Our study compared blood lactate and hormonal data from a device initially intended as an in-flight countermeasure for knee extensor mass and strength losses. Each dependent variable shows a certain degree of statistical significance. Peak angular velocity and TW gender differences are likely because of the higher body and/or muscle mass of male subjects (18,40). Peak angular velocity data showed no interworkout differences, which suggests that intensity was similar across protocols. Total work results show a workout effect, an expected outcome of the current design. CE3 and CO6 workouts had twice the TW volume of CO3, yet CO6 was solely a muscle shortening effort, whereas CE3 included equal numbers of concentric and eccentric actions. A comparison of current study workout data allows exercise volume and contractile mode influences upon lactate and hormonal responses to be examined.
Exercise entails reliance on anaerobic glycolysis whose products include lactate and H+, which are each correlated to [GH] (20). Limited in-flight data suggest that [GH] are not elevated, which contribute to muscle mass and strength losses (37,45). Although in-flight cycle ergometry (n = 1) led to higher [GH] vs. a similar exercise workload done 3 weeks before space travel (36), the reverse was true from exercise (n = 4) on a torque-velocity dynamometer (37). It was concluded that a minimal level of neuromuscular activity and mechanical loading was needed for in-flight exercise; if that threshold was not met, lower [GH] ensued (37). Table 4 shows that women had higher initial [GH], likely from estrogen-based sensitization to somatotrophs (19,34,44,47). Yet compared with pre-exercise data, men had higher percentage increases over time. This likely occurred because men (a) had much lower initial [GH] and (b) yielded higher TW (Table 2) values from FERG exercise (1,2,19,20,34,42,46). Table 4 includes a workout × time effect, with CO6 > CE3, CO3 at 1 minute postexercise but CE3 > CO3 after 30 minutes. Postexercise [GH] usually peak within 15 minutes because of the volume of work performed (1,2,19,34,44). Although the CO6 workout provided an expected response, whereby a higher TW volume evoked a greater [GH] at 1 minute postexercise and the latter declined (33,45), CE3 had the opposite effect, which suggests that eccentric actions delay postexercise [GH] increases.
Current [T] yielded a 3-way effect, with male CE3 and CO6 data at 1 and 30 minutes postexercise shown as the interaction source. Gender-related [T] differences concur with prior studies (26,34). Less understood is the impact of various resistive exercise protocols on [T]. A comparison of concentric- vs. eccentric-induced changes showed that either muscle action, whether against an absolute (14) or relative (29) load, evoked greater [T] postexercise. Yet [T] returned to pre-exercise values 15 minutes after workouts (14,29). Postexercise differences among the current and prior studies may be because of intensity-mediated changes in sympathetic activity (15). Perhaps the current study, which entailed a single exercise done at a higher intensity than earlier studies (14,29), led to more localized fatigue and greater sympathetic activity that augmented postworkout [T] (14). CE3 and CO6 protocols, each with a higher TW volume than CO3, evoked greater [T] (34). Yet among protocols with similar TW values, those done at higher intensities elicit greater [T] (1). A comparison of 2 exercise protocols with similar work volumes, whereby one included spotter-assisted repetitions performed beyond momentary failure, whereas the other did not, showed that the former yielded higher [T] (1). Thus, current results, whereby CE3 and CO6 protocols yielded similar PAV values and higher TW than CO3, evoked higher [T] to concur with prior studies (26,34).
[C] shows gender, time, and workout effects. The high current pre-exercise values may be because of greater sympathoadrenal activity that evoked epinephrine secretion (15). To abate in-flight muscle mass and strength losses, protocols should be devised that blunt [C]. Gender-based [C] differences from exercise are likely because of higher male workout intensities. For instance, PAV had a gender (men > women) effect that parallels current [C] changes. The rigor of FERG workouts also contributed to the current time effect. Research notes workouts against heavy loads evoke time effects, because athletes will incur higher [C] than sedentaries who perform similar protocols (23,25,30-33,38). Finally multisprint (32,41) and 2-hour running protocols (12) each elevated [C], because the former produced values like current results. Thus, the stress of current workouts performed at similar intensities yielded results that concur with prior studies (12,23,25,30-33,38,41).
