Effects of inter-synergistic mechanical interactions on the mechanical behaviour of activated spastic semitendinosus muscle of patients with cerebral palsy

Previous physiological experiments and finite element modelling indicate that inter-synergistic epimuscular myofascial force transmission (EMFT) between co-activated muscles has a potential to affect healthy muscle's contribution to joint moment and joint range of movement. This is quite relevant for patients with cerebral palsy (CP) since, amplitude of spastic muscle's force and the joint range of force exertion are central to the joint movement limitation. Stiffness of activated spastic muscle is also a determinant for pathological joint movement. However, assessments of effects of inter-synergistic EMFT on the mechanical behaviour of spastic muscle are lacking. Those assessments require measurement during surgery of activated spastic muscle's forces directly at its tendon and as a function of joint angle. Employing this methodology, the aim was to test the following study hypotheses: added activation of semimembranosus (SM) and gracilis (GRA) muscles of patients with CP changes (1) force, (2) stiffness and (3) joint range of force exertion of activated spastic semitendinosus (ST) due to inter-synergistic EMFT. Isometric spastic ST forces were measured intraoperatively (12 limbs of 7 patients (mean age 8 years 9 months) for knee angles from flexion (120°) to full extension (0°). Conditions I and II: spastic ST was activated alone, and simultaneously with its synergists SM and GRA muscles, respectively. Condition II did increase activated spastic ST's forces significantly (by 33.3%), but did not change its stiffness and joint range of force exertion, confirming only study hypothesis 1. Therefore, we conclude that inter-synergistic EMFT affects forces exerted at spastic ST tendon, but not other characteristics of its angle-force relationship.