Increased strength is associated with lower trunk muscle activation during loaded back squats and dynamic body weight jumps

This study measured how back squat strength (1RM) affected trunk muscle activation in performing squats, squat jump (SJ), and countermovement jump (CMJ). Fifty males, completed two test sessions. Squat 1RM was tested first. Participants were assigned to three groups: (a) strong group (SG), (b) middle group (MG), or (c) weak group (WG), based on relative squat 1RM. Test 2: EMG data were collected for four trunk muscle sites; rectus abdominus, external oblique, lumbar sacral erector spinae, and upper lumbar erector spinae while performing (3 reps) SJ, CMJ, and squats at 65%, 75%, and 95% 1RM. Squat and jump phases were determined from a linear transducer and 30° tertiles for each phase, from a knee goniometer. Normalized root mean square RMS increased significantly with load for each muscle site in both squat phases. Trunk muscle activation was significantly lower in SG vs WG in eccentric and concentric squat phases. Concentric and flight phase RMS in both jumps was lower in SG vs WG. RMS increased significantly for each eccentric tertile and first concentric tertile. Greater squat strength is associated with lower trunk muscle activation in squats and jumps and trunk muscle activation was highest in the two deepest 30° squat segments. In conclusion, back squat strength training to parallel, where top of thighs are horizontal, is an effective method of developing dynamic trunk stability.