Acute effect of exercise on electromechanical delay
Zhou, S 1995, 'Acute effect of exercise on electromechanical delay', in K Hakkinen, KL Keskinen, PV Komi & A Mero (eds), XVth Congress of the International Society of Biomechanics: book of abstracts, Jyvaskyla, Finland, 206 July, Gummerus Kirjapaino O.Y., Helsinki, Finland, pp. 1034-1035.
It has been suggested that electromechanical delay (EMD) measurement is essential to a proper understanding of the type of central nervous system commands required for the execution of different movements, the role and coordination of muscles in a movement, and the apparent anomalies between electromyogram (EMG) activity and body segment motion (Norman and Komi, 1979; Zhou et al., 1995a). The average EMD in isometric knee extension of young adults is approximately 43 ms (Zhou et al., 1995b). Any factors that affect muscle fibre conductive, contractile, and elastic properties may influence EMD time. Controversial results about the effects of exercise and fatigue on EMD have been reported in the literature. The purposes of the present study were to investigate the EMD changes during and after repeated maximal isometric voluntary contractions (MVC) and the relationships between EMD and muscle fibre type composition, metabolite concentration, temperature and fibre conduction velocity (MFCV) changes. METHODS
Subjects were 11 physical education students with average age of 20.6 yr., height 1.778 m, mass 69.1 kg. Subject sat in a testing chair with knee joint angle of 90° and hip joint angle of 120°. Surface EMG was recorded from vastus lateralis (VL) and rectus femoris (RF) muscles of the right leg. Two channels of EMG were recorded from VL to determine MFCV. Muscle contraction force was recorded using a load cell which was strapped to subject's lower leg. EMD was determined as the time lag between the onset of EMG and muscle tension development when the subject responded to a light signal. Subject performed 25 MVCs, each lasted 8 s followed by a 2 s recovery. EMD was measured for each contraction, and at 5, 10, 15, and 20 min after the exercise. Seven of the subjects repeated the exercise protocol on a separate day, during which muscle biopsy samples were obtained from VL before and immediately after the exercise, to determine muscle fibre type composition and metabolite concentrations. The muscle temperature was also measured before and immediately after the exercise using a needle thermistor. RESULTS
A significant elongation in EMD (17-19 ms) was found during the exercise along with reductions in MVC (57%), rate of force development (RFD, 56%) and MFCV (20%; Figure 1). The impaired EMD, MVC and MFCV recovered after 10 min of rest, however, RFD did not recover within 20 min. Moderate correlations (p<0.05) were found between EMD and MVC (r=-0.54), RFD (r=-0.39) or MFCV (r=-0.34) during the exercise. The concentrations of PCr and glycogen decreased and lactate increased (all p<0.05) after the exercise (Figure 2). The muscle temperature increased from resting 33.8degC to post exercise 35.9degC on (p<0.05). The subjects possessed an average of 57.7 (SEM 4.5) % FT muscle fibres. There was a significant correlation found between %FT and post-exercise lactate concentration (r=0.91, p<0.05). There was no evidence of selective glycogen depletion in different types of muscle fibres. The relative decrease in RFD was significantly correlated to the predicted muscle pH change (r=0.94, p<0.05). However, no significant correlations were found among EMD, %FT and other contractile property and metabolite changes.
Horita and Ishiko (1987) reported an increased EMD of VL after 60 s maximal isokinetic knee extensions, in which the peak torque decreased 59%. However, Vos et al. (1991) found no significant change in EMD of the same muscle group during 150 submaximal (50%MVC) isometric contractions. The results of the present study support that EMD time increases after a fatiguing exercise with maximal effort. The elongated EMD required about 10 min to recover. In another investigation in our laboratory (Zhou 1994), four bouts of 30-s all-out sprint cycling also produced an elongation of EMD in a similar extent (approx. 20 ms). The information of EMD elongation and recovery could be useful to evaluate the effects of a strenuous exercise on electro-mechanical properties of the muscle in a subsequent exercise. The mechanisms underlying the EMD elongation may include the reduced MFCV, the impaired E-C coupling process due to the accumulation of lactic acid and reduced PCr storage. The increased muscle temperature could also influence muscle conductive, contractile and elastic properties, therefore increase EMD. REFERENCES
Horita T. and Ishiko T. (1987) Relationships between muscle lactate accumulation and surface EMG activities during isokinetic contractions in man. Eur. J. Appl. Physiol. 56: 18 23.
Norman R.W. and Komi P.V. (1979) Electromechanical delay in skeletal muscle under normal movement conditions. Acta Physiol. Scand. 106: 241 248.
Vos E.J., Harlaar J. and Van Ingen Schenau G.J. (1991) Electromechanical delay during knee extensor contractions. Med. Sci. Sports Exerc. 23: 1187-1193.
Zhou S. (1994) Electromechanical delay of knee extensors: the normal range and the effects of exercise. Unpublished Ph.D. thesis, The University of Melbourne.
Zhou S., Lawson D.L., Morrison W.E. and Fairweather I. (1995a) Electromechanical delay in isometric muscle contractions evoked by voluntary, reflex and electrical stimulation. Eur. J. Appl. Physiol. 70: 138-145.
Zhou S., Lawson D.L., Morrison W.E. and Fairweather I. (1995b) Electromechanical delay of knee extensors: the normal range and the effects of age and gender. J. Human Movement Studies. 28: 127-146.