Four hours of normobaric hypoxia reduces Achilles tendon reflex inhibition.

Acute exposure to hypoxia increases postural sway, but the underlying neurophysiological factors are unclear. Golgi tendon organs (GTOs), located within the musculotendinous junction (MTJ), provide inhibitory signals to plantar flexor muscles that are important for balance control; however, it is uncertain if GTO function is influenced by hypoxia. The aim of this study was to determine how normobaric hypoxia influences lower limb tendon-evoked inhibitory reflexes during upright stance. We hypothesized that tendon-evoked reflex area and duration would decrease during hypoxia, indicating less inhibition of postural muscles compared to normoxia. At baseline (BL; 0.21 fraction of inspired oxygen, FIO2) and at ~2 (H2) and 4 (H4) hours of normobaric hypoxia (0.11 FIO2) in a normobaric hypoxic chamber, sixteen healthy participants received electrical musculotendinous stimulation (MTstim) to the MTJ of the left Achilles tendon. The MTstim was delivered as two sets of 50 stimuli while the participant stood on a force plate with their feet together. Tendon-evoked inhibitory reflexes were recorded from the surface electromyogram of the ipsilateral medial gastrocnemius, and center of pressure (CoP) variables were recorded from the force plate. Normobaric hypoxia increased CoP velocity (p ≤ 0.002) but not CoP standard deviation (p ≥0.12). Compared to BL, normobaric hypoxia reduced tendon-evoked inhibitory reflex area by 45% at H2 and 53% at H4 (p ≤ 0.002). In contrast, reflex duration was unchanged during hypoxia. The reduced inhibitory feedback from the GTO pathway could likely play a role in the increased postural sway observed during acute exposure to hypoxia.

Differential Modulation of Motor Unit Behaviour when a Fatiguing Contraction is Matched for Torque versus EMG.

INTRODUCTION: When an isometric contraction is sustained at a submaximal torque, activation of the motoneuron pool increases, making it difficult to measure neural excitability alterations. Thus, more recently, isometric contractions with maintained electromyographic activity (matched-EMG) are being used to induce fatigue; however, little is known about the neurophysiological adjustments that occur to satisfy the requirements of the task. METHODS: For our study, 16 participants performed a 10-min sustained isometric elbow flexion contraction at 20% maximal voluntary contraction (MVC) torque or the level of integrated biceps brachii EMG recorded at 20% MVC torque. Surface EMG was used to assess global median frequency, and four fine-wire electrode pairs were used to obtain motor unit (MU) discharge rate from biceps brachii. Torque or EMG steadiness was also assessed throughout the fatiguing contractions. RESULTS: MU discharge rate increased and torque steadiness decreased during the matched-torque contraction; however, MU discharge rate decreased during the matched-EMG contraction and no changes occurred for EMG steadiness. Data pooled for the two contractions revealed a decrease of global median frequency. Lastly, a greater loss of MVC torque was observed immediately after the matched-torque compared to matched-EMG contraction. CONCLUSIONS: These findings indicate that, during a matched-torque fatiguing contraction, the nervous system increases MU discharge rates at the cost of poorer steadiness in order to maintain the requisite torque. In contrast, during a matched-EMG fatiguing contraction, a reduction of MU discharge rates allows for a maintenance of EMG steadiness.

Neuromechanical characterization of the abductor hallucis and its potential role in upright postural control.

There is growing evidence to support a role for the abductor hallucis (AH) in standing balance control; however, functional properties of the muscle that may provide more insight into AH's specific contribution to upright posture have yet to be characterized. This study was conducted to quantify functional neuromechanical properties of the AH and correlate the measures with standing balance variables. We quantified strength and voluntary activation during maximal voluntary isometric contractions of the great toe abductor in nine (3 females and 6 males) healthy, young participants. During electrically evoked twitch and tetanic contractions, we measured great toe abduction peak force and constructed a force-frequency curve. We also evaluated peak abduction force, contraction time (CT), half-relaxation time (HRT), rate of force development (RFD), and relaxation rate (RR) from twitch contractions evoked using doublet stimuli. Strength, VA, CT, HRT, RFD, and RR were correlated to centre of pressure standard deviation (COP SD) and velocity (COP VEL) variables of the traditional COP trace and its rambling and trembling components during single-legged stance. AH twitch properties (e.g., CT: 169.8 ± 32.3 ms; HRT: 124.1 ± 29.2 ms) and force-frequency curve were similar to other slow contractile muscles. Contractile speed related negatively with COP VEL, suggesting AH may be appropriate for slow, prolonged tasks such as ongoing postural balance control. Correlation coefficient outcomes for all variables were similar between rambling and trembling components. Our results provide further evidence for the importance of AH neuromechanical function for standing balance control, at least during a challenging single-legged posture.

