Muscle-joint unit transfer function derived from torque and surface mechanomyogram in humans using different stimulation protocols

Torque and laser detected surface mechanomyogram (MMG) analysis after electrical stimulation of human tibialis anterior (TA) of 14 male subjects was aimed to: (a) obtain the dynamic responses of TA muscle-joint unit from a long (LP, about 1 h) and short (SP, 12.5 s) stimulation protocol; (b) compare the resulting transfer function parameters from the two signals.The sinusoidal amplitude modulation of a 30 Hz stimulation train (SST) changed the number of the recruited motor units, and hence the isometric torque and the TA surface position in the same fashion. Subject instrumentation and SST amplitude range definition took about 25 min. SP: seven consecutive modulation frequencies (0.4, 6.0, 1.0, 4.5, 1.8, 3.0, and 2.5 Hz). LP: fourteen 5 s long isolated frequencies (0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.5, 3.0, 4.0, 5.0, and 6.0 Hz), 5 min rest in between. Poles position (Hz) and added delay (ms) for phase correction with respect to the input sine (parameters of a critically damped II order system) were: torque 2.44 ± 0.27 Hz (SP) or 2.32 ± 0.33 Hz (LP) and 18.3 ± 2.2 ms (SP) or 17.2 ± 4.5 ms (LP); MMG 2.28 ± 0.30 Hz (SP) or 2.30 ± 0.44 Hz (LP) and 17.4 ± 5.6 ms (SP) or 17.4 ± 6.4 ms (LP). Differences were never statistically significant. Conclusion: it is possible to characterise the in vivo mechanics of muscle-joint unit with a short (few seconds) stimulation protocol affordable in clinical environment using both torque and MMG signals.