Clark, DR, Lambert, MI, and Hunter, AM. Trunk muscle activation in the back and hack squat at the same relative loads. J Strength Cond Res 33(7S): S60-S69, 2019-The hack squat (HS) is likely to produce a greater 1 repetition maximum (1RM) compared with the back squat (BS). This can be attributed to the support of the trunk during the HS compared with no support during BS. This support, however, may compromise trunk muscle activation (TMA), therefore producing different training adaptations. Accordingly, the purpose of this study was to compare 1RM in BS and HS and TMA at 4 relative loads, 65, 75, 85, and 95% of maximal system mass. Ten males completed 3 test sessions: (a) BS and HS 1RM, (b) HS and BS neuromuscular test familiarization, and (c) neuromuscular test for 3 reps at 4 loads for BS and HS. Back squat TMA was significantly greater (p ≤ 0.05) than HS for all muscles and phases except rectus abdominus in concentric phase. Trunk muscle activation increased (p ≤ 0.05) with load in all muscles for both exercises and phases apart from lumbar sacral erector spinae in HS eccentric phase. Mean HS 1RM and submaximal loads were significantly (p < 0.0001) higher than the equivalent BS loads. Duration of the eccentric phase was higher (p < 0.01) in HS than BS but not different in concentric phase. Duration increased significantly (p < 0.01) with load in both exercises and both phases. Despite higher absolute tests loads in HS, TMA was higher in BS. Trunk muscle activation is sensitive to load in both exercises. Back squat is more effective than HS in activating the muscles of the trunk and therefore, arguably more effective in developing trunk strength and stability for dynamic athletic performance.