Quantification of muscle fatigue in cerebal palsy and its relationship to impairments and function

Abstract

Three experiments were designed to explore the measurement of muscle fatigue in people with cerebral palsy (CP). The four aims were to 1) develop a feasible and reliable isokinetic protocol to assess muscle fatigue of the knee flexors and extensors in this population, 2) determine if muscle fatigue of the knee flexors and extensors in people with CP differs from subjects without a motor disorder, 3) determine whether muscle fatigue is related to functional measures of activity and participation, and 4) investigate possible contributing factors of muscle fatigue. Results show that muscle fatigue can be reliably assessed through an isokinetic protocol consisting of 35 consecutive knee extension and flexion repetitions at 60 degrees/second by calculation of a fatigue index (FI) and the slope of the decline in peak torque. When compared to a control group of age-matched peers without motor disorder, the knee flexors and extensors in subjects with CP were observed to be less fatigable. Furthermore, muscle fatigue of the knee extensors and flexors in the group with CP was positively correlated with transfers and basic mobility. Muscle fatigue of the knee extensors was also positively correlated with overall global functioning, participation in sports and physical function, and fast walking velocity. Lower Gross Motor Function Classification System Levels (GMFCS) (i.e. less involved subjects) were also associated with higher levels of muscle fatigability. Strength was directly related to muscle fatigability, where weaker subjects had lower levels of fatigue, regardless of muscle. Cocontraction and quadriceps stiffness, on the other hand, were inversely related to muscle fatigability. The strongest predictors of hamstring fatigability were hamstrings strength and quadriceps stiffness, whereas the strongest predictor of quadriceps fatigability was hamstring cocontraction. The presence of spasticity, regardless of muscle group, was associated with lower fatigability compared to control subjects. In summary, the results indicate that the knee flexors and extensors of people with CP are less fatigable than age-matched peers without motor disability. In addition, lower levels of muscle fatigability are associated with lower levels of function and participation. Furthermore, weakness, spasticity, stiffness, and cocontraction are possible contributing factors to the observed fatigue resistance