(10) MUSCLE RECRUITMENT RESPONSES TO INCREMENTAL EXERCISE IN FLYWHEEL-BASED INERTIAL TRAINING (FIT) SQUATS

Flywheel-based inertial training (FIT) is a gravity-independent form of exercise known to induce rapid and robust muscle hypertrophy and improved performance which is thought to arise from eccentric overload. FIT recently gained popularity among athletes of various disciplines. We recently adapted this modality for aerobic exercise. 

Purpose: To characterize muscle recruitment responses to submaximal-effort FIT with varying moment of inertias (MOI).

Methods: Twenty healthy, physically active participants (10M, 10F; age: 19-39y) completed FIT squats (Exxentric Kbox 4Pro). Testing entailed 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 and synchronized to surface electromyography (sEMG) of the gluteus maximus (GM), vastus lateralis (VL), biceps femoris (BF), and the soleus (SOL) in accordance with SENIAM guidelines (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. Peak EMG amplitude, integrated EMG (iEMG) amplitude, mean of concentric (CON) and eccentric (ECC) EMG amplitude and the ECC/CON ratio were examined. Mixed model 2x2 ANOVAs assessed muscle recruitment responses for the VL, BF, GM, and SOL to MOI (within subjects) for both sexes (between subjects).

Results: There was no effect of sex or sex x MOI interaction. There was a significant main linear effect of MOI on peak EMG amplitude, iEMG, and mean EMG amplitude during the eccentric phase for all muscles (p< 0.05). There was no main linear effect of MOI on mean EMG amplitude during the concentric phase for the VL. Yet mean CON EMG amplitude rose with increasing MOI for the BF, GM, and the SOL. There was a main linear effect of MOI on the ECC/CON ratio for only the GM and SOL (p< 0.05).

Conclusions: Increasing MOI during submaximal FIT squats done at a standardized pace 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.  PRACTICAL APPLICATIONS: Increasing MOI during submaximal FIT quarter squats increases EMG activity of ankle, hip, and 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. 

Acknowledgements: None