Ten healthy subjects, three males and seven females, (mean age 28.5 ± 0.7, mean height 171 cm ±8.5, mean weight 64 kg ±15.6) participated in the study. Subjects were excluded if they had a current or previous record of knee pain, trauma, surgery, or other joint disease or were involved in competitive sports. The tests were performed in agreement with the Declaration of Helsinki and informed written consent was obtained from the subjects. The study was approved by the institutional research ethic committee.
EMG activity of vastus medialis obliquus (VMO), vastus lateralis (VL), vastus medialis longus (VML), and RF was recorded with surface electrodes (5 mm disks, Grass, U.S.) placed approximately in parallel with the muscle fibers over the muscle bellies, based on a modification of standard proposed by Zipp (30). The distances and angles were measured for optimal electrode placement (Fig. 1). The skin was carefully prepared by rubbing with abrasive gel and alcohol. EMG data were amplified 2000 times, filtered between 20 and 1000 Hz (Neurolog, UK) and sampled at 2 kHz using Power1401 and Spike2 software (CED, UK).
Knee extension force was measured with a strain gauge (Validyne, U.S.). Force data were amplified and sampled at 1 kHz with the EMG data.
Subjects sat on a firm plinth with the hip flexed to 90° and knee flexion 30° from full extension. Ankle joint position was kept at 90°. The pelvis was firmly strapped to the plinth. This position was used as it represented a mid range position and allowed the joint position to be kept constant between tasks. Knee extension efforts were performed as a reaction-time task in two different conditions. For OKC, the strain gauge was connected from the plinth to a strap around the ankle, approximately 10 cm proximal to the malleoli and isometric knee extension efforts were made against the resistance of the cable. In the CKC task, the strain gauge was incorporated into an inelastic belt that passed around the trunk support of the plinth and under the sole of the foot (Fig. 2). Isometric extension efforts were performed by pushing the foot into the belt. Subjects were instructed to respond as quickly as possible (by either extending the knee or pushing into the belt depending on condition) in response to an auditory stimulus and to use a moderate effort. Twenty repetitions in sets of 10 were performed for each condition and subjects were allowed 0–30 s of rest between each repetition and 2–3 min of rest between sets of 10. Subjects were encouraged to relax their quadriceps between each repetition. Experimenters observed EMG activity with high gain to ensure activity was minimal during the rest period. The order of task presentation was randomized between sets of OKC and CKC. Subjects performed a single maximal voluntary contraction for 5 s against manual resistance and with loud verbal encouragement for each task after completion of 20 repetitions.
The onset of EMG activity of each muscle and the onset of force measure were identified visually for each trial. To remove observer bias, data were presented for each individual trial in random order with no reference to muscles or order of repetition. The time of onset of force was identified in a similar manner. EMG amplitude was calculated for the initial 100 ms of the response and normalized to the amplitude recorded during the maximum voluntary contraction. Data were presented as the difference between EMG onsets of muscle pairs (VMO:VML, VMO:VL, VMO:RF, VML:VL, VML:RF, VL:RF), difference between onset of EMG and onset of force, and difference in peak amplitude.
Differences in EMG onset latency for muscle pairs and EMG amplitude between the open and closed chain tasks were evaluated with a repeated-measures ANOVA; two factors, condition (OKC and CKC) and muscle portion (N = 4). Differences between onset of EMG and onset of force between the open and closed chain tasks were evaluated with a repeated-measures ANOVA two factors; condition (OKC and CKC) and latency (force-muscle portion (N = 4)). Values where corrected for sphericity (Greenhouse-Geisser). Paired t-tests were used to evaluate specific differences. The level of probability chosen as statistically significant was p < 0.05.
When subjects performed rapid knee extension efforts in response to an auditory stimulus, there were differences in the pattern of recruitment of the portions of the quadriceps muscles between OKC and CKC (condition × muscle interaction:P < 0.001). The onset of activity was more simultaneous in the CKC task than in OKC (Fig. 3). Figure 4 illustrates the EMG onset data expressed relative to the initiation of the force for each muscle and shows that there was no difference between muscles for CKC, that is, the onsets of EMG of all muscles were simultaneous. In contrast, for OKC, there was a difference in latency between EMG onset and onset of force increase between muscles. The latency was greatest for RF (mean 62 ms ± 20) and shortest for VMO (mean 55 ms ± 22). The data indicate that for the OKC task the EMG onsets of all muscles occurred before that of VMO. The relative latency between all pairs of muscles is significantly different between tasks for all pairs (RF:VML P < 0.05, for all the rest P < 0.001) except VMO:VL.
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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