Effects of antagonistic and synergistic muscles' co-activation on mechanics of activated spastic semitendinosus in children with cerebral palsy

Objectives: Most activities involve co-activation of several muscles and epimuscular myofascial force transmission (EMFT) can affect their mechanics. This can be relevant for spastic muscles of cerebral palsy (CP) patients. Isometric spastic semitendinosus (ST) forces vs. knee angle (ICA-Fsr) data were collected intra-operatively to test the following hypotheses: (i) Inter-antagonistic EMFT elevates FST, (ii) changes the shape of KA-FST characteristics, (iii) reduces the muscle's joint range of force exertion (Range-Far) and (iv) combined inter-antagonistic and synergistic EMFT further changes those effects. Methods: 11 limbs of 6 patients with CP (mean (SD) = 7.7 (4.7) years; GMFCS levels = II IV) were tested in 3 conditions from 120 to full extension: ST activated (I) exclusively, (11) simultaneously with an antagonist, and (III) with added activation of synergists. Results: Condition II increased FST (e.g., peak force = 87.6 N (30.5 N)) significantly (by 33.6%), but condition III caused no further change. No condition changed the muscle's wide Range-FsT (100.7 (15.9)) significantly. Therefore, only the first hypothesis was confirmed. Conclusions: Co-activating its antagonist elevates forces of activated spastic ST substantially, but does not change its joint range of force exertion. Added activation of its synergists causes no further effects. Therefore, EMFT effects in CP can be relevant and need to be tested in other knee flexors.