Phase- and Workload-Dependent Changes in Corticospinal Excitability to the Biceps and Triceps Brachii during Arm Cycling

Abstract

This is the first study to examine corticospinal excitability (CSE) to antagonistic muscle groups during arm cycling. Transcranial magnetic stimulation (TMS) of the motor cortex and transmastoid electrical stimulation (TMES) of the corticospinal tract were used to assess changes in supraspinal and spinal excitability, respectively. TMS induced motor evoked potentials (MEPs) and TMES induced cervicomedullary evoked potentials (CMEPs) were recorded from the biceps and triceps brachii at two positions, mid-elbow flexion and extension, while cycling at 5% and 15% of peak power output. While phase-dependent modulation of MEP and CMEP amplitudes occurred in the biceps brachii, there was no difference between flexion and extension for MEP amplitudes in the triceps brachii and CMEP amplitudes were higher during flexion than extension. Furthermore, MEP amplitudes in both biceps and triceps brachii increased with increased workload. CMEP amplitudes increased with higher workloads in the triceps brachii, but not biceps brachii, though the pattern of change in CMEPs was similar to MEPs. Differences between changes in CSE between the biceps and triceps brachii suggest that these antagonistic muscles may be under different neural control during arm cycling. Putative mechanisms are discussed.

Intensity matters: effects of cadence and power output on corticospinal excitability during arm cycling are phase and muscle dependent

“…Responses were evoked at two positions during arm cycling: 6 and 12 o'clock, defined relative to a clock face. Similar to our previous arm cycling studies, 6 o'clock was specified as "bottom dead center" and 12 o'clock was specified as "top dead center" (Forman et al 201420152016;Spence et al 2016). These two positions were examined because they occur during mid-elbow flexion (6 o'clock) and extension (12 o'clock) during arm cycling ( Fig.…”

“…More specifically, we reported that corticospinal excitability to the biceps brachii was enhanced throughout arm cycling with increased cadence, while spinal excitability was phase dependent . When cycling power output was manipulated, overall corticospinal excitability to the biceps and triceps brachii increased during both phases of arm cycling, whereas the pattern of spinal excitability to the biceps and triceps brachii tended to increase with power output (Spence et al 2016). Thus, during arm cycling, alterations in intensity (i.e., changes in cadence or power output) have differential effects and may be important in determining corticospinal excitability.…”

“…Substantially less information is currently available regarding the modulation of excitability in the corticospinal pathway during locomotor output of varying intensities Spence et al 2016;Weavil et al 2015). The excitability of the corticospinal pathway can be assessed by measuring the amplitude of responses evoked by transcranial magnetic stimulation (TMS) known as motorevoked potentials (MEPs).…”

“…Therefore, when TMS and TMES are both used, it can be logically deduced whether a change in MEP is likely due to either supraspinal or spinal factors. Using these techniques, we recently demonstrated cadence )-and power output-dependent (Spence et al 2016) changes in supraspinal and spinal excitability to muscles of the upper limb during arm cycling. More specifically, we reported that corticospinal excitability to the biceps brachii was enhanced throughout arm cycling with increased cadence, while spinal excitability was phase dependent .…”

“…We assessed corticospinal excitability to the biceps and triceps brachii at two positions during arm cycling (mid-elbow flexion and mid-elbow extension) at combinations of two different cadences (60 and 90 rpm) at three relative power outputs (20, 40 and 60% of peak power output). It was hypothesized, based on our previous work Spence et al 2016) that 1) corticospinal excitability to both the biceps and triceps brachii would increase in each position examined (flexion and extension; see METHODS) as cycling intensity (both cadence and power output) increased; 2) supraspinal excitability would account for increases in corticospinal excitability during the less active phase of each muscle (i.e., elbow extension for the biceps and elbow flexion for the triceps brachii), while spinal excitability would be largely responsible for the increase in excitability during the most active phase of each muscle (i.e., elbow flexion for the biceps and elbow exten-sion for the triceps brachii); and 3) supraspinal and spinal excitability would be modulated differently with changes in cadence vs. changes in power output. Thus we hypothesized that supraspinal and spinal excitability would be differentially modulated depending on the muscle, phase, and type of intensity assessed.…”