The objective of this study was to assess the electromyographic (EMG) activity of rectus abdominis (upper and lower part) and external oblique during sit-ups performed on BOSU ball(s). 24 men participated in a familiarization session and in the next session performed the experimental tests in randomized order. The sit-ups were performed with 10 repetitions with body-weight and with 10 repetitions maximum (10-RM) using elastic bands as external resistance in four different conditions: 1) on a stable surface, 2) with BOSU ball under feet (dome side down, lower-body instability), 3) BOSU ball under back/gluteus (dome side up, upper-body instability) and 4) with BOSU balls under both feet and back/gluteus (dual instability). The feet were not attached to the surface. We observed that with body-weight, external oblique activation was decreased by upper-body instability and dual instability by 22-24% (P=0.002-0.006), whereas rectus abdominis was not affected by surface. Using 10-RM loads, the upper and lower rectus abdominis activities were increased by upper body and dual instability by 21-24% compared to stable surface (P=<0.001-0.036). Further, lower-body instability did not affect muscle activities significantly with either load for any condition. Hence, BOSU balls under back/gluteus can both increase and decrease abdominal muscle activation depending on the load, whereas placing a BOSU ball under feet with the dome side down had little impact.
Copyright (C) 2014 by the National Strength & Conditioning Association.