The primary finding of this investigation was that some differences do exist between the acute endocrine response patterns of a bilateral and a dominant-arm unilateral upper-body resistance exercise protocol. The bilateral resistance exercise protocol produced a greater anabolic hormonal response, which was likely a result of the higher volume of work and therefore a larger metabolic demand. The findings of this study indicate that the volume of total work reflected by the amount of muscle mass used will dictate the endocrine signaling patterns to the body tissues. Furthermore, the use of only an upper-body exercise protocol, even when using 80% of 1RM loading, may not be enough to stimulate endocrine release of testosterone from the testicular Leydig cells.
Testosterone may be an important and rapid-signaling molecule for priming the other mechanisms needed for strength increases and muscle hypertrophy from resistance training in men (16,21). When testosterone was reduced to castrate levels, the resistance training-induced increase in isometric strength was eliminated and muscle hypertrophy was attenuated in young men (16). In the present study, testosterone concentrations were not increased for either protocol, yet this may have in part been a result of acute circadian decreases over the hour time frame examined. Other higher volume exercise studies have still observed increases to occur (7,12). A lack of a testosterone response has been shown previously for upper-body unilateral exercise (8,29), but no study has yet examined the testosterone response to bilateral upper-body resistance exercise using dumbbells. In untrained young men, heavy upper-body bilateral barbell resistance exercise can induce an increase in testosterone concentrations (11,28). Barbell exercises allow for a higher absolute load to be used and thus for a higher volume of work to be performed compared to dumbbell exercises. A theoretic threshold for volume or metabolic demand that must be reached before increases in testosterone are observed may exist but has not been directly quantified (7,26,27). In an examination of various combinations of repetition, set, and rest duration schemes it was demonstrated that, generally, higher volumes of work induce a greater testosterone response (27). It appears likely that the volume of work performed in the present study was too low to stimulate a significant endocrine increase in testosterone concentrations.
Circulating growth hormones have important metabolic properties that have been associated with their function and support muscular and skeletal growth (9). In the present study, the iGH concentration (i.e., 22 kD form) was significantly elevated following exercise in both conditions. However, the increase was significantly greater for the bilateral protocol. The iGH moiety has been shown to be responsive to the metabolic demands of exercise and as reflected by the lactic acid response patterns (12). It has been speculated that growth hormone has a permissive or synergistic effect on testosterone's priming and promotion of protein synthesis and that a minimum concentration of growth hormone is needed for the anabolic actions of testosterone (19). Although increases were observed in iGH, they are lower than has been observed following whole-body or large muscle group resistance exercise protocols with a similar rest period length (12,15).
Circulating iGH concentrations are related to volume of work and the amount of muscle mass used during exercise (7,12,14,18,20,27). The addition of a lower-body exercise to a unilateral upper-body resistance exercise protocol has demonstrated an augmented iGH response (8). In the present study, the increase in iGH followed a similar pattern to the increase in lactate, which has already been demonstrated in full-body resistance exercise protocols (12). The reduction in blood pH occurring concurrently with increases in circulating lactate concentrations during exercise has been shown to increase iGH secretion from the anterior pituitary (3). The larger increase in iGH found with the bilateral protocol could be explained by the larger metabolic demand and accompanying augmented reduction in pH compared to the unilateral protocol.
Insulin, another hormone with potentially anabolic interactions, is regulated by blood glucose and amino acid concentrations in the blood (14). The unilateral and bilateral protocol exhibited a similar exercise-induced increase in insulin concentrations. To optimize the potent anabolic effects of insulin, blood glucose or amino acids must be increased by ingestion of these nutrients prior to the exercise session (1,15). Subjects in the present study fasted for 12 hours prior to exercise; thus a direct effect of nutritional intake did not affect the circulating insulin concentrations. Potentially, the increases in insulin seen in this study (while the subjects were in a fasted state) might be related to the trend for an increase in glucose induced by the exercise protocols. Insulin appears to play a greater role in protein synthesis after resistance exercise compared to after nonresistance exercise. This could be a result of the increased muscle damage that resistance exercise's eccentric component can produce (5). Thus, there might also be an as yet unidentified feedback mechanism from the tissue involved in the exercise resulting in insulin release from the pancreas.
Cortisol was largely unaffected by the 2 different resistance exercise protocols. In the unilateral protocol cortisol decreased from Pre to IP exercise and thus appeared to have been elevated at Pre for this condition. This apparently elevated concentration at Pre might have been a result of psychological factors or arousal for the subsequent exercise protocol. It is unclear, however, why the same effect was not observed during the bilateral protocol. A balanced cross-over design was used to assign the order of the exercise protocols so an equal number of subjects completed each condition first. The lack of a cortisol response was likely a result of the training status of the subjects and the low volume in both exercise sessions. Although there were no exercise-induced increases in cortisol for either protocol, cortisol concentrations for the bilateral condition were maintained at Pre values during recovery, whereas cortisol concentrations were reduced for the unilateral condition during recovery. This indicates that the cortisol responses differed between the 2 protocols and dramatically differed from whole-body or large muscle mass protocols where dramatic increases occur (10,12-15), whereas the involvement of small muscle groups does not induce a cortisol response (8). It is pure speculation that the lack of a cortisol response could also be a result of the training status of the participants. This is because cortisol increases from resistance exercise are more pronounced at the beginning of a resistance training program and have been shown to decrease with chronic resistance training (20). Additionally, a circadian decrease might have occurred, yet one would have expected the same effects for both protocols and, as noted before, dramatic increases have been observed over this same time period with large muscle group and whole-body resistance exercise workouts (14).
The results of this investigation demonstrate that when using a 1-arm-only exercise protocol, whether for experimental manipulation or for training purposes, the endocrine signaling is different from a 2-arm protocol. Furthermore, the magnitude of the endocrine responses are lower than has been observed with full-body and or large muscle mass squat workout protocols using rest periods of 2 minutes or less. The findings of this investigation also support the creative use of the acute program variable, “order of exercise,” in that the consummate principle of starting with large muscle group exercises first, such as the squat, leg press, or power cleans, may in fact optimize the workout. This would be accomplished by increasing the concentrations of hormones prior to the smaller muscle group exercises being performed. This would dramatically increase the molar concentrations in blood perfusing the smaller muscles and augment a variety of signaling effects than when starting with the small muscle group exercises. Finally, for experimental purposes, the use of a 1-arm-only exercise protocol along with a nonexercising arm as a control, the single-arm model would most likely rely on other redundant signaling mechanisms for anabolic stimuli in addition to neural factors in mediating changes in strength and size of the muscle (25). Therefore, the underlying mediating mechanism(s) for adaptations in strength and hypertrophy that are observed in the experiment even at the level of genetic expression would be potentially different when using 1-arm versus 2-arm protocols because of differences in endocrine releases.
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