Role Of Coactivation During An Isometric Leg-extensor Fatigue Test In Children With Cerebral Palsy

Abstract

High levels of coactivation have been identified during multi-joint movements such as walking in children with cerebral palsy (CP) compared to able-bodied control subjects. It is not entirely known how the amount of coactivation changes in children and adolescents with CP during a single-joint fatiguing exercise. PURPOSE The purpose of this study was to quantify coactivation of the quadriceps and hamstrings during an intermittent isometric fatigue test in children and adolescents with and without CP. METHODS Five children and adolescents with spastic CP (mean=13.0±3 yr, GMFCS=3) and five able-bodied control children (CON), matched for age, sex, body mass and stature (mean: 12.6±2.9 yr) participated in an intermittent isometric fatiguing protocol for 1 min (5 sec contract/ 5 sec rest) of the knee extensors. Voluntary torque (Nm) and EMG activity of the vastus lateralis (VL), vastus medialis (VM), rectus femoris (RF) and the biceps femoris (BF) were measured during the fatiguing exercise. A ratio of the agonist-to-antagonist muscle activity (coactivation) was calculated by dividing the mean VL, VM, and RF EMG activity by the BF EMG activity. RESULTS The CP subjects generated less voluntary isometric force (p=.03, effect size=1.49) and experienced greater amounts of coactivation (p=.00, effect size=2.93) throughout the protocol than the CON group. The CP subjects experienced a 26% decrease in force during the fatigue test and the CON group had an 8% decrement in force (p=.056, effect size= 1.35). There was no significant decrease in the ratio of quadriceps to hamstring activity (increased coactivation) during the fatigue test in the CP subjects (16%) compared to the CON group (11.6%), (p=0.32, effect size=0.38). CONCLUSION Although the children with CP experienced greater amounts of leg extensor fatigue than the CON group, this does not appear to be mediated by increases in coactivation. Therefore, other mechanisms, such as deficits in muscle metabolism and fiber type alterations, should be examined in the CP population.