Effect of shortening velocity on work output and energy cost during repeated contractions of the rat EDL muscle

Summary

The effect of shortening velocity on the reduction in work output, energy consumption and efficiency during repetitive contractions has been determined in rat extensor digitorum longus muscle. Muscles in situ (with occluded blood flow, 37° C) were stimulated to perform 40 successive contractions (at 4 Hz) with a total duration of the exercise period of 10 s and shortening velocities of either 25, 50 or 75 mm·s⁻¹ (whole muscle-tendon complex). Care was taken that work output during the shortening phase of the first contraction was the same for the different velocities used. Total work output of the 40 contractions was not significantly different between the three groups with different shortening velocities; nor was there a significant difference in the high-energy phosphate consumption over this 10-s exercise period. However, when the ratio of total work output and total energy consumption was calculated a significantly higher efficiency (25–30% in comparison with the efficiency of the other two velocities) was found with the shortening velocity of 50 mm · s⁻¹. There was no significant difference in efficiency between shortening velocities of 25 and 75 mm · s⁻¹. This suggests that with this protocol efficiency showed a velocity-dependent pattern that may have the same shape as the power/velocity curve. Whereas total work output during the 10-s exercise period was not significantly different between the velocities studied, the time course of the changes in work output was quite different. With shortening velocities of 50 and 75 mm · s⁻¹ work output initially increased by maximally 6% and 12% respectively in contrast to a steady level in the contractions with a velocity of 25 mm · s⁻¹. Between the 20th and 30th contractions, however, work output steadily decreased by approximately 2% each contraction with shortening velocities of 25 and 50 mm · s⁻¹. In contrast, there was a much faster drop in work output at 75 mm · s⁻¹, averaging 6% each contraction over the same period. These data have been discussed with respect to the mechanisms of fatigue and potentiation in relation to fibre-type distribution.