Changes in onset times of muscular activity during a side hop task

Abstract

IntroductionRecovery of functional muscle force after anterior cruciate ligament (ACL) injury is critical prior returning to sport. Beside restored muscle power, adequately timed motor control is crucial. Since fatigue alters neuromuscular control and is suggested to increase ACL injury risk, we hypothesized that the development of fatigue during a standardized 30-second hop test would modify the onset times of EMG activity in healthy individuals. MethodsEight healthy participants (4 males, 4 females; aged 26±6years) performed a 30-second one-legged side hop test across a distance of 40cm using their non-dominant leg. Synchronized kinetics (Arsalis force plates sampled with 1kHz) and surface electromyography (Noraxon TeleMyo 8-channel EMG system sampled with 1.5kHz) signals were recorded. Successful hops, defined as jumping side-to-side across the two parallel lines without touching the marked area, were identified by video analysis. The technique described by Santello and McDonagh (1998) was used to obtain the onset times of muscular activity from corresponding electromyographic signals of vastus medialis (VM), vastus lateralis (VL), biceps femoris (BF) and semitendinosus (ST). Vertical ground reaction force exceeding 10N indexed landing events. Preceding muscular activity onset times were expressed in milliseconds (ms). To discriminate between non-fatigued and fatigued conditions, the first and last five successful hops of the test were compared with paired samples t-tests. ResultsAveraged onset times of muscular activity before landing during the first five hops were as follows: 56±11ms in VM, 55±11ms in VL, 77±14ms in BF and 86±13ms in ST. The last five hops showed significantly earlier onset times for VM (74±15ms, P=0.020) and VL (66±12ms, P=0.046), but not for BF (90±12ms, P=0.143) and ST (91±20ms, P=0.455). Males and females displayed similar onset times for all muscles in all conditions. DiscussionIn the course of a 30-second side hop task healthy individuals display earlier activation onset times of knee extensor muscles prior to landing. This strategy likely stiffens the knee joints and may help anticipate impact loads under fatigue. The reason why this was not observed in knee flexor muscles is unclear. However, assuming that the main role of the quadriceps is to deliver the knee extension power while hamstrings account more for the stabilization, the effect of fatigue could be identified more easily in the former muscle group. Further research is needed to elucidate if the evaluation of muscle onset timing may be of interest for the return to sports decision after ACL injury.