The effects of strength training on isometric force production symmetry in recreationally trained males

Objectives: The purpose of this investigation was to determine what effect a bilateral strength training regimen has on isometric force production symmetry and if changes in force production symmetry can be accounted for by differences in pre-intervention strength levels. Design: Sixteen recreationally trained males (1-RM squat: 146.8 ± 23.0 kg.) were assigned to two groups for the 7-week training intervention: strong (S) and weak (W) based on pre-training squat isometric peak force allometrically scaled (IPFa) at 120° knee angle. Methods: Subjects completed a 7-week training intervention following a block-periodized model and were tested on measures of dynamic (1RM squat) and isometric (isometric squat at 90° and 120° knee angle) strength preand post-intervention. The degree of bilateral lower limb asymmetry was calculated as a percentage where 0% symmetry index (SI) indicates perfect symmetry on the isometric squat. Results: ANCOVA results showed no statistical difference between groups for all dependent variables when pre-intervention IPFa 120° scores were used as the covariate. Paired t-tests results showed both groups statistically improved 1RM squat and IPFa 120° (p < 0.05). IPFa 120° SI decreased statistically from pre-training in the W group (p = 0.03). Independent t-test results showed the W group had statistically larger pre-intervention SI scores for IPFa 90° (p = 0.045) and IPFa 120° (p = 0.007); however this difference was no longer present following strength training. There was a strong inverse relationship between pooled IPFa 120° and IPFa 120° SI (r = -0.64, p = 0.004). Conclusions: The findings of the current study support the notion that weaker individuals can augment lower limb symmetry with strength training. The same does not seem to be true for stronger individuals who already have a low symmetry index score. These findings indicate that strength training improves force production symmetry in relatively weak males, which may be important for bilateral tasks and injury potential reduction. (Journal of Trainology 2014;3:6-10)