1. Abnormal morphological and enzymatic patterns in the lateral vastus muscle have been found in women with corticosteroid treated rheumatoid arthritis. By means of biopsies from the lateral heads of right gastrocnemius muscles, the histology and enzyme activities were compared with those found in right vastus lateralis biopsies. The findings were correlated with isometric and isokinetic strength of the plantar flexors.
2. The relative occurrence of type I fibres in the gastrocnemius muscle was 46.4 ± 18.7 (sd) %, which is significantly higher than found in the vastus lateralis [35.7 ± 13.3 (sd) %] (P < 0.03).
3. The relatively lower percentage of type II fibres in the gastrocnemius muscle was due to a relatively low percentage of type II A fibres [mean 27.9 ± 16.4 (sd) %] (P < 0.05).
4. The area of type I fibres in the gastrocnemius muscle was 26.1 × 102 ± 10.0 (sd) μm2, which is 74% of the mean area for type I fibres found in the vastus lateralis (P < 0.01).
5. The area of type II fibres in the gastrocnemius was 14.9 × 102 ± 7.1 (sd) μm2, which is 77% of the mean area for type II fibres found in the vastus lateralis.
6. The isokinetic muscle strength of the plantar flexors in corticosteroid treated patients with rheumatoid arthritis was reduced to less than 50% at all angular velocities when compared with healthy women. The same difference was found in the knee extensors.
7. Both type I and type II fibre areas correlated positively with the activity of β-hydroxyacyl-CoA dehydrogenase (r = 0.72, P < 0.02; r = 0.77, P < 0.01). The activity of the oxidative enzyme citric acid synthase (but not β-hydroxyacyl-CoA dehydrogenase) correlated positively with the isokinetic muscle strength of the plantar flexors at all the angular velocities (r = 0.75, P < 0.01; r = 0.57, P < 0.05).
8. The isokinetic strength as well as the type I and II fibre areas in the vastus lateralis biopsies correlated well with the same parameters in the gastrocnemius muscle (r approx. 0.70, P < 0.001). The vastus lateralis and the gastrocnemius muscles had similar morphology in the same subjects. In the control subjects and in the patients a close correlation was found between the isokinetic strength of the knee extensors and the plantar flexors.
9. It is concluded that the corticosteroid effect on muscles seems to occur in both the proximal and distal skeletal muscles.