Normobaric hypoxia does not influence the sural nerve cutaneous reflex during standing.

Hypoxia increases postural sway compared to normoxia, but the underlying sensorimotor factors remain unclear. An important contributor to balance control is cutaneous feedback arising from the feet, which can be partially characterized by electrically evoking a reflex from a purely cutaneous nerve (i.e., sural) and sampling the subsequent motor activity of a muscle. The purpose of the present study was to determine how normobaric hypoxia influences sural nerve reflex parameters during a standing posture. It was hypothesized that normobaric hypoxia would reduce cutaneous reflex area compared to normoxia. Participants (n = 16; 5 females, 11 males) stood with their feet together while receiving two trials of 50 sural nerve stimulations (200-Hz, 5-pulse train, presented randomly every 3-6 s) at baseline (BL; normoxia), and at 2 (H2) and 4 (H4) h of normobaric hypoxia (~ 0.11 fraction of inspired oxygen in a hypoxic chamber). The sural nerve reflex was recorded using surface electromyography from the left medial gastrocnemius, and characterized by area and duration of the initial positive and negative peaks of the response. When normalized to pre-stimulus electromyography, the area of the peak-to-peak cutaneous reflex was not different than BL (p ≥ 0.14) for up to 4 h of normobaric hypoxia (BL: 0.26 ± 0.22, H2: 0.19 ± 0.19, H4: 0.22 ± 0.20 A.U.). Furthermore, the duration of the response was not different during hypoxia (BL: 73.2 ± 42.4; H2: 75.2 ± 47.0; H4: 77.6 ± 54.6 ms; p ≥ 0.13) than BL. Thus, reflexes arising from cutaneous afferents of the lateral border of the foot are resilient to at least 4 h of normobaric hypoxia.

A violation of Fitts' Law occurs when a target range is presented before and during movement.

According to Fitts' Law, the time to reach a target (movement time, MT) increases with distance. A violation of Fitts' Law occurs when target positions are outlined before and during movement, as MTs are not different when reaching to the farthest and penultimate targets. One hypothesis posits that performers cognitively process the edges of a target array before the center, allowing for corrective movements to be completed more quickly when moving to edge targets compared to middle targets. The objective of this study was to test this hypothesis by displaying a target range rather than outlines of individual targets in an effort to identify the effects of array edges. Using a touch-screen laptop, participants (N = 24) were asked to reach to one of three targets which would appear within a presented range. Separately, targets were also presented without a range to determine if the display protocol could evoke Fitts' Law. Movements were assessed with the touch screen and optical position measurement. A main effect was found for relative position within a range (touch: F2,44 = 15.4, p < 0.001, η2p = 0.412; position: F2,40 = 15.6, p < 0.001, η2p = 0.439). As hypothesised, MT to the farthest target in a range was not significantly different than MT to the middle target (touch: p = 0.638, position: p = 0.449). No violation was found when a target range was not presented (touch: p = 0.003, position: p = 0.001). Thus, a target range reproduces the Fitts' Law violation previously documented with individually outlined targets, which supports and extends the discussed hypothesis.

The orderly recruitment of motor units may be modified when a muscle is acting as an antagonist.

