Cadence-dependent changes in corticospinal excitability of the biceps brachii during arm cycling

Abstract

This is the first study to report the influence of different cadences on the modulation of supraspinal and spinal excitability during arm cycling. Supraspinal and spinal excitability were assessed using transcranial magnetic stimulation of the motor cortex and transmastoid electrical stimulation of the corticospinal tract, respectively. Transcranial magnetic stimulation-induced motor evoked potentials and transmastoid electrical stimulation-induced cervicomedullary evoked potentials (CMEPs) were recorded from the biceps brachii at two separate positions corresponding to elbow flexion and extension (6 and 12 o'clock relative to a clock face, respectively) while arm cycling at 30, 60 and 90 rpm. Motor evoked potential amplitudes increased significantly as cadence increased during both elbow flexion (P < 0.001) and extension (P = 0.027). CMEP amplitudes also increased with cadence during elbow flexion (P < 0.01); however, the opposite occurred during elbow extension (i.e., decreased CMEP amplitude; P = 0.01). The data indicate an overall increase in the excitability of corticospinal neurons which ultimately project to biceps brachii throughout arm cycling as cadence increased. Conversely, changes in spinal excitability as cadence increased were phase dependent (i.e., increased during elbow flexion and decreased during elbow extension). Phase- and cadence-dependent changes in spinal excitability are suggested to be mediated via changes in the balance of excitatory and inhibitory synaptic input to the motor pool, as opposed to changes in the intrinsic properties of spinal motoneurons.

Keywords: CMEP; MEP; motoneuron; transcranial; transmastoid.

Fig. 1.

A: rectified electromyographic (EMG) values of the biceps brachii and triceps brachii from a single participant throughout one full revolution of arm cycling. The 1 denotes the 6 o'clock position, where the elbow is flexing and the biceps brachii is active, and 2 occurs during elbow extension, when the biceps brachii is relatively inactive. Averaged EMG activity of the biceps brachii (B) and triceps brachii (C) throughout arm cycling at varying cadences is shown. The dashed, light gray lines represent cycling at 30 rpm; dark, solid gray lines, cycling at 60 rpm; and black solid lines are cycling at 90 rpm. Each crank position represents 8.3% of a full cycling revolution. EMG rectified averages were normalized to the largest EMG elicited between the three different cadences.

Fig. 2.

Average motor evoked potential (MEP; A and C) and cervicomedullary evoked potential (CMEP; B and D) traces following 8 stimulations during arm cycling at 30 rpm (solid gray line), 60 rpm (dotted black line), and 90 rpm (solid black line) at the 6 o'clock (A and B) and 12 o'clock (C and D) positions. Amplitudes are expressed as a percentage of maximal M-wave (M_max).

Fig. 3.

Group data (means ± SE, n = 10) at the 6 o'clock position during the flexion phase for MEP amplitude (A), background EMG (bEMG) of the biceps brachii prior to transcranial magnetic stimulation (TMS; B), and bEMG of the triceps brachii prior to TMS (C), as well as group data (means ± SE, n = 7) at the 6 o'clock position for CMEP amplitude (D), bEMG of the biceps brachii prior to TMES (E), and bEMG of the triceps brachii prior to TMES (F). MEP and CMEP amplitudes are expressed relative to the M_max taken during cycling at the same cadence, and bEMG is expressed relative to the maximum EMG found during the 90 rpm trial. *Significant difference (P < 0.05) between cadences. #Differences that approached significance.

Fig. 4.

Group data (means ± SE, n = 10) at the 12 o'clock position during the extension phase for MEP amplitude (A), bEMG of the biceps brachii prior to TMS (B), and bEMG of the triceps brachii prior to TMS (C), as well as group data (means ± SE, n = 7) at the 12 o'clock position for CMEP amplitude (D), bEMG of the biceps brachii prior to TMES (E), and bEMG of the triceps brachii prior to TMES (F). MEP and CMEP amplitudes are expressed relative to the M_max taken during cycling at the same cadence, and bEMG is expressed relative to the maximum EMG found during the 90 rpm trial. *Significant difference (P < 0.05) between cadences.