Fatigability and Recovery of Arm Muscles with Advanced Age for Dynamic and Isometric Contractions

This study determined whether age-related mechanisms can increase fatigue of arm muscles during maximal velocity dynamic contractions, as it occurs in the lower limb. We compared elbow flexor fatigue of young (n = 10, 20.8 ± 2.7 years) and old men (n = 16, 73.8 ± 6.1 years) during and in recovery from a dynamic and an isometric postural fatiguing task. Each task was maintained until failure while supporting a load equivalent to 20% of maximal voluntary isometric contraction (MVIC) torque. Transcranial magnetic stimulation (TMS) was used to assess supraspinal fatigue (superimposed twitch, SIT) and muscle relaxation. Time to failure was longer for the old men than for the young men for the isometric task (9.5 ± 3.1 vs. 17.2 ± 7.0 min, P = 0.01) but similar for the dynamic task (6.3 ± 2.4 min vs. 6.0 ± 2.0 min, P = 0.73). Initial peak rate of relaxation was slower for the old men than for the young men, and was associated with a longer time to failure for both tasks (P \u3c 0.05). Low initial power during elbow flexion was associated with the greatest difference (reduction) in time to failure between the isometric task and the dynamic task (r = − 0.54, P = 0.015). SIT declined after both fatigue tasks similarly with age, although the recovery of SIT was associated with MVIC recovery for the old (both sessions) but not for the young men. Biceps brachii and brachioradialis EMG activity (% MVIC) of the old men were greater than that of the young men during the dynamic fatiguing task (P \u3c 0.05), but were similar during the isometric task. Muscular mechanisms and greater relative muscle activity (EMG activity) explain the greater fatigue during the dynamic task for the old men compared with the young men in the elbow flexor muscles. Recovery of MVC torque however relies more on the recovery of supraspinal fatigue among the old men than among the young men

Aging and Muscle Fatigability in the Upper Extremity

Aging is accompanied by reductions in strength and contraction velocity, and increased fatigability of limb muscles during high-velocity dynamic contractions. These age-related changes affect functional tasks and are well described for the lower limb, with less known about the upper limb muscles. The aims of the thesis were to compare in young and old men and women: (1) maximal torque and power of the elbow flexor muscles across a range of isokinetic velocities, and (2) the neural (supraspinal) and muscular mechanisms of fatigue induced by high-velocity dynamic contractions of the elbow flexor muscles. 28 young (23.2 ± 2.6 years) men (n = 14) and women (n = 14) and 33 (72.6 ± 5.6 years) old men (n = 18) and women (n = 15) with the elbow flexor muscles performed: (1) maximal isokinetic contractions at 15 velocities to assess strength and power (0-450°/s), and (2) a dynamic fatiguing task involving 80 fast, maximal-effort contractions with a load equivalent to 20% of maximal voluntary isometric torque (MVIC). Before and after the fatiguing task the following were assessed: voluntary activation using motor cortical stimulation as a measure of supraspinal fatigue, and contractile properties evoked with electrical stimulation as a measure of muscular mechanisms. The elbow flexor muscles of the old adults were weaker and less powerful than young adults across all the velocities assessed (P\u3c0.01), although voluntary activation was similar between the age groups (P\u3e0.05). Some young and old adults were not able attain higher contraction velocities, primarily driven by the women. Old adults were more fatigable than young adults (P\u3c0.001, 15% difference) with now sex differences (P\u3e0.05). Old adults exhibited a larger reduction in voluntary activation (P=0.036, 7.5% age difference) and greater increase in relaxation in the old adults (55%) than the young (36%) following the fatiguing task. The elbow flexor muscles of old men and women were weaker and less powerful than young, but this was not due to differences in voluntary activation. The greater fatigability of elbow flexor muscles in the old adults however, was due to both supraspinal mechanisms and slowing of the muscle that occurs with aging

Effects of Elevated H\u3csup\u3e+\u3c/sup\u3e And P\u3csub\u3ei\u3c/sub\u3e on The Contractile Mechanics of Skeletal Muscle Fibres From Young and Old Men: Implications for Muscle Fatigue in Humans

