Full length articleWalking mechanics for patellofemoral pain subjects with similar self-reported pain levels can differ based upon neuromuscular activation
Introduction
Knee pain is a common problem that often limits physical function and decreases quality of life [1]. More specifically, patellofemoral pain (PFP) is a common musculoskeletal impairment in physically-active individuals [2], [3]. Approximately 25% of all knee problems that result from physical activity are attributed to PFP [4]. Further, although PFP incidence rates are not consistent across populations [5], PFP was shown to affect 15% and 12% all physically-active females and males, respectively, in a military cadet population [6]. PFP often lasts for long durations [7], involves high recurrence rates [8], restricts physical activity [7] and might be associated with the genesis and development of patellofemoral osteoarthritis [9].
The PFP population is diverse and likely contains etiological subgroups [9]. Abnormal morphology is one hypothesized contributor to PFP: abnormal knee alignment, especially in the frontal plane, and trochlear morphology has been associated with PFP [10]. Muscle weakness is another factor that may contribute to PFP; in particular, quadriceps weakness appears to influence PFP, as well as decreased strength for more proximal muscles like the gluteus medius and minimus [11], [12]. Abnormal movement mechanics are also thought to cause PFP. For example, individuals with PFP demonstrate abnormal net sagittal-plane knee torque during stair ascent and descent [13]; further, patellofemoral osteoarthritis patients demonstrate altered net knee torque during the stance phase of gait [14].
Abnormal lower-extremity neuromuscular activation is likely related to PFP [15], [16], [17]. The central activation ratio (CAR) is one of several methods that has been used to quantify voluntary quadriceps activation [18], [19]. The CAR is based on the idea that additional muscle force which can be elicited beyond a maximal voluntary isometric contraction, via a superimposed electrical stimulus, reflects a muscle’s inability to produce maximal voluntary force. Various methods to quantify lower-extremity neuromuscular activation exist, including the CAR, however, PFP has often been studied by researchers who classify PFP research subjects using only self-reported pain levels. For example, researchers have often used a generic visual analog scale (VAS; [20]) or the Kujala Anterior Knee Pain Scale (AKPS; [21]) to classify PFP subjects. Also, researchers have often used self-reported pain level during various physical activities (e.g., stair descent or ascent, squatting, kneeling, prolonged sitting, running and jumping) to classify PFP research subjects [17], [22], as well as PFP duration [23], [24]. When studying PFP in a clinical setting that is absent of sophisticated laboratory measures like the CAR, classifying PFP using other measures (e.g., pain and functional ability) is appropriate.
The purpose of this study was to compare several oft-studied mechanical variables, during a common movement (walking), between two subdivisions of one sample of PFP research subjects that appeared to be homogenous based on similar self-reported pain levels. These subjects were similar, based on self-reported pain level, but different, based on neuromuscular activation level. We hypothesized that the present results would show that PFP subjects who classify similarly, based on self-reported pain level, would exhibit significantly different walking mechanics that depend upon neuromuscular activation. The overall intent of this manuscript was to demonstrate the importance of objectively considering neuromuscular activation characteristics, in addition to self-reported pain level, when researching PFP; this idea is important because the study of PFP subjects, classified by self-reported pain level alone, might be confounded due to varied motor function within the PFP subject population.
Section snippets
Subjects
Thirty subjects who participated in sport-related physical activity for at least 30 min per day, for at least 3 days per week, were recruited to participate. Subjects qualified to participate if they had experienced anterior knee pain (≥3/10 on a VAS) during stair descent, and at least two other activities (including stair ascent, squatting, kneeling, prolonged sitting, running or jumping) for at least four weeks prior to data collection. Subjects were also required to have experienced gradually
Results
No between-group difference existed for walking speed (p = 0.48): preferred walking speeds were 1.37 ± 0.12 m/s and 1.39 ± 0.15 m/s for the QD and NQD groups, respectively. CAR was significantly (p < 0.01) greater for the QD group, relative to the NQD group (QD = 0.91 ± 0.03; NQD = 0.97 ± 0.01). No between-group differences existed, however, for age (p = 0.46; QD = 23 ± 3 years; NQD = 22 ± 2 years), height (p = 0.55; QD = 1.74 ± 0.08 m; NQD = 1.76 ± 0.10 m), or mass (p = 0.40; QD = 70 ± 10 kg; NQD = 75 ± 17 kg). Similarly, the two groups did not
Discussion
The purpose of this project was to compare several commonly measured mechanical characteristics of walking, between individuals with similar levels of perceived PFP, but different quadriceps activation levels. We hypothesized that walking mechanics would differ, based on quadriceps activation, and the results supported this hypothesis: PFP subjects who classified similarly based on self-reported pain level exhibited different walking mechanics that depended on quadriceps activation level. The
Conflict of interest
There are no conflicts of interest to report.
Acknowledgement
There are no other individual contributors or funding sources to acknowledge.
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