This work estimates the influence of the single twitch (ST) parameters changes on specific regions of the force-frequency relationship (FFR) in fatigued human tibialis anterior (TA). In 20 subjects (age 20-40) the TA underwent three stimulation phases: (a) five STs at 1 Hz followed by 5 s stimulation with increasing rate (1-50 Hz, to obtain FFR); (b) fatiguing stimulation (35 Hz for 40 s); (c) same as in "a". By the average STs (mean of the five responses) of a and c phases, the peak twitch (Pt) was calculated. Moreover, after ST normalization to Pt, the maximum contraction rate (MCR) and the maximum relaxation rate (MRR) were computed. By the FFR, normalized to the 50 Hz force, we first defined the threshold frequency (TF) when the force oscillation presented the same value in (a) and (c), and then the areas below the FFR in the 1 Hz-TF and in the TF-50 Hz ranges.
Results: In unfatigued and fatigued muscle Pt, and MRR changed from 6.12 +/- 3.08 to 3.27 +/- 1.16 N and from 0.87 +/- 0.13 to 0.65 +/- 0.09% Pt/ms, respectively. MCR did not change significantly. The 1 Hz-TF area ratio (c/a) was > 1 for muscles having fatigued Pt > 60% of its basal value. The TF-50 Hz area ratio (c/a) was mostly below 1.
Conclusions: At fatigue, MRR reduction, leading to a better fusion of muscle mechanical output, is able to compensate, in the 1 Hz-TF frequency range, up to 40% Pt loss; beyond TF, the changes of FFR are related to the degree of force loss indicated by the fatigued Pt.