Despite the perceived importance of antagonist muscle activity, it is unknown if motor unit (MU) behavior at recruitment differs when a muscle acts as an antagonist versus agonist. Fourteen healthy participants performed ramped, isometric elbow flexor or extensor contractions to 50% or 100% maximal voluntary contraction (MVC) torque. Surface and fine-wire intramuscular electromyographic (EMG) recordings were sampled from biceps and triceps brachii. During agonist contractions, low-threshold MUs (recruited at <10% MVC torque) were sampled in all participants, with a total of 107 and 90 for biceps and triceps brachii, respectively. For ramped MVCs, antagonist surface EMG coactivation (% amplitude during agonist MVC) was 8.3 ± 6.6% for biceps and 15.2 ± 7.3% for triceps brachii. However, antagonist single MU activity was recorded from only four participants, with only one of these individuals having antagonist MUs recorded from both muscles. All antagonist MUs were successfully detected during agonist contractions, but many (∼40%) had a recruitment threshold >10% MVC torque. For MUs recorded during both agonist and antagonist contractions, discharge rate at recruitment was seemingly lower for antagonist than agonist contractions. Coexistence of typical levels of surface EMG-derived coactivation with scant antagonist MU recordings suggests that coactivation in these muscles is primarily the result of cross talk. Based on the limited antagonist MU data detected, MUs recruited early during an agonist contraction are not necessarily among those first recruited during an antagonist contraction. These findings highlight the possibility of a modification of orderly recruitment when a motoneuron pool is acting as an antagonist.NEW & NOTEWORTHY Modest levels of coactivation are widely considered essential for appropriate motor control; however, minimal attention has been given to recruitment patterns of motor units (MUs) from antagonist muscles. Despite the successful recording of many low-threshold MUs during agonist contractions, we recorded no antagonist MUs in most participants. Of the units recorded, only ∼60% matched those recruited at <10% of maximal torque when the muscle acted as an agonist, which suggests a modified recruitment order for antagonist MUs.

Visual feedback-dependent modulation of arousal, postural control, and muscle stretch reflexes assessed in real and virtual environments.

Introduction: The mechanisms regulating neuromuscular control of standing balance can be influenced by visual sensory feedback and arousal. Virtual reality (VR) is a cutting-edge tool for probing the neural control of balance and its dependence on visual feedback, but whether VR induces neuromodulation akin to that seen in real environments (eyes open vs. closed or ground level vs. height platform) remains unclear. Methods: Here we monitored 20 healthy young adults (mean age 23.3 ± 3.2 years; 10 females) during four conditions of quiet standing. Two real world conditions (eyes open and eyes closed; REO and REC) preceded two eyes-open virtual 'low' (ground level; VRL) and 'high' (14 m height platform; VRH) conditions. We measured arousal via electrodermal activity and psychosocial questionnaires rating perceived fear and anxiety. We recorded surface electromyography over the right soleus, medial gastrocnemius, and tibialis anterior, and performed force plate posturography. As a proxy for modulations in neural control, we assessed lower limb reflexive muscle responses evoked by tendon vibration and electrical stimulation. Results: Physiological and perceptual indicators of fear and anxiety increased in the VRH condition. Background soleus muscle activation was not different across conditions; however, significant increases in muscle activity were observed for medial gastrocnemius and tibialis anterior in VRH relative to REO. The mean power frequency of postural sway also increased in the VRH condition relative to REO. Finally, with a fixed stimulus level across conditions, mechanically evoked reflexes remained constant, while H-reflex amplitudes decreased in strength within virtual reality. Discussion: Notably, H-reflexes were lower in the VRL condition than REO, suggesting that these ostensibly similar visual environments produce different states of reflexive balance control. In summary, we provide novel evidence that VR can be used to modulate upright postural control, but caution that standing balance in analogous real and virtual environments may involve different neural control states.

Intermuscular coherence of plantar and dorsiflexor muscles in older adults with Parkinson's disease and age-matched controls during bipedal and unipedal stance.

