Exercise Responses to Gravity-Independent Flywheel Aerobic and Resistance Training

Abstract

Background: Although several exercise systems have been developed to mitigate the physiological deconditioning that occurs in microgravity, few have the capacity to positively impact multiple physiological systems and still meet the volume/mass requirements needed for missions beyond low Earth orbit. The purpose of this study was to test the gravity-independent Multi-Mode Exercise Device (M-MED) for both resistance (RE) and aerobic (AE) training stimuli.

Methods: Eight men and nine women (mean age 22.0 ± 0.4 yr) completed 5 wk of training on the M-MED: RE 4 × 7 squats 2 d/wk, and AE 4 × 4-min rowing bouts at ∼90% Vo2max 3 d/wk. Pre- and post-training data collection included an aerobic capacity test, MR imaging, strength testing, and vastus lateralis muscle biopsy.

Results: Vo2max increased 8%, 3RM strength 18%, and quadriceps femoris cross-sectional area (CSA) 10%. Knee extensor strength increased at all isokinetic speeds tested. Subjects also demonstrated improved fatigue resistance in knee extension. At the cellular and molecular level, the biopsy revealed increases in mixed myofiber CSA (13%), citrate synthase activity (26%), total RNA concentration (24%), IGF-I mRNA (77%), and Type IIa myosin heavy chain (MHC) mRNA (8%), and a concomitant decrease in Type IIx MHC mRNA (-23%). None of the changes were gender-specific.

Discussion: Both the functional outcomes and biomarker changes indicate that a very low volume of M-MED exercise results in robust adaptation in the cardiovascular and musculoskeletal systems. The M-MED has the potential to provide a wide range of countermeasure exercises and should be considered for testing in ground-based spaceflight simulation.

Figure 1

M-MED can operate in both aerobic (AE) and resistance (RE) modes. (a) AE mode training approximates rowing exercise. (b) RE mode training (as shown) approximates supine squats. In AE mode, the flywheel is accelerated by concentric contraction of knee extensors (KE), and decelerated by a magnetic brake (23). In RE mode, the flywheel is accelerated by concentric KE contraction and decelerated by eccentric KE contraction. In this study, efficacy of the M-MED was tested strictly on KE, but the device can be reconfigured to target other muscle groups. Straight arrows show linear translation of the subject with respect to the flywheel. Arced arrows show acceleration (arrows getting longer) and deceleration (arrows getting shorter) of the flywheel.

Figure 1

M-MED can operate in both aerobic (AE) and resistance (RE) modes. (a) AE mode training approximates rowing exercise. (b) RE mode training (as shown) approximates supine squats. In AE mode, the flywheel is accelerated by concentric contraction of knee extensors (KE), and decelerated by a magnetic brake (23). In RE mode, the flywheel is accelerated by concentric KE contraction and decelerated by eccentric KE contraction. In this study, efficacy of the M-MED was tested strictly on KE, but the device can be reconfigured to target other muscle groups. Straight arrows show linear translation of the subject with respect to the flywheel. Arced arrows show acceleration (arrows getting longer) and deceleration (arrows getting shorter) of the flywheel.

Figure 2

Effects of five weeks of concurrent AE and RE mode training. In both men and women subjects, M-MED-based training increased VO₂max (a), citrate synthase activity (b), quadriceps CSA (c), and muscle strength (d). PRE (open symbols) and POST (filled symbols) data points show mean±s.e.m. All differences (POST vs. PRE) significant at p<0.05 by RM-ANOVA.

Figure 3

Speed-dependent muscle strength increased in men and women subjects under eccentric, isometric and concentric conditions following the five-week training on the M-MED. PRE (open symbols) and POST (filled symbols) data points show mean±s.e.m. All differences (POST vs. PRE) significant at p<0.05 by RM-ANOVA.

Figure 4

M-MED training improved knee extensor endurance. Subjects completed three sets of 30 maximum knee extension actions, at 180°/s, separated by 1min rest. (a) While fatigue ensued with duration of exercise, improvements in muscle strength and fatigue resistance of both men and women subjects are evident in POST measurements. Each datum depicts a quintet of maximum contraction repetitions. (b) Relative improvement in muscle strength and fatigue resistance due to M-MED training was greater in women than men, and more pronounced with duration of exercise (last vs. first quintet). PRE (open symbols) and POST (filled symbols) data points show mean±s.e.m. All differences (POST vs. PRE) significant at p<0.05 by RM-ANOVA.