The purpose of this study was to evaluate any differences in quadriceps activation during functional activity between patients with patellofemoral problems and normal subjects. 24 patients and 11 controls were assessed. Surface EMG amplitudes were recorded from three parts of the quadriceps, vastus medialis obliquus (VMO), lower fibres of vastus lateralis (VLO) and rectus femoris (RF), whilst subjects stepped on and off a stool. These amplitudes were normalised to those from a maximal isometric voluntary contraction at 90° of knee flexion. The patients activated their VMO significantly less effectively than controls (mean + SEM normalised peak amplitude was 1.06 + 0.09 in patients, compared to 1.41 + 0.12 in the control subjects). In the controls, most of the activity was seen with the knee in flexion, with very little activity with the knee in extension. In comparison, patients recorded less activity with the knee in flexion and more when the knee approached extension. EMG amplitudes in isometric contractions at 60° of knee flexion were compared with those at 90°. VMO and VLO showed less activation at 60° in both groups of subjects, but in RF there was no difference in the EMG, between these two angles. This suggests that the motor control of VMO/VLO may be different from the bulk of the quadriceps. We have shown that high activity in the VMO and VLO parts of the quadriceps appears to be important in an exercise that involved taking the body weight on one knee at around 90° of flexion. This high activity is likely to have two effects: it is important for the stability of the knee itself, and by increasing the area of contact, it reduces contact pressures. Patients with patellofemoral problems were unable to produce this needed activity in VMO. The VMO and VLO may have a different motor control from the bulk of the quadriceps, and in normal function, work synchronously. The loss of this synchronicity in patients with patellofemoral problems, could well help to explain the retropatellar pain that these patients experience.
