Razi Journal of Medical Sciences

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New studies in neuromuscular control have indicated that the strength of muscle contraction isn’t the only important factor in a task. But the speed at which a muscle responds, preparation of muscle and synergistic pattern at which a group of muscles recruit to a task or respond to a condition are much more important in constant joint stabilization and potential injury prevention. The purpose of this study was to assess the timing and scaling of knee related muscles’ electrical activity. Included muscles were: vastus medialis, vastus lateralis, rectus femoris, gastrocnemius, lateral hamstring and medial hamstring. The electrical activity was collected with the use of surface electromyography during selected active and reactive movement patterns. This research was a quasi-experimental design on 30 healthy young men (assigned by the sample of convenience) between 20-30 years of age (avg: 25.36). Our subjects sequentially and randomly were asked to perform different movement patterns and during these tests EMG signals were collected from each muscle. These signals were collected to assess onset time and the amount of muscle electrical activity (IAV) in each task. All movement patterns were in three main categories. Active movements included forward step up and lateral step up abrupt movements included vertical jump and distance jump and reactive movements (angular perturbation of base of support) included anterior and posterior perturbation with knee straight and knee bend. In this research both muscles in reactive movement patterns were significantly activated before active movements. Vastus medialis in anterior perturbation was activated 182.6 ms sooner than FSU (P<0.0001), and medial hamstring was 279.5 ms earlier under the same situation. The gastrocnemius muscle activity was 23.154 µv in distance jump, but 5.46 µv in FSU. In anterior perturbation VM and VL were significantly activated faster than posterior limb muscles, for example, VM was 23.3 ms before medial hamstring. However, in posterior perturbation the findings were vise versa and posterior muscles were faster. In all reactive movements medial hamstring had greater level of muscle activity, related to lateral hamstring and itself, in active movements. In posterior perturbation MH was activated 7.2 µv more than LH. Results indicated that learned movements had more efficiency in CNS decision for choosing a pattern of movement among possible ways of doing a task, and unnecessary muscle activity was much less in learned movements. Gastrocnemius muscle had a great role in abrupt and posterior perturbation movements in comparison with VM and VL muscles. The role of medial hamstring in reactive movements was clear and this role was not seen in lateral hamstring. This muscle can act as a propericeptive and postural muscle trigger, especially in proximal stability for trunk and pelvic region, and in distal perturbation we don't expect to see an ascending muscle synergy all the time.

Keywords: Key Words: 1) Surface electromyography 2) Postural perturbation 3) Neuromuscular control

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