Assessment of Muscle Endurance of the Knee Extensor Muscles in Adolescents With Spastic Cerebral Palsy Using a Submaximal Repetitions-to-Fatigue Protocol

Objective: To compare muscle endurance in adolescents with spastic cerebral palsy (CP) with typically developing (TD) peers using a submaximal repetitions-to-fatigue (RTF) protocol. Design: Cross sectional. Setting: Human motion laboratory. Participants: Adolescents with spastic CP (n=16; Gross Motor Function Classification System levels I or II) and TD adolescents (n = 18) within the age range of 12 to 19 years old. Interventions: Not applicable. Main Outcome Measures: Each participant performed 3 RTF tests at different submaximal loads, ranging from 50% to 90% of their maximal voluntary knee extension torque. The relation between the number of repetitions (repetition maximum [RM]) and imposed submaximal relative (percent of maximal voluntary torque [%MVT]) and absolute (Nm/kg) torque was quantified. To compare adolescents with CP with TD adolescents, a mixed linear model was used to construct load endurance curves. Surface electromyography of quadriceps muscles was measured to assess changes in normalized amplitude and median frequency (MF) as physiological indicators of muscle fatigue. Results: Adolescents with CP showed a larger decrease in %MVT per P.M than TD adolescents (P<.05). TD adolescents showed substantial higher absolute (Nm/kg) load endurance curves than adolescents with CP (P<.001), but they did not show a difference in slope. Electromyographic normalized amplitude increased significantly (P<.05) in the quadriceps muscles in all tests for both groups. Electromyographic MF decreased significantly (P<.05) in tests with the low and medium loads. Electromyographic responses did not differ between groups, indicating that similar levels of muscle fatigue were reached. Conclusions: Adolescents with CP show slightly lower muscle endurance compared with TD adolescents on a submaximal RTF protocol, which is in contrast with earlier findings in a maximal voluntary fatigue protocol. Accordingly, adolescents with CP have a reduced capacity to endure activities at similar relative loads compared with TD adolescents. (C) 2014 by the American Congress of Rehabilitation Medicine