Differences in EMG amplitude between tasks were also identified. The mean amplitude for the normalized EMG was significantly larger for RF (P < 0.001) in the OKC task compared with CKC, whereas the mean amplitude for VMO was significantly larger (P < 0.05) in the CKC task than in OKC. Amplitude of activity was not significantly different between the tasks for other muscles (Fig. 5). The differences between the EMG amplitudes within a task showed that in CKC, the VMO amplitude was greater than that of VML and RF, but less than VL. In the OKC task the VMO EMG amplitude was less than for VL. RF was less active than VL in OKC and was in CKC less active than all other muscle portions. In OKC, VML was least active.
The present study shows that there is a difference in time of onset and amplitude of EMG for the different knee extensors in open and closed kinetic chain tasks. Most notably, the near-simultaneous onset of activity of the quadriceps muscles during closed chain knee extension was not apparent when the task was performed in open chain. In general, there was agreement between the temporal and spatial EMG parameters. In CKC where VMO is activated early (Fig. 3), its amplitude was greater compared with OKC, in which its onset of activity was later. Rectus femoris had greater EMG amplitude in OKC when it was the first muscle active compared with a smaller amplitude in CKC where its EMG onset was later. This may suggest that the initial relative contribution of muscles with early onset of activity is larger than for the muscles with later onset of activity.
The differences in EMG onset and amplitude for RF in the two conditions may be explained by its nature as a two-joint muscle. In OKC where the force is directed upward, the contribution of RF is increased, presumably as a result of its dual function as a knee extensor and hip flexor. In CKC, where the force is directed downward, this is more akin to hip and knee extension. Indeed the subjects had to be firmly strapped down during testing conditions, to prevent extension at the hip in CKC. On the contrary, in OKC there was less tendency to extend at the hip.
The result from our study shows that CKC provides more simultaneous activity in the different portions of the quadriceps muscle than OKC, with earlier onset and greater amplitude of EMG activity in VMO. Because muscle function has significant impact on the biomechanics of the knee joint, CKC tasks may provide more optimal loading conditions for the patellofemoral joint due to more central tracking of the patella (20). A mediolateral muscular imbalance in force production (3,7,24) and timing (4,26,29) has been suggested by several authors as important factors contributing to malalignment of the patella. Malalignment affects the pressure distribution between patella and femur. In vitro and modeling studies of forces show increased lateral pressure as tension from the VMO is decreased (20). The main cause for patellofemoral pain syndrome (PFPS) is believed to be lateral tracking and/or tilt of patella in the femoral groove. Weakness of the knee extensors and atrophy of vastus medialis muscle are common findings (25). Patients with this syndrome also show a decrease in VMO activity relative to VL. In knee extension the ratio between VMO and VL activity increases closer to full extension, whereas the ratio in nonsymptomatic subjects remains steady (3,24). For onset of muscle activity, PFPS patients show a delayed onset of activity in VMO relative to VL, when ascending and descending stairs, by 16 and 19 ms, respectively (4). In nonsymptomatic subjects, there is no difference in onset time for VMO and VL in these same tasks. These findings are supported by other similar studies, however, with smaller time differences (7,26,29). Degree of decreased reflex response time in VMO and duration of symptoms have been reported to be the only factors that significantly predict the outcome of training intervention for this patient group. Shorter reflex time of VMO predicts a better functional outcome (28).
Seemingly small time differences (5–10 ms) appear significant for the central nervous system to coordinate muscle activity for a certain task. Even with the same joint configuration, the net mechanical effect of different loading conditions requires the central nervous system to adjust the strategy accordingly (9). For instance, recent biomechanical studies have indicated that a delay in VMO onset of 5 ms has significant consequences for patellofemoral joint mechanics in terms of increased peak and average lateral contact force (17). In addition increased relative contribution of VMO force produces a reduction in lateral patellofemoral joint loading (17). The findings from the present study, particularly regarding onset and activity of the VMO may have clinical implications for how to design training intervention programs for patients suffering from PFPS. For knee rehabilitation in general, CKC exercises have been promoted in favor over OKC, because CKC exercises are considered more functional, safe, and effective (19,21). Exercises designed to remedy muscular imbalances as described for PFPS should be particularly aimed at VMO. Our study shows in healthy subjects that CKC promotes more simultaneous quadriceps activity and earlier onset and greater amplitude in EMG activity for VMO than does OKC. To what extent this also applies to PFPS needs to be investigated. We compared OKC and CKC tasks under isometric conditions in identical positions, seated with the hip in 90° and 30° knee flexion from full extension, with moderate force exertion. However, activation patterns may be different for OKC and CKC as other biomechanical conditions apply for dynamic conditions with different joint angles and loading conditions. Evaluation of CKC training intervention has showed that for patients with patellofemoral pain, more selective VMO activation can be obtained in closed kinetic chain exercises at 60° knee flexion (23). Hodges and Richardson (10) reported greater VMO activity in CKC, which could be further augmented by additional hip adduction. Even though CKC in PFPS may elicit earlier and greater VMO activity than OKC exercises, this may not guarantee a normalization of VMO activity in other activities. It also remains to be investigated whether and to what extent an eventual normalization of VMO activity in an exercise condition has a carry over effect to daily activity with improvement of physical function and reduced pain.
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