Fatiguing intermittent lower limb exercise influences corticospinal and corticocortical excitability in the nonexercised upper limb.

BACKGROUND: It has recently been reported that unilateral fatiguing exercise affects not only the motor area innervating the exercising muscle but also the ipsilateral motor area innervating homologous nonexercised muscle.

OBJECTIVE: This study was designed to clarify the effects of fatiguing intermittent lower limb exercise on the excitability of the motor cortex representation of nonexercised muscles in the arm.

METHODS: Eight subjects performed an intermittent leg press exercise composed of three bouts of 5-minute leg press (T1, T2, and T3) at 50% of maximal voluntary contraction separated by a 2-minute rest. Motor-evoked potentials (MEP), short interval intracortical inhibition (SICI), and intracortical facilitation (ICF), using paired-pulse transcranial magnetic stimulation, were assessed in two nonexercised arm muscles (first dorsal interosseous muscle: FDI, n = 8; biceps brachii muscle: BB, n = 6) and one exercised leg muscle (quadriceps femoris muscle: QF, n = 6) before and immediately after each bout of exercise and for 30 minutes during recovery after the end of the third exercise bout (Experiment 1). Experiment 2 was the same as Experiment 1, except that the test pulse intensity was adjusted to produce a given amplitude of MEP(TEST) at each time point.

RESULTS: MEPs and SICI in the exercised QF muscle were depressed at all time points during and after fatigue. In contrast, MEPs in nonexercised arm muscles were facilitated from T1-T3 (T3, only FDI), but were then depressed for up to 20 minutes in the recovery period. SICI was reduced in both muscles during T1-T3 and remained depressed until 20 minutes into recovery. ICF was unchanged in arm muscles but depressed in QF over T1-T3.

CONCLUSIONS: The current study indicates that muscle fatigue induced by exercise of a large lower limb muscle group has powerful effects on the excitability of both SICI and the corticospinal projection to muscles of the nonexercised upper limb.