IS ELECTROMECHANICAL DELAY MISINTERPRETED?

Abstract

INTRODUCTION: In all publications on this subject the major portion of electromechanical delay (EMD), with direct or indirect references to Cavanagh/ Komi (1979), is referred to as the time it takes to stretch the series elastic elements (SE) by the contractile elements (CE). This seems to be confirmed by investigations showing shorter EMD in stretched muscle (SM) than in unstretched muscle (UM), whereby there are also contrary examples. In physiological positions a permanent tension exists in SE comparable with a stretched rubber band, and they do not slacken. A force generated by the CE at one end of the SE can only have a stretching effect if the other end of the SE is held back by the same amount of counter-force. It is of decisive importance that this happens at the same time without any delay. It was the aim of the present study to show experimentally that EMD cannot be determined by the time for stretching the SE. METHODS: For this purpose 18 male subjects performed four explosive maximal isometric voluntary contractions with their hamstrings in six different length positions from stretched to unstretched. EMG- and force-time-curves were digitally recorded with a sampling rate of 2 kHz. EMD was estimated by means of a specially computer program as the delay between the onset of EMG and force. RESULTS: The results showed that the shortest EMD, 38ms, occurred in an optimal muscle length (OML), and longer EMDs, 47ms for SM and 44ms for UM respectively, were seen. The results in OML were in good accordance with the shortest values reported for the hip flexors (32ms, SYMONS et al. 1988). Comparing the resulting EMDs in a stretched with those in an unstretched position, the investigation showed contrary results as those reported in most of the previous studies. Adding the four intermediate positions this trend was continued. CONCLUSIONS: Comparison of all six length positions showed the shortest EMD in OML and not in SM, as should be expected on the basis of earlier results. In contrast to previous results and assumptions, the EMD-length-curve showed the same, but inverse, course as the force-length curve. This latter phenomena could be attributed to the rate of force development, which depends on the grade of overlapping of the sarcomeres in different length positions. Both results impressively underlined the assumption that EMD cannot be determined by the time for stretching the SE, and it may be necessary to redefine EMD. REFERENCES: Cavanagh, P. R., Komi, P. V. (1979). Electromechanical Delay in Human Skeletal Muscle under Concentric and Eccentric Contractions. Eur. J. Appl. Phys. 42, 159- 163. Symons, J. D. et al. (1988). Electromechanical Response Times and Muscle Strength after Sleep Deprivation. Canadian Journal of Sport Science 13, 225-230.