Force steadiness as a predictor of time to complete a pegboard test of dexterity in young men and women

Explaining the influence of practice on the grooved pegboard times of older adults: role of force steadiness

The purpose was to identify the variables that can explain the variance in the grooved pegboard times of older adults categorized as either fast or slow performers. Participants (n = 28; 60-83 years) completed two experimental sessions, before and after 6 practice sessions of the grooved pegboard test. The 2 groups were identified based on average pegboard times during the practice sessions. Average pegboard time during practice was 73 ± 11 s for the fast group and 85 ± 13 s for the slow group. Explanatory variables for the pegboard times before and after practice were the durations of 4 peg-manipulation phases and 12 measures of force steadiness (coefficient of variation [CV] for force) during isometric contractions with the index finger abductor and wrist extensor muscles. Time to complete the grooved pegboard test after practice decreased by 25 ± 11% for the fast group and by 28 ± 10% for the slow group. Multiple regression models explained more of the variance in the pegboard times for the fast group before practice (Adjusted R2 = 0.85) than after practice (R2 = 0.51), whereas the variance explained for the slow group was similar before (Adjusted R2 = 0.67) and after (Adjusted R2 = 0.64) practice. The explanatory variables differed between before and after practice for the fast group but only slightly for the slow group. These findings indicate that performance-based stratification of older adults can identify unique adjustments in motor function that are independent of chronological age.

Influence of visual feedback and cognitive challenge on the age-related changes in force steadiness

Force steadiness can be influenced by visual feedback as well as presence of a cognitive tasks and potentially differs with age and sex. This study determined the impact of altered visual feedback on force steadiness in the presence of a difficult cognitive challenge in young and older men and women. Forty-nine young (19-30 yr; 25 women, 24 men) and 25 older (60-85 yr; 15 women; 10 men) performed low force (5% of maximum) static contractions with the elbow flexor muscles in the presence and absence of a cognitive challenge (counting backwards by 13) either with low or high visual feedback gain. The cognitive challenge reduced force steadiness (increased force fluctuation amplitude) particularly in women (cognitive challenge × sex: P < 0.05) and older individuals (cognitive challenge × age: P < 0.05). Force steadiness improved with high-gain visual feedback compared with low-gain visual feedback (P < 0.01) for all groups (all interactions: P > 0.05). Manipulation of visual feedback had no influence on the reduced force steadiness in presence of the cognitive challenge for all groups (all P > 0.05). These findings indicate that older individuals and women have greater risk of impaired motor performance of the upper extremity if steadiness is required during a low-force static contraction. Manipulation of visual feedback had minimal effects on the reduced force steadiness in presence of a difficult cognitive challenge.

The application of heating film to hands reduces the decline in manual dexterity performance associated with cold exposure

PURPOSE

Exposure to cold temperatures decreases finger temperature (Tfing) and dexterity. Decreased manual function and dexterity can be serious safety risks, especially in tasks that require fine motor movements that must be performed outdoors. The aim of this study was to determine whether hand heating with a minimal power requirement (14.8 W) results in a smaller reduction in Tfing and manual dexterity performance during mild cold exposure compared to a non-heated control condition.

METHODS

In a randomized crossover design, twenty-two healthy participants were exposed to a moderately cold environment (5  ºC) for 90 min. One condition had no intervention (CON), while the other had the palmar and dorsal hands heated (HEAT) by using electric heating films. Tfing and cutaneous vascular conductance (CVC) were continuously monitored using laser Doppler flowmetry. Manual dexterity performance and cognitive function were assessed by the Grooved Pegboard Test (GPT) and Stroop Color-Word (SCW) test, respectively, during the baseline period and every 30 min during the cold exposure.

RESULTS

After the cold exposure, Tfing was higher in HEAT relative to CON (CON 9.8 vs. HEAT 13.7 ºC, p < 0.0001). GPT placing time, as an index of dexterity performance, was also shorter in HEAT by 14.5% (CON 69.10 ± 13.08 vs. HEAT 59.06 ± 7.99 s, p < 0.0001). There was no difference in CVC between the two conditions during the cold exposure (p > 0.05 for all). Cognitive function was similar between two conditions (p > 0.05 for all).

CONCLUSION

The proposed hand heating method offers a practical means of heating fingers to maintain dexterity throughout prolonged cold exposure.

Short-term effects of running exercise on pinch strength, grip strength, and manual dexterity of the dominant and non-dominant hands

Occupations including first responders and military require manual tasks; therefore changes in hand strength and dexterity could affect performance. We hypothesised that pinch strength, grip strength, and dexterity will change after unloaded and loaded exercise. Twenty-four male (25 ± 4.0 yrs; 86.3 ± 9.3 kg) and 10 female (25 ± 6.0 yrs; 62.1 ± 5.9 kg) participants completed 3 conditions for 5 minutes: (1) no exercise (2) run with no load at 3.0 m/s and (3) run wearing a 9.1 kg belt. Heart rate was different among conditions (p ≤ 0.05). Pinch strength was significantly different for the non-dominant hand after exercise (p = 0.005) for male participants, but not for the dominant hand. Grip strength was significantly different for the non-dominant hand between loaded and unloaded run (p = 0.035) for male participants. Pinch and grip strength did not change after exercise for female participants. Dexterity times were not different after exercise, but female participants were significantly faster (p ≤ 0.039) than male participants.

Short term effects of contralateral tendon vibration on motor unit discharge rate variability and force steadiness in people with Parkinson's disease

Background

Vibration of one limb affects motor performance of the contralateral limb, and this may have clinical implications for people with lateralized motor impairments through vibration-induced increase in cortical activation, descending neural drive, or spinal excitability.

Objective

The objective of this study was to evaluate the effects of acute biceps brachii tendon vibration on force steadiness and motor unit activity in the contralateral limb of persons with Parkinson's disease.

Methods

Ten participants with mild to moderate Parkinson's disease severity performed a ramp, hold and de-ramp isometric elbow flexion at 5% of maximum voluntary contraction with the more-affected arm while vibration was applied to the distal biceps brachii tendon on the contralateral, less-affected arm. Using intramuscular fine wire electrodes, 33 MUs in the biceps brachii were recorded across three conditions (baseline, vibration, and post-vibration). Motor unit recruitment & derecruitment thresholds, discharge rates & variability, and elbow flexion force steadiness were compared between conditions with and without vibration.

Results

Coefficient of variation of force and discharge rate variability decreased 37 and 17%, respectively in post-vibration compared with baseline and vibration conditions. Although the motor unit discharge rates did not differ between conditions the total number of motor units active at rest after de-ramp were fewer in the post-vibration condition.

Conclusion

Contralateral tendon vibration reduces MU discharge rate variability and enhances force control on the more affected side in persons with Parkinson's disease.