Introduction: Postural instability increases with age and is exacerbated in neurological disorders such as Parkinson's disease (PD). Reducing the base of support from bipedal to unipedal stance increases center of pressure (CoP) parameters and intermuscular coherence in lower-leg muscles of healthy older adults. To further develop an understanding of postural control in an altered state of neurological impairment, we explored intermuscular coherence in lower-leg muscles and CoP displacement in older adults with PD. Methods: This study measured surface EMG from the medial (MG) and lateral (LG) gastrocnemii, soleus (SOL), and tibialis anterior (TA), and examined EMG amplitude and intermuscular coherence during bipedal and unipedal stance on a force plate with firm (no foam) and compliant (standing on foam) surface conditions in nine older adults with PD (70±5 years, 6 females) and 8 age-matched non-Parkinsonian older adults (5 females). Intermuscular coherence was analyzed between agonist-agonist and agonist-antagonist muscle pairs in the alpha (8-13 Hz) and beta (15-35 Hz) frequency bands. Results: CoP parameters increased from bipedal to unipedal stance in both groups (p < 0.01), but did not increase from the firm to compliant surface condition (p > 0.05). During unipedal stance, CoP path length was shorter in older adults with PD (2027.9 ± 1074.1 mm) compared to controls (3128.5 ± 1198.7 mm) (p < 0.01). Alpha and beta agonist-agonist and agonist-antagonist coherence increased by 28% from bipedal to unipedal stance (p > 0.05), but did not differ between older adults with PD (0.09 ± 0.07) and controls (0.08 ± 0.05) (p > 0.05). The older adults with PD also had greater normalized EMG amplitude of the LG (63.5 ± 31.7%) and TA (60.6 ± 38.4%) during the balance tasks (p > 0.05) than the non-Parkinsonian counterparts. Discussion: Older adults with PD had shorter path lengths during unipedal stance and required greater muscle activation than older adults without PD to perform the tasks, but intermuscular coherence did not differ between the groups. This may be attributable to their early disease stage and high motor function.

Eccentric exercise-induced muscle weakness abolishes sex differences in fatigability during sustained submaximal isometric contractions.

BACKGROUND: Females are typically less fatigable than males during sustained isometric contractions at lower isometric contraction intensities. This sex difference in fatigability becomes more variable during higher intensity isometric and dynamic contractions. While less fatiguing than isometric or concentric contractions, eccentric contractions induce greater and longer lasting impairments in force production. However, it is not clear how muscle weakness influences fatigability in males and females during sustained isometric contractions. METHODS: We investigated the effects of eccentric exercise-induced muscle weakness on time to task failure (TTF) during a sustained submaximal isometric contraction in young (18-30 years) healthy males (n = 9) and females (n = 10). Participants performed a sustained isometric contraction of the dorsiflexors at 35° plantar flexion by matching a 30% maximal voluntary contraction (MVC) torque target until task failure (i.e., falling below 5% of their target torque for ≥2 s). The same sustained isometric contraction was repeated 30 min after 150 maximal eccentric contractions. Agonist and antagonist activation were assessed using surface electromyography over the tibialis anterior and soleus muscles, respectively. RESULTS: Males were ∼41% stronger than females. Following eccentric exercise both males and females experienced an ∼20% decline in maximal voluntary contraction torque. TTF was ∼34% longer in females than males prior to eccentric exercise-induced muscle weakness. However, following eccentric exercise-induced muscle weakness, this sex-related difference was abolished, with both groups having an ∼45% shorter TTF. Notably, there was ∼100% greater antagonist activation in the female group during the sustained isometric contraction following exercise-induced weakness as compared to the males. CONCLUSION: This increase in antagonist activation disadvantaged females by decreasing their TTF, resulting in a blunting of their typical fatigability advantage over males.

Effects of sleep deprivation on perceived and performance fatigability in females: An exploratory study.

