HAMSTRING STRENGTH ONLY AFFECTED BY STRETCHING AT SHORT MUSCLE LENGTHS
McHugh, M. P., & Johnson, A. (2006). Strength loss following static stretching: The role of muscle length. Medicine and Science in Sports and Exercise, 38(5), Supplement abstract 2122.
The purpose of this study was to examine whether the muscle length at which strength was measured affected strength loss following stretching. Isometric and isokinetic (eccentric and concentric) knee flexion strength was measured in males (N = 10) before and after six static hamstring stretches. For strength tests patients were seated upright (90º trunk flexion) with the test thigh flexed 15º. Two maximum isometric contractions were performed at 80º, 65º, 50º, 35º, 20º and 5º of knee flexion. Then followed four maximal eccentric and concentric contractions were performed from 80º- 0º at 60º/s (reciprocal contractions with 1 s pause). Following the initial strength tests, the knee was passively extended to full extension and held for 90 seconds (this hamstring stretch was repeated six times). Passive resistance to stretch was recorded for the first and sixth stretch.
Stretching resulted in an 8% decrease in resistance to stretch at the end range of motion. It also led to a decrease in peak isometric torque with the muscle in a shortened position (-15% at 80º) but not in a lengthened position (+7% at 5º). Stretching did not decrease peak isokinetic eccentric or concentric torque or change the angle of peak torque.
Implication. Isometric data indicated that strength loss following stretching was limited to the short muscle lengths. The lack of effect of stretching on isokinetic strength can be explained by the fact that peak torque occurred at 10-15º for eccentric contractions and 20-25º for concentric contractions, that is, with the muscle in a lengthened position. The practical significance of these findings is that stretching did not appear to have a detrimental effect on hamstring strength at muscle lengths where muscle strain injuries are thought to occur (in a stretched position).
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