PURPOSE: To examine the effect of unstable conditions on muscle activation patterns during deep squats.
METHODS: Nineteen healthy male (20±1 years, height: 1.7±4m, mass: 63±7kg) volunteered for the experiment. One repetition maximum (1RM) tests (75.47±6.9 kg) were performed for squats on stable surface by all subjects. A minimum of 48 hours post 1RM, subjects returned to perform 5 consecutive squats with two different loads (zero (0) and 30% 1RM (30)) on 3 unstable surfaces and ground (G). The first-two unstable surfaces (state A and B) were performed on a platform with six degrees of freedom which could be programmed to rotate around any axis of rotation. For state A the six axes of rotation were found to be at x=10, y=2, z=2, ax=2, ay=2, and az=2cm; vibration frequency were set at x=0.8, y=0.2, z=0.2, ax=0.2, ay=0.2, and az=0.2 Hz. For state B, the only changes were x=15 cm comparing to state A. State C is on a core stability disk. Surface electromyography (sEMG) of eight muscles were measured, rectus abdominis (RA), lower erector spinae (LES), rectus femoris (RF), biceps femoris (BF), tensor fascia (TF), tibialis anterior (TA), gluteus maximus (GMa), and gluteus medius (GMe), during deep squats. The sequence of the eigth conditions was randomized and each subject had 2 minutes rest between conditions to avoid fatigue. A polar heart rate monitor was used to ensure subjects’ heart rate were less than 100 beats per minute during the process. The relative values of the average root mean square (RMS) were analyzed with paired T-test.
RESULTS: No significant difference was observed for all muscle normalized RMS of EMG (Table1) between unstable and stable surface during all weight conditions (p>.05). Muscle EMG significantly increased with load (p<.05).
CONCLUSIONS: Compared with the stable conditions, muscle activation did not increase to adapt to the unstable conditions.