Changes in muscle architecture after hemispheric stroke can adversely affect efficiency of muscle contraction in pennate muscles: an analysis of muscle gear ratios
Rymer WZ & Son J
1. Arms and Hands Lab, Shirley Ryan AbilityLab, Chicago, Illinois, USA
After hemispheric stroke, there is routinely weakness for voluntary movement in contralesional limbs. While much of this weakness is attributable to a loss of descending excitatory neural commands from cerebral cortex and brainstem, there are also progressive changes in muscle architecture, associated with changes in muscle material properties. Our objective here is to evaluate the relative impact of these muscular and architectural changes on the generation of voluntary force in stroke-impaired muscles.
Our specific hypothesis is that changes in the muscle matrix of pennate muscles following stroke may limit rotation of fascicles in pennate muscles, forcing fascicles to stay aligned with the shortening direction of the muscle, a suboptimal mechanical state. In particular, we plan to evaluate changes in fascicle “gear ratio”, in which we estimate the ratio between muscle shortening velocity and fascicle shortening velocity (1) using B mode ultrasound imaging.
We report here on differences in gear ratio between impaired and contralateral medial gastrocnemius (MG) muscles of four hemispheric stroke survivors. Subjects were required to perform isometric contractions of the MG muscle at predetermined activation levels, over a range of ankle joint angles. Fascicle length and fascicle rotation were assessed using advanced ultrasound techniques.
Our findings were that there was a systematic reduction in fascicle rotation in impaired MG muscles as voluntary force increased. Comparisons of fascicle rotation between the two sides showed a consistent 6 to 12 difference, which would contribute to a significant change in force generating capacity of the plantar flexor muscles.
- Azizi E, Brainerd EL, Roberts TJ, “Variable gearing in pennate muscles,” Proc Natl Acad Sci, 105(5):1745-50, 2008.