Anita Christie

Skeletal Muscle Fatigue

Skeletal muscle fatigue is defined as the fall of force or power in response to contractile activity. Both the mechanisms of fatigue and the modes used to elicit it vary tremendously. Conceptual and technological advances allow the examination of fatigue from the level of the single molecule to the intact organism. Evaluation of muscle fatigue in a wide range of disease states builds on our understanding of basic function by revealing the sources of dysfunction in response to disease.

Systematic Review and Meta-Analysis of Skeletal Muscle Fatigue in Old Age

UNLABELLED Despite intense interest in understanding how old age may alter skeletal muscle fatigability, a quantitative examination of the impact of study design on age-related differences in muscle fatigue does not exist. PURPOSE The purpose of this study was to conduct a systematic review of the differences in muscle fatigue between young and older adults, with specific examination of moderator variables suggested to contribute to discrepancies across studies: contraction intensity, contraction mode, duty cycle, fatigue index, sex, muscle group, and contraction type. METHODS The standardized effect of age on muscle fatigue was computed for 37 studies (60 standardized effects). Standardized effects were coded as positive when less fatigue was reported in older individuals compared with young individuals. RESULTS The overall standardized effect of age on muscle fatigue was positive (0.56). In studies using dynamic contractions or using muscle power as the index of fatigue, the standardized effect was negative (-0.12 and -2.5, respectively). The standardized effect for all other moderator categories was positive (range = 0.09-0.90), indicating less fatigue in older adults under all other methodological conditions. CONCLUSION This review provides the first quantitative analysis of the effect of study design on age-related differences in muscle fatigue. The results indicate that older individuals develop less muscle fatigue than young individuals, particularly during isometric contractions of the elbow flexor and knee extensor muscles. However, the results also suggest that older adults develop greater fatigue during dynamic contractions, particularly when the decline in power is assessed. Studies that verify this latter outcome are needed, as are studies designed to elucidate the mechanisms of fatigue.

Short‐term training adaptations in maximal motor unit firing rates and afterhyperpolarization duration

In this study we investigated age‐ and training‐related adaptations in maximal motor unit firing rates and the duration of the motoneuron afterhyperpolarization (AHP) in the dorsiflexor muscles of the foot. Subjects included 30 young (21.9 ± 3.1 years) and 30 older (72.9 ± 4.6 years) individuals, who were randomly assigned to a control or training group. Maximal voluntary force (MVC), maximal motor unit firing rate, and motoneuron AHP duration were measured on two occasions. The training group participated in six dorsiflexor exercise training sessions between test and retest. At baseline, young subjects had higher MVC force and maximal motor unit firing rate, and shorter AHP duration, compared with older subjects. Young and older subjects in the trained group demonstrated a respective 17.4% and 19.8% increase in MVC force, a 6.8% and 24.3% increase in maximal MUFR, and a 7.4% and 14.2% decrease in AHP duration. These results indicate that age‐related changes in neuromuscular function are not a necessary consequence of aging. Muscle Nerve, 2010

Reliability of motor-evoked potentials in the ADM muscle of older adults

The purpose of this study was to determine the reliability of motor-evoked potentials (MEPs) produced in the abductor digiti minimi (ADM) muscle of male and female older adults, both within and between sessions. The number of trials within a session required to obtain reliable results was also examined. The investigation was conducted on 30 elderly individuals (15 M, 15 F; mean age 76 years). With the ADM at rest, MEPs were evoked at intensities of 1.1, 1.3, and 1.5 times motor threshold (MT). Ten stimuli were delivered at each intensity, with 10-12s between stimuli. The MEP responses were blocked with two, three, four, and five-stimuli means in each block. An intraclass correlation (ICC) reliability analysis of variance model was used to assess reliability of the MEP amplitude, using a variable number of trials per block. A subset of 10 subjects repeated the protocol following 20min of rest to assess the reliability between sessions. As expected, MEP amplitudes were significantly higher as stimulus intensity increased. There were no significant differences between blocks, nor was there a significant gender effect. ICC reliability coefficients ranged between 0.09 with two trials per block and 0.97 with five trials per block. Between session reliability ranged from 0.65 to 0.83. Highly reliable MEP amplitudes can be obtained in older individuals using two blocks of TMS stimuli with five trials per block.

