Whereas there is an abundance of data on the effects of muscle fibre type composition on metabolic and contractile properties during isometric and concentric contractions, there is little information regarding the effects during eccentric action. Rat medial gastrocnemius (GM) muscle consists of a proximal part which contains mostly oxidative fibres and a distal part which is dominated by glycolytic fibres. Using selective stimulation of branches of the GM nerve, it has been shown that the distal part generates more power but fatigues faster than the proximal part (de Ruiter et al. 1995). Studying fibre type effects on contractile characteristics within the same muscle has the advantage that morphological and mechanical differences are smaller than when two different muscles are compared. The goal of the present study was to investigate whether, similar to shortening contractions, oxidative and glycolytic parts of the GM differ also in their responses to active stretch at different velocities.

In situ experiments were performed on GM muscleÐtendon complexes of anaesthetised (urethane, 1.5 g kg^-1 I.P.) male Wistar rats (n = 12, body mass 243Ð302 g). Proximal and distal muscle parts (each n = 6) were activated maximally. The muscles were subjected to eccentric contractions at eight different velocities (5, 10, 15, 20, 40, 60, 100, 150 mm s^-1) in random order. Stretches (3 mm) started from a maximal isometric force plateau (F_before) at 2.5 mm below optimum length. After reaching peak eccentric force (F_peak), stimulation was continued for 100 ms to obtain isometric force after stretch (F_after). Normalised eccentric force was calculated as (F_peak – [F_after – F_before])/F_before (de Ruiter et al. 2000) and expressed as a percentage. After the experiments, the animals were humanely killed.

Maximal isometric forces were 5.1 ± 1.7 and 5.4 ± 1.8 N (mean ± S.D.) for the proximal and distal part (P > 0.05, Student’s unpaired t test), respectively. There was a significant effect of velocity on eccentric force, with no differences between muscle parts (P < 0.05, ANOVA repeated measures). Maximum normalised forces were obtained at 60 mm s^-1 and were 157 ± 3 and 153 ± 6 % for the proximal and distal part, respectively. In conclusion, oxidative and glycolytic parts of rat GM have similar eccentric forceÐvelocity relationships despite differences in metabolic, isometric and concentric contractile properties. Apparently, fibre type composition affects muscle function more when muscles are used as a motor than when used as a brake.

All procedures accord with current National guidelines.