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

Fatigue has been hypothesized to alter the biomechanical and neuromuscular factors associated with the risk of sustaining musculoskeletal injury. 1 Biomechanical and neuromuscular factors include muscle strength, rate of force production, muscle activation strategies, movement strategies, and stiffness properties of the muscle and joint complex. 2 These variables are believed to be of particular importance because they involve active muscle contributions. After a fatiguing exercise bout, biomechanical and neuromuscular factors such as muscle activation patterns, 3–7 coactivation, 8–10 kinematics and kinetics, 4, 5, 11–14 and stiffness properties 5, 15, 16 are altered. In addition, epidemiologic research suggests a relationship between fatigue and musculoskeletal injury rates. 17, 18 Investigators have demonstrated a greater incidence of injuries occurring during the later stages of athletic competitions and practices. 17, 18 The psychosocial perception of fatigue has also been implicated as a factor influencing injury, because the perception of fatigue measured during preseason and midseason strongly predicted injury rates over the course of a season. 19 Based on these research findings, fatigue does appear to influence musculoskeletal injury rates during athletic activities. Preexisting sex differences in various biomechanical and neuromuscular factors 20–25 may be amplified by fatigue. For example, females have been shown to have less muscle strength than males 24 and may respond differently to a fatiguing bout of exercise because of this inherent strength difference. However, the effects of fatigue in males and females have not been compared. If females respond differently than males to fatigue, this may be an additional risk factor for anterior cruciate ligament (ACL) injury. 1 Most of the previous authors investigating the influence of fatigue on biomechanical and neuromuscular factors have studied open kinetic chain exercise fatigue protocols that were performed under maximal effort conditions for a short duration of time. 1, 9–12 14, 16 Unfortunately, these types of fatigue protocols do not mimic the loading conditions sustained by the lower extremity. It may be important to assess the effects of fatigue using movement patterns similar to those experienced by the lower extremity (closed kinetic chain) and performed under submaximal loading conditions for sustained periods of time. In addition, no previous investigators have assessed the influence of sex and fatigue induced via closed kinetic chain exercise on vertical leg stiffness (K VERT) and control strategies for modulating K VERT. Control strategies for modulating K VERT may be operationally defined as the multijoint coordination (joint kinematics) and muscular recruitment plan (muscle activation) an individual executes to modulate joint torsional stiffness and lower extremity stiffness, hence satisfying the objectives of the functional task. 26–28 Therefore, our primary purpose was to investigate the effect of lower extremity muscle fatigue induced through closed kinetic chain exercise on K VERT and control strategies for modulating K VERT (muscle activation peak amplitude, coactivation, and sagittal-plane knee and ankle joint kinematics) during a functional hopping task. Our secondary purpose was to determine whether the response to lower extremity fatigue differed between healthy males and females.