Submaximal isometric fatiguing exercise of the elbow flexors has no age-related effect on GABAB mediated inhibition

Age-related changes in the neuromuscular system can result in differences in fatigability between young and older adults. Previous research has shown that single joint isometric fatiguing exercise of small muscle results in an age-related compensatory decrease in GABAB mediated inhibition. However, this has yet to be established in a larger muscle group. In 15 young (22 ± 4 years) and 15 older (65 ± 5 years) adults, long interval cortical inhibition (LICI; 100 ms ISI) and corticospinal silent period (SP) were measured in the biceps brachii during a 5% EMG contraction using transcranial magnetic stimulation (TMS) before, during and after a submaximal contraction (30% MVC force) held intermittently to task failure. Both age groups developed similar magnitude of fatigue (~24% decline in MVC; P = 0.001) and ~28% decline in LICI (P = 0.001) post fatiguing exercise. No change in SP duration was observed during and immediately following fatigue (P = 0.909) but ~ 6% decrease was seen at recovery in both age groups (P<0.001)." Contrary to previous work in a small muscle, these findings suggest no age-related differences in GABAB mediated inhibition following single joint isometric fatiguing exercise of the elbow flexors, indicating that GABAB modulation with ageing may be muscle group dependent. Furthermore, variations in SP duration and LICI modulation during and post fatigue in both groups suggest that these measures are likely mediated by divergent mechanisms.

Analysis of electromyographic changes in gastrocnemius muscle on exposure to 24 h of dry supine immersion

Introduction: Under microgravity, changes are observed in both structure and content of the gravity-dependent muscles. This may result in disuse atrophy and muscle weakening. However, changes have not been described in the short term exposure of 24 h. Examination of changes in electromyographic activity of the gastrocnemius muscle, on exposure to 24 h of simulated microgravity using dry supine immersion (DSI), was the desired objective of the study. Material and Methods: Ten healthy volunteers were exposed to 24 h of simulated microgravity using DSI. The force generated by maximal voluntary contraction of isometric plantar flexion of ankle was recorded. Electromyography (EMG) of the gastrocnemius corresponding to more than 80% of maximal voluntary isometric contraction (labeled as submaximal contraction) was recorded pre- and post-exposure to 24 h of DSI. Results: Time domain analysis of the surface EMG of gastrocnemius during submaximal contraction revealed a significant increase in mean integrated EMG (iEMG) amplitude (effect size = 0.73, P = 0.031) following 24 h DSI. Power spectral analysis showed a significant decrease in mean frequency (MNF) (P = 0.043) and median frequency (MDF) (P = 0.024) after 24 h DSI. No significant changes were observed in total power, mean power (MNP), and maximal voluntary contraction. A very strong negative correlation was noted between iEMG, MNF, and MDF for the duration of submaximal voluntary contraction (R = −0.827 and −0.810, P = 0.003 and 0.004, respectively); whereas, a very strong positive correlation was noted between iEMG and MNP (R = 0.911, P = 0.002). Conclusion: The findings of the study point toward muscle weakening seen by an early onset of muscle fatigue in anti-gravity muscles as early as 24 h of exposure to microgravity. The same may be borne in mind even during very short duration human space missions.

Alternating Current Is More Fatigable Than Pulsed Current in People Who Are Healthy: A Double-Blind, Randomized Crossover Trial

Objective Tolerance level and rapid fatigue onset are limitations in the use of neuromuscular electrical stimulation (NMES) as an electrotherapeutic resource in rehabilitation and training protocols; however, it is unclear if pulsed current (PC) and alternating current (AC) produce different a fatigue level when applied at submaximal contraction level. The purpose of this study was to compare fatigue and discomfort levels between PC and AC during a submaximal contraction protocol in people who are healthy. Methods In this double-blind, randomized crossover trial conducted in a laboratory setting, there were 30 participants (male; 23.23 years of age (SD = 4.59). Participants performed 2 submaximal fatigue protocols (with a 7-day interval) in a randomized order: PC (pulse duration = 2 milliseconds, pulse frequency = 100 Hz) and AC (2.5 kHz, pulse duration = 0.4 milliseconds, burst frequency = 100 Hz). NMES currents were applied to the knee extensor motor point of the dominant limb. The NMES protocol consisted of 80 evoked contractions (time on:off = 5:10 seconds) and lasted 20 minutes. The current was maintained at a constant intensity throughout the NMES protocol. The primary outcome measures evaluated were: maximal voluntary isometric contraction (MVIC), fatigue index (evoked torque decline), fatigability (number of contractions for a 50% drop in evoked-torque from the protocol start), total evoked torque-time integral (TTI), decline in TTI, and discomfort level. Results AC at 2.5 kHz demonstrated higher MVIC decline post-fatigue, higher fatigue index, higher fatigability (ie, fewer contractions to reach the 50% evoked torque decline from the protocol start), smaller total TTI, and higher TTI decline when compared with PC. No between-currents difference was observed in mean discomfort. Conclusion PC is less fatigable than AC at 2.5 kHz. Impact Based on this study, PC is the preferred current choice when the NMES goal is to generate higher muscle work, higher mechanical load, and smaller fatigability during training both for athletes who are healthy and for rehabilitation programs for people with disease or injury.

