In experiment I ten young men (29 +/- 3 yrs; M30), 12 middle-aged men (50 +/- 4 yrs; M50) and 12 women (48 +/- 5 yrs; W50), 12 elderly men (67 +/- 4 yrs; M70) and 12 women (68 +/- 4 yrs; W70) volunteered for subjects for examination of maximal 1 RM strength and electromyographic activity of the knee extensor muscles during the bilateral and unilateral concentric contraction on a variable resistance knee extension dynamometer. In experiment II 10 young (Y) men (29 +/- 5 yrs) and 10 older (O) men (61 +/- 4 yrs) were examined for their maximal voluntary isometric force and force-time curves and electromyographic activity of the knee extensor muscles during the bilateral and unilateral contractions. The bilateral 1 RM of 165.5 +/- 25.5 kg in M30 was greater (p < 0.01) than that of 127.7 +/- 24.5 kg recorded for M50 the latter being also greater (p < 0.05) than that of 109.0 +/- 17.7 kg recorded for M70. The bilateral value of 87.4 +/- 13.4 kg in W50 was greater (p < 0.05) than that of 69.9 +/- 15.0 kg recorded for W70. The bilateral 1 RM values were slightly greater than the summed unilateral 1 RM values in all groups M50, W50 and W70 showing a significant (p < 0.05) difference. All groups showed slightly (ns.) greater mean maximal integrated EMG values during the bilateral conditions in comparison to that of the corresponding unilateral condition. The maximal isometric forces in Y men were 25% greater (p < 0.001) than in O men. In both groups the bilateral forces were somewhat greater (p < 0.05) than the summed unilateral forces and the bilateral IEMG values slightly (ns.) greater than the corresponding unilateral IEMG values. The early forces on the force-time curve were much greater (p < 0.05-0.001) in Y than O men in both conditions. The present findings suggest that both maximal voluntary isometric and concentric force, and especially explosive strength of the knee extensors decrease greatly with increasing age probably due to selective muscle atrophy and/or possible decreases in the amount or rate of voluntary activation of the muscles. However, no bilateral deficit could be found indicating that the central nervous system in a simple single joint isometric and maximal 1 RM concentric force production of the knee extensors was capable of activation of the two bilateral muscle groups simultaneously independent of age and sex of the subject. To which extent the activation and force production of the muscles would be different in terms of the bilateral deficit during various multijoint exercises utilizing isometric and higher velocity concentric, eccentric and various stretch shortening cycle exercise needs to be examined in the future.