levels were already elevated. Nevertheless, the correlations in this study only approached moderate strength and notwithstanding the fact that they still only reflect casual relationships between variables. Other research on weightlifters have also reported no discernable relationships between these variables (4,13), possibly resulting from differences in training procedures and study design.

The mechanism(s) by which elevated C levels may improve athlete performance is multifactorial. Cortisol may contribute to human and muscle performance by regulating or controlling energy metabolism (40), motor cortex function (34), the electrophysiological properties of muscle (7), and intracellular signals (30). Additionally, C can affect brain neural activity (29) and cognitive function (24,33), and this has further implications for the expression of human movement in sport. Further research is needed to elucidate those C-related mechanisms contributing to athletic performance. This information would help in the management, assessment, and training of athletes within their sporting environment.

Other limitations of this study include the lack of a nontraining control group and the small number of subjects recruited, although the pool of Olympic weightlifters available for research is limited. Indeed, the size of our study population is consistent with that of other studies in this area (1,4,14,17-19,21). We also acknowledge the possible confounding effects of treatment order, or any interactions thereof, given the current study design (i.e., simulated competitions before the actual competitions) and the differences in training volume before the respective competitions (i.e,. high- and low-volume weeks). However, these are inherent problems when working with elite athletic groups so the observed findings do reflect the actual sporting environment. Finally, we acknowledge that other hormones (e.g., T, growth hormone, insulin-like growth factor 1, catecholamines) may help to regulate athlete performance, but their discussion is beyond the scope of this article.

In conclusion, actual competitions produced greater Sal-C responses than simulated competitions, and this appeared to benefit the 1RM performance of Olympic weightlifters. Individuals with higher Sal-C concentrations also tended to exhibit superior 1RM lifts during the simulated competitions.

Practical Applications

The results of this study suggest that higher C concentrations may benefit weightlifting performance. Consequently, greater emphasis should be placed upon the monitoring of C to establish normative values and training standards for individual weightlifters or weight-trained athletes, and to assist with performance prediction in this sport. Researchers and practitioners could potentially use different strategies (e.g., nutrition, physical and psychological activities), or address the timing of exercise (i.e., diurnal variation), to modify C concentrations during training and competition, thereby reducing the potential for overreaching and overtraining while augmenting weightlifting performance.