Inability to increase the neural drive to muscle is associated with task failure during submaximal contractions

Motor unit firing and contractile properties during a submaximal contraction until failure were assessed with a new tracking technique. Two distinct phases in firing behavior were observed, which compensated for changes in twitch area and predicted time to failure. However, the late increase in firing rate was below the rates attained in the absence of fatigue, which points to an inability of the central nervous system to sufficiently increase the neural drive to muscle with fatigue.

Foot sole cutaneous stimulation mitigates neuromuscular fatigue during a sustained plantar flexor isometric task

Cutaneous coupling with lower limb motor neurons has long been known. We set out to establish whether this pathway could serve a purpose other than muscular modulation during standing and walking. We found that during a submaximal contraction of the plantar flexor muscles, the addition of intermittent cutaneous stimulation to the skin of the foot sole resulted in an increase in time to task failure by 15%, which was over a minute longer in duration. We conclude that skin stimulation may serve as a mechanism to mitigate fatigue.

Effect of Cutaneous Heat Pain on Corticospinal Excitability of the Tibialis Anterior at Rest and during Submaximal Contraction

Previous studies have shown that pain can interfere with motor control. The neural mechanisms underlying these effects remain largely unknown. At the upper limb, mounting evidence suggests that pain-induced reduction in corticospinal excitability is involved. No equivalent data is currently available at the lower limb. The present study therefore examined the effect of thermal pain on the corticospinal drive to tibialis anterior (TA) at rest and during an isometric submaximal dorsiflexion. Transcranial magnetic stimulation was used to induce motor-evoked potentials (MEPs) in the TA at rest and during contraction in the presence or absence of cutaneous heat pain induced by a thermode positioned above the TA (51°C during 1 s). With similar pain ratings between conditions (3.9/10 at rest and 3.6/10 during contraction), results indicate significant decreases in MEP amplitude during both rest (−9%) and active conditions (−13%) (main effect of pain, p=0.02). These results therefore suggest that cutaneous heat pain can reduce corticospinal excitability in the TA muscle and that such reduction in corticospinal excitability could contribute to the interference of pain on motor control/motor learning.

Matching of postcontraction perfusion to oxygen consumption across submaximal contraction intensities in exercising humans

Studying the magnitude and kinetics of blood flow, oxygen extraction, and oxygen consumption at exercise onset and during the recovery from exercise can lead to insights into both the normal control of metabolism and blood flow and the disturbances to these processes in metabolic and cardiovascular diseases. The purpose of this study was to examine the on- and off-kinetics for oxygen delivery, extraction, and consumption as functions of submaximal contraction intensity. Eight healthy subjects performed four 1-min isometric dorsiflexion contractions, with two at 20% MVC and two at 40% MVC. During one contraction at each intensity, relative perfusion changes were measured by using arterial spin labeling, and the deoxyhemoglobin percentage (%HHb) was estimated using the spin- and gradient-echo sequence and a previously published empirical calibration. For the whole group, the mean perfusion did not increase during contraction. The %HHb increased from ∼28 to 38% during contractions of each intensity, with kinetics well described by an exponential function and mean response times (MRTs) of 22.7 and 21.6 s for 20 and 40% MVC, respectively. Following contraction, perfusion increased ∼2.5-fold. The %HHb, oxygen consumption, and perfusion returned to precontraction levels with MRTs of 27.5, 46.4, and 50.0 s, respectively (20% MVC), and 29.2, 75.3, and 86.0 s, respectively (40% MVC). These data demonstrate in human subjects the varied recovery rates of perfusion and oxygen consumption, along with the similar rates of %HHb recovery, across these exercise intensities.

Abstract 241: Endothelial Function is Preserved with Endoscopic Vein Harvest for Lower Extremity Bypass

Objective: Endoscopic vein harvest for lower extremity arterial bypass technique has been questioned due to concern for endothelial damage during procurement. We sought to compare NO mediated endothelial dependent relaxation (EDR) in vein segments harvested with open surgical (OH) versus endoscopic (EH) techniques. Methods: Saphenous vein segments were harvested for lower extremity bypass. 3-4mm vein rings were mounted on force transducers. Segments were mounted in 37° oxygenated Krebs solution and maximally contracted using KCl. NE was used to achieve submaximal contraction. EDR was determined using increasing concentrations of bradykinin (BDK). Endothelial independent relaxation was confirmed using sodium nitroprusside. Two-way ANOVA was used to analyze differences between harvest techniques across BDK concentration. Student t-test was used to examine nitrite levels in each cohort. Results: Vein segments harvested from patients (n=13) led to 28 rings (11 rings; 5 patients EH v. 17;8 OH). Both cohorts achieved moderate relaxation to maximal BDK concentration, [10 -6 M]; (49.5% EH vs. 40.55%, OH, P = .270). Analysis by two way ANOVA for mean % relaxation for BDK concentration [10 -11 - 10 -6 M] showed improved EDR in EH samples compared to OH (P =.029). Mean nitrite tissue bath concentration measurements post-BDK were 279 nM (EH) v. 194 nM (OH) (P = .264). Histology and IHC confirmed intact endothelium by morphometric analysis and CD31 staining. Conclusion: Endothelial function is preserved when utilizing endoscopic harvesting techniques. The advantages of minimally invasive vein procurement for lower extremity bypass can be obtained without concern for damaging venous endothelium.