Clara J. Mitchinson1, Stuart Best1, John Caruso2, Lance M. Bollinger1. 1University of Kentucky, Lexington, KY. 2University of Louisville, Louisville, KY.

BACKGROUND: Spaceflight induces rapid loss of aerobic capacity. Flywheel-based inertial training (FIT) - a gravity-independent form of exercise previously used as a countermeasure against unloading-induced muscle atrophy - provides external resistance through moment of inertia (MOI) of a rotating disk in a velocity-dependent manner. Optimizing FIT for cardiorespiratory fitness may improve exercise efficiency and reduce hardware needs associated with deep space exploration. The purpose of this study was to determine the cardiorespiratory responses to FIT with varying MOIs. METHODS: 20 healthy, physically active participants (10M, 10F; age: 19-39y) completed two bouts of FIT squats (Exxentric Kbox 4Pro) separated by ≥7d. Testing consisted of quarter-squats (3 min per stage; 50 repetitions per minute) with increasing MOI increments of 0.005 kg·m2 until volitional fatigue. Squat depth (60° knee flexion) was monitored by wireless electrogoniometer in real-time. Heart rate (HR) and gas exchange data were averaged over the final 60s of each stage. Inter-session reliability (intra-class correlation coefficients; ICC, Standard Error of the Mean; SEM, and Minimal Difference; MD, and Coefficient of Variation; CV) and reproducibility (Bland-Altman plots) were assessed. A 2x2 mixed model ANOVA was used to assess cardiorespiratory responses to MOI (within subjects) for both sexes (between subjects). RESULTS: HR (ICC = 0.81, SEM = 11.13bpm, MD = 30.84bpm, CV = 1.92%) and VO2 (ICC = 0.81, SEM = 2.00 mL·kg-1·min-1, MD = 5.53 mL·kg-1·min-1, CV = 2.53%) demonstrated good reliability. RER (ICC = 0.59, SEM = 0.06, MD = 0.16, CV = 0.95%) demonstrated poor reliability. Bland-Altman plots revealed biases of HR = -1.60%, VO2 = -4.61%, RER = -2.76% where responses tended to be lesser on trial 2 than trial 1. Mixed model ANOVA revealed a main effect of MOI on HR (p<0.001), VO2 (p<0.001) and RER (p<0.001). A main effect of sex was noted for VO2. There was no significant MOI x sex interaction for any of the observed variables. CONCLUSION: FIT squats provide reliable cardiorespiratory data between testing sessions. However, a small test-retest effect suggests the need for greater familiarization. Standardizing food intake may improve reliability and reproducibility of RER. Increasing MOI during FIT evokes a linear response in HR and VO2 in males and females. Future work should aim to determine how sustained FIT affects CRF.

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