[C] shows a workout (CO6 > CE3, CO3) effect. Despite similar CE3 and CO6 TW values, only the latter yielded higher [C] than CO3, which suggests eccentric actions blunt [C]. Prior work shows a lower metabolic cost from eccentric actions (9,13). Perhaps [C] changes from eccentric actions are dictated by metabolic factors in addition to TW. Metabolism research showed eccentric actions evoked 1/7th the net energy cost of muscle shortening from leg presses done on standard equipment (13). In a similar fashion, eccentric FERG leg press actions added no net caloric cost beyond that from concentric exercise (9). It was concluded the CE3 protocol uses stored elastic energy to evoke muscle potentiation and stretch reflexes with eccentric FERG actions (9). Future research on the impact of eccentric actions on metabolism and [C] is warranted.
[T/C] ratios assess the relative anabolism/catabolism incurred from various treatments. Table 7 shows large post-workout cortisol increases in men, while women's values show insignificant changes. [T/C] ratios were previously monitored in male athletes before and after a sprint protocol with repetitive 20-second runs separated by 100-second rests (41). The repetitive design of the sprint protocol is similar to the current study, yet sprint results led to a 21% [T/C] increase, unlike current outcomes (41). Differences between the current and sprint studies are in part because of variations in exercise intensity. Current subjects were told to exert maximal voluntary effort, whereas sprinters ran at a myriad (56-100%) of intensities (41). Lower sprint intensities imposed less stress and [C] that in turn yielded an elevated [T/C] (41). Yet lower PAV data from our female subjects suggest that a reduced exercise intensity yielded smaller [C] and left [T/C] unchanged. Thus, exercise intensity variations may account for [T/C] differences in the current and prior studies (41).
Our study design sought to identify the best workout, in terms of mechanical loads imposed and the resultant hormonal milieu, for baseline knowledge of future in-flight resistive exercise programs. Workouts on standard resistive exercise devices showed that hormonal responses generally differed little by contractile mode (29-31). A resistive exercise study that compared hormonal responses done against absolute loads showed greater [GH] from concentric actions, yet the effect was attributed to intensity rather than contractile mode (14). CE3 and CO6 workouts yielded similar [T] changes; yet in terms of [GH], the former protocol evoked the highest 30 minute postexercise value and did not contribute to the gender × workout interaction. In addition to 30 minute postexercise [C] data (CO6 > CE3, CO3), results imply CE3 is the best of the examined workouts. Future International Space Station workouts hope to use a modified FERG that offers less eccentric resistance than the Figure 1 prototype to limit spacecraft vibration (17).
Although minimization of in-flight vibration is a valid concern, current data suggest that reduced eccentric resistance may compromise the resultant hormonal milieu. Current results should be interpreted with caution, because continued research is needed to ascertain an ideal resistive exercise protocol to abate in-flight mass and strength losses. Future research may include microgravity simulation to make results more applicable to space flight.
Based upon the mechanical loads imposed and the resultant hormonal milieu, current results suggest that FERG workouts should include eccentric actions. Many athletic endeavors and conditioning programs include eccentric actions as a regular part of workouts. In contrast to standard resistive exercise devices, the FERG provides greater eccentric resistance. Programmatic needs of athletes will differ based upon the requirements of their chosen sport, but for those who wish to include FERG protocols for added muscle mass and strength, eccentric actions should be used.
We wish to thank our subjects for their participation. Funding was provided through a University Faculty Development Fellowship Program. John F. Caruso, Michael A. Coday, and Julie K. Monda are participants in the TURC (Tulsa Undegraduate Research Challenge) at The University of Tulsa.
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