Influence of Proprioceptive Inputs and Force Feedback Modality on Force Reproduction Performance

The sense of force can be assessed using a force reproduction task (FRT), which consists of matching a target force with visual feedback (TARGET phase) and reproducing it without visual feedback (REPRODUCTION phase). We investigated the relevance of muscle proprioception during the TARGET phase (EXP1) and the influence of the sensory source used for the force feedback (EXP2). Accordingly, EXP1 compared the force reproduction error (RE) between trials with (LV) and without (NoLV) local tendon vibration applied on the first dorsal interosseous during the TARGET phase, while EXP2 compared RE between trials performed with visual (VISIO) or auditory (AUDIO) feedback. The FRT was performed with the index finger at 5% and 20% of the maximal force (MVC). RE was greater with LV compared with NoLV at 5% (p = 0.004) but not 20% MVC (p = 0.65). The involvement of muscle proprioception in RFT was further supported by the increase in RE with LV frequency (supplementary experiment). RE was greater for VISIO than AUDIO at 5% (p < 0.001) but not 20% MVC (p = 0.054). This study evidences the relevance of proprioceptive inputs during the target PHASE and the influence of the force feedback modality on RE, and thereby on the assessment of the sense of force.

Practice-Induced Changes in Manual Dexterity of Older Adults Depend on Initial Pegboard Time

INTRODUCTION

The purpose of our study was to determine the influence of practice on the pegboard times and peg-manipulation phases of older adults who were classified as having either slow or fast initial pegboard times.

METHODS

Participants ( n = 26, 70 ± 6.6 yr) completed two evaluation sessions and six practice sessions in which they performed 25 trials (5 blocks of 5 trials) of the grooved pegboard test. All practice sessions were supervised, and the time to complete each trial was recorded. In each evaluation session, the pegboard was mounted on a force transducer so that the downward force applied to the board could be measured.

RESULTS

Participants were stratified into two groups based on the initial time to complete the grooved pegboard test: a fast group (68.1 ± 6.0 s) and a slow group (89.6 ± 9.2 s). Both groups exhibited the classic two-phase profile (acquisition + consolidation) for learning a de novo motor skill. Despite the similar learning profile for the two groups, there were differences between groups in the phases of the peg-manipulation cycle that became faster with practice. The fast group seemed to reduce trajectory variability when transporting the peg, whereas the slow group seemed to exhibit both a decrease in trajectory variability and greater precision when inserting pegs into the holes.

CONCLUSIONS

The changes underlying practice-induced decreases in grooved pegboard time differed for older adults who initially had either a fast or a slow pegboard time.

Measuring the nonselective effects of motor inhibition using isometric force recordings

Inhibition is a key cognitive control mechanism humans use to enable goal-directed behavior. When rapidly exerted, inhibitory control has broad, nonselective motor effects, typically demonstrated using corticospinal excitability measurements (CSE) elicited by transcranial magnetic stimulation (TMS). For example, during rapid action-stopping, CSE is suppressed at both stopped and task-unrelated muscles. While such TMS-based CSE measurements have provided crucial insights into the fronto-basal ganglia circuitry underlying inhibitory control, they have several downsides. TMS is contraindicated in many populations (e.g., epilepsy or deep-brain stimulation patients), has limited temporal resolution, produces distracting auditory and haptic stimulation, is difficult to combine with other imaging methods, and necessitates expensive, immobile equipment. Here, we attempted to measure the nonselective motor effects of inhibitory control using a method unaffected by these shortcomings. Thirty male and female human participants exerted isometric force on a high-precision handheld force transducer while performing a foot-response stop-signal task. Indeed, when foot movements were successfully stopped, force output at the task-irrelevant hand was suppressed as well. Moreover, this nonselective reduction of isometric force was highly correlated with stop-signal performance and showed frequency dynamics similar to established inhibitory signatures typically found in neural and muscle recordings. Together, these findings demonstrate that isometric force recordings can reliably capture the nonselective effects of motor inhibition, opening the door to many applications that are hard or impossible to realize with TMS.

Dual-hemisphere anodal transcranial direct current stimulation improves bilateral motor synergies

Transcranial direct current stimulation (tDCS) is one of the non-invasive brain stimulation techniques that can improve motor functions. As bimanual motor actions require high motor cortical activations between hemispheres, applying bilateral anodal stimulation on left and right sides of primary motor cortex (M1) can improve for improvements in bimanual motor tasks. This study investigated which bilateral tDCS protocol effectively improves bimanual hand-grip force control capabilities in healthy young adults. We used three different bilateral tDCS protocols: (a) dual-anodal stimulation on the M1 of bilateral hemispheres (Bi-AA), (b) anodal-cathodal stimulation on the M1 of dominant and nondominant hemispheres (Bi-AC), and (c) sham stimulation (Sham). The results indicated that applying the Bi-AA significantly improved bilateral motor synergies estimated by uncontrolled manifold analysis relative to Sham. However, these differences were not observed in the comparison between Bi-AA and Bi-AC as well as between Bi-AC and Sham. These findings suggest that facilitating motor cortical activations between both hemispheres may be an additional option for advancing interlimb motor coordination patterns.

Sex differences in torque steadiness, accuracy and activation of the shoulder girdle muscles during isometric shoulder scaption

Females present more neck/shoulder musculoskeletal disorders and have different activation strategies of the shoulder girdle muscles than males. However, the sensorimotor performance and potential sex differences are still largely unexplored. The aim of this study was to investigate sex differences in torque steadiness and accuracy during isometric shoulder scaption. We also examined the amplitude and variability of the activation of the trapezius, serratus anterior (SA), and anterior deltoid muscles during torque output evaluation. Thirty-four asymptomatic adults (17 females) participated. Torque steadiness and accuracy were evaluated during submaximal contractions at 20 % and 35 % of peak torque (PT). There was no sex difference in torque coefficient of variation, but females had significantly lower torque standard deviation (SD) values than males at the two intensities evaluated (p < 0.001) and lower torque median frequency values compared to males, regardless of intensity (p < 0.01). Females had significantly lower absolute error values than males for torque output at 35 %PT (p < 0.01) and lower constant error values compared to males, regardless of intensity (p = 0.01). Females had significantly higher muscle amplitude values than males, except for SA (p = 0.10) and in general, females showed higher muscle activation SD values compared to males (p < 0.05). Females may require more complex muscle activation patterns to achieve a more stable and accurate torque output. Therefore, these sex differences may reflect control mechanisms that may also be at play when explaining the greater risk of neck/shoulder musculoskeletal disorders in females than males.

Cognitive challenge as a probe to expose sex- and age-related differences during static contractions

Despite activities of daily living being frequently performed simultaneously with a cognitive task, motor function is often investigated in isolation, which can hinder the applicability of findings. This brief review presents evidence that 1) performing a cognitive challenge simultaneously with a motor task can negatively impact force steadiness and fatigability of limb muscles during a static contraction, 2) the negative impact on old adults (>65 years old), particularly older women is greater than young when a cognitive challenge is simultaneously performed with a static motor task, 3) age-related mechanisms potentially explain impairments in motor performance in the presence of a cognitive challenge, and 4) the mechanisms for the age-related decrements in motor performance can be distinct between men and women. These observations are highly relevant to the older adults, given the increased risk of accidents and injury when a motor task is performed with a high cognitive-demand task, especially in light of the expanding reliance on an aging workforce.

