Influence of age on force and re-lengthening dynamics after tetanic stimulation withdrawal in the tibialis anterior muscle

Purpose During alternate movements across a joint, the changeover from one direction of rotation to the opposite may be influenced by the delay and rate of tension reduction and the compliance to re-lengthening of the previously active muscle group. Given the aging process may affect the above-mentioned factors, this work aimed to compare the dynamics of both the ankle torque decline and muscle re-lengthening, mirrored by mechanomyogram (MMG), in the tibialis anterior because of its important role in gait. Methods During the relaxation phase, after a supramaximal 35 Hz stimulation applied at the superficial motor point, in 20 young (Y) and 20 old (O) subjects, the torque (T) and MMG dynamics characteristics were measured. Results The T and MMG analysis provided: (I) the beginning of the decay after cessation of stimulation (T: 22.51 +/- 5.92 ms [Y] and 51.35 +/- 15.21 ms [O]; MMG: 27.38 +/- 6.93 ms [Y] and 61.41 +/- 18.42 ms [O]); (II) the maximum rate of reduction (T: - 110.4 +/- 45.56 Nm/s [Y] and - 52.72 +/- 32.12 Nm/s [O]; MMG: - 24.47 +/- 10.95 mm/s [Y] and - 13.76 +/- 6.54 mm/s [O]); (III) the muscle compliance, measuring the MMG reduction of every 10% reduction of torque (bin 20-10%: 15.69 +/- 7.5[Y] and 10.8 +/- 3.3 [O]; bin 10-0%: 22.12 +/- 10.3 [Y] and 17.58 +/- 5.6 [O]). Conclusion Muscle relaxation results are different in Y and O and can be monitored by a non-invasive method measuring physiological variables of torque and re-lengthening dynamics at the end of the electromechanical coupling previously induced by the neuromuscular stimulation.