By Matt Greig, MPhil, PhD
Hamstring strain incidence in soccer increases during the latter stages of match-play, consistent with experimental studies that report reduced eccentric hamstring strength during soccer-specific fatigue.
The biarticular function of the hamstring is such that fatigue-induced changes in strength are also likely to have an impact on knee-joint stabilization, with the knee a common site of severe injury in soccer. The aim of this study was to consider concurrent changes in eccentric hamstring strength and knee-joint kinematics, with isokinetic testing speed matched to the kinematic demands of the task. In selecting an appropriate functional task, jump landings and hopping tasks are commonly used because they replicate the mechanism of knee ligamentous injury.
To investigate the influence of soccer-specific fatigue on concurrent changes in knee-joint kinematics and hamstring strength, 10 male professional soccer players completed reactive inversion, eversion, and neutral hop tasks at 15-minute intervals during a soccer-specific treadmill protocol. Knee-joint kinematics in frontal and sagittal planes were calculated every 15 minutes. In a separate trial, players completed maximal eccentric knee flexions at 160° s-1 (reflecting average knee angular velocity in the functional task) at 15-minute intervals, quantifying peak torque.
All trials were characterized by knee flexion and varus at touchdown. Knee flexion of approximately 30° was maintained throughout the exercise protocol, but a main effect for task highlighted greater flexion at touchdown in the inversion trial (P = .01). This task-dependent observation in the degree of knee flexion at landing most likely indicates the greater mechanical challenge imposed by this task, with inversion movements incurring a greater risk of ligamentous injury.
Knee varus at touchdown (Figure) was sensitive to exercise duration, with approximately 4° greater malalignment elicited over the final 15 minutes of the protocol (P ≤ .05). Although knee flexion at touchdown serves as a protective mechanism, varus displacement of the knee might be a risk factor for ligamentous injury. Genu varum in these elite male players is in marked contrast to the knee valgus commonly observed in female players, and has been identified as a risk factor for patellofemoral pain syndrome, deterioration of the articular cartilage in the medial tibiofemoral compartment of the knee, and osteoarthritis.
Peak eccentric hamstring strength was significantly (P = .05) reduced throughout the second half of the trial, compared to pre-exercise values. This reduced strength might provide the mechanism for increased varus displacement because hamstring musculature acts to stabilize the knee joint during such functional tasks. Conversely, the knee in varus would increase tension on the biceps femoris, creating an increased risk of hamstring strain injury with fatigue—a finding that would support epidemiological observations.
Coincident impairment of eccentric hamstring strength and increased knee varus at touchdown predisposes the player to injury—again, supporting epidemiological observations. The association between strength and joint kinematics implicated in injury risk provides clinical merit for isokinetic assessments, provided that the isokinetic assessment has functional relevance to the demands of the sport or the mechanism of injury (or both). In this study, functional relevance was aligned to speed-matched analyses of strength and knee-joint motion.
Matt Greig, MPhil, PhD, is Associate Head for Sports Therapy, and Sports Injuries Research Group Lead at Edge Hill University, Ormskirk, Lancashire, England. His research focuses on the physical (biomechanical and physiological) response to soccer-specific activity, with implications for performance enhancement and injury prevention.
Source: Greig M. Concurrent changes in eccentric hamstring strength and knee joint kinematics induced by soccer-specific fatigue. Phys Ther Sport. 2019:37:21-26.