The Forces Generated by Agonist Muscles during Isometric Contractions Arise from Motor Unit Synergies

The purpose of our study was to identify the low-dimensional latent components, defined hereafter as motor unit modes, underlying the discharge rates of the motor units in two knee extensors (vastus medialis and lateralis, eight men) and two hand muscles (first dorsal interossei and thenars, seven men and one woman) during submaximal isometric contractions. Factor analysis identified two independent motor unit modes that captured most of the covariance of the motor unit discharge rates. We found divergent distributions of the motor unit modes for the hand and vastii muscles. On average, 75% of the motor units for the thenar muscles and first dorsal interosseus were strongly correlated with the module for the muscle in which they resided. In contrast, we found a continuous distribution of motor unit modes spanning the two vastii muscle modules. The proportion of the muscle-specific motor unit modes was 60% for vastus medialis and 45% for vastus lateralis. The other motor units were either correlated with both muscle modules (shared inputs) or belonged to the module for the other muscle (15% for vastus lateralis). Moreover, coherence of the discharge rates between motor unit pools was explained by the presence of shared synaptic inputs. In simulations with 480 integrate-and-fire neurons, we demonstrate that factor analysis identifies the motor unit modes with high levels of accuracy. Our results indicate that correlated discharge rates of motor units that comprise motor unit modes arise from at least two independent sources of common input among the motor neurons innervating synergistic muscles.SIGNIFICANCE STATEMENT It has been suggested that the nervous system controls synergistic muscles by projecting common synaptic inputs to the engaged motor neurons. In our study, we reduced the dimensionality of the output produced by pools of synergistic motor neurons innervating the hand and thigh muscles during isometric contractions. We found two neural modules, each representing a different common input, that were each specific for one of the muscles. In the vastii muscles, we found a continuous distribution of motor unit modes spanning the two synergistic muscles. Some of the motor units from the homonymous vastii muscle were controlled by the dominant neural module of the other synergistic muscle. In contrast, we found two distinct neural modules for the hand muscles.

Knee extensor force control as a predictor of dynamic balance in healthy adults

BACKGROUND

Previous research has demonstrated that force control in various muscles of the lower limb (measured according to the magnitude of force fluctuations) explains significant variance in static balance. Given the dynamic nature of many functional activities and sports, assessment of balance and its determinants under dynamic conditions is of importance.

RESEARCH QUESTION

Does muscle force control explain significant variance in dynamic balance, as measured using the Y balance test (YBT)?

METHODS

YBT performance and knee extensor muscle force control were measured in 28 healthy participants. The YBT involved stance on the right leg and attempting maximal reach with the left leg in the anterior, posteromedial, and posterolateral directions. Force control was assessed during isometric knee extension contractions of the right leg at 10%, 20% and 40% maximal voluntary contraction (MVC) and was quantified according to the magnitude (using the coefficient of variation [CV]), and the temporal structure (using sample entropy, SampEn; and detrended fluctuation analysis α), of force fluctuations.

RESULTS

Significant correlations were observed for YBT anterior reach and muscle force CV (r = -0.44, P = 0.02) and SampEn (r = 0.47, P = 0.012) during contractions at 40% MVC. A subsequent regression model demonstrated that muscle force CV and SampEn at 40% MVC significantly explained 54% of variance in YBT anterior reach. Significant correlations were also observed for YBT posteromedial reach and MVC (r = 0.39, P = 0.043) and muscle force CV during contractions at 40% MVC (r = -0.51, P = 0.006). The regression model demonstrated that MVC and muscle force CV at 40% MVC significantly explained 53.9% of variance in YBT posteromedial reach.

SIGNIFICANCE

These results are the first to indicate that a moderate amount of variance in dynamic balance can be explained by measures of isometric force control.

The Neuromuscular Fatigue-Induced Loss of Muscle Force Control

Neuromuscular fatigue is characterised not only by a reduction in the capacity to generate maximal muscle force, but also in the ability to control submaximal muscle forces, i.e., to generate task-relevant and precise levels of force. This decreased ability to control force is quantified according to a greater magnitude and lower complexity (temporal structure) of force fluctuations, which are indicative of decreased force steadiness and adaptability, respectively. The “loss of force control” is affected by the type of muscle contraction used in the fatiguing exercise, potentially differing between typical laboratory tests of fatigue (e.g., isometric contractions) and the contractions typical of everyday and sporting movements (e.g., dynamic concentric and eccentric contractions), and can be attenuated through the use of ergogenic aids. The loss of force control appears to relate to a fatigue-induced increase in common synaptic input to muscle, though the extent to which various mechanisms (afferent feedback, neuromodulatory pathways, cortical/reticulospinal pathways) contribute to this remains to be determined. Importantly, this fatigue-induced loss of force control could have important implications for task performance, as force control is correlated with performance in a range of tasks that are associated with activities of daily living, occupational duties, and sporting performance.

Finger dexterity measured by the Grooved Pegboard test indexes Parkinson's motor severity in a tremor-independent manner

Fine motor impairments are frequent complaints in people with Parkinson's disease (PD). While they may develop at an early stage of the disease, they become more problematic as the disease progresses. Tremors and fine motor symptoms may seem related, but evidence suggests two distinct phenomena. The purpose of our study was to investigate the relationships between fine motor skills and clinical characteristics of PD patients. We hypothesized worse fine motor skills to be associated with greater motor severity that is independent of tremor. We measured fine motor abilities using the Grooved Pegboard test (GPT) in each hand separately and collected clinical and demographics data in a cohort of 82 persons with PD. We performed regression analyses between GPT scores and a range of outcomes: motor severity, time from diagnosis, age and tremors. We also explored similar associations using finger and hand dexterity scores from a standardized PD rating scale. Our results indicate that scores on the GPT for each hand, as measures of manual dexterity, are associated with motor severity and time from diagnosis. The presence of tremors was not a confounding factor, as hypothesized, but age was associated with GPT scores for the dominant hand. Motor severity was also associated with hand and finger dexterity as measured by single items from the clinical Parkinson's rating scale. These findings suggest that the GPT to be useful tool for motor severity assessments of people with PD.

Effect of New Zealand Blackcurrant Extract on Force Steadiness of the Quadriceps Femoris Muscle during Sustained Submaximal Isometric Contraction

Intake of anthocyanin-rich New Zealand blackcurrant (NZBC) can alter physiological responses that enhance exercise performance. In two studies, we examined the effects of NZBC extract on force steadiness during a sustained submaximal isometric contraction of the quadriceps femoris muscle. With repeated measures designs, male participants in study one (n = 13) and study two (n = 19) performed a 120 s submaximal (30%) isometric contraction of the quadriceps femoris muscle following a 7-day intake of NZBC extract and placebo (study one) and following 0 (control), 1-, 4- and 7-day intake of NZBC extract (study two). Participants for both studies were different. In study one, NZBC extract enhanced isometric force steadiness during the 120 s contraction (placebo: 6.58 ± 2.24%, NZBC extract: 6.05 ± 2.24%, p = 0.003), with differences in the third (60-89 s) and fourth quartile (90-120 s) of the contraction. In study two, isometric force steadiness was not changed following 1 and 4 days but was enhanced following 7-day intake of NZBC extract in comparison to control. In study two, the enhanced isometric force steadiness following 7-day intake did occur in the second (30-59 s), third (60-89 s) and fourth (90-120 s) quartiles. Daily supplementation of anthocyanin-rich NZBC extract can enhance force steadiness of the quadriceps femoris muscle during a sustained submaximal isometric contraction. Our observations may have implications for human tasks that require postural stability.

