Measurement of trunk muscle activation (TMA) using surface electromyography (sEMG) in BS is novel and has focussed on squats on unstable surfaces. The hack squat (HS) has more support than BS and is performed at greater absolute loads but lower trunk muscle activation. This has implications for the application of both exercises in exercise training programmes. The purpose of this study was to compare 1RM and TMA in BS and HS at 4 relative loads, 65, 75, 85 and 95% of system mass max (SM). Ten males completed three test sessions: 1) BS and HS 1RM; 2) TMA test familiarization, and; 3) TMA for 3 reps at 4 loads for BS and HS. Kinematics measured by linear transducer and sEMG for rectus abdominus (RA), external oblique (EO), upper lumbar erector spinae (ULES) and lumbar sacral erector spinae (LSES). Vastus lateralis (VL) sEMG reference lower limb muscle activation. sEMG amplitude determined by root mean square (RMS) processing. Mean RMS for each phase of BS and HS at 75, 85 and 95% SM were normalized to mean concentric (con) BS RMS at 65% SM (Balshaw & Hunter, JEK 2012,22,308–319). Mean HS 1RM was 18.2% greater than BS, hence 4 test loads in HS were significantly (p<0.01) higher than BS. Eccentric (ecc) bar displacement was significantly (p<0.001) less in HS than BS for 4 loads. Duration of ecc phase was shorter (p<0.001) in BS than HS. Duration increased (p<0.01) in both phases and exercises with load. BS TMA was greater than HS for all muscles, phases and loads. This was significant (p<0.05) in 14/24 instances (3 loads x 4 muscles x 2 phases). TMA increased significantly (p<0.05) with load in all muscles for both exercises and phases apart from HS LSES in ecc phase. VL RMS was greater in BS than HS for all loads but only significant (p<0.05) in con phase. Despite higher absolute test loads and force in HS, TMA was higher in BS. BS is an effective method of developing trunk strength and TMA is sensitive to load in both BS and HS.
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