References

1. Ahtiainen, JP, Pakarinen, A, Alén, M, Kraemer, WJ, and Häkkinen, K. Muscle hypertrophy, hormonal adaptations and strength development during strength training in strength-trained and untrained men. Eur J Appl Physiol 89: 555-563, 2003.
2. Arregger, AL, Contreras, LN, Tumilasci, OR, Aquilano, DR, and Cardoso, EML. Salivary testosterone: A reliable approach to the diagnosis of male hypogonadism. Clin Endocrinol 67: 656-662, 2007.
3. Crewther, B, Keogh, J, Cronin, J, and Cook, C. Possible stimuli for strength and power adaptation: Acute hormonal responses. Sports Med 36: 215-238, 2006.
4. Crewther, BT and Cook, C. Relationships between salivary testosterone and cortisol concentrations and resistance training performance in Olympic weightlifters. J Sports Med Phys Fit 50: 371-375, 2010.
5. Crewther, BT, Lowe, T, Weatherby, RP, and Gill, N. Prior sprint cycling did not enhance training adaptation, but resting salivary hormones were related to workout power and strength. Eur J Appl Physiol 105: 919-927, 2009.
6. Crewther, BT, Lowe, T, Weatherby, RP, Gill, N, and Keogh, J. Neuromuscular performance of elite rugby union players and relationships with salivary hormones. J Strength Cond Res 23: 2046-2053, 2009.
7. Dlouhá, H and Vyskočil, F. The effect of cortisol on the excitability of the rat muscle fibre membrane and neuromuscular transmission. Physiol Bohemoslov 28: 485-494, 1979.
8. Drust, B, Waterhouse, J, Atkinson, G, Edwards, B, and Reilly, T. Circadian rhythms in sports performance-an update. Chronobiol Int 22: 21-44, 2005.
9. Dunn, JF, Nisula, BC, and Rodbard, D. Transport of steroid hormones: Binding of 21 endogenous steroids to both testosterone-binding globulin and corticosteroid-binding globulin in human plasma. J Clin Endocrinol Metab 53: 58-68, 1981.
10. Durnin, JV and Rahaman, MM. The assessment of the amount of fat in the human body from measurements of skinfold thickness. Br J Nutr 21: 681-689, 1967.
11. Edwards, DA, Wetzel, K, and Wyner, DR. Intercollegiate soccer: Saliva cortisol and testosterone are elevated during competition, and testosterone is related to status and social connectedness with teammates. Physiol Behav 87: 135-143, 2006.
12. Filaire, E, Sagnol, M, Ferrand, C, Maso, F, and Lac, G. Psychophysiological stress in judo athletes during competitions. J Sports Med Phys Fit 41: 263-268, 2001.
13. Fry, AC, Kraemer, WJ, Stone, MH, Koziris, LP, Thrush, JT, and Fleck, SJ. Relationships between serum testosterone, cortisol, and weightlifting performance. J Strength Cond Res. 14: 338-343, 2000.
14. Fry, AC, Kraemer, WJ, Stone, MH, Warren, BJ, Fleck, SJ, Kearney, J, and Gordon, SE. Endocrine responses to overreaching before and after 1 year of weightlifting. Can J Appl Physiol 19: 400-410, 1994.
15. Gibson, LE, Checkley, S, Papadopoulos, A, Poon, L, Daley, S, and Wardle, J. Increased salivary cortisol reliability induced by a protein-rich midday meal. Psychosom Med 61: 214-224, 1999.
16. Gozansky, WS, Lynn, JS, Laudenslager, ML, and Kohrt, WM. Salivary cortisol determined by enzyme immunoassay is preferable to serum total cortisol for assessment of dynamic hypothalamic-pituitary-adrenal axis activity. Clin Endocrinol 63: 336-341, 2005.
17. Haff, GG, Carlock, JM, Hartman, MJ, Kilgore, LJ, Kawamori, N, Jackson, JR, Morris, RT, Sands, WA, and Stone, MH. Force-time curve characteristics of dynamic and isometric muscle actions of elite women Olympic weightlifters. J Strength Cond Res 19: 741-748, 2005.
18. Haff, GG, Jackson, JR, Kawamori, N, Carlock, JM, Hartman, MJ, Kilgore, LJ, Morris, RT, Ramsey, MW, Sands, WA, and Stone, MH. Force-time curve characteristics and hormonal alterations during an eleven-week training period in elite women weightlifters. J Strength Cond Res 22: 433-446, 2008.
19. Häkkinen, K, Keskinen, KL, Alén, M, Komi, PV, and Kauhanen, H. Serum hormone concentrations during prolonged training in elite endurance-trained and strength-trained athletes. Eur J Appl Physiol Occup Physiol 59: 233-238, 1989.
20. Häkkinen, K and Pakarinen, A. Serum hormones in male strength athletes during intensive short term strength training. Eur J Appl Physiol Occup Physiol 63: 194-199, 1991.
21. Häkkinen, K, Pakarinen, A, Alén, M, Kauhanen, H, and Komi, PV. Relationships between training volume, physical performance capacity, and serum hormone concentrations during prolonged training in elite weight lifters. Int J Sports Med 8: S61-S65, 1987.
22. Häkkinen, K, Pakarinen, A, Alén, M, Kauhanen, H, and Komi, PV. Neuromuscular and hormonal adaptations in athletes to strength training in two years. J Appl Physiol 65: 2406-2412, 1988.
23. Häkkinen, K, Pakarinen, A, Alén, M, and Komi, P. Serum hormones during prolonged training of neuromuscular performance. Eur J Appl Physiol Occup Physiol 53: 287-293, 1985.
24. Hsu, FC, Garside, MJ, Massey, AE, and McAllister-Williams, RH. Effects of a single dose of cortisol on the neural correlates of episodic memory and error processing in healthy volunteers. Psychopharmacology 167: 431-442, 2003.
25. Kivlighan, KT, Granger, DA, and Booth, A. Gender differences in testosterone and cortisol response to competition. Psychoneuroendocrinology 30: 58-71, 2005.
26. Linder, EE. The effect of heavy resistance exercise on the circadian rhythm of salivary cortisol in women. UWL: J Undergrad Res VI: 1-4, 2003.
27. McGuigan, MR, Egan, AD, and Foster, C. Salivary cortisol responses and perceived exertion during high intensity and low intensity bouts of resistance exercise. J Sports Sci Med 3: 8-15, 2004.
28. McGuigan, MR and Kane, MK. Reliability of performance of elite Olympic weightlifters. J Strength Cond Res 18: 650-653, 2004.
29. Papir-Kricheli, D and Feldman, S. Modifications in single cell activity in the rat midbrain during the iontophoretic application of cortisol. Exp Neurol 79: 576-581, 1983.
30. Passaquin, AC, Lhote, P, and Rüegg, UT. Calcium influx inhibition by steroids and analogs in C2C12 skeletal muscle cells. Br J Pharmacol 124: 1751-1759, 1998.
31. Passelergue, P and Lac, G. Saliva cortisol, testosterone and T/C ratio variations during a wrestling competition and during the post-competitive recovery period. Int J Sports Med 20: 109-113, 1999.
32. Passelergue, P, Robert, A, and Lac, G. Salivary cortisol and testosterone variations during an official and a simulated weight-lifting competition. Int J Sports Med 16: 298-303, 1995.
33. Putman, P, Antypa, N, Crysovergi, P, and Van Der Does, WAJ. Exogenous cortisol acutely influences motivated decision making in healthy young men. Psychopharmacology 208: 257-263, 2010.
34. Sale, MV, Ridding, MC, and Nordstorm, MA. Cortisol inhibits neuroplasticity induction in human motor cortex. J Neurosci 28: 8285-8293, 2008.
35. Salvador, A. Coping with competitive situations in humans. Neurosci Biobehav Rev 29: 195-205, 2005.
36. Salvador, A, Suay, F, González-Bono, E, and Serrano, MA. Anticipatory cortisol, testosterone and psychological responses to judo competition in young men. Psychoneuroendocrinology 28: 364-375, 2003.
37. Snegovskaya, V and Viru, A. Elevation of cortisol and growth hormone levels in the course of further improvement of performance capacity in trained rowers. Int J Sports Med 14: 202-206, 1993.
38. Suay, F, Salvador, A, González-Bono, E, Sanchís, C, Martínez, M, Martínez-Sanchis, S, Simón, VM, and Montoro, JB. Effects of competition and its outcome on serum testosterone, cortisol and prolactin. Psychoneuroendocrinology 24: 551-566, 1999.
39. Vining, RF, McGinley, RA, Maksvytis, JJ, and Ho, KY. Salivary cortisol: A better measure of adrenal cortical function than serum cortisol. Ann Clin Biochem 20: 329-335, 1983.
40. Viru, A and Viru, M. Cortisol-Essential adaptation hormone in exercise. Int J Sports Med 25: 461-464, 2004.
Keywords:

adrenal; athletic; neuromuscular; strength

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