The present study aimed to identify the mechanisms responsible for the loss in muscle power and increased fatigability with ageing by integrating measures of whole‐muscle function with single fibre contractile mechanics. After adjusting for the 22% smaller muscle mass in old (73–89 years, n = 6) compared to young men (20–29 years, n = 6), isometric torque and power output of the knee extensors were, respectively, 38% and 53% lower with age. Fatigability was ∼2.7‐fold greater with age and strongly associated with reductions in the electrically‐evoked contractile properties. To test whether cross‐bridge mechanisms could explain age‐related decrements in knee extensor function, we exposed myofibres (n = 254) from the vastus lateralis to conditions mimicking quiescent muscle and fatiguing levels of acidosis (H+) (pH 6.2) and inorganic phosphate (Pi) (30 mm). The fatigue‐mimicking condition caused marked reductions in force, shortening velocity and power and inhibited the low‐ to high‐force state of the cross‐bridge cycle, confirming findings from non‐human studies that these ions act synergistically to impair cross‐bridge function. Other than severe age‐related atrophy of fast fibres (−55%), contractile function and the depressive effects of the fatigue‐mimicking condition did not differ in fibres from young and old men. The selective loss of fast myosin heavy chain II muscle was strongly associated with the age‐related decrease in isometric torque (r = 0.785) and power (r = 0.861). These data suggest that the age‐related loss in muscle strength and power are primarily determined by the atrophy of fast fibres, but the age‐related increased fatigability cannot be explained by an increased sensitivity of the cross‐bridge to H+ and Pi

Knee joint neuromuscular activation performance during muscle damage and superimposed fatigue

This study examined the concurrent effects of exercise-induced muscle damage and superimposed acute fatigue on the neuromuscular activation performance of the knee flexors of nine males (age: 26.7 ± 6.1yrs; height 1.81 ± 0.05m; body mass 81.2 ± 11.7kg [mean ± SD]). Measures were obtained during three experimental conditions: (i) FAT-EEVID, involving acute fatiguing exercise performed on each assessment occasion plus a single episode of eccentric exercise performed on the first occasion and after the fatigue trial; (ii) FAT, involving the fatiguing exercise only and; (iii) CON consisting of no exercise. Assessments were performed prior to (pre) and at lh, 24h, 48h, 72h, and 168h relative to the eccentric exercise. Repeated-measures ANOVAs showed that muscle damage within the FAT-EEVID condition elicited reductions of up to 38%, 24%) and 65%> in volitional peak force, electromechanical delay and rate of force development compared to baseline and controls, respectively (F[io, 80] = 2.3 to 4.6; p to 30.7%>) following acute fatigue (Fp; i6] = 4.3 to 9.1; p ; Fp, iq = 3.9; p <0.05). The safeguarding of evoked muscle activation capability despite compromised volitional performance might reveal aspects of capabilities for emergency and protective responses during episodes of fatigue and antecedent muscle damaging exercise

Central and peripheral determinants of fatigue in acute hypoxia

This thesis was submitted for the degree of Docter of Philosophy and awarded by Brunel University on 24th March 2011.Fatigue is defined as an exercise-induced decrease in maximal voluntary force produced by a muscle. Fatigue may arise from central and/or peripheral mechanisms. Supraspinal fatigue (a component of central fatigue) is defined as a suboptimal output from the motor cortex and measured using transcranial magnetic stimulation (TMS). Reductions in O2 supply (hypoxia) exacerbate fatigue and as the severity of hypoxia increases, central mechanisms of fatigue are thought to contribute more to exercise intolerance. In study 1, the feasibility of TMS to measure cortical voluntary activation and supraspinal fatigue of human knee-extensors was determined. TMS produced reliable measurements of cortical voluntary activation within- and between-days, and enabled the assessment of supraspinal fatigue. In study 2, the mechanisms of fatigue during single-limb exercise in normoxia (arterial O2 saturation [SaO2] ~98%), and mild to severe hypoxia (SaO2 93-80%) were determined. Hypoxia did not alter neuromuscular function or cortical voluntary activation of the knee-extensors at rest, despite large reductions in cerebral oxygenation. Maximal force declined by ~30% after single-limb exercise in all conditions, despite reduced exercise time in severe-hypoxia compared to normoxia (15.9 ± 5.4 vs. 24.7 ± 5.5 min; p < 0.05). Peripheral mechanisms of fatigue contributed more to the reduction in force generating capacity of the knee-extensors following single-limb exercise in normoxia and mild- to moderate-hypoxia, whereas supraspinal fatigue played a greater role in severe-hypoxia. In study 3, the effect of constant-load cycling exercise to the limit of tolerance in hypoxia (SaO2 ~80%) and normoxia was investigated. Time to the limit of tolerance was significantly shorter in hypoxia compared to normoxia (3.6 ± 1.3 vs. 8.1 ± 2.9 min; p < 0.001). The reductions in maximal voluntary force and knee-extensor twitch force at task-failure were not different in hypoxia compared to normoxia. However, the level of supraspinal fatigue was exacerbated in hypoxia, and occurred in parallel with reductions in cerebral oxygenation and O2 delivery. Supraspinal fatigue contributes to the decrease in whole-body exercise tolerance in hypoxia, presumably as a consequence of inadequate O2 delivery to the brain

The relationship between perception of effort and physiological responses to an acute fatiguing task of the elbow flexors. Evaluation of a new rating scale of perception of effort

