Lower-limb resistance training reduces exertional dyspnea and intrinsic neuromuscular fatigability in individuals with chronic obstructive pulmonary disease.

Skeletal muscle atrophy, dysfunction, and fatigue are important complications of chronic obstructive pulmonary disease (COPD). Greater reliance on glycolytic metabolism and increased type III/IV muscle afferent activity increase ventilatory drive, promote ventilatory constraint, amplify exertional dyspnea, and limit exercise tolerance. To investigate whether muscular adaptation with resistance training (RT) could improve exertional dyspnea, exercise tolerance, and intrinsic neuromuscular fatigability in individuals with COPD ( = 14, FEV = 62 ± 21% predicted), we performed a proof-of-concept single-arm efficacy study utilizing 4 wk of individualized lower-limb RT (3 times/wk).

Intrinsic Neuromuscular Fatigability in Humans: The Critical Role of Stimulus Frequency.

Electrically evoked contractions provide insight into intrinsic neuromuscular fatigability and also represent a valuable technique to maintain muscle mass in a clinical setting. To appropriately investigate intrinsic fatigability and design optimal stimulation protocols, it would seem to be crucial to stimulate the muscle at a frequency equivalent to the mean motor unit discharge rate expected at the target force level.

The inclusion of interstimulus interval variability does not mitigate electrically-evoked fatigue of the knee extensors.

Purpose: Transcutaneous electrical stimulation (TES) is used to activate muscles when volitional capacity is impaired but potential benefits are limited by rapid force loss (fatigue). Most TES fatigue protocols employ constant-frequency trains, with stimuli at a fixed interstimulus interval (ISI); however, a brief ISI between the first two pulses (variable-frequency train, VFT) to maximize the catchlike property of muscle can attenuate fatigue development. The purpose of this study was to investigate if a VFT that simulates intrinsic variability of voluntary motor unit discharge rates would also mitigate fatigue, owing to the sensitivity of muscle to acute activation history.

Electrically evoked force loss of the knee extensors is equivalent for young and old females and males.

Data are scant on sex-related differences for electrically evoked contractions, which assess intrinsic contractile properties while limiting spinal and supraspinal adaptations to mitigate fatigue. Furthermore, the few studies that exist use stimulus frequencies considerably higher than the natural motor unit discharge rate for the target force. The purpose of this study was to compare force loss to electrically evoked contractions at a physiological stimulus frequency among young females ( = 12), young males ( = 12), old females ( = 11), and old males ( = 11).

The Sexes Do Not Differ for Neural Responses to Submaximal Elbow Extensor Fatigue.

Purpose: To investigate possible sex-related differences in group III/IV muscle afferent feedback with isometric fatigue, we aimed to assess the effect of a sustained submaximal elbow extensor contraction on motoneuronal excitability (cervicomedullary motor evoked potential [CMEP]) and voluntary activation (VA).

Methods: Twenty-four participants (12 females) performed a 15-min contraction at the level of EMG activity recorded at 15% of maximal torque. Each minute, CMEP were elicited by cervicomedullary stimulation with and without conditioning transcranial magnetic stimulation (TMS) delivered 100 ms earlier.

The effects of forearm position and contraction intensity on cortical and spinal excitability during a submaximal force steadiness task of the elbow flexors.

Elbow flexor force steadiness is less with the forearm pronated (PRO) compared with neutral (NEU) or supinated (SUP) and may relate to neural excitability. Although not tested in a force steadiness paradigm, lower spinal and cortical excitability was observed separately for biceps brachii in PRO, possibly dependent on contractile status at the time of assessment. This study aimed to investigate position-dependent changes in force steadiness as well as spinal and cortical excitability at a variety of contraction intensities.

The Effects of Sex and Motoneuron Pool on Central Fatigue.

Purpose: It is uncertain if sex influences central fatigue because the reduction in voluntary activation (VA) has been reported as not different between the sexes for elbow flexors (EF) but greater in males compared with females for knee extensors. This disparity could result from the facilitatory and inhibitory effects of group III/IV muscle afferents on flexor versus extensor motoneurons, respectively. The purpose of this study was to examine central fatigue and motoneuron responsiveness of EF and elbow extensors (EE) in males and females.

UBC-Nepal expedition: acclimatization to high-altitude increases spinal motoneurone excitability during fatigue in humans.

Key Points: Acute exposure and acclimatization to hypoxia are associated with an impairment and partial recovery, respectively, of the capability of the central nervous system to drive muscles during prolonged efforts. Motoneurones play a vital role in muscle contraction and in fatigue, although the effect of hypoxia on motoneurone excitability during exercise has not been assessed in humans. We studied the impact of fatigue on motoneurone excitability in normoxia, acute and chronic exposure (5050 m) to hypoxia.

The effect of muscle length on transcranial magnetic stimulation-induced relaxation rate in the plantar flexors.

Transcranial magnetic stimulation (TMS) of the motor cortex during a maximal voluntary contraction (MVC) permits functionally relevant measurements of muscle group relaxation rate (i.e., when muscles are actively contracting under voluntary control).

Motoneuron responsiveness to corticospinal tract stimulation during the silent period induced by transcranial magnetic stimulation.

Based on H-reflex data, spinal mechanisms are proposed to be responsible for the first 50-80 ms of the transcranial magnetic stimulation (TMS)-induced silent period. As several methodological issues can compromise H-reflex validity as a measure of motoneuron excitability, this study used transmastoid stimulation to elicit cervicomedullary motor evoked potentials (CMEPs) during the silent period. Eleven subjects made 1-3 visits which involved 32 or 44 brief (~3 s) isometric elbow flexor contractions at 25 % of maximal torque.