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

BackgroundIt 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.ObjectiveThis 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.MethodsEight 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 MEPTEST at each time point.ResultsMEPs 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.ConclusionsThe 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.