Mechanomyogram amplitude correlates with human gastrocnemius medialis muscle and tendon stiffness both before and after acute passive stretching

The study aimed to assess the level of correlation between muscle-tendon unit (MTU) stiffness andmechanomyogram (MMG) signal amplitude of the human gastrocnemius medialismuscle, both before and after acute passive stretching. The passive torque (T_pass), electrically evoked peak torque (pT) and myotendinous junction displacement were determined at different angles of dorsiflexion (0, 10 and 20 deg), while maximum voluntary isometric torque (T_max) was assessed only at 0 deg. Measurements were repeated after a bout of passive stretching. From the MMG signal, the root mean square (RMS) and peak to peak (p-p) were calculated. The MTU, muscle and tendon stiffness were determined by ultrasound and T_pass measurements. Before stretching, correlations between MMG RMS and MTU, muscle and tendon stiffness were found (R² = 0.22-0.46). After stretching, T_pass, T_max, pT and MTU, muscle and tendon stiffness decreased by 25 ± 7, 16 ± 2, 9 ± 2, 22 ± 7, 23 ± 8 and 28 ± 5%, respectively (P <0.05). During voluntary and electrically evoked contractions, MMG p-p decreased by 9 ± 2 and 5 ± 1%, while MMG RMS increased by 48 ± 7 and 50 ± 8%, respectively (P <0.05). Correlations between MMG RMS and MTU, muscle and tendon stiffness were still present after stretching (R² = 0.44-0.60). In conclusion, correlations between MMG RMS and stiffness exist both before and after stretching, suggesting that a slacker MTU leads to larger muscle fibre oscillations. However, care must be taken in using MMG amplitude as an indirect index to estimate stiffness owing to the relatively small R² values of the investigated correlations.