Acute effects of ankle plantar flexor force-matching exercises on postural strategy during single leg standing in healthy adults

BACKGROUND

Ankle plantar flexor force steadiness, assessed by measuring the fluctuation of the force around the submaximal target torque, has been associated with postural stability.

RESEARCH QUESTION

To investigate whether a force-matching exercise, where submaximal steady torque is maintained at the target torque, can modulate postural strategy immediately.

METHODS

Twenty-eight healthy young adults performed ankle plantar flexor force-matching exercises at target torques of 5%, 20%, and 50% of maximum voluntary contraction (MVC), in a randomized crossover trial. Participants with their ankle in a neutral position were instructed to maintain isometric contraction at each target torque, as measured by a dynamometer, for 20 s with 3 sets of 5 contractions. Before and after the force-matching exercises, the anterior-posterior velocities and standard deviation of the center of pressure (COP) on the stable platform and the tilt angle of the unstable platform during 20-seconds single-leg standing were measured. The velocities and standard deviations of the COP and tilt angle before and after the exercises were compared using paired t-tests.

RESULTS

The tilt angle velocity of an unstable platform significantly decreased after the force-matching exercise at a target torque of 5% MVC (p = 0.029), whereas it was unchanged after the exercises at target torques of 20% and 50% MVC. The standard deviations of the tilt angle of unstable platform test did not change significantly after any exercise. Furthermore, no significant differences were observed in the COP velocities or standard deviations on the stable platform test after any exercise.

SIGNIFICANCE

Our findings suggest that repeated exertion training at low-intensity contractions can affect postural stability in an unstable condition. Particularly, force-matching exercise at very low-intensity torque, such as 5% of MVC, may be an effective method to improve postural control in the unstable condition, but not in a stable condition.

Upper limb manual dexterity, strength and blood flow after walking with backpack load

This study aimed to characterize the effects of walking with backpack load on upper limb function. Fifteen males participated in 3 conditions: no load, 40% body weight loaded backpack (BP) and loaded backpack with simulated rifle (BRC). Pinch strength, grip strength, sensory threshold, blood flow volume, and a manual dexterity test were assessed before and after a 45-min walking trial. Pinch strength in the BP condition was significantly different than the control (p < 0.05). Grooved pegboard times were faster after a seated recovery (p = 0.026) than immediately after walking with load. Blood flow was significantly decreased to <53% of baseline (p ≤ 0.001) in BP and BRC immediately after donning the backpack. No significant changes in grip strength or sensory threshold were measured among conditions or time points. In conclusion, pinch strength, manual dexterity and blood flow were affected by backpack carriage, but other upper limb measures remained unaffected.

The role of eye movements, attention, and hand movements on age-related differences in pegboard tests

Well-documented manual dexterity impairments in older adults may critically depend on the processing of visual information. The purpose of this study was to determine age-related changes in eye and hand movements during commonly used pegboard tests and the association with manual dexterity impairments in older adults. The relationship between attentional deficits and manual dexterity was also assessed. Eye movements and hand kinematics of 20 young (20-38 yr) and 20 older (65-85 yr) adults were recorded during 9-Hole Pegboard, Grooved Pegboard, and a visuospatial dual test. Results were compared with standardized tests of attention (The Test of Everyday Attention and Trail Making Test) that assess visual selective attention, sustained attention, attentional switching, and divided attention. Hand movement variability was 34% greater in older versus young adults when placing the pegs into the pegboard and this was associated with decreased pegboard performance, providing further evidence that increased movement variability plays a role in dexterity impairments in older adults. Older adults made more corrective saccades and spent less time gazing at the pegboard than young adults, suggesting altered visual strategies in older compared with young adults. The relationship between pegboard completion time and Trail Making Test B demonstrates an association between attentional deficits and age-related pegboard impairments. Results contribute novel findings of age-associated changes in eye movements during a commonly used manual dexterity task and offer insight into potential mechanisms underlying hand motor impairments in older adults.NEW & NOTEWORTHY This eye tracking study contributes novel findings of age-associated changes in eye movements during the commonly used pegboard tests of manual dexterity, including a greater number of corrective saccades and lesser time gazing at the pegboard holes in older compared with young adults. An association between attentional deficits and dexterity impairments in older adults is also highlighted. Results shed light on potential mechanisms underlying well-documented motor deficits in older adults.

Force Steadiness: From Motor Units to Voluntary Actions

Voluntary actions are controlled by the synaptic inputs that are shared by pools of spinal motor neurons. The slow common oscillations in the discharge times of motor units due to these synaptic inputs are strongly correlated with the fluctuations in force during submaximal isometric contractions (force steadiness) and moderately associated with performance scores on some tests of motor function. However, there are key gaps in knowledge that limit the interpretation of differences in force steadiness.

Dynamic Wrist Flexion and Extension Fatigue Induced via Submaximal Contractions Similarly Impairs Hand Tracking Accuracy in Young Adult Males and Females

We evaluated the effects of muscle fatigue on hand-tracking performance in young adults. Differences were quantified between wrist flexion and extension fatigability, and between males and females. Participants were evaluated on their ability to trace a pattern using a 3-degrees-of-freedom robotic manipulandum before (baseline) and after (0, 1, 2, 4, 6, 8, and 10 mins) a submaximal-intensity fatigue protocol performed to exhaustion that isolated the wrist flexors or extensors on separate days. Tracking tasks were performed at all time points, while maximal voluntary contractions (MVCs) were performed at baseline, and 2, 6-, and 10-mins post-task termination. We evaluated movement smoothness (jerk ratio, JR), shape reproduction (figural error, FE), and target tracking accuracy (tracking error, TE). MVC force was significantly lower in females (p < 0.05), lower than baseline for all timepoints after task termination (p < 0.05), with no muscle group-dependent differences. JR did not return to baseline until 10-mins post-task termination (most affected), while FE returned at 4-mins post-task termination, and TE at 1-min post-task termination. Males tracked the target with significantly lower JR (p < 0.05), less TE (p < 0.05), and less FE (p < 0.05) than females. No muscle group-dependent changes in hand-tracking performance were observed. Based on this work, hand tracking accuracy is similarly impaired following repetitive submaximal dynamic wrist flexion or extension. The differences between male and female fatigability was independent of the changes in our tracking metrics.

