The loaded barbell back squat (BS) (Fig. 1) is an established method for development of strength and power in lower limb (1,2). Trunk muscle activation (TMA) using surface electromyography (sEMG) in BS is novel and clarity on the role of BS in developing dynamic trunk strength and stability is required. BS performed on an unstable surface will result in greater TMA, but compromises the load and therefore primary purpose (1,2). TMA in BS is load sensitive (3) and greater in free barbell version than the more supported Smith machine squat. How does TMA in a more stable squat, hack squat (HS) (Fig. 2), at the same relative, but higher absolute load compare to BS? The centre of gravity of BS system, person and external load, must remain over base of support or feet (4) to prevent failure or injury. In HS the trunk is supported by a 45o angled board and feet are placed anterior to line force (4). Hypothesis, TMA in BS will be greater than HS at the same relative loads but greater absolute load in HS. Aims of the study: 1) determine max strength in BS and HS, 2) compare TMA in BS and HS, and 3) assess TMA response to load increases in BS and HS. Ethical approval according to Helsinki Declaration (2013) was granted. 3 test sessions (n=10 males): 1) BS and HS 1 rep max (RM) test, 2) EMG test familiarization, 3) EMG tests for 3 reps of BS and HS at 65, 75, 85 and 95% of system mass max (SM). SM = 1RM + (0.886 x body mass) (kg), where 0.886 is body mass minus shanks. Kinematics measured by a linear transducer and sEMG (SENIAM guidelines) for rectus abdominus (RA), external oblique (EO), upper lumbar erector spinae (ULES) and lumbar sacral erector spinae (LSES). Vastus lateralus (VL) sEMG as reference lower limb muscle. sEMG was root mean square (RMS) processed. Mean RMS for each phase of BS and HS at 75, 85 and 95% SM were normalized to mean concentric BS RMS at 65% SM (5). Mean HS 1RM was 28.5 kg (18.24%) greater than BS, hence 4 test loads in HS were significantly higher than BS (F(1, 9) = 19.94 p<0.01). Eccentric displacement was 21.5 cm less in HS than BS for 4 test loads. Force was higher in HS than BS at each load and increased with each load in both exercises. BS TMA was greater than HS for all muscles, both phases for all test loads. Difference was significant (p<0.05) in 14/24 instances (3 loads x 4 muscles x 2 phases). TMA increased with load in all muscles for both exercises and phases apart from HS LSES in eccentric phase. VL RMS was greater in BS than HS for all loads but only significant in concentric phase. There was a load effect for VL in both exercises. This study demonstrated a greater 1RM for HS vs BS for well-trained cohort. Despite higher absolute tests loads and force in HS, TMA was higher in BS. This study suggests the BS is an effective method of developing trunk strength and that TMA is sensitive to load in both BS (3) and HS.