Activity in three parts of the quadriceps recorded isometrically at two different knee angles and during a functional exercise.

The purpose of this study was to evaluate individual differences in three parts of the quadriceps activated isometrically at 60 degrees and 90 degrees of knee flexion, and during a functional activity involving both concentric and eccentric muscle work. Surface EMG amplitudes were therefore recorded from oblique parts of vastus medialis (VMO) and vastus lateralis (VLO) and from rectus femoris (RF). VMO and VLO showed less activation at 60 degrees than at 90 degrees, but in RF there was no difference between the two angles. In the second experiment, where 11 subjects stepped on and off a stool; these amplitudes were compared with those from a maximal isometric voluntary contraction at 90 degrees of knee flexion. For VMO & VLO the normalised peak amplitude in stepping up was 1.41 +/- 0.12, & 1.46 +/- 0.15 respectively, showing that higher activity is necessary during concentric contractions. These two results suggest that the motor control of VMO/VLO may be different from the bulk of quadriceps. Our findings have implications for patellofemoral function.

A comparison of the activation of muscles moving the patella in normal subjects and in patients with chronic patellofemoral problems.

The activation of the oblique fibres of vastus medialis, the postero-lateral fibres of vastus lateralis, and rectus femoris was studied by surface electromyography, during the force development of a maximal isometric contraction, performed near full extension (20 degrees of flexion), by 49 patients with chronic patellofemoral problems and 20 normal subjects. In the normal subjects activation of oblique portions of the vasti was in advance of force rise, during the time for 80% tension development. In the patient group, however, the activation of these lagged behind force rise. Force rise was slower in the patients even though the contraction was generally pain free. In all groups the activation of these two sections of the muscle remained approximately synchronous, suggesting that they have a reciprocal action in controlling patellar position, disruption of which might contribute to patellofemoral problems.