Importance of Maximal Strength and Muscle-Tendon Mechanics for Improving Force Steadiness in Persons with Parkinson's Disease

Although plantar flexion force steadiness (FS) is reduced in persons with Parkinson’s disease (PD), the underlying causes are unknown. The aim of this exploratory design study was to ascertain the influence of maximal voluntary contraction (MVC) force and gastrocnemius-Achilles muscle-tendon unit behaviour on FS in persons with PD. Nine persons with PD and nine age- and sex-matched non-PD controls (~70 years, 6 females per group) performed plantar flexion MVCs and sub-maximal tracking tasks at 5, 10, 25, 50 and 75% MVC. Achilles tendon elongation and medial gastrocnemius fascicle lengths were recorded via ultrasound during contraction. FS was quantified using the coefficient of variation (CV) of force. Contributions of MVC and tendon mechanics to FS were determined using multiple regression analyses. Persons with PD were 35% weaker during MVC (p = 0.04) and had 97% greater CV (p = 0.01) with 47% less fascicle shortening (p = 0.004) and 38% less tendon elongation (p = 0.002) than controls. Reduced strength was a direct contributor to lower FS in PD (ß = 0.631), and an indirect factor through limiting optimal muscle-tendon unit interaction. Interestingly, our findings indicate an uncoupling between fascicle shortening and tendon elongation in persons with PD. To better understand limitations in FS and muscle-tendon unit behavior, it is imperative to identify the origins of MVC decrements in persons with PD.

Can Resistance Training Improve Upper Limb Postural Tremor, Force Steadiness and Dexterity in Older Adults? A Systematic Review

BACKGROUND

The ageing process and several health conditions may increase tremor and reduce force steadiness and dexterity, which can severely impact on function and quality of life. Resistance training can evoke a range of neuromuscular adaptions that may significantly reduce tremor and/or increase force steadiness and/or dexterity in older adults, irrespective of their health condition.

OBJECTIVES

The objective of this study was to systematically review the literature to determine if a minimum of 4 weeks' resistance training can reduce postural tremor and improve force steadiness and/or dexterity in older adults, defined as aged 65 years and over.

METHODS

An electronic search using Ovid, CINAHL, SPORTDiscus and EMBASE was performed. Risk of bias was assessed using the Cochrane Risk of Bias Tool.

RESULTS

Fourteen studies met the eligibility criteria, including six randomised controlled trials and two quasi-randomised controlled trials. All eight studies that recruited healthy older adults reported significant reductions in postural tremor and/or improvements in force steadiness and dexterity. Five out of seven studies that examined older adults with a particular health condition reported some improvements in force steadiness and/or dexterity. Specifically, significant benefits were observed for older adults with chronic obstructive pulmonary disease and essential tremor; however, small or no changes were observed for individuals with osteoarthritis or stroke.

CONCLUSIONS

Resistance training is a non-pharmacological treatment that can reduce tremor and improve force steadiness and dexterity in a variety of older adult populations. Future research should employ randomised controlled trials with larger sample sizes, better describe training programme methods, and align exercise prescription to current recommendations for older adults.

Force control during submaximal isometric contractions is associated with walking performance in persons with multiple sclerosis

Individuals with multiple sclerosis (MS) experience progressive declines in movement capabilities, especially walking performance. The purpose of our study was to compare the amount of variance in walking performance that could be explained by the functional capabilities of lower leg muscles in persons with MS and a sex- and age-matched control group. Participants performed two walking tests (6-min walk and 25-ft walk), strength tests for the plantar flexor and dorsiflexor muscles, and steady submaximal (10% and 20% maximum) isometric contractions. High-density electromyography (EMG) was recorded during the steady contractions, and the signals were decomposed to identify the discharge times of concurrently active motor units. There were significant differences between the two groups in the force fluctuations during the steady contractions (force steadiness), the strength of the plantar flexor and dorsiflexor muscles, and the discharge characteristics during the steady contractions. Performance on the two walking tests by the MS group was moderately associated with force steadiness of the plantar flexor and dorsiflexor muscles; worse force steadiness was associated with poorer walking performance. In contrast, the performance of the control group was associated with muscle strength (25-ft test) and force steadiness of the dorsiflexors and variance in common input of motor units to the plantar flexors (6-min test). These findings indicate that a reduction in the ability to maintain a steady force during submaximal isometric contractions is moderately associated with walking performance of persons with MS.NEW & NOTEWORTHY The variance in walking endurance and walking speed was associated with force control of the lower leg muscles during submaximal isometric contractions in individuals with multiple sclerosis (MS). In contrast, the fast walking speed of a sex- and age-matched control group was associated with the strength of lower leg muscles. These findings indicate that moderate declines in the walking performance of persons with MS are more associated with impairments in force control rather than decreases in muscle strength.

Differences in postural sway among healthy adults are associated with the ability to perform steady contractions with leg muscles

Upright standing involves small displacements of the center of mass about the base of support. These displacements are often quantified by measuring various kinematic features of the center-of-pressure trajectory. The plantar flexors have often been identified as the key muscles for the control of these displacements; however, studies have suggested that the hip abductor and adductors may also be important. The purpose of our study was to determine the association between the force capabilities of selected leg muscles and sway-area rate across four balance conditions in young (25 ± 4 years; 12/19 women) and older adults (71 ± 5 years; 5/19 women). Due to the marked overlap in sway-area rate between the two age groups, the data were collapsed, and individuals were assigned to groups of low- and high-sway area rates based on a k-medoid cluster analysis. The number of participants assigned to each group varied across balance conditions and a subset of older adults was always included in the low-sway group for each balance condition. The most consistent explanatory variable for the variance in sway-area rate was force control of the hip abductors and ankle dorsiflexors as indicated by the magnitude of the normalized force fluctuations (force steadiness) during a submaximal isometric contraction. The explanatory power of the regression models varied across conditions, thereby identifying specific balance conditions that should be examined further in future studies of postural control.

Poor estimates of motor variability are associated with longer grooved pegboard times for middle-aged and older adults

Goal-directed movements that involve greater motor variability are performed with an increased risk that the intended goal will not be achieved. The ability to estimate motor variability during such actions varies across individuals and influences how people decide to move about their environment. The purpose of our study was to identify the decision-making strategies used by middle-aged and older adults when performing two goal-directed motor tasks and to determine if these strategies were associated with the time to complete the grooved pegboard test. Twenty-one middle-aged (48 ± 6 yr; range 40-59 yr, 15 women) and 20 older adults (73 ± 4 yr; range 65-79 yr, 8 women) performed two targeted tasks, each with two normalized target options. Decision-making characteristics were not associated with time to complete the test of manual dexterity when the analysis included all participants, but slower pegboard times were associated with measures of greater movement variability during the target-directed actions. When the data were clustered on the basis of pegboard time rather than age, relatively longer times for the faster group were associated with greater motor variability during the prescribed tasks, whereas longer times for the slower group were associated with increased risk-seeking behavior (α) and greater variability in the targeted actions. NEW & NOTEWORTHY This study was the first to examine the association between decision-making choices and an NIH Toolbox test of manual dexterity (grooved pegboard test) performed by middle-aged and older adults. Significant associations were observed between decision-making choices and time to complete the test when the analyses were based on pegboard times rather than chronological age. This result indicates that decision-making choices of middle-aged and older adults, independent of age, were associated with time to complete a test of manual dexterity.

