RELATION BETWEEN INITIAL MUSCLE LENGTH AND ELECTROMECHANICAL D
Keywords: electromechanical delay, hamstrings, muscle length, explosive force
AbstractINTRODUCTION: The exact knowledge of electromechanical delay (EMD) can provide important conclusions concerning the mechanical effectiveness of a particular muscle, which becomes evident via the corresponding EMG. Recent results show an EMD of 24ms for the hamstrings (HS) in an optimal muscle length (OML). Therefore these values are distinctly shorter than those ever determined for the hip flexors (32-118ms). However athletic movement normally starts in an initial muscle length (IML) which is not necessarily identical with OML. Previous publications comparing 3 IML at the most show both positive and negative relations to EMD. Because existing results are not very satisfactory it was the aim of the present study to elucidate the relation between IML and EMD more exact. METHODS: For this purpose 18 male subjects performed four explosive maximal isometric voluntary contractions with their HS in six different length positions. EMG- and force-time-curves were digitally recorded with a sampling rate of 2 kHz. EMD was estimated by means of a specially designed computer program as the delay between the onset of EMG and force. RESULTS: The results showed a dependence of EMD on IML. While OML showed the shortest EMD (37-40ms) an increment of EMD was to be seen in direction of both stretched (45-48ms) and unstretched (42-46ms) IML. The results in OML were greater than in a recent study (24ms, JÖLLENBECK, 1997) for the same muscle group but in good accordance with the shortest values reported for the hip flexors (32ms, SYMONS et al., 1988). CONCLUSIONS: Maximum differences of about 8ms or 20% of EMD in different IML are not negligible in exact motion analysis. Furthermore the EMD-lengthcurve in course seems to correspond with the inverse force-length-curve. It must be supposed that EMD is not directly dependent on IML but on the maximum force and the rate of force development which depends on the grade of overlapping of the sarcomeres in different length positions. These results and the enormous differences in existing EMD values underline the importance of further investigation of the EMD and suggest to redefine the EMD in different IML for all muscles involved in athletic motions. 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. Jöllenbeck, T. (1997). Electromechanical Delay of the Hamstrings and its Significance for the Analysis and Optimization of Sprint Running. Congress: Sport Kinetics ’97, Magdeburg. In Press. Symons, J. D. et al. (1988). Electromechanical Response Times and Muscle Strength after Sleep Deprivation. Canadian Journal of Sport Science 13, 225-230.
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