Low frequency fatigue and changes in muscle fascicle length following eccentric exercise of the knee extensors.

Abstract

What is the central question of this study? Does low frequency muscle fatigue indicate a failure of excitation-contraction coupling after eccentric exercise, or is it simply due to a change in muscle length? What is the main finding and its importance? The low to high frequency muscle fatigue ratio was relatively insensitive to changes in muscle length, and any changes in length following eccentric exercise were far too small to account for the high degree of low frequency fatigue. The results strengthen the suggestion that the early loss of force following eccentric exercise is due to a deficit of excitation-contraction coupling. Development of long lasting fatigue (low frequency fatigue; LFF), assessed as the ratio of forces at 20 and 100Hz stimulation, suggests the early phase of muscle damage caused by eccentric exercise is due to a deficit of excitation-contraction coupling. However, this could be caused by a change of muscle length. Eleven men (21.3±2.0years) performed 200 maximum eccentric knee extensions (30-110deg flexion). Force generated by 20 and 100Hz stimulation and maximum isometric force (MIF) were determined at knee angles 50, 70 and 90deg before and immediately after the exercise. Vastus lateralis fascicle length (FL) was measured by ultrasound of resting and contracting muscle. Peak MIF (829±119N) was at 70deg knee flexion, falling to 486±180N (P<0.001) after exercise, but with no change in optimum angle. FLs at rest were unaffected by eccentric exercise, but during contraction they were on average 8.8% (95% CI: 4.1, 13.5%, P=0.002) longer after exercise. Before exercise, the 20/100 ratio increased with muscle length, from 0.69±0.09 at 50deg, 0.72±0.05 at 70deg and 0.80±0.08 at knee angle 90deg (P<0.001). After eccentric exercise the 20/100 ratio was reduced to 0.29±0.08 at 50deg, 0.27±0.04 at 70deg and 0.34±0.04 at 90deg (P<0.001). The 20/100 ratio was relatively insensitive to changes in muscle length and the decrease following eccentric exercise was far greater than might be caused by any changes in muscle length after eccentric exercise. The results show that LFF following eccentric exercise is not due to change in muscle length and strengthen the suggestion that it represents a deficit in excitation-contraction coupling.