Is there sufficient evidence to explain the cause of sexually dimorphic behaviour in force steadiness?

Neuromuscular noise is a determining factor in the control of isometric force steadiness (FS), quantified as coefficient of variation (CV) of force around a preestablished target output. In this paper we examine sex-related differences of neural, muscular, and tendon influences on neuromuscular noise to understand FS in females and males. We use evidence from the literature to identify that CV of force is higher in females compared with males in the upper and lower body, with sex-related differences becoming less apparent with increasing age. Evaluation of sex-related physiology in tandem with results from FS studies indicate that differences in fibre type, contractile properties, and number of motor units (MUs) are unlikely contributors to differences in FS between females and males. MU type, behaviour of the population (inclusive of number of active MUs from the population), agonist–antagonist activity, maximal strength, and tendon mechanics are probable contributors to sexually dimorphic behaviour in FS. To clearly determine underlying causes of sex-related differences in FS, further study and reporting between females and males is required. Females and males are included in many studies; however, rich data on sexually dimorphic behaviour is lost when data are collapsed across sex or identified as nonsignificant without supporting values. This poses a challenge to identifying the underlying cause of females having higher CV of force than males. This review provides evidence of sexually dimorphic behaviour in FS and suggests that physiological differences between females and males effect neuromuscular noise, and in-turn contribute to sex-related differences in FS.

Impaired Trunk Flexor Strength, Fatigability, and Steadiness in Postpartum Women

PURPOSE

To determine whether postpartum women (vaginal and cesarean delivery) have deficits in trunk flexor strength, fatigability and steadiness, compared with nulligravid women, up to 26 wk postpartum. We hypothesized that postpartum women would be weaker, more fatigable, and have greater torque fluctuations than controls, with cesarean delivery showing greater deficits than vaginal delivery.

METHODS

Twenty-two control women (nulligravid) and 29 postpartum women (20-40 yr, 19 who delivered via vaginal birth, 13 via Caesarian section) participated. Postpartum women were tested 8 to 10 wk and 24 to 26 wk postpartum. Control women were tested 16 to 18 wk apart. Maximal voluntary isometric contractions (MVC) were performed at multiple trunk positions with the trunk flexor muscles. To determine trunk flexor fatigability, subjects performed intermittent isometric contractions at 50% MVC (6-s contraction, 4-s rest) in upright sitting until task failure. An MVC was performed during the fatiguing task (one per minute) and at 10 and 20 min of recovery.

RESULTS

At 8 and 26 wk, postpartum women (groups pooled) were weaker at all trunk angles (38% and 44% respectively, P < 0.05) than controls despite no differences in handgrip strength. Postpartum women were more fatigable (71% and 52% respectively) and had greater torque fluctuations than controls (P < 0.05). At 8 wk postpartum, women who had a cesarean delivery, were 59% more fatigable (P = 0.004) than the vaginal delivery group, with no difference between delivery types at 26 wk postpartum.

CONCLUSIONS

Musculoskeletal recovery, including trunk flexor muscle strength and fatigability, is incomplete at 26 wk postpartum. These findings provide a rationale for future studies to address outcomes of rehabilitation programs specifically targeted at improving strength and fatigability of the trunk flexor muscles after pregnancy and childbirth.

Variability in common synaptic input to motor neurons modulates both force steadiness and pegboard time in young and older adults

KEY POINTS

The fluctuations in force during a steady isometric contraction (force steadiness) are associated with oscillations in common synaptic input to the involved motor neurons. Decreases in force steadiness are associated with increases in pegboard times in older adults, although a mechanism for this link has not been established. We used a state-space model to estimate the variability in common synaptic input to motor neurons during steady, isometric contractions. The estimate of common synaptic input was derived from the discharge times of motor units as recorded with high-density surface electrodes. We found that the variability in common synaptic input to motor neurons modulates force steadiness for young and older adults, as well as pegboard time for older adults.

ABSTRACT

We investigated the associations between grooved pegboard times, force steadiness (coefficient of variation for force) and variability in an estimate of the common synaptic input to motor neurons innervating the wrist extensor muscles during steady contractions performed by young and older adults. The discharge times of motor units were derived from recordings obtained with high-density surface electrodes when participants performed steady isometric contractions at 10% and 20% of maximal voluntary contraction force. The steady contractions were performed with a pinch grip and wrist extension, both independently (single action) and concurrently (double action). The variance in common synaptic input to motor neurons was estimated with a state-space model of the latent common input dynamics. There was a statistically significant association between the coefficient of variation for force during the steady contractions and the estimated variance in common synaptic input in young (r2  = 0.31) and older (r2  = 0.39) adults, although not between either the mean or the coefficient of variation for interspike interval of single motor units with the coefficient of variation for force. Moreover, the estimated variance in common synaptic input during the double-action task with the wrist extensors at the 20% target was significantly associated with grooved pegboard time (r2  = 0.47) for older adults but not young adults. These findings indicate that longer pegboard times of older adults were associated with worse force steadiness and greater fluctuations in the estimated common synaptic input to motor neurons during steady contractions.

Peg-manipulation capabilities of middle-aged adults have a greater influence on pegboard times than those of young and old adults

Declines in manual dexterity are frequently quantified as the time it takes to complete the grooved pegboard test. The test requires individuals to manipulate 25 pegs, one at a time, by removing them from a well and inserting them into a prescribed hole. The manipulation of each peg involves four phases: selection, transport, insertion, and return. The purpose of our study was to compare the times to complete the four phases of peg manipulation and the forces applied to the pegboard during peg insertion as young, middle-aged, and old adults performed the grooved pegboard test. The relative significance of the peg-manipulation attributes for 30 young (24.0 ± 4.4 years), 15 middle-aged (46.5 ± 6.5 years), and 15 old (70.4 ± 4.0 years) adults was assessed with a multiple-regression analysis. The grooved pegboard test was performed on a force plate. Pegboard times for the old adults (81 ± 17 s) were longer than those for young (56 ± 7 s) and middle-aged (58 ± 11 s) adults. Regression analysis indicated that the explanatory variables for the pegboard times of young (R² = 0.33) and middle-aged (R² = 0.78) adults were the times for the peg insertion and return phases, whereas the predictors for old adults (R² = 0.49) were the times for the peg selection and transport phases. The relative influence of peg-manipulation capabilities on a pegboard test of manual dexterity was greater for middle-aged adults than for young and old adults.

Pulse Width Does Not Influence the Gains Achieved With Neuromuscular Electrical Stimulation in People With Multiple Sclerosis: Double-Blind, Randomized Trial

BACKGROUND

Multiple sclerosis (MS) eventually compromises the walking ability of most individuals burdened with the disease. Treatment with neuromuscular electrical stimulation (NMES) can restore some functional abilities in persons with MS, but its effectiveness may depend on stimulus-pulse duration.

OBJECTIVE

To compare the effects of a 6-week intervention with narrow- or wide-pulse NMES on walking performance, neuromuscular function, and disability status of persons with relapsing-remitting MS.

