Interhemispheric inhibition is different during arm cycling than a position- and intensity-matched tonic contraction.

Task-dependent changes in inhibition may explain why supraspinal excitability is higher during arm cycling than an intensity- and position-matched tonic contraction. The present study investigated whether interhemispheric inhibition (IHI) associated with biceps brachii activity was different during arm cycling, a locomotor output, compared to a tonic contraction. IHI was quantified using an ipsilateral silent period (iSP) evoked via transcranial magnetic stimulation (TMS) of the ipsilateral motor cortex. TMS was delivered at 120% resting motor threshold during the mid-elbow flexion phase of arm cycling (6 o'clock position, made relative to a clock face) and during a position- and intensity-matched tonic contraction. In total, 36 participants took part in the study. However, only 14 participants demonstrated IHI during arm cycling and 10 participants during tonic contraction. Of these participants, eight displayed clear iSPs during arm cycling and tonic contraction. The iSP duration was longer during arm cycling than tonic contraction (p < 0.05), while iSP EMG amplitude and area were not different between tasks (p > 05 for both comparisons). The main finding from this study is that IHI appears to be stronger during arm cycling than an intensity- and position-matched tonic contraction. This does not support previous findings of higher supraspinal excitability during arm cycling.

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

The use of transcranial magnetic stimulation to assess the excitability of the central nervous system to further understand the neural control of human movement is expansive. The majority of the work performed to-date has assessed corticospinal excitability either at rest or during relatively simple isometric contractions. The results from this work are not easily extrapolated to rhythmic, dynamic motor outputs, given that corticospinal excitability is task-, phase-, intensity-, direction-, and muscle-dependent (Power KE, Lockyer EJ, Forman DA, Button DC. Appl Physiol Nutr Metab 43: 1176-1185, 2018). Assessing corticospinal excitability during rhythmic motor output, however, involves technical challenges that are to be overcome, or at the minimum considered, when attempting to design experiments and interpret the physiological relevance of the results. The purpose of this narrative review is to highlight the research examining corticospinal excitability during a rhythmic motor output and, importantly, to provide recommendations regarding the many factors that must be considered when designing and interpreting findings from studies that involve limb movement. To do so, the majority of work described herein refers to work performed using arm cycling (arm pedaling or arm cranking) as a model of a rhythmic motor output used to examine the neural control of human locomotion.

Two weeks of arm cycling sprint interval training enhances spinal excitability to the biceps brachii.

The present study aimed to investigate whether a 2-wk arm cycling sprint interval training (SIT) program modulated corticospinal pathway excitability in healthy, neurologically intact participants. We employed a pre-post study design with two groups: 1) an experimental SIT group and 2) a nonexercising control group. Transcranial magnetic stimulation (TMS) of the motor cortex and transmastoid electrical stimulation (TMES) of corticospinal axons were used at baseline and post-training to provide indices of corticospinal and spinal excitability, respectively. Stimulus-response curves (SRCs) recorded from the biceps brachii were elicited for each stimulation type during two submaximal arm cycling conditions [25 watts (W) and 30% peak power output (PPO)]. All stimulations were delivered during the mid-elbow flexion phase of cycling. Compared with baseline, performance on the time-to-exhaustion (TTE) test at post-testing was improved for members of the SIT group but was not altered for controls, suggesting that SIT improved exercise performance. There were no changes in the area under the curve (AUC) for TMS-elicited SRCs for either group. However, the AUC for TMES-elicited cervicomedullary motor-evoked potential SRCs were significantly larger at post-testing in the SIT group only (25 W: P = 0.012, d = 0.870; 30% PPO: P = 0.016, d = 0.825). This data shows that overall corticospinal excitability is unchanged following SIT, whereas spinal excitability is enhanced. Although the precise mechanisms underlying these findings during arm cycling at post-SIT are unknown, it is suggested that the enhanced spinal excitability may represent a neural adaptation to training.NEW & NOTEWORTHY Two weeks of arm cycling sprint interval training (SIT) improves subsequent aerobic exercise performance and induces changes within the descending corticospinal pathway. Specifically, spinal excitability is enhanced following training, whereas overall corticospinal excitability does not change. These results suggest that the enhanced spinal excitability may represent a neural adaptation to training. Future work is required to discern the precise neurophysiological mechanisms underlying these observations.

