Lower Extremity Symmetry and Normative Values During Functional Performance Tests in Entry-Level U.S. Marines

The limb symmetry index (LSI) is a common tool for evaluating functional performance across a variety of populations. An LSI of ≥90% is a common, generally accepted clinical threshold for patients with musculoskeletal injuries to achieve before returning to activity. This study's aims were to: (1) describe normative symmetry characteristics and evaluate the influence of sex and limb dominance during functional tasks in healthy, entry-level U.S. Marines, and (2) describe the relationship between symmetry performance across all functional tasks.

This was a cross-sectional study including 651 active duty U.S. Marines with no current injuries (187F/464M, 19.9 ± 2.4 years, limb dominance 59L/592R, 4.8 ± 3.2 months of service). Limb dominance was defined as the preferred limb to kick a soccer ball. Participants completed 5 repetitions of bodyweight bilateral squats and maximal effort vertical jumps using a natural arm swing. Peak eccentric and concentric force (N) were recorded bilaterally on dual force plates for the squat and jump. Participants completed an isometric midthigh pull (IMTP) by standing atop force plates with their knees and hips flexed at approximately 40-50° and 35°, respectively. A fixed bar was positioned at the midpoint between the hips and knees. Participants pulled up as hard and as fast as possible and held a maximal effort for 3-5 seconds for 3 repetitions. Peak vertical force (N) was recorded bilaterally. Force values were normalized to participants' body mass (N/kg). Limb symmetry index was calculated as nondominant limb/dominant limb*100%. Independent sample t-tests determined significant differences in LSI force variables and sexes. Paired t-tests evaluated significant differences in loading between dominant and nondominant limbs. Effect sizes were evaluated via Cohen's d values. Pearson correlations coefficients (r) described the relationship between all dependent variables across all tasks.

Across all tasks and variables, there were no differences in LSI values across sexes (P > .05). Regardless of sex, participants loaded more of their body mass on their dominant limb compared to their nondominant limb with weak-to-small effect sizes (d: 0.22-0.34). Squat LSI had a weak-to-moderate positive relationship with LSI during the vertical jump (r: 0.13-0.69). No other significant relationships were observed (P > .05).

Our study provides stakeholders (e.g., military personnel, clinicians, researchers, etc.) normative ranges of LSI performance values during a bodyweight bilateral squat, vertical jump, and IMTP tasks in a large healthy active duty population. Sex does not appear to influence LSI performance; however, limb dominance may influence loading patterns during these specific functional tasks. Given the similarities between the squat and vertical jump performance, future work should be conducted to determine if stakeholders should eliminate one of these tasks from the testing battery to mitigate the temporal burden required when completing functional task assessments in healthy active individuals.