METHODS

Individuals with MS (52.6 ± 7.4 years) were randomly assigned to either the narrow-pulse (n = 13) or wide-pulse (n = 14) group. The NMES intervention was performed on the dorsiflexor and plantar flexor muscles of both legs (10 minutes each muscle, 4 s on and 12 s off) at a tolerable level for 18 sessions across 6 weeks. Outcomes were obtained before (week 0) and after (week 7) the intervention and 4 weeks later (week 11).

RESULTS

There was no influence of stimulus-pulse duration on the outcomes ( P > .05); thus, the data were collapsed across groups. The NMES intervention improved ( P < .05) gait speed and walking endurance, dorsiflexor strength in the more-affected leg, plantar flexor strength in the less-affected leg, force control for plantar flexors in the less-affected leg, and self-reported levels of fatigue and walking limitations.

CONCLUSION

There was no influence of stimulus-pulse duration on the primary outcomes (gait speed and walking endurance). The 6-week NMES intervention applied to the lower leg muscles of persons with mild to moderate levels of disability can improve their walking performance and provide some symptom relief.

Motor unit discharge characteristics and walking performance of individuals with multiple sclerosis

Walking performance of persons with multiple sclerosis (MS) is strongly influenced by the activation signals received by lower leg muscles. We examined the associations between force steadiness and motor unit discharge characteristics of lower leg muscles during submaximal isometric contractions with tests of walking performance and disability status in individuals who self-reported walking difficulties due to MS. We expected that worse walking performance would be associated with weaker plantar flexor muscles, worse force steadiness, and slower motor unit discharge times. Twenty-three individuals with relapsing-remitting MS (56 ± 7 yr) participated in the study. Participants completed one to three evaluation sessions that involved two walking tests (25-ft walk and 6-min walk), a manual dexterity test (grooved pegboard), health-related questionnaires, and measurement of strength, force steadiness, and motor unit discharge characteristics of lower leg muscles. Multiple regression analyses were used to construct models to explain the variance in measures of walking performance. There were statistically significant differences (effect sizes: 0.21-0.60) between the three muscles in mean interspike interval (ISI) and ISI distributions during steady submaximal contractions with the plantar flexor and dorsiflexor muscles. The regression models explained 40% of the variance in 6-min walk distance and 47% of the variance in 25-ft walk time with two or three variables that included mean ISI for one of the plantar flexor muscles, dorsiflexor strength, and force steadiness. Walking speed and endurance in persons with relapsing-remitting MS were reduced in individuals with longer ISIs, weaker dorsiflexors, and worse plantar flexor force steadiness. NEW & NOTEWORTHY The walking endurance and gait speed of persons with relapsing-remitting multiple sclerosis (MS) were worse in individuals who had weaker dorsiflexor muscles and greater force fluctuations and longer times between action potentials discharged by motor units in plantar flexor muscles during steady isometric contractions. These findings indicate that the control of motor unit activity in lower leg muscles of individuals with MS is associated with their walking ability.

Voluntary activation and variability during maximal dynamic contractions with aging

Whether reduced supraspinal activation contributes to age-related reductions in maximal torque during dynamic contractions is not known. The purpose was to determine whether there are age differences in voluntary activation and its variability when assessed with stimulation at the motor cortex and the muscle during maximal isometric, concentric, and eccentric contractions. Thirty young (23.6 ± 4.1 years) and 31 old (69.0 ± 5.2 years) adults performed maximal isometric, shortening (concentric) and lengthening (eccentric) contractions with the elbow flexor muscles. Maximal isometric contractions were performed at 90° elbow flexion and dynamic contractions at a velocity of 60°/s. Voluntary activation was assessed by superimposing an evoked contraction with transcranial magnetic stimulation (TMS) or with electrical stimulation over the muscle during maximal voluntary contractions (MVCs). Old adults had lower MVC torque during isometric (- 17.9%), concentric (- 19.7%), and eccentric (- 9.9%) contractions than young adults, with less of an age difference for eccentric contractions. Voluntary activation was similar between the three contraction types when assessed with TMS and electrical stimulation, with no age group differences. Old adults, however, were more variable in voluntary activation than young (standard deviation 0.99 ± 0.47% vs. 0.73 ± 0.43%, respectively) to both the motor cortex and muscle, and had greater coactivation of the antagonist muscles during dynamic contractions. Thus, the average voluntary activation to the motor cortex and muscle did not differ with aging; however, supraspinal activation was more variable during maximal dynamic and isometric contractions in the old adults. Lower predictability of voluntary activation may indicate subclinical changes in the central nervous system with advanced aging.

A framework for identifying the adaptations responsible for differences in pegboard times between middle-aged and older adults

Time to complete two tests of manual dexterity, the 9-hole Peg Test and Grooved Pegboard Test, increases with advancing age. However, the adaptations responsible for the differences in pegboard times between middle-aged and older adults are largely unknown. Potential mechanisms include neuromuscular characteristics, cognitive function, and cutaneous sensation. To provide a tractable framework to address these gaps in knowledge, the purpose of the current study was to identify the latent variables underlying age-associated differences in time to complete the 9-hole and grooved pegboard tests. The approach involved an independent component analysis that identified associations between the two pegboard times for the two groups of participants with two to six secondary outcomes. The common association across three of the four conditions (two groups and two pegboard tests) was features derived from force-matching tasks requiring submaximal isometric contraction. In addition, there were significant associations for older adults between age, measures of cognitive function, and pegboard times. Nonetheless, the significant associations were unique for each age group and pegboard test. The results provide a framework for subsequent mechanistic studies to identify the adaptations underlying age-associated declines in manual dexterity.

Rate Coding and the Control of Muscle Force

The force exerted by a muscle during a voluntary contraction depends on the number of motor units recruited for the action and the rates at which they discharge action potentials (rate coding). Over most of the operating range of a muscle, the nervous system controls muscle force by varying both motor unit recruitment and rate coding. Except at relatively low forces, however, the control of muscle force depends primarily on rate coding, especially during fast contractions. This review provides five examples of how the modulation of rate coding influences the force exerted by muscle during voluntary actions. The five examples comprise fast contractions, lengthening and shortening contractions, steady isometric contractions, fatiguing contractions, and contractions performed after a change in the daily level of physical activity.

Peg-manipulation capabilities during a test of manual dexterity differ for persons with multiple sclerosis and healthy individuals

Manual dexterity declines with advancing age and the development of neurological disorders. Changes in manual dexterity are frequently quantified as the time it takes to complete the grooved pegboard test, which requires individuals to manipulate 25 pegs. The manipulation of each peg involves four phases: selection, transport, insertion, and return. The purpose of the study was to compare the times to complete the four phases of manipulating each peg and the forces applied to the pegboard during peg selection and insertion in persons with multiple sclerosis (MS) and age- and sex-matched healthy adults. Multiple-regression models that could explain the variance in pegboard times for each group of participants were compared to assess the relative significance of the peg-manipulation attributes. The performance of 17 persons with MS (52.2 ± 8.3 years) was compared with 17 control subjects (52.2 ± 11.5 years). The grooved pegboard test was performed on a force plate. Pegboard times for the MS group (104 ± 40 s) were longer than those for the Control group (61 ± 15 s). Regression analysis indicated that the pegboard times for the MS group could be predicted by the time for the peg-selection phase (R ² = 0.78), whereas the predictors for Control group (R ² = 0.77) were the times for the peg-transport (partial r = 0.80) and selection (partial r = 0.58) phases. The variance in the time it took the MS participants to complete the grooved pegboard test was strongly related to the time required to select each peg, whereas the pegboard times for the Control subjects depended mostly on the duration of the transport phase but also on the time to select each peg.

