Musculoskeletal Bibliography

PURPOSE/HYPOTHESIS: To determine differences in hip and knee strength, EMG activity, and kinematics in subjects diagnosed with and without patellofemoral pain syndrome (PFPS).

SUBJECTS: Twenty females diagnosed with PFPS and 20 age-matched controls.

MATERIALS/METHODS: Subjects completed a 10-cm visual analogue pain scale. Surface electrodes were donned to the gluteus medius (GM), vastus medialis (VM), and vastus lateralis (VL) of the symptomatic lower extremity for PFPS subjects and the right lower extremity for controls. Subjects performed 3 repetitions of isometric hip abduction, hip external rotation (HER), and knee extension against a hand-held dynamometer. EMG data were also simultaneously collected to determine the maximal voluntary isometric contraction (MVIC) for each muscle. Reflective markers were then placed on each subject prior to the stair-stepping task. Prior to testing, subjects practiced ascending and descending 2 20-cm high steps (lifting and lowering the body with the test extremity). They performed 5 test repetitions. Kinematic data were sampled at 60 Hz and low pass filtered at 6 Hz. EMG data were amplified, band pass filtered from 20-500 Hz, and sampled at 960 Hz with video data. Seven control subjects were retested within 1 week to establish reliability. Average isometric torque generated for each strength test was normalized to height and weight. Hip adduction, internal rotation, and knee valgus angles during stair descent were ensemble averaged. GM, VM, and VL EMG activity during stair descent were ensemble averaged and expressed as a percent MVIC. Average onset timing differences between the GM and vasti muscles at the beginning of stair descent were also determined. Mann-Whitney U tests identified group differences. Spearman rho coefficients quantified associations between pain and all variables.

RESULTS: Intraclass correlation coefficients exceeded .78. PFPS subjects generated less muscle torque (P<.02) but greater EMG activity (P<.02) during stair ascent that controls. No differences existed for kinematic and onset timing variables. A moderate association existed between pain and HER strength (r = -0.62) and GM (r = 0.56) and VM (r = 0.64) EMG activity during stair descent.

CONCLUSIONS: Contemporary rehabilitation for PFPS patients has focused on quadriceps strengthening. Results from this study inferred that hip weakness was related to PFPS as evidenced by a stronger association between pain and HER strength and significant GM activity during stair descent. However, we were unable to determine if hip weakness was a cause or a result from PFPS. Our results showed no kinematic differences, which may have resulted from difficulty measuring transverse and frontal plane motion. EMG onset timing differences between the GM and vasti muscles were not found, implying that delayed VM activity was not a significant factor.