Bilateral Differences In Normalized Vertical And Lateral Ground Reaction Forces In Back Squats With Different Loads

Abstract

The back squat is an exercise used in training for almost every sport that desires an increase in lower extremity strength. The average vertical ground reaction forces (GRFs) between the right and left legs have been compared and bilateral asymmetries have been identified (Flanagan and Salem, 2007). However, this comparison used self-defined leg dominance and did not consider the frontal plane. Additionally, the forces were not represented in terms of the combined system load (subject plus weights). PURPOSE: To determine if bilateral differences in normalized vertical and lateral GRFs exist when comparing the R and L legs, as well as legs defined by preference, dominance and strength. METHODS: Twenty subjects (10 males, 10 females) were recruited to perform a set of 5 back squat reps at 50% 1RM, 5 reps at 70% 1RM, and 3 reps at 90% 1RM with each foot on a different Kistler force plate. The middle repetition of each set was used for analysis. Knee extensor strength was determined using a Biodex System 3 isokinetic dynamometer at 60°/s. Vertical forces (Fz) were normalized by the combined subject plus load weight. RESULTS: The peak and average normalized vertical GRFs were not different based on load for any of the comparisons. The right leg created a larger normalized peak Fz (64.7% vs. 62.1%; p = 0.010) and average Fz (50.6% vs. 48.0%; p = 0.014) compared to the left leg. There was no difference in peak or average Fz based on leg preference. Dominance was determined by which foot created a larger average Fz value during the concentric phase of the squat. The dominant side created a significantly larger peak Fz (65.2% vs. 61.6%; p < 0.001) and average Fz (51.4% vs. 47.2%; p < 0.001). The stronger side created less Fz than the weaker side, but the difference was not significant. There were differences in the average lateral force based on load for all comparisons and for peak lateral force for all but leg dominance. The dominant leg created a larger average lateral force than the non-dominant leg (p = 0.009). CONCLUSION: The differences in vertical GRF based on load that have been identified previously do not exist when the forces are normalized. When comparing legs, it is important to identify the legs in terms of preference, dominance or strength. These findings can assist trainers in noting asymmetries which can increase injury risk.