Sex influences the relationship between hamstrings-to-quadriceps strength imbalance and co-activation during walking gait

Background: While traditionally viewed as a beneficial adaptation to preserve stability in the presence of knee pathology, excessive muscle co-activation may be detrimental for joint health when extrapolated to repetitive movement patterns over time. Lesser hamstrings strength relative to the quadriceps (low H:Q strength ratio) may influence neuromuscular patterns about the knee, as it is reported to increase risk for lower extremity injury among healthy females. Research question: Does the relationship between H:Q strength ratio and H:Q co-activation differ between sexes during walking and jogging? Methods: We used a descriptive laboratory study to assess hamstrings and quadriceps strength and muscle activity patterns during the loading response of treadmill walking gait (1.34 m/s) and jogging (2.68 m/s) in healthy males (n = 11) and females (n = 12). Concentric-concentric peak isokinetic torque (60 degrees /s) was used to derive the H:Q strength ratio, which was treated as an explanatory variable for H:Q co-activation indices (medial, lateral, composite) and constituent EMG amplitudes. Bivariate correlations (Pearson r or Spearman rho) were used for analysis. Results: In females, lesser H:Q strength ratios were associated with greater lateral co-activation (r=-.715, P = .007) and biceps femoris EMG amplitude (rho=- .532, P = .046) during the loading response of walking gait. When controlling for sex differences in knee flexion, the relationship between lesser H:Q strength ratios and greater lateral co-activation was preserved (partial r=-.699, P = .012); yet, biceps femoris EMG was no longer correlated (partial r=-.331, P = .175). Significant relationships were not observed among male participants during walking or in either sex during jogging (all P > .05). Significance: Collectively, these data provide evidence of a sex-specific neuromuscular pattern with implications for joint health. Excessive lateral co-activation may consequently promote a greater valgus moment and ligamentous strain. Future investigations would benefit from understanding the influence of hamstrings-dominant exercise programs on the neuromuscular patterns of the knee.