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.While fatigue is a common daily phenomenon, the exact relationship between perception of effort and fatigue is still unknown. Existing tools for assessing perception of effort are effectively limited to whole body exercise, while current methods for assessing voluntary activation are painful and not feasible for clinical application. The main aims of this thesis were to evaluate existing methodologies for their appropriateness in assessing perception of effort and voluntary activation following isolated muscle function testing, and to examine the relationship between subjective perception of effort and objective changes in the healthy motor control system. The implementation of reliable and valid assessment tools in clinical practice may enable clarification of the pathogenesis of many neurological conditions that have chronic fatigue as a key feature. Four studies of within-subjects repeated measures design have been conducted. Sixtynine healthy volunteers were recruited among staff and students of Brunel University. Magnetic stimulation was tested as a valid alternative to electrical stimulation in the conventional single-pulse Twitch Interpolation Technique. The 0–10 Numeric Rating Scale (NRS) was also tested for its reliability and validity in assessing the perception of effort during isometric exercise of elbow flexors. The changes of perception of effort following a submaximal elbow flexion fatiguing task, as well as following transcranial direct current stimulation (tDCS) over the motor cortex were also tested. The main findings showed significant differences between peripheral and magnetic stimulation in conventional single-pulse Twitch Interpolation Technique. The 0–10 NRS demonstrated linear properties and reported excellent test-retest reliability and good concurrent criterion validity in recording perception of effort under repeated isometric contractions of elbow flexors. Ten minutes of a submaximal intermittent isometric fatiguing exercise produced a significant elevation in rating of perceived effort, which was associated with central and peripheral neurophysiological changes of the motor control system. In contrast, perception of effort did not change significantly following 10 minutes of tDCS. The major findings of this thesis suggest the 0–10 NRS is a valid and reliable scale for rating perception of effort in healthy individuals. Further testing of the scale on patients is needed to establish its validity in clinical settings. Additionally, the findings indicate a substantial role of perception of effort in the voluntary motor control system. However, further research towards revealing the underlying mechanisms of perceived effort regulation in both health and disease is required

The Role of Dopamine on Central Neuromuscular Activation during Passive Hyperthermia

Acute methylphenidate (MPH) (dopamine reuptake inhibitor) ingestion improves cycling time trial performance and power output in hot conditions (30 C), while also allowing for tolerance of higher core temperatures. However, the mechanisms for why this occurs have not been isolated. One potential explanation for this ergogenic benefit is that MPH intake was enhancing neuromuscular activation. Thus, this research project examined the influence of MPH on neuromuscular activation during hyperthermia. Participants ingested either placebo (PLA; 20mg) or MPH (Ritalin; 20mg) 1 hour prior to a passive heating protocol. 6 participants were passively heated until volitional cessation, or after 3 hours of heating had passed. Neuromuscular responses, as indicated by maximal voluntary contraction (MVC) force, and voluntary activation (VA) percentage were assessed prior to drug ingestion, 1 hour after MPH wash-in, throughout the heating protocol and at cessation of heating. A primary non-significant finding of this research project was that participants reached higher rectal temperatures (Tre) by ~0.3 C in trials where they ingested MPH (p = 0.065). This effect occurred in absence of any differences in thermal comfort or sensation ratings or heating durations. However, while MPH improves thermal tolerance, it was not able to attenuate the decreases in MVC force and VA that occurred during passive heating. Therefore, the aforementioned ergogenic benefits that MPH has in hot conditions are not occurring as a result of enhanced neuromuscular activation

Stressor-Induced Increase in Muscle Fatigability of Young Men and Women is Predicted by Strength but Not Voluntary Activation

This study investigated mechanisms for the stressor-induced changes in muscle fatigability in men and women. Participants performed an isometric-fatiguing contraction at 20% maximal voluntary contraction (MVC) until failure with the elbow flexor muscles. Study one (n = 55; 29 women) involved two experimental sessions: 1) a high-stressor session that required a difficult mental-math task before and during a fatiguing contraction and 2) a control session with no mental math. For some participants (n = 28; 14 women), cortical stimulation was used to examine mechanisms that contributed to muscle fatigability during the high-stressor and control sessions. Study two (n = 23; nine women) determined the influence of a low stressor, i.e., a simple mental-math task, on muscle fatigability. In study one, the time-to-task failure was less for the high-stressor session than control (P \u3c 0.05) for women (19.4%) and men (9.5%): the sex difference response disappeared when covaried for initial strength (MVC). MVC force, voluntary activation, and peak-twitch amplitude decreased similarly for the control and high-stressor sessions (P \u3c 0.05). In study two, the time-to-task failure of men or women was not influenced by the low stressor (P \u3e 0.05). The greater fatigability, when exposed to a high stressor during a low-force task, was not exclusive to women but involved a strength-related mechanism in both weaker men and women that accelerated declines in voluntary activation and slowing of contractile properties