The electrically evoked isometric properties of the triceps surae have been studied in young (22 yr) and elderly (69 yr) men and women. The results show that the triceps surae of elderly subjects had an increased time to peak tension (TPT) of the twitch, a lower specific tension (force/cross section area), and showed a greater relative force loss when subjected to a standard "fatigue" test procedure than young triceps surae. The respective figures for TPT, specific tension, and fatigue index (FI) were 147 +/- 15 ms (male), 143 +/- 8 ms (female), 19.5 N/cm2 and 0.52 +/- 0.16 (male), 0.37 +/- 0.09 (female) in the elderly, compared with 118 +/- 14 ms (male), 132 +/- 11 ms (female), 32.9 N/cm2 and 0.67 +/- 0.11 (male), 0.62 +/- 0.09 (female) in their younger counterparts. The peak rates of rise (delta P50, delta P200) and relaxation (delta PR50, delta PR200) of tetani at 50 Hz and 200 Hz were slower (p less than 0.05) in the elderly group. The delta P50 and delta P200 were inversely associated (r = 0.76 and -0.50 respectively) with TPT, and the half relaxation time (1/2RT) of the maximal twitch was related to delta PR50 (r = 0.53). The decrease of tetanus relaxation rate during the 2 min fatigue test (delta PRF) was greater in the elderly than the young and was positively associated (r = 0.74) with FI. It is concluded that elderly muscle is weaker, more slowly contracting and fatigable than that of the young. However, it is suggested that the reduced specific tension and enhanced fatiguability of the elderly may in part be due respectively to the inadequacies of our estimates of the effective cross-sectional area CSA) of contractile tissue in the lower leg and the influence of blood flow which may be compromised during a 2 min test.