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

New Findings: What is the central question of this study? The aim of this study was to determine the possible correlations between the mechanomyogram signal amplitude and the stiffness at the muscle-tendon unit, muscle and tendon levels, which were only hypothesised in previous investigations. What is the main finding and its importance? The main findings of this study are that the amplitude of the mechanomyogram was correlated inversely, albeit with small r2 values, with muscle-tendon unit, muscle and tendon stiffness both before and after application of stretching. The study aimed to assess the level of correlation between muscle-tendon unit (MTU) stiffness and mechanomyogram (MMG) signal amplitude of the human gastrocnemius medialis muscle, both before and after acute passive stretching. The passive torque (Tpass), 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 (Tmax) 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 Tpass measurements. Before stretching, correlations between MMG RMS and MTU, muscle and tendon stiffness were found (R2 = 0.22-0.46). After stretching, Tpass, Tmax, 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.