The Neuromuscular and Mechanical Control of the Knee Joint in Patellofemoral Pain Sufferers

Abstract

Results
Grouped Tape Response: The mean motor unit firing exhibited an increase in the VM firing rate and a decrease in the VL firing rate within the tape condition, across both subject groups, suggesting a modification in the load bias across the Vasti muscles. The common drive, a physiological phenomenon describing common fluctuations in the motor unit firings and consequently a measure of the nervous systems signals to control pools of motor units, increased in the VM and decreased for the VL in response to the tape condition in symptomatic subjects showing that the tape may be providing enhanced feedback to the nervous system that responded by modifying the ‘control’ to the motor units. There were no changes however in the non-symptomatic subject group, perhaps suggesting their motor units were already controlled efficiently. Motor unit recruitment analyses, through regression analysis of the motor unit firing rate and knee joint moment, showed the VM motor units in the non-symptomatic subjects were recruitment at a higher firing rate in the tape condition but interestingly no change in the VL and no change in the symptomatic subjects. Both non- symptomatic and symptomatic subjects demonstrated significant reductions in transverse plane knee joint range of moment, illustrating a more controlled rotational knee joint after the application of tape. There were no significant changes found in the coronal or sagittal joint mechanics. Symptomatic subjects demonstrated a propensity to consciously control their movements, suggestively disrupting automatic motor control tasks. Symptomatic subjects presented with pain scores of 4.2/10 on the numerical rating scale.
Individual Tape Response: Exploration of individuals’ response to tape, opposed to pooling data and treating as homogenous groups, exhibited a non-uniform response with variable increases, decreases and no changes across the different measurements taken. The exploration of the data with this method is in line with common clinical presentation of PFP subjects and presents rationale for new ways to view the data as to not mask the true physiological behaviours.
Comparison of groups: Motor unit recruitment analyses comparing the subject groups demonstrated that symptomatic subjects had a significantly different motor unit recruitment strategy for the Vasti muscles, where the larger motor units were firing faster compared to the non-symptomatics larger motor units in both VM and VL for the same level of force. Symptomatic subjects demonstrated a lower common drive to the VM and higher common drive in the VL compared to non-symptomatic subjects, which after the application of tape became the same level as the non-symptomatics. Symptomatic subjects also exhibited significantly lower muscle activity in the GAS, BF, RF and GMed, thus suggestively increasing the muscle activity bias to the Vasti muscles. No changes were seen in the joint biomechanics or mean motor unit firing rate between the groups.

Discussion
These findings suggest that the nervous system offers a portfolio of solutions to control and distribute force, which can be manipulated through a common taping intervention. The results show that the motor unit firing rate in the VM increases and decreases in the VL, coupled with altered motor unit recruitment strategies thus inferring that there may be a re-distribution of force across the Vasti with the application of tape. This is interestingly complimented with an increase of common drive in the VM and decrease in the VL between subject groups. The presence and then change of common drive within the Vasti so that the motor units are firing in unison, more so with tape, and providing a tantalising prospect that the VM muscle is acting more efficiently and controlled with tape. The novel findings of the neuromuscular system and its modification were alongside the increase in torsional joint control. However, it is evident that the motor unit firing rate, common drive and motor unit recruitment present variable responses amongst individuals, offering different solutions to achieve the same goal; increasing the force and its control within the muscle. The underlying mechanism for the observed findings are unable to be expressed definitively, however it can be deduced that the application of tape presents proprioceptive feedback to the muscle that alters the motor unit pool; consequently adjusting the force and its control within and across muscles leading to an increase in knee stability.

Conclusion
The key implications from this work is that the application of tape can offer clinically meaningful changes to the sensory-motor control system, through the manipulation and alteration of the motor unit pool, suggestively from an enhanced proprioceptive feedback mechanism. However, researchers and clinicians should consider the individualistic responses and the potential to mask true physiological findings by assuming homogeneity within patient populations with data analyses and clinical decision processes respectively. This work offers unique and novel insights into both the behaviour of patients with Patellofemoral Pain and also the effects of a taping intervention, thus providing additional clinical understanding and also tantalizing opportunities for future work exploring musculoskeletal or neurological disorders and insight into the sensory-motor control strategies.