Motor unit firing behavior during prolonged 50% MVC dorsiflexion contractions in young and older adults

The purpose of this study was to investigate changes in motor unit firing behavior during prolonged contractions in young and older adults. Motor unit activity was recorded from the tibialis anterior of 16 subjects (8 young and 8 older), while they performed isometric dorsiflexion at 50% MVC until task failure. Mean motor unit firing rate, the standard deviation (SD), and coefficient of variation (CV) of the interspike intervals, and number of doublet discharges were calculated for a total of 52 motor units, tracked for an average of 92.9+/-68.6s. There was no age-related difference in the time to task failure. A modest decline in firing rate was observed in 71% of the motor units, with no significant age-related difference. The SD and CV of the interspike interval had a positive slope in 65% and 69% of the motor units, respectively, with no significant age-related differences. The number of doublet discharges remained stable throughout the contraction. Both groups exhibited motor unit dropout (discharge cessation) during the contraction. Thus, a fatiguing task producing modest changes in firing rate in young and older adults is accompanied by an appreciable increase in firing rate variability. The incidence of doublet discharges is not increased during fatiguing contractions.

Acute and longitudinal changes in motor cortex function following mild traumatic brain injury.

Abstract Primary objective: To evaluate excitability and inhibition of the motor cortex acutely and longitudinally following mild traumatic brain injury (mTBI). Research design: A longitudinal paired case-control design was used to examine cortical excitability and inhibition in 15 adults who had sustained an mTBI (mean age = 20.8 ± 1.2 years) and 15 matched control participants (mean age = 21.1 ± 1.3 years). Methods and procedures: Participants visited the lab within 72 hours of injury and again at 1, 2, 4 and 8 weeks post-injury. During each visit, transcranial magnetic stimulation was used to examine resting motor threshold (RMT), motor evoked potential peak-to-peak amplitude (MEPamp) and cortical silent period (CSP) duration of the first dorsal interosseous muscle. Main outcomes and results: There were no differences between groups in RMT (p = 0.10) or MEPamp (p = 0.22) at 72 hours post-injury or across the 2-month testing period (p ≥ 0.68), indicating similar cortical excitability. However, the CSP duration was higher in individuals with mTBI, indicating greater intra-cortical inhibition compared with the control group at 72 hours post-injury (p = 0.03) and throughout the 2 months of recovery (p = 0.009). Conclusions: mTBI appeared to have little effect on cortical excitability, but an acute and long-lasting effect on intra-cortical inhibition.

Human skeletal muscle metabolic economy in vivo: effects of contraction intensity, age, and mobility impairment

We tested the hypothesis that older muscle has greater metabolic economy (ME) in vivo than young, in a manner dependent, in part, on contraction intensity. Twenty young (Y; 24±1 yr, 10 women), 18 older healthy (O; 73±2, 9 women) and 9 older individuals with mild-to-moderate mobility impairment (OI; 74±1, 7 women) received stimulated twitches (2 Hz, 3 min) and performed nonfatiguing voluntary (20, 50, and 100% maximal; 12 s each) isometric dorsiflexion contractions. Torque-time integrals (TTI; Nm·s) were calculated and expressed relative to maximal fat-free muscle cross-sectional area (cm2), and torque variability during voluntary contractions was calculated as the coefficient of variation. Total ATP cost of contraction (mM) was determined from flux through the creatine kinase reaction, nonoxidative glycolysis and oxidative phosphorylation, and used to calculate ME (Nm·s·cm(-2)·mM ATP(-1)). While twitch torque relaxation was slower in O and OI compared with Y (P≤0.001), twitch TTI, ATP cost, and economy were similar across groups (P≥0.15), indicating comparable intrinsic muscle economy during electrically induced isometric contractions in vivo. During voluntary contractions, normalized TTI and total ATP cost did not differ significantly across groups (P≥0.20). However, ME was lower in OI than Y or O at 20% and 50% MVC (P≤0.02), and torque variability was greater in OI than Y or O at 20% MVC (P≤0.05). These results refute the hypothesis of greater muscle ME in old age, and provide support for lower ME in impaired older adults as a potential mechanism or consequence of age-related reductions in functional mobility.