Interhemispheric inhibition to the biceps brachii during arm cycling.

This is the first demonstration of interhemispheric inhibition (IHI) during a locomotor output, arm cycling. IHI was quantified by assessing the depth of the ipsilateral silent period (iSP) evoked via transcranial magnetic stimulation of the motor cortex. There was a significant reduction in electromyography (EMG) amplitude of the iSP during cycling compared with the control EMG (16.8% ± 17.1%; p < 0.001). Depth and area for measuring the iSP during arm cycling are discussed. Novelty: This is the first study to demonstrate activation of the cortical circuit, interhemispheric inhibition, during a locomotor output.

Upper and lower body responses to repeated cyclical sprints.

PURPOSE: To compare the physiological and perceptual responses of the upper and lower body to all-out cyclical sprints with short or long rest periods between sprints. METHODS: Ten recreationally trained males completed four 10 × 10 s sprint protocols in a randomized order: upper body with 30 s and 180 s of rest between sprints, and lower body with 30 s and 180 s of rest between sprints. Additionally, maximum voluntary contractions (MVC) were measured at pre-sprint and post-sprints 5 and 10. Normalized (% of first sprint) peak power, MVC, heart rate (HR) and rating of perceived exertion (RPE) were compared between upper and lower body within the same recovery period, and absolute values (Watts, bpm, RPE scores) were compared within the same body part and between recovery periods. RESULTS: Trivial differences were identified in normalized peak power, HR and RPE values between the upper and lower body in both recovery conditions (<2%, d ≤ 0.1), but MVC forces were better maintained with the upper body (∼9.5%, d = 1.0) in both recovery conditions. Absolute peak power was lower (∼147 Watts, d = 1.3), and HR was higher (∼10 bpm, d = 0.73) in the 30 s compared to 180 s condition in both the upper and lower body whereas RPE scores were similar (<0.6 RPE units, d ≤ 0.1). Despite the reductions in peak power, MVC forces were better maintained in the 30 s condition in both upper (2.5 kg, d = 0.4) and lower (7.5 kg, d = 0.7) body. CONCLUSIONS: Completing a commonly used repeated sprint protocol with the upper and lower body results in comparable normalized physiological and perceptual responses.

Repeated sprint ability but not neuromuscular fatigue is dependent on short versus long duration recovery time between sprints in healthy males.

OBJECTIVES: During maximal intensity leg cycling sprints, previous research has shown that central and peripheral fatigue development occurs with various (<30s) short-duration recovery periods between sprints. The aim of the current study was to compare the development of neuromuscular fatigue during maximal intensity lower-body sprints interspersed with short and longer duration recovery periods. DESIGN: Crossover study. METHODS: Ten participants completed 10, 10s sprints interspersed with either 30 or 180s of recovery. Peak power outputs were measured for each sprint. Maximal force, voluntary activation (VA) and evoked contractile properties of the knee extensors were measured at pre-sprint 1, post-sprint 5 and post-sprint 10. Perceived pain was also measured immediately following each sprint. RESULTS: Peak power output was significantly lower by 16.1±4.2% (p<0.001) during sprint 10 with 30 compared to 180s of recovery. Irrespective of recovery time, maximal force, VA and potentiated twitch force decreased by 26.7±7.2% (p<0.005), 5.8±1.2% (p=0.025), 38.7±6.1% (p=0.003) respectively, from pre-sprint 1 to post-sprint 10. MVC and PT decreased by 17±4% (p<0.003) and 23±9% (p<0.002) respectively, from pre-sprint 1 to post-sprint 5. CONCLUSIONS: Although decreases in peak power and increases in perceived pain were greater when sprints were interspersed with 30 compared to 180s of recovery, the development of neuromuscular fatigue of the knee extensors was similar. The results illustrate that peripheral fatigue developed early whereas central fatigue developed later in the sprint protocol, however the effect of recovery time on neuromuscular fatigue could be task specific.