Changes in muscle activity during the flexion and extension phases of arm cycling as an effect of power output are muscle-specific

Abstract

Arm cycling is commonly used in rehabilitation settings for individuals with motor impairments in an attempt to facilitate neural plasticity, potentially leading to enhanced motor function in the affected limb(s). Studies examining the neural control of arm cycling, however, typically cycle using a set cadence and power output. Given the importance of motor output intensity, typically represented by the amplitude of electromyographic (EMG) activity, on neural excitability, surprisingly little is known about how arm muscle activity is modulated using relative workloads. Thus, the objective of this study was to characterize arm muscle activity during arm cycling at different relative workloads. Participants (n = 11) first completed a 10-second maximal arm ergometry sprint to determine peak power output (PPO) followed by 11 randomized trials of 20-second arm cycling bouts ranging from 5–50% of PPO (5% increments) and a standard 25 W workload. All submaximal trials were completed at 60 rpm. Integrated EMG amplitude (iEMG) was assessed from the biceps brachii, brachioradialis, triceps brachii, flexor carpi radialis, extensor carpi radialis and anterior deltoid of the dominant arm. Arm cycling was separated into two phases, flexion and extension, relative to the elbow joint for all comparisons. As expected, iEMG amplitude increased during both phases of cycling for all muscles examined. With the exception of the triceps brachii and extensor carpi radialis, iEMG amplitudes differed between the flexion and extension phases. Finally, there was a linear relationship between iEMG amplitude and the %PPO for all muscles during both elbow flexion and extension.

Moving forward: methodological considerations for assessing corticospinal excitability during rhythmic motor output in humans

“…The assumption made when the bEMG between tasks is similar is that the effort level to produce those tasks is also similar. In our experience and inline with leg cycling work (108), the EMG produced during arm cycling at a relatively high workload, such as 35% of maximal power output (48), is often greater than can be produced by the same muscle during a submaximal tonic contraction. In other words, the effort required to generate equivalent EMG during an isometric contraction as that produced during a submaximal bout of arm cycling may be substantially higher.…”

“…One of the main factors for consideration when setting stimulation parameters during locomotor outputs is the phase-dependent changes in corticospinal excitability. During locomotor outputs, corticospinal excitability is largely modulated in a phase-dependent manner that closely parallels the phasic modulation of the EMG (25,31,48,55,(84)(85)(86)(87). In other words, when muscle activity is high during specific phases of the movement, the threshold to elicit evoked responses is generally low and the amplitude of these responses is typically large.…”

“…Second, arm cycling may not be performed similarly for each individual. For example, we typically collect corticospinal excitability measures from the biceps (elbow flexor) and triceps brachii (elbow extensor) during mid-elbow flexion, while the arm is in the pulling phase of arm cycling (3 o'clock to 9 o'clock) and the opposite arm engages in elbow extension or a pushing phase (9 o'clock to 3 o'clock) (48). In recent work from our laboratory (unpublished), not every participant adopted the same strategy to arm cycle.…”

“…Although a generally accepted method to match tasks, there are caveats and questions that must be considered in data interpretation. Rhythmic motor outputs include relatively predictable muscle activity patterns that involve both activation and inactivation (48), as opposed to tonic contractions, whereby the muscle actively is generally constant or performed in a controlled, ramp-like fashion. Thus, the time window in which the EMG is measured encompasses a changing EMG pattern and raises an important question, "when during the rhythmic motor output is neural excitability examined?"…”

The Effect of Crank Resistance on Arm Configuration and Muscle Activation Variances in Arm Cycling Movements

“…Given a cadence, an increase in crank resistance is known to require an increased muscle activation. Arm muscle activities during arm cycling at different workloads were characterized in ( Chaytor et al, 2020 ) and it was found that there was a linear relationship between EMG amplitude and power output for individual muscles. On the other hand, an increase in muscle activation also produces an increased signal dependent motor noise.…”