Is the Interpolated-Twitch Technique-derived Voluntary Activation just Neural? Novel Perspectives from Mechanomyographic Data

Purpose: Voluntary activation determined by interpolation-twitch technique (VA) 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 (VAMMG) has been proposed. We examined the changes in VA and VAMMG 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 plantar flexors (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 VAMMG were calculated in the stretched and contralateral non-stretched limb. Joint passive stiffness was also defined. Results: In the stretched limb, passive stretching increased dorsiflexion range [+18(10)% (P < 0.001, ES: 1.54)], whereas reduced joint passive stiffness [-22(8)% (P < 0.001; ES: -1.75)], MVC [-15(7)% (P < 0.001, ES:-0.87)], VA [-7(3)% (P < 0.001; ES:-2.32)], and VAMMG [~-5(2)% (P < 0.001; ES: -1.26/-1.14)]. In the contralateral non-stretched limb, passive stretching increased dorsiflexion range [+10(6)% (P < 0.001, ES: 0.80)], whereas reduced joint passive stiffness [-3(2)% (P = 0.041; ES: -0.27)], MVC [-4(3)% (P = 0.035, ES:-0.24)], VA [-4(2)% (P < 0.001; ES:-1.77)], and VAMMG [~-2(1)% (P < 0.05; ES: -0.54/-0.46)]. The stretch-induced changes in VA correlated with VAMMG (R ranging from 0.447 to 0.583 considering all muscles) and with joint passive stiffness (stretched limb: R = 0.503; contralateral non-stretched limb: R = 0.530). Conclusions: VA output is overall influenced by both neural and mechanical factors, not distinguishable using the interpolated-twitch technique. VAMMG is a complementary index to assess the changes in VA not influenced by mechanical factors and to examine synergistic muscles.