The time delay between the onset of muscle activation and the onset of force or motion is commonly referred to as electromechanical delay, motor time, or motor execution time. This time has been used in the study of reaction time, of physiological properties of muscle, and of population differences. In this study, we show that electromechanical delay is comprised of two components. The first is transport time (tt) which is very brief (perhaps 10 ms). The second is the time to generate detectable changes in force (tf). The absolute duration of electromechanical delay is usually dominated by the second component which is influenced by four separate factors that are related in the following way: f = g T R Zd Zm That is, tf is a function (g) of the product of two ratios. One is between the threshold T of the measuring device and the rate R at which muscle force rises. The other is the ratio of the mechanical impedances of the measuring device (Zd) and the muscle (Zm). We conclude that the measured absolute value of electromechanical delay has no physiological or psychological meaning and that task and performance induced changes need great care in their interpretation. © 1992.225968Agarwal, Gottlieb, Mathematical modeling and simulation of the postural control loop (1984) CRC Crit Rev Biomed Eng, 11, pp. 113-154. , part IIBell, Jacobs, Electro-mechanical response times and rate of force development in males and females (1986) Med Sci Sports Exer, 18, pp. 31-36Carlton, Carlton, Newell, Reaction time and response dynamics (1987) The Quarterly Journal of Experimental Psychology Section A, 39 A, pp. 337-360Cavanagh, Komi, Electromechanical delay in human skeletal muscle under concentric and eccentric contractions (1979) Eur J Appl Physiol, 42, pp. 159-163Corcos, Agarwal, Flaherty, Gottlieb, Organizing principles for single joint movement. IV. Implications for isometric contractions (1990) J Neurophysiol, 64, pp. 1033-1042Corcos, Gottlieb, Agarwal, Organizing principles for single joint movements. II. A speed-sensitive strategy (1989) J Neurophysiol, 62, pp. 358-368Crochetiere, Vodovnik, Reswick, Electrical stimulation of skeletal muscle. A study of muscle as an actuator (1967) Med Biol Eng, 5, pp. 111-125Di Fabio, Reliability of computerized surface electromyography for determining the onset of muscle activity (1987) Phys Ther, 67, pp. 43-48Gottlieb, Corcos, Agarwal, Organizing principles for single joint movements. I. A speed-insensitive strategy (1989) J Neurophysiol, 62, pp. 342-357Hakkinen, Komi, Electromyographic and mechanical characteristics of human skeletal muscle during fatigue under voluntary and reflex conditions (1983) Electroencephalography and Clinical Neurophysiology, 55, pp. 436-444Hathaway, An action study of neuromuscular relations (1935) J Exp Psychol, 18, pp. 285-298Hill, The Dynamic Constants of Human Muscle (1940) Proceedings of the Royal Society B: Biological Sciences, 128, pp. 263-274. , 5th EditionKarrer, Input, central and motor segments of response time in mentally retarded and normal children (1986) Motor Skill Acquisition of the Mentally Handicapped: Issues in Research and Training, pp. 167-187. , MG Wade, Elsevier (North Holland), AmsterdamNorman, Komi, Electromechanical delay in skeletal muscle under normal movement conditions (1979) Acta Physiol Scand, 106, pp. 241-248Popianov, Computer detection of EMG edges for synchronization of movement-related brain potentials (1986) Electroencephalography and Clinical Neurophysiology, 64, pp. 171-176Rios, Pizarro, Voltage sensor in excitation-contraction coupling in skeletal muscle (1991) Physiol Rev, 71, pp. 849-908Siegel, Fractionated reaction time and the rate of force development (1988) Q J Exp Psychol, 40 A, pp. 545-560Stelmach, Teasdale, Phillips, Worringham, Force production characteristics in Parkinson's disease (1989) Exp Brain Res, 76, pp. 165-172Tomberg, Levarlet-Joyce, Desmedt, Reaction time recording methods: Reliability and EMG analysis of patterns of motor commands (1991) Electroencephalogr Clin Neurophysiol, 81, pp. 269-278Walter, Temporal quantification of electromyography with reference to motor control research (1984) Hum Move Sci, 3, pp. 155-162Weiss, The locus of reaction time changes with set, motivation and age (1965) J Gerontol, 20, pp. 60-64Winter, Pathologic gait diagnosis with computer-averaged electromyographic profiles (1984) Arch Phys Med Rehabil, 65, pp. 393-398