Experimental and modelling investigation of surface EMG spike analysis

A pattern classification method based on five measures extracted from the surface electromyographic (sEMG) signal is used to provide a unique characterization of the interference pattern for different motor unit behaviours. This study investigated the sensitivity of the five sEMG measures during the force gradation process. Tissue and electrode filtering effects were further evaluated using a sEMG model. Subjects (N=8) performed isometric elbow flexion contractions from 0 to 100% MVC. The sEMG signals from the biceps brachii were recorded simultaneously with force. The basic building block of the sEMG model was the detection of single fibre action potentials (SFAPs) through a homogeneous, equivalent isotropic, infinite volume conduction medium. The SFAPs were summed to generate single motor unit action potentials. The physiologic properties from a well-known muscle model and motor unit recruitment and firing rate schemes were combined to generate synthetic sEMG signals. The following pattern classification measures were calculated: mean spike amplitude, mean spike frequency, mean spike slope, mean spike duration, and the mean number of peaks per spike. Root-mean-square amplitude and mean power frequency were also calculated. Taken together, the experimental data and modelling analysis showed that below 50% MVC, the pattern classification measures were more sensitive to changes in force than traditional time and frequency measures. However, there are additional limitations associated with electrode distance from the source that must be explored further. Future experimental work should ensure that the inter-electrode distance is no greater than 1cm to mitigate the effects of tissue filtering.

Return to Physical Activity Timing and Dual-Task Gait Stability Are Associated 2 Months Following Concussion.

Objective:The effect of physical activity resumption on functional recovery following concussion is poorly understood. We sought to examine the relationship between physical activity resumption timing and gait stability 2 months following concussion. Setting:Motion analysis laboratory. Participants:Twenty-nine individuals who sustained a concussion and were subsequently allowed to return to preinjury levels of physical activity within 2 months postinjury. Design:Prospective cohort study. Main Measures:The return-to-activity day (RTA-day) was identified as the first day postinjury that each participant was medically cleared to resume regular physical activity. Whole-body center-of-mass (COM) motion during single-task and dual-task walking was assessed 2 months postinjury. Correlation coefficients were calculated between the RTA-day and single/dual-task gait stability as well as symptom severity measurements 2 months postinjury. Results:Dual-task medial-lateral COM displacement (r = −0.52, P = .004) and peak medial-lateral COM velocity (r = −0.37, P = .048) were significantly correlated with RTA-day. Dual-task peak forward velocity and single-task gait stability measures were not significantly correlated with RTA-day. Conclusions:The time of RTA-day clearance, within a 2 months postinjury period, is significantly correlated with dual-task medial-lateral gait stability measured at the end of that period, suggesting that frontal plane gait stability recovery is sensitive to the timing of RTA-day.

A Comparison of Statistical Models for Calculating Reliability of the Hoffmann Reflex.

The Hoffmann reflex is obtained through surface electromyographic recordings, and it is one of the most common neurophysiological techniques in exercise science. Measurement and evaluation of the peak-to-peak amplitude of the Hoffmann reflex has been guided by the observation that it is a variable response that requires multiple trials to obtain a stable mean. As a result, reliability assessment of the Hoffmann reflex has thus far been limited to trial-to-trial variability or the mean of multiple trials across test days to evaluate day-to-day variability, without considering how the two dimensions of measurement interact to affect test–retest reliability. This article compares two analysis of variance models for reliability assessment of the peak-to-peak amplitude of the Hoffmann reflex: one model incorporated both days and trials, and the other used the mean of multiple trials on each test day. The intraclass correlation coefficient calculated from an analysis of variance model that incorporated both trial-to-trial and day-to-day error variances was R = .93. Using the mean of the trials for each test day as a single score resulted in a decrease in the intraclass correlation coefficient (R 2,1 = .76). This study demonstrated that an analysis of variance model that accounts for both trial-to-trial and day-to-day error variances provided a higher estimate of reliability. Further, reliability estimation based on the interaction between both trials and days revealed that a schedule with additional test days had a disproportionate impact upon securing a reliable measure of Hoffmann reflex amplitude.