The results of the present study showed medium to high reliability for the determination of VMO, VL onset, high to very high for determination of MH, LH onset during stair ambulation. Moreover, the EMG amplitude showed moderate to high reliability during stair descending.
It is difficult to have a comparison in reliability between this study and the previous ones, due to different experimental design, EMG onset identification, electrode placement and statistical analysis. We performed the current study to determine the activation pattern of muscles around the knee in PFPS subjects. Our results indicate that patients with PFPS have a different muscle activation pattern when descending stairs compared to healthy subjects. Subjects with PFPS indicated an increased onset latency for all the muscles assessed during stair descending task except for LH.
We observed a significant difference in LH-MH OTD between the groups. The results revealed that hamstring muscle activity pattern was different in patients with PFPS compared to control group.
Quadriceps activation pattern
previous studies have reported that during stair stepping, medial-lateral quadriceps imbalance is a contributing factor to PFPS,(6, 9) (32)while others have demonstrated no significant difference between VMO-VL onset times.(5, 7, 8, 12) Tiggelen et al. used back rockers on the heels to investigate onset time difference of medial and lateral heads of quadriceps muscle using EMG. They found VL activation preceded VMO activation in PFP patients, so they reported that altered temporal characteristics in VMO-VL activation might be a contributing factor for PFPS.(11)
Our findings are consistent with those of previous studies, which found no significant difference between VMO-VL onset activities. Weight bearing tasks challenge neuromuscular control by alteration in muscle activation pattern to provide stabilization of the joint.(33) Also, Stair stepping rate maybe a source of difference in the obtained results of the current study and those by Cowan et al.(9) and Boling et al.(34). In previous studies stepping rate was synchronized with an external metronome at a rate of 96 steps per minute, while in the current study the subjects performed stair negotiation with a self-selected pace. Furthermore, physical activity level may cause controversial results as explained by Briani et al. Detected differences in VMO-VL timing were similar to those patients with PFP who practiced moderate level of physical activity in Briani study. They considered EMG onset during stair ascending in PFP patients who had high to moderate level of physical activity. They found significant difference between PFPS and pain free women who practiced an intense physical activity. They found a correlation between physical activity level and muscle activation onset.(32) Functional methodological approach that we use in this study might explain the disparity of the results.
Patients with PFPS demonstrated a significantly lower amplitude of VMO, VL compared to healthy participants. Force modification is a compensatory mechanism to reduce patellofemoral compressive force during pain provoking activities. These results are in agreement with those of Santos et al. who assessed the influence of open and close kinetic chain activities on the electrical activity of VMO, VL longus and VL obliqus muscles. They reported significantly lower values for VMO/VLL ratios in individuals with PFPS.(35)
Hamstrings activation pattern
We found that patients with PFPS executed stair descending by activating the lateral hamstring earlier than medial hamstring. Our finding concurred with previous research by Patil et al.(22) who reported lateral-medial hamstring imbalance as a contributing factor that might cause abnormal knee external rotation moments in patients with PFPS. Sheehan et al.(18) showed that external rotation of tibia would increase the patellofemoral contact pressure in response to lateral translation of patellar tendon. Moreover, in vitro studies have indicated that hamstring loading produced tibial external rotation and posterior translation, which have led to increased pressure on lateral patellar cartilage.(17, 36–38) According to Shultz et al.(23) medial-lateral hamstrings synchronous activity during an external perturbation stabilize the knee joint against rotatory function of these muscles in contrast to unilateral activation of them.
Secondary analysis demonstrated a substantially lower activation level of hamstring in patients with PFPS compared to the control group. This result was similar to those reported by Liebensteiner et al. (24) who concluded that reduced activation of hamstring during eccentric contraction might be a compensatory strategy to limit the harmful loading of patellofemoral joint. Elias et al.(17) suggested exercises that minimize the hamstrings activation, as the hamstrings contraction exacerbated the PFPS due to lateral malalignment.
It was suggested that reduction in hamstring activity would be associated with pain inhibitory response that influenced not only agonist muscles but also antagonist group. Dieter et al. reported higher BF but lower ST activation level in PFPS patients compared to control group.(21) Difference of our study with theirs might be due to different nature of the evaluated movements.
Our study had some limitations. First, as neuromuscular control could be affected by sex hormones, we only investigated female population in this study. Therefore, generalizing our results to entire population with PFPS must be made with caution. Second, it is possible that abnormal tibial kinematics caused by medial hamstring time delay, also, it is plausible that tibial external rotation caused medial-lateral hamstrings imbalance as a compensatory strategy. Given that, we could not establish a cause and effect relationship.
This is the first study to demonstrate hamstrings muscle imbalance in PFPS patients during a functional task. This study established the use of a functional rehabilitation in weight bearing conditions focusing on neuromuscular control of the knee especially hamstring muscles. Future studies should include men, kinematic and kinetic assessments to identify whether altered hamstrings activation can affect abnormal tibial torsion and to understand whether hamstrings onset time difference might affect spatiotemporal measures of stair ambulation.