Influence of the Amount of Instability on the Leg Muscle Activity During a Loaded Free Barbell Half-Squat

Abstract

This study aimed to understand the acute responses on the muscular activity of primary movers during the execution of a half-squat under different unstable devices. Fourteen male and female high-standard track and field athletes were voluntarily recruited. A repeated measures design was used to establish the differences between muscle activity of the primary movers, the body centre of mass acceleration and the OMNI-Perceived Exertion Scale for Resistance Exercise (OMNI-Res) in a half-squat under four different stability conditions (floor, foam, BOSU-up and BOSU-down). A significant correlation was found between the highest performance limb muscle activity and body centre of mass acceleration for half-squat floor (r = 0.446, p = 0.003), foam (r = 0.322, p = 0.038), BOSU-up (r = 0.500, p = 0.001), and BOSU-down (r = 0.495, p = 0.001) exercises. For the exercise condition, the half-squat BOSU-up and BOSU-down significantly increased the muscle activity compared to half-squat floor (vastus medialis: p = 0.020, d = 0.56; vastus lateralis: p = 0.006, d = 0.75; biceps femoris: p = 0.000-0.006, d = 1.23-1.00) and half-squat foam (vastus medialis: p = 0.005-0.006, d = 0.60-1.00; vastus lateralis: p = 0.014, d = 0.67; biceps femoris: p = 0.002, d = 1.00) activities. This study contributes to improving the understanding of instability training, providing data about the acute muscular responses that an athlete experiences under varied stability conditions. The perturbation offered by the two BOSU conditions was revealed as the most demanding for the sample of athletes, followed by foam and floor executions.

Keywords: electromyography; perturbation; squatting; unstable.

Figure 1

Exercise conditions: (a) half-squat floor, (b) half-squat foam, (c) half-squat BOSU-up, and (d) half-squat BOSU-down.

Figure 2

Body centre of mass acceleration signal (Y–axis). The signal shows all the changes in the body centre of mass acceleration (BCMA) during one repetition (entire phase) of the half-squat performed on the floor (a) = The shaded area shows the total number of amplitudes in the entire phase; (b) the magnified zone details each of the maximum BCMA values (red circle). These values were summed to determine the value of BCMA in the entire phase.

Figure 3

Comparison of the collected data under half-squat conditions: (a) global activity ^§, (b) body centre of mass acceleration, and (c) OMNI-Perceived Exertion Scale for Resistance Exercise (OMNI-Res). Each bar represents the mean, and the error bar represents the standard error of the mean (SE). ^§ = Sum of the activity of the vastus medialis, lateralis and biceps femoris; sEMG = surface electromyography; mV = microvolts; BCMA = body centre of mass acceleration; A.U. = Arbitrary units; † Significantly different from half-squat BOSU-up; ‡ Significantly different from half-squat BOSU-down.