Electromechanical delay in weight lifters and endurance trained athletes

INTRODUCTION Electromechanical delay (EMD) has been found to be significantly correlated to maximal contraction force, rate of force development and muscle fibre type composition (Viitasalo and Komi, 1981). It has been suggested that 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., 1995). However, limited information was found in the literature about the EMD adaptation to specific sport training. The purpose of this study was to compare EMD of knee extensors between weight lifters and endurance trained athletes. METHODS Subjects consisted of 6 weight lifters (W), 6 endurance trained athletes (E), who had been trained in the specific types of sport for at least two years, and 6 college students (C) who were physically active but not specifically trained in sports. The average age, height, and mass of the subjects were 22.9 yr., 1.757 m and 68.4 kg, respectively. Subject was tested when sitting in a testing chair with the knee joint angle of 90° and hip joint angle of 120°. Surface EMGs were recorded from the vastus lateralis (VL) and rectus femoris (RF) muscles of the right leg. Two channels of EMG were recorded from VL to determine muscle fibre conduction velocity (MFCV). Muscle contraction force was recorded using a load cell which was strapped to subject\u27s lower leg. EMD of maximal voluntary contraction (VC) was determined as the time lag between the onset of EMG and muscle tension development when the subject responded to a light signal. EMD in patellar tendon reflex evoked contraction (TR) and of electrically stimulated contraction (ES, stimulated by square wave of 150 V and 1 ms, applied percutaneously over the muscle) were also measured. The longer EMD among VL and RF muscles was defined as the true latency in a contraction (EMDmax). RESULTS The EMDmax in VC, TR and ES contractions are shown in Figure 1. The maximal contraction force (MVC), rate of force development (RFD) and MFCV in VC are shown in Figure 2. There were no significant MVC, RFD and MFCV differences found between the subject groups in TR and ES contractions. When pooling the data of the three groups together, there were significant correlations (p\u3c0.05) found between EMD and MVC (r=-0.65) or RFD (r=-0.60) in voluntary contractions. DISCUSSION The weight lifters demonstrated a shorter EMD in VC compared with endurance athletes and non-etheless. Kamen et al. (1981) suggested that the motor time (MT, defined similar to EMD) was shorter in athletes than that of non-etheless, however, no MT difference was reported between weight lifters and distance runners. In our laboratory, a seven-week sprint cycling training program did not alter EMD of knee extensors (Zhou, 1994). However, the present study suggests that a long term of specific training could induce adaptations in EMD. Apparently, the shorter EMD found in W athletes could attributed to their greater MVC, RFD and MFCV. Other influencing factors speculated include alterations in muscle fibre type composition, pattern of motor unit recruitment, and increased muscle-tendon stiffness associated with training. It was interesting to see that the EMDs in TR, and ES contractions were shorter than that in VC. These differences might reflect the effects of motor unit recruitment on EMD (Viitasalo and Komi, 1981; Zhou et al., 1995). The significantly longer EMD found in TR of E athletes might be due to the adaptations in muscle spindle activity, in the function of a certain type of motor unit, or in the motor unit recruitment pattern of the central nervous system. REFERENCES Kamen G., Kroll W., Clarkson P.M. and Zigon S.T. (1981) Fractional reaction time in power trained and endurance trained athletes under conditions of fatiguing isometric exercise. J. Motor Behav. 13: 117-129. Norman R.W. and Komi P.V. (1979) Electromechanical delay in skeletal muscle under normal movement conditions. Acta Physiol. Scand. 106: 241 248. Viitasalo J.T. and Komi P.V. (1981) Interrelationships between electromyographical, muscle structure and reflex time measurements in man. Acta Physiol. Scand. 111: 97 103. 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. (1995) Electromechanical delay in isometric muscle contractions evoked by voluntary, reflex and electrical stimulation. Eur. J. Appl. Physiol. 70: 138-145