Patellar Kinematics, Part I: The Influence of Vastus Muscle Activity in Subjects With and Without Patellofemoral Pain
Background and Purpose. Reduced motor unit activity of the vastus medialis muscle relative to the vastus lateralis muscle has been implicated as a cause of lateral patellar subluxation. The purpose of this study was to assess the influence of vastus musclemotor unit activity on patellar kinematics. Subjects. Twenty-three women (mean age 26.8 years, SD 8.5, range 14 – 46) with a diagnosis of patellofemoral pain and 12 women (mean age 29.1 years, SD 5.0, range 24 –38) without patellofemoral pain participated. Only female subjects were studied because of potential biomechanical differences between sexes. Methods. Patellar kinematics (kinematic magneticresonance imaging) and vastus muscle electromyographic (EMG) activity using indwelling electrodes were measured during resisted knee extension. Measurements of medial and lateral patellar displacement and tilt obtained from magnetic resonance images were correlated with normalized vastus lateralis:vastus medialis oblique muscle and vastus lateralis:vastus medialis longus muscle EMG ratios at 45, 36,27, 18, 9, and 0 degrees of knee flexion using a stepwise regression procedure. Results. The vastus lateralis:vastus medialis longus muscle EMG ratio contributed to the prediction of lateral patellar displacement at 27 degrees of knee flexion (r -.48), with increased vastus medialis longus muscle activity being associated with greater lateral patellar displacement. A similar inverse relationshipwas evident with lateral patellar tilt at 36, 27, 18, and 9 degrees of knee flexion. Conclusion and Discussion. These results suggest that increased motor unit activity of the vastus medialis muscle appears to be associated with abnormal patellar kinematics in women, but it is not necessarily a cause of abnormal patellar kinematics. [Powers CM. Patellar kinematics, part I: the influence of vastusmuscle activity in subjects with and without patellofemoral pain. Phys Ther. 2000;80:956 –964.]
Key Words: Electromyography, Magnetic resonance imaging, Patellar kinematics, Patellofemoral joint, Quadriceps femoris muscle. Christopher M Powers
956 Physical Therapy . Volume 80 . Number 10 . October 2000
atellar instability is commonly thought to be the result of unequal activity of thevarious components of the quadriceps femoris muscle.1–5 More specifically, lateral patellar subluxation has been attributed to reduced motor unit activity of the vastus medialis muscle.6 – 8 Lieb and Perry9 separated the vastus medialis muscle of cadavers into 2 functional components based on fiber orientation, with the proximal longitudinal fibers being termed the vastus medialis longus muscle(VML) and the distal oblique fibers being designated the vastus medialis oblique muscle (VMO). As a result of its more horizontal fiber orientation, they considered the VMO to be the primary medial stabilizer of the patella. This premise has formed the theoretical basis for exercises for patellofemoral pain (PFP) because improving VMO force is thought by some authors3,10,11 to be essential inovercoming the lateral pull of the much larger vastus lateralis muscle (VL). Despite the large emphasis on the VMO in the treatment of PFP, assessment of VMO force production in vivo is not possible. In lieu of this limitation, electromyography (EMG) has been used to establish the activity patterns of the vastus muscles with the rationale that decreased activity of the VMO relative to the VL isindicative of compromised medial patellar stability. Numerous researchers7,8,12–15 have compared the EMG activity of the VMO with that of the VL. There is no general consensus, however, as to whether reduced motor unit activity of the VMO exists in people with PFP or, more
importantly, whether it is predictive of abnormal patellofemoral joint function. With the advent of kinematic magnetic resonance...