Twelve middle-aged men and 12 middle-aged women in the 50-year-old age group (M50; range 44-57 years; W50; 43-57), and 12 elderly men and 12 elderly women in the 70-year-old age group (M70; 59-75; W70; 62-75) volunteered as subjects in order to examine effects of 12-week progressive heavy resistance strength training on electromyographic activity (EMG), muscle cross-sectional area (CSA) of the quadriceps femoris and maximal concentric force in a one repetition maximum (1 RM) test of the knee extensor muscles. One half of the subjects in each group performed the knee extension (and flexion) exercises only bilaterally (BIL), while another half performed the exercises only unilaterally (UNIL). None of the subject groups demonstrated statistically significant changes in any of the 1 RM values during the 2 week control period with no training (between week -2 and 0) preceding the actual experimental training. However, the 12-week training resulted in increases (P < 0.05-0.001) in 1 RM values in each group so that the average relative increase of 19 +/- 12% (P < 0.001) in bilateral 1 RM in all BIL trained subjects was greater (P < 0.05) than that of 13 +/- 8% (P < 0.001) recorded for all UNIL trained subjects. The average relative increases of 17 +/- 11% (P < 0.001) and 14 +/- 14% (P < 0.001) in unilateral 1 RM values of the right and left leg in all UNIL trained subjects were greater (P < 0.05) than those of 10 +/- 18% (P < 0.001) and 11 +/- 11% (P < 0.001) recorded for all BIL trained subjects, respectively. The relative average increase of 19 +/- 19% (P < 0.001) observed in the maximum averaged IEMG of both legs during the bilateral actions in all BIL trained subjects was greater (P < 0.05) than that of 10 +/- 17% (P < 0.05) recorded for all UNIL trained subjects. The relative increases of 14 +/- 12% (P < 0.001) and 11 +/- 6% (P < 0.001) recorded for the CSA in all BIL and UNIL trained subjects did not differ significantly from each others. The present findings suggest that progressive heavy resistance strength training leads to great increases in maximal dynamic strength of the trained subjects accompanied by both considerable neural adaptations and muscular hypertrophy not only in middle-aged but also in elderly men and women. Both bilateral and unilateral exercises are effective to produce functional and structural adaptations in the neuromuscular system, although the magnitude of functional strength increase seems to be specific to the type of exercise used, further supporting the principle of specificity in the design of strength programmes.