Cardiorespiratory and Muscle Recruitment Responses to Incremental Exercise in Flywheel-Based Inertial Training (FIT) Squats
Year of Publication
Master of Science (MS)
Kinesiology and Health Promotion
Dr. Lance Bollinger
BACKGROUND: Spaceflight induces rapid loss of aerobic capacity and muscle strength. 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. This modality of exercise has also been of interest among athletes of various disciplines due to its purported benefits on athletic performance. PURPOSE: To determine the cardiorespiratory and muscular recruitment responses to FIT with varying MOIs. METHODS: Twenty 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 a wireless electrogoniometer in real-time. Heart rate (HR) and gas exchange data were averaged over the final 60s of each stage. Surface electromyography (EMG) of the gluteus maximus (GM), vastus lateralis (VL), biceps femoris (BF), and soleus (SOL) were measured. EMG and electrogoniometer data were time-synchronized throughout testing and transmitted to a Trigno Avanti EMG system (Delsys, Natick, MA). Prior to FIT testing, subjects performed three reference isometric contractions in a squat position; all EMG data were normalized to this level of EMG activity. Inter-session reliability and reproducibility (Bland-Altman plots) were assessed. Mixed model 2x2 ANOVAs assessed cardiorespiratory and muscle recruitment responses to MOI (within subjects) for both sexes (between subjects). RESULTS: Mixed model ANOVA revealed a main effect of MOI on HR (p2 (p2 with males demonstrating greater values than females. When normalized to peak VO2, no effect of sex was noted. There was no significant MOI x sex interaction for any of the observed variables. There was a significant main linear effect of MOI on peak EMG amplitude, integrated EMG, and mean EMG amplitude during the eccentric phase for all muscles (p2 responses at any stage between trials one and two. CONCLUSION: FIT squats provide reliable cardiorespiratory data between testing sessions when performed at a standardized pace. Increasing MOI during FIT evokes a linear response in HR, VO2, and RER in males and females with no apparent effects of sex when normalized to peak values. The RER response (>1.0) at a relatively modest oxygen uptake (~60% VO2peak) suggests this modality stimulus compared to other forms of aerobic exercise. Increasing MOI during submaximal FIT squats evokes a linear response in peak EMG amplitude, mean ECC EMG amplitude, and iEMG of knee extensors, hip flexors, and plantar flexors. Increasing MOI increases mean CON EMG amplitude and ECC/CON ratio of the plantar flexors and hip extensors, but not the knee extensors. However, ECC overload was only evident in the knee extensors. At higher MOI, muscle recruitment of hip extensors and plantar flexors is preferentially increased, but eccentric overload is only achieved in the knee extensors. Future work should aim to determine how sustained FIT affects cardiorespiratory fitness.
Digital Object Identifier (DOI)
Mitchinson, Clara Joyce, "Cardiorespiratory and Muscle Recruitment Responses to Incremental Exercise in Flywheel-Based Inertial Training (FIT) Squats" (2023). Theses and Dissertations--Kinesiology and Health Promotion. 94.
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