The role played by the vastus lateralis and vastus medialis obliquus in their alignment of the patella during maximal isometric quadriceps contractions was studied by electromyography in 28 patients with unilateral symptoms of the patellofemoral pain syndrome. Eleven knees were classified as subluxation of the patella and 17 as idiopathic chondromalacia patellae. The muscular activity pattern was the same in the two groups but decreased as compared with the contralateral nonsymptomatic knees. However, none of the groups revealed differences in the activity of vastus lateralis and vastus medialis obliquus suggesting muscular imbalance.

The influence of quadriceps contraction on patellar alignment was investigated in tangential views of 31 patients with patellofemoral complaints and compared with 25 controls. The congruence angle decreased significantly following quadriceps contraction in patients with patellar subluxation in contrast to those with chondromalacia and normals. Electromyography (EMG) of the vastus lateralis (VL) and vastus medialis obliquus (VMO) during maximal quadriceps contraction was obtained prior to and after 3 months of isometric quadriceps exercises in 15 knees with patellar subluxation and in 11 with idiopathic chondromalacia of the patella. The EMG after 3 months revealed an increase in the activity of both VL and VMO only in patients with patellar subluxation, but the muscular balance of the patella remained unchanged. Only seven patients were improved after 3 months. A more selective training of the VMO is proposed.