Differential Effects Of Sex And ACLR Status On Quantity And Quality Of Force Output

After ACL reconstruction (ACLR), females exhibit greater deficits in the quantity of quadriceps force and fatigue response than males. Sex differences have not been explored for measures of force quality, such as sample entropy (SampEn), which quantifies the change in signal structure, during a fatiguing contraction. PURPOSE: To investigate sex-based differences in quantity and quality of quadriceps force output during a fatiguing isometric contraction in those with ACLR (n = 66 [31 males], months from surgery = 36 ± 42) and uninjured controls (n = 28 [12 males]). METHODS: Subjects performed a 30-s maximal isometric knee extension contraction in an isokinetic dynamometer. Peak torque in the first 5-s of the trial was identified and the subsequent 25-s were used for analysis. SampEn was calculated with m = 2, r = 0.05*SD. The start (first 5-s mean) and end (last 5-s mean) of the trial were used to determine percent change (%Δ SampEn). Quadriceps fatigue was quantified via fatigue index (FI). Pearson’s correlations were used to test for association between %Δ SampEn and FI. Separate two-way ANOVAs tested for effects of group and sex. Independent t-tests examined comparisons of interest post-hoc. Effect sizes (d) were used to quantify the magnitude of differences. RESULTS: SampEn was higher in females (p = .004, d = 0.91) and males (p = 0.019, d = 0.83) with ACLR than controls (Table 1). SampEn was higher in females than males in ACLR (p = .005, d = 0.71) and control groups (p = .028, d = 0.89). FI was higher in females compared to males in the control group only (p = <.001, d = 1.43). %Δ SampEn did not differ between sexes (p = .182) or groups (p = .822). %Δ SampEn did not associate with FI (All p > .05, r = -.13-.12). CONCLUSIONS: Sex and ACLR status affect quadriceps force quality and quantity differently, and changes in SampEn and FI during a contraction do not correlate, indicating the two measures may explain unique constructs of external force expression in patients recovering from ACLR.