Muscle Activation Deficits During Fatigue after Limb Disuse

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1890 PURPOSE: To determine the extent of plantarflexor muscle activation deficits during fatigue in patients after immobilization. METHODS: Six patients with surgically stabilized, unilateral ankle fractures and 6 healthy subjects participated. Patients were immobilized for 8 weeks, followed by 10 weeks of rehabilitation. Isometric plantarflexion torque was measured at 1 week (1w-R), 5 weeks (5w-R) and 10 weeks (10w-R) of rehabilitation. Subjects performed maximal voluntary isometric contractions (MVIC) with burst superimposition (100ms; 100pps) to assess muscle activation. Fatigue testing involved a 90 second, sustained MVIC of the plantarflexors, with burst superimposition at the beginning, middle, and end. Muscle activation was quantified using the central activation ratio (CAR), which was calculated by dividing the maximum voluntary force by the maximum force produced with electrical stimulation. A CAR of 1.0 indicates full muscle activation by the central nervous system. A fatigue index (final/initial force) was used quantify the extent of muscle fatigue. T-tests were used for all comparisons. RESULTS: At 1 wk-R, the involved leg fatigued less than 5%, while the uninvolved leg fatigued almost 50% (p<0.05). At 5w-R and 10w-R, the involved leg fatigued between 20 and 25%. During the nonfatigued state at 1wk-R, there were significant differences in muscle activation between the involved and uninvolved limbs (CAR = 0.98 vs. 0.79) (p<0.05), which resolved by 5w-R (0.93) and 10w-R (0.96). Decreases in CAR during the fatigue test were minimal (<0.1) for all subjects for both the involved and uninvolved legs. Torque deficits present in the involved legs at 1w-R, recovered to normal by 10w-R (p<0.05). CONCLUSION: Patients experienced less muscle fatigue in their involved leg, early after limb disuse. Fatigability did not recover to normal levels even after 10 weeks of rehabilitation, despite the complete restoration of torque and muscle activation deficits. Initial muscle activation deficits can only account for some of the decreased fatigue in patients early after limb disuse because activation deficits resolved by 5w-R. Clinically, these results suggest that traditional measures of strength may not completely reflect the resolution of a patient's impairments. Supported by NIH-RO1HD37645, NIHRO1HD40850, 2P41RR02305.