Tension reduction and re-lenghtening of muscle- tendon unit in young and old tibialis anterior

Purpose: During an alternating movement of a joint, the inversion of the rotational component of the joint movement happens for the contraction of the agonist muscle, and for the relaxation/re-lengthening of the previously active muscle that is now being configured as antagonist. Gait can be considered as a global alternating movement resulting by the combination of several joints alternating flex-extension sequences with an ordered sequence of agonist and antagonist muscles contraction/relaxation. The aim of the work is to compare young and old muscle- tendon unit deactivation/re-lengthening process which can affect the biomechanics of a daily life crucial gesture such as gait. Methods: The static contraction of tibialis anterior muscle of 20 young (Y) subjects (age 21–33 years old) and 20 older (O) adults (65–80 years old) was studied. A 3 s train of supra-maximal electrical stimulations (35 Hz) was delivered on tibialis anterior motor point while recording the output torque (T) and the muscle transverse dimensional variation, mirroring the re-lengthening process, thanks the laser detected surface mechano-myogram (MMG). During the relaxation phase a delay (D) can be observed between the end of the stimulation and the beginning of signals (T and MMG) decay: DT and DMMG. Using a 20 ms moving window along the collected signals during the off phase the maximum rates of torque and MMG reduction were calculated (RRT and RRMMG). These parameters were calculated also on the normalized signals (NRRT and NRRMMG). Results: DT in O and Y was 51.35 ± 15.21 ms and 22.51 ± 5.92 ms, respectively (p \0.001). DMMG in O and Y was 61.41 ± 18.42 ms and 27.38 ± 6.93 ms, respectively (p \0.001). RRT in O and Y was - 52.72 ± 32.12 Nm/s and 110.4 ± 45.56 Nm/s, respectively (p\ 0.001). RRMMG in O and Y was - 13.76 ± 6.54 mm/s and - 24.47 ± 10.95 mm/s, respectively (p\0.001). NRRT in O and Y was -1026.26 ± 267.76%/s and - 1256.16 ± 333.36%/s, respectively (p = 0.02). NRRMMG in O and Y was - 710.35 ± 178.84%/s and - 867.79%/s ± 148.67, respectively (p = 0.004). Conclusions: These functional data, easy to be obtained during the relaxation phase after static contraction, suggest that the reduction of the tension and the re-lengthening process take place later and slower in O vs Y and provide biomechanical evidences that may contribute to explain the longer gait phases and the reduction of the walking speed typical of the elderly subjects.