Submaximal fatigue of the hamstrings impairs specific reflex components and knee stability.
ABSTRACT Rupture of the anterior cruciate ligament (ACL) is one of the most serious sports-related injuries and requires long recovery time. The quadriceps and hamstring muscles are functionally important to control stability of the knee joint complex. Fatigue, however, is an important factor that may influence stabilizing control and thus cause ACL injuries. The objective of this study was therefore to assess how submaximal fatigue exercises of the hamstring muscles affect anterior tibial translation as a direct measure of knee joint stability. While 15 test participants were standing upright with the knees in 30 degrees of flexion, anterior tibial translation was induced by a force of 315 N. Two linear potentiometers placed on the tibial tuberosity and the patella recorded tibial motion relative to the femur. Reflex latencies and neuromuscular hamstring activity were determined using surface electromyography (EMG). Muscle fatigue produced a significant longer latency for the monosynaptic reflex latencies, whereas no differences in the latencies of the medium latency component were found. Fatigue significantly reduced EMG amplitudes of the short and medium latency components. These alterations were in line with significantly increased anterior tibial translation. Our results suggest that hamstring fatigue is effectively associated with mechanical loss of knee stability. This decrease in joint stability may at least in part explain higher risk of ACL injury, especially in fatigued muscles. Furthermore, we discuss why the present findings indicate that reduced motor activity rather than the extended latency of the first hamstring response is the reason for possible failure.
Article: Effect of fatigue on hamstring reflex responses and posterior-anterior tibial translation in men and women[show abstract] [hide abstract]
ABSTRACT: Anterior cruciate ligament (ACL) rupture ranks among the most common injuries in sports. The incidence of ACL injuries is considerably higher in females than in males and the underlying mechanisms are still under debate. Furthermore, it has been suggested that muscle fatigue can be a risk factor for ACL injuries. We investigated gender differences in hamstring reflex responses and posterior-anterior tibial translation (TT) before and after fatiguing exercise. We assessed the isolated movement of the tibia relative to the femur in the sagittal plane as a consequence of mechanically induced TT in standing subjects. The muscle activity of the hamstrings was evaluated. Furthermore, isometric maximum voluntary torque (iMVT) and rate of torque development (RTD) of the hamstrings (H) and quadriceps (Q) were measured and the MVT H/Q as well as the RTD H/Q ratios were calculated. After fatigue, reflex onset latencies were enhanced in women. A reduction of reflex responses associated with an increased TT was observed in females. Men showed no differences in these parameters. Correlation analysis revealed no significant associations between parameters for TT and MVT H/Q as well as RTD H/Q. The results of the present study revealed that the fatigue protocol used in this study altered the latency and magnitude of reflex responses of the hamstrings as well as TT in women. These changes were not found in men. Based on our results, it is conceivable that the fatigue-induced decrease in neuromuscular function with a corresponding increase in TT probably contributes to the higher incidence of ACL injuries in women.PLoS ONE 02/2013; 8(2):1-8. · 4.09 Impact Factor
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ABSTRACT: Descent of a ramp has been shown to induce large anterior shear forces on the knee joint. Compensatory muscle responses observed in individuals following an anterior cruciate ligament (ACL) injury are believed to be adopted for the purpose of reducing these forces at the knee, in the absence of the mechanical restraint previously provided by the ACL. As such, examining the kinematics and muscle responses of ACL deficient individuals during ramp descent may provide further insight into strategies used by this population to compensate for anterior shear forces at the knee. Eight ACL deficient individuals were studied, in comparison to a healthy CONTROL group (N=8), during the descent of a 20 degrees ramp. Kinematics and electromyography were recorded for the injured lower limb of ACLD and matched limb of healthy control individuals. ACLD individuals produced altered knee kinematics at heel contact only. Knee motion through stance and swing were similar to CONTROL individuals. ACLD individuals produced significantly greater vastus lateralis and gastrocnemius total muscle activity, but decreased total biceps femoris activity. No significant differences were observed for the timing of peak muscle activity or the magnitude at this point between ACLD and CONTROL. Greater total muscle activity of vastus lateralis implies that greater force contributions from this muscle were used by ACLD in comparison to CONTROL in response to the ramp. These observations reinforce that quadriceps avoidance is not used by ACLD individuals to reduce anterior shear forces at the knee joint. Rather, vastus lateralis may be used to reduce internal tibial rotation in extreme loading situations.The Knee 04/2008; 15(2):117-24. · 1.74 Impact Factor