Effects of an Unstable Load on Force and Muscle Activation During a Parallel Back Squat

Stability training has become commonplace in the strength and conditioning field. Although unstable surface training has been investigated, little is known regarding the efficacy of performing resistance exercises with an unstable load. The purpose of this study was to determine if performing a parallel back squat with an unstable load (weights suspended from the bar by an elastic band) produces greater ground reaction forces (GRFs) and muscle activation in the trunk and lower extremities than a stable condition (a normally loaded barbell). Fifteen resistance-trained males (age: 24.2 ± 3.4 years, mass: 83.4 ± 18.7 kg) completed 10 repetitions of the back squat, with 60% of their 1 repetition maximum in both stable and unstable conditions. Peak vertical GRF and the integrated muscle activity of the rectus femoris, vastus lateralis, vastus medialis, biceps femoris, soleus, rectus abdominis, external oblique, and erector spinae muscles on the right side of the body were determined The unstable load resulted in a small (3.9%) but significant decrease in peak vertical GRF. The unstable load also produced greater muscle activation in the rectus abdominis, external oblique, and soleus. The findings of this study suggest that squatting with an unstable load will increase activation of the stabilizing musculature; and while force decrements were statistically significant, the decrease was so small it may not be relevant to practitioners.