Control of force during rapid visuomotor force-matching tasks can be described by discrete time PID control algorithms

Force trajectories during isometric force-matching tasks involving isometric contractions vary substantially across individuals. In this study, we investigated if this variability can be explained by discrete time proportional, integral, derivative (PID) control algorithms with varying model parameters. To this end, we analyzed the pinch force trajectories of 24 subjects performing two rapid force-matching tasks with visual feedback. Both tasks involved isometric contractions to a target force of 10% maximal voluntary contraction. One task involved a single action (pinch) and the other required a double action (concurrent pinch and wrist extension). 50,000 force trajectories were simulated with a computational neuromuscular model whose input was determined by a PID controller with different PID gains and frequencies at which the controller adjusted muscle commands. The goal was to find the best match between each experimental force trajectory and all simulated trajectories. It was possible to identify one realization of the PID controller that matched the experimental force produced during each task for most subjects (average index of similarity: 0.87 ± 0.12; 1 = perfect similarity). The similarities for both tasks were significantly greater than that would be expected by chance (single action: p = 0.01; double action: p = 0.04). Furthermore, the identified control frequencies in the simulated PID controller with the greatest similarities decreased as task difficulty increased (single action: 4.0 ± 1.8 Hz; double action: 3.1 ± 1.3 Hz). Overall, the results indicate that discrete time PID controllers are realistic models for the neural control of force in rapid force-matching tasks involving isometric contractions.

Muscle function and fatigability of trunk flexors in males and females

BACKGROUND

Optimal function of the abdominal muscles is necessary for several life functions including lifting and carrying tasks. Sex differences in strength and fatigability are established for many limb muscles and back extensor muscles, but it is unknown if sex differences exist for the abdominal muscles despite their functional importance.

METHODS

Eighteen females (24.3 ± 4.8 years) and 15 males (24.1 ± 6.6 years) performed (1) isometric trunk flexion maximal voluntary contractions (MVCs) in a range of trunk positions to establish a torque-angle curve and (2) submaximal (50% MVC), intermittent isometric contraction (6 s on, 4 s off) until task failure to determine fatigability of the trunk flexor muscles. Dual X-ray absorptiometry quantified body fat and lean mass. Physical activity levels were quantified with a questionnaire. Torque-angle curves, electromyography (EMG), MVC torque, and torque steadiness were compared with repeated measures ANOVA with sex as a between-subjects factor.

RESULTS

For the torque-angle curve, MVC torque was reduced as the trunk angle increased toward flexion (p < 0.001). Males had greater MVC torque than females at the extended positions (31% difference), with no sex differences in torque in upright sitting (p > 0.05). Time-to-task failure for the submaximal fatigability task in upright sitting was similar between males and females (12.4 ± 7 vs 10.5 ± 6 min). Time-to-task failure was positively associated with strength (r = 0.473, p = 0.005) and self-reported physical activity (r = 0.456, p = 0.030). Lean mass in the trunk was positively associated with trunk flexor strength (r = 0.378, p = 0.011) and self-reported physical activity (r = 0.486, p = 0.007). Finally, torque steadiness [coefficient of variation of torque (CV)] during submaximal isometric contractions decreased with contraction intensity and was similar for males and females across all intensities.

CONCLUSIONS

Unlike many limb muscle groups, males and females had similar fatigability and torque steadiness of the trunk flexor muscles during isometric contractions. Stronger individuals, however, exhibited less fatigability. Lower self-reported physical activity was associated with greater fatigability of trunk flexor muscles. The relationship between strength and fatigability of the trunk flexor muscles and physical activity supports the importance of abdominal muscle strengthening to offset fatigability in both males and females.

Age differences in dynamic fatigability and variability of arm and leg muscles: Associations with physical function

INTRODUCTION

It is not known whether the age-related increase in fatigability of fast dynamic contractions in lower limb muscles also occurs in upper limb muscles. We compared age-related fatigability and variability of maximal-effort repeated dynamic contractions in the knee extensor and elbow flexor muscles; and determined associations between fatigability, variability of velocity between contractions and functional performance.

METHODS

35 young (16 males; 21.0±2.6years) and 32 old (18 males; 71.3±6.2years) adults performed a dynamic fatiguing task involving 90 maximal-effort, fast, concentric, isotonic contractions (1 contraction/3s) with a load equivalent to 20% maximal voluntary isometric contraction (MVIC) torque with the elbow flexor and knee extensor muscles on separate days. Old adults also performed tests of balance and walking endurance.

RESULTS

Old adults had greater fatigue-related reductions in peak velocity compared with young adults for both the elbow flexor and knee extensor muscles (P<0.05) with no sex differences (P>0.05). Old adults had greater variability of peak velocity during the knee extensor, but not during the elbow flexor fatiguing task. The age difference in fatigability was greater for the knee extensor muscles (35.9%) compared with elbow flexor muscles (9.7%, P<0.05). Less fatigability of the knee extensor muscles was associated with greater walking endurance (r=-0.34, P=0.048) and balance (r=-0.41, P=0.014) among old adults.

CONCLUSIONS

An age-related increase in fatigability of a dynamic fatiguing task was greater for the knee extensor compared with the elbow flexor muscles in males and females, and greater fatigability was associated with lesser walking endurance and balance.

The aging neuromuscular system and motor performance

Age-related changes in the basic functional unit of the neuromuscular system, the motor unit, and its neural inputs have a profound effect on motor function, especially among the expanding number of old (older than ∼60 yr) and very old (older than ∼80 yr) adults. This review presents evidence that age-related changes in motor unit morphology and properties lead to impaired motor performance that includes 1) reduced maximal strength and power, slower contractile velocity, and increased fatigability; and 2) increased variability during and between motor tasks, including decreased force steadiness and increased variability of contraction velocity and torque over repeat contractions. The age-related increase in variability of motor performance with aging appears to involve reduced and more variable synaptic inputs that drive motor neuron activation, fewer and larger motor units, less stable neuromuscular junctions, lower and more variable motor unit action potential discharge rates, and smaller and slower skeletal muscle fibers that coexpress different myosin heavy chain isoforms in the muscle of older adults. Physical activity may modify motor unit properties and function in old men and women, although the effects on variability of motor performance are largely unknown. Many studies are of cross-sectional design, so there is a tremendous opportunity to perform high-impact and longitudinal studies along the continuum of aging that determine 1) the influence and cause of the increased variability with aging on functional performance tasks, and 2) whether lifestyle factors such as physical exercise can minimize this age-related variability in motor performance in the rapidly expanding numbers of very old adults.