Muscle function of knee extensors and flexors after stroke is selectively impaired at shorter muscle lengths.

OBJECTIVE To investigate whether muscle strength is selectively more affected at shorter lengths of thigh muscles as result of stroke and to determine whether this is associated with impaired neural activation at shorter muscle lengths. DESIGN Case-control study. SUBJECTS Fourteen patients with sub-acute stroke, and 12 able-bodied controls. METHODS In patients (bilaterally) and able-bodied controls torque and activation was measured during maximal voluntary isometric contractions of knee extensors and flexors at 30 degrees , 60 degrees and 90 degrees knee flexion. RESULTS The paretic knee extensors showed lower normalized maximal torques (73%) and lower normalized activation (71%) compared with control at 30 degrees . The paretic knee flexors showed lower normalized maximal torques at 60 degrees (64%) and 90 degrees (45%) with non-significantly lower normalized activation (~79% and ~67%, respectively). CONCLUSION As a result of stroke, lower muscle torque at shorter muscle lengths of the knee extensors is associated with a length-dependent lower voluntary activation, which may also be the case for the flexor muscles, but not with altered co-activation. We recommend investigating the role of specific training of paretic knee extensors and flexors in especially shortened positions to improve recovery of function after stroke.