An age-related shift in the force-frequency relationship affects quadriceps fatigability in old adults

We examined the effect of an age-related leftward shift in the force-frequency relationship on the comparative quadriceps fatigability of nine young (27 +/- 1 yr old) and nine old men (78 +/- 1 yr old) during low-frequency electrical stimulation. Two different protocols of intermittent trains (6 pulses on, 650 ms off) of electrical stimulation at 25% maximum voluntary contraction were performed by both groups: 1) 180 trains at 14.3 Hz [constant frequency (CF) protocol], and 2) 180 trains at the frequency corresponding to 60% of each subject's force-frequency curve [normalized frequency (NF) protocol; young 14.9 +/- 0.4 vs. old 12.7 +/- 0.5 Hz; P < 0.05]. The quadriceps of the old men were weaker (approximately 31%) and relaxation was slower compared with the young men, as assessed by the maximal relaxation rate constant of the 50-Hz tetanus (young 12.1 +/- 0.2 vs. old 9.2 +/- 0.5 s(-1); P < 0.05) and a leftward shift in the force-frequency relationship. The NF protocol revealed a decreased fatigability in the quadriceps with old age (percentage of 1st contraction force remaining at 180th: old 63.4 +/- 1.5 vs. young 58.2 +/- 1.7%; P < 0.05) that was masked during the CF protocol (old 60.7 +/- 1.6 vs. young 58.6 +/- 2.3%; P > 0.05). Irrespective of the protocol, the maximal relaxation rate was reduced to approximately 73 and approximately 57% of the prefatigue value in the young and old men, respectively. The age-related leftward shift in the force-frequency relationship of the quadriceps contributed to an underestimation of the fatigue resistance with old age during the CF protocol. However, when the stimulation frequency used in the NF protocol was adjusted to account for the age-related shift in the force-frequency relationship, the quadriceps muscles of the old men were less fatigable than those of the young men. Thus we suggest that whole muscle fatigability is better examined by electrical stimulation protocols that are adjusted for inter- and intragroup differences in the force-frequency relationship.