M. Chamberlain, S. Anderson, S. Clay, S. Musgrove, J. Sanburg, R. McCulloch

Gonzaga University, Spokane, WA

PURPOSE: An increased quadriceps to hamstring muscle activation ratio (Q:H) is a significant predictor of anterior cruciate ligament injuries for females. To reduce the risk of injury, instability exercise has been implemented in training programs for the perceived benefits of increased neuromuscular activation. In particular, instability systems using suspension straps to create unstable environments for either the upper or lower extremities have become increasingly popular, although the direct implications remain unclear. Our objective was to determine the effect of instability introduced by a suspension system during single-leg side lunges and if that instability would increase the activation of the hamstrings, subsequently decreasing the Q:H as hypothesized. METHODS: Seventeen physically fit female subjects (18-23 years) were asked to volunteer for the study. Two different types of single-leg side lunges were performed; one using a suspension exercise system (SLSUS) and the other using a stationary box (SLBOX). Two sets of three repetitions of each lunge were performed to maximal knee flexion. Electromyography (EMG) was used to measure muscle activation during both the eccentric and concentric phases of the lunge. Motion capture analysis was used to evaluate frontal plane motion and knee flexion angle in the sagittal plane. Medial/lateral (M/L) knee motion in the frontal plane was normalized as a percentage of hip motion. Paired t-tests were conducted between trials to compare frontal and sagittal plane postural sway and knee flexion angle. EMG SLSUS trials were normalized to SLBOX trials and compared using one sample t-tests. RESULTS: Participants reached a greater degree of knee flexion during the SLSUS compared to the SLBOX (75.03° ± 8.7° vs. 70.3°, ± 8.7° p = 0.005; all values reported as mean ± SD). Greater M/L normalized knee motion was observed when performing SLSUS compared to SLBOX (267% ± 22% of hip motion vs. 172% ± 13% of hip motion, p = 0.021). The rectus femoris and the semimembranosus/semitendinosus showed greater peak activation during the suspension trials (1.19 % ± 0.29%, p = 0.014; 1.30% ± 0.37%, p = 0.004). No differences were found for Q:H ratio between lunge modalities. The biceps femoris showed more muscle activation during the concentric phase of the suspension trials versus the box trials (1.13 % ± .21%, p = 0.021). CONCLUSION: Although there was no decrease in the Q:H ratio, the instability created by the suspension exercise increased hamstring activation more than quadriceps activation. Our findings suggest suspension exercise can be a beneficial exercise based on its ability to achieve higher activation of knee stabilizing muscles.

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