Is the Interpolated-Twitch Technique-Derived Voluntary Activation Just Neural? Novel Perspectives from Mechanomyographic Data

Abstract

Purpose: Voluntary activation (VA) determined by interpolation-twitch technique could be affected by the characteristics of the in-series elastic components. To overcome this possible bias, a novel approach based on the mechanomyographic (MMG) signal to detect voluntary activation (VA MMG ) has been proposed. We examined the changes in VA and VA MMG after passive stretching to check the influence of neural and mechanical factors in the force output. Methods: Twenty-six healthy men underwent VA assessment using the interpolated-twitch technique before and after unilateral passive stretching of the plantarflexors (five 45-s on + 15-s off ). In addition to the force signal, the MMG signal was detected on gastrocnemius medialis, gastrocnemius lateralis, and soleus. From the force and MMG signal analysis, VA and VA MMG were calculated in the stretched and contralateral nonstretched limbs. Joint passive stiffness was also defined. Results: In the stretched limb, passive stretching increased dorsiflexion range (mean ± SD = +18% ± 10%, P < 0.001, ES = 1.54) but reduced joint passive stiffness (−22% ± 8%, P < 0.001, ES = −1.75), maximum voluntary contraction (−15% ± 7%, P < 0.001, ES = −0.87), VA (−7% ± 3%, P < 0.001, ES = −2.32), and VA MMG (~−5% ± 2%, P < 0.001, ES = −1.26/−1.14). In the contralateral nonstretched limb, passive stretching increased dorsiflexion range (+10% ± 6%, P < 0.001, ES = 0.80) but reduced joint passive stiffness (−3% ± 2%, P = 0.041, ES = −0.27), maximum voluntary contraction (−4% ± 3%, P = 0.035, ES = −0.24), VA (−4% ± 2%, P < 0.001, ES = −1.77), and VA MMG (~− 2% ± 1%, P < 0.05, ES = −0.54/−0.46). The stretch-induced changes in VA correlated with VA MMG (R ranging from 0.447 to 0.583 considering all muscles) and with joint passive stiffness (stretched limb: R = 0.503; contralateral nonstretched limb: R = 0.530). Conclusions: VA output is overall influenced by both neural and mechanical factors, not distinguishable using the interpolated-twitch technique. VA MMG is a complementary index to assess the changes in VA not influenced by mechanical factors and to examine synergistic muscles.

Synergistic difference in the effect of stretching on electromechanical delay components

“…Overall high inter-session reliability and low MDC 95% were observed for the dependent variables. The sEMG and MMG signal crosstalk values spanned from 5% to 11%, in agreement with previous literature [27,36,37]. Altogether, these results suggest that the EMD and R-EMD components were minimally influenced by crosstalk.…”

“…The maneuver was performed with the participant lying prone on an experimental bed, with the knee fully extended. The difference between the ankle neutral position and the angle at the point of discomfort was considered the joint ROM [ 27 ].…”