Fatigue, vertical leg stiffness, and stiffness control strategies in males and females.

CONTEXT Fatigue appears to influence musculoskeletal injury rates during athletic activities, but whether males and females respond differently to fatigue is unknown. OBJECTIVE To determine the influence of fatigue on vertical leg stiffness (K (VERT)) and muscle activation and joint movement strategies and whether healthy males and females respond similarly to fatigue. DESIGN Repeated-measures design with all data collected during a single laboratory session. SETTING Laboratory. PATIENTS OR OTHER PARTICIPANTS Physically active males (n = 11) and females (n = 10). INTERVENTION(S) Subjects performed hopping protocols at 2 frequencies before and after fatigue, which was induced by repeated squatting at submaximal loads. MAIN OUTCOME MEASURE(S) We measured K (VERT) with a forceplate and peak muscle activity of the quadriceps, hamstrings, gastrocnemius, soleus, and anterior tibialis muscles with surface electromyography. Sagittal-plane kinematics at the knee and ankle were recorded with an electrogoniometer. RESULTS After fatigue, K (VERT) was unchanged for all subjects. However, both males and females demonstrated reduced peak hamstrings ( P = .002) and anterior tibialis ( P = .001) activation, coupled with increased gastrocnemius ( P = .005) and soleus ( P = .001) peak activity, as well as increased quadriceps-hamstrings ( P = .005) and gastrocnemius/soleus-anterior tibialis coactivation ratios ( P = .03) after fatigue. Overall, females demonstrated greater quadriceps-hamstrings coactivation ratios than males, regardless of the fatigue condition ( P = .026). Only females showed increased knee flexion at initial contact after fatigue during hopping ( P = .03). CONCLUSIONS Although K (VERT) was unaffected, the peak muscle activation and joint movement strategies used to modulate K (VERT) were affected after fatigue. Once fatigued, both males and females used an ankle-dominant strategy, with greater reliance on the ankle musculature and less on the knee musculature. Also, once fatigued, all subjects used an antagonist inhibition strategy by minimizing antagonist coactivation. Overall, females used a more quadriceps-dominant strategy than males, showing greater quadriceps activity and a larger quadriceps-hamstrings coactivation ratio. Changes in muscle activation and coactivation ratios because of fatigue and sex are suggested to alter knee joint stability and increase anterior cruciate ligament injury risk.