Sleep deprivation (SD) is prevalent and impairs motor function; however, little is known about its effect on perceived and performance fatigability, especially in females. To examine the effects of 24 h of SD on these attributes of fatigue, nine females completed a 20-min isometric, sustained elbow flexion contraction, followed by 10 min of recovery. The superimposed twitch (SIT) elicited via transcranial magnetic stimulation (TMS) assessed supraspinal drive. Biceps brachii electromyographic data indicated neural excitability in response to stimulation over the motor cortex (motor evoked potential; MEP), corticospinal tract (cervicomedullary motor evoked potential; CMEP), and brachial plexus (maximal M-wave; Mmax). MEPs and CMEPs were recorded during a TMS-induced silent period. At baseline, ratings of perceived effort (RPE; 2.9 vs. 1.6) and fatigue (RPF; 6.9 vs. 2.9), were higher for SD than control. Across the 20-min contraction, RPE increased from 2.2 to 7.6, SIT and MEP/CMEP increased by 284 and 474%, respectively, whereas maximal voluntary isometric contraction (MVC) torque and CMEP/Mmax decreased by 26 and 57%, respectively. No differences were found across conditions for MVC, SIT, Mmax, CMEP/Mmax, or MEP/CMEP prior to, during, and after the fatiguing task. During recovery, RPE (4.9 vs. 3.4), RPF (7.6 vs. 2.8), and perception of task difficulty (5.5 vs. 4.5) were greater for SD than control. Acute SD does not appear to alter performance fatigability development and subsequent recovery; however, it increases perceptions of fatigue, effort, and task difficulty. Thus, the disconnect between perceived and actual neuromuscular capacity following a sustained, submaximal isometric task is exacerbated by SD.HighlightsSleep deprivation did not alter supraspinal drive or neural excitability during and after a 20-min submaximal elbow flexion contractionSleep deprivation increased perceived fatigue and perception of task difficultyThe disconnect between perceived and performance fatigability is exacerbated in a sleep-deprived state.

The influence of reduced foot dorsum cutaneous sensitivity on the vestibular control of balance.

PURPOSE: Foot sole cooling increases vestibular-evoked balance responses, but less is known about foot dorsum temperature alterations. The purpose was to determine whether decreasing cutaneous receptor sensitivity via foot dorsum cooling modulates the vestibular control of balance. METHODS: Eighteen participants (9 males; 9 females) stood quietly on a force plate with feet together, eyes closed, and head rotated leftward during 4, 90-s trials (2 control; 2 cooled) of continuous electrical vestibular stimulation (EVS). Icepacks placed on the dorsum of both feet for 15 min induced cooling and remained throughout the EVS trials. Monofilament testing was performed at multiple locations before and after cooling to determine tactile detection thresholds. T-type thermocouples monitored skin temperature over the tibialis anterior, soleus, foot dorsum and arch of the right leg. Vestibular-evoked balance responses were characterized using time (cumulant density) and frequency (coherence and gain) domain analyses to determine the relationship between the EVS input and motor output (anteroposterior force-AP force; right medial gastrocnemius electromyography-MG EMG). RESULTS: Skin temperature of the foot dorsum and arch decreased ~ 70 and 15%, respectively during cooling (p < 0.05), but was unaltered at other locations (p ≥ 0.10). Detection thresholds for the foot dorsum increased following cooling (p < 0.05). Surprisingly, cooling reduced EVS-AP force and EVS-MG EMG coherence and gain at multiple frequencies, and peak-to-peak amplitude compared to control (p < 0.05). CONCLUSION: Our results indicate that vestibular-driven balance responses are reduced following foot dorsum cooling, likely owing to alterations in cutaneous mechanoreceptor sensitivity and subsequent alterations in the transformation of vestibular cues for balance control.

Age-related performance fatigability: a comprehensive review of dynamic tasks.

Adult aging is associated with a myriad of changes within the neuromuscular system, leading to reductions in contractile function of old adults. One of the consequences of these age-related neuromuscular adaptations is altered performance fatigability, which can limit the ability of old adults to perform activities of daily living. Whereas age-related fatigability during isometric tasks has been well characterized, considerably less is known about fatigability of old adults during dynamic tasks involving movement about a joint, which provides a more functionally relevant task compared with static contractions. This review provides a comprehensive summary of age-related fatigability during dynamic contractions, where the importance of task specificity is highlighted with a brief discussion of the potential mechanisms responsible for differences in fatigability between young and old adults. The angular velocity of the task is critical for evaluating age-related fatigability, as tasks that constrain angular velocity (i.e., isokinetic) produce equivocal age-related differences in fatigability, whereas tasks involving unconstrained velocity (i.e., isotonic-like) consistently induce greater fatigability for old compared with young adults. These unconstrained velocity tasks, which are more closely associated with natural movements, offer an excellent model to uncover the underlying age-related mechanisms of increased fatigability. Future work evaluating the mechanisms of increased age-related fatigability during dynamic tasks should be evaluated using contraction modes that are specific to the task (i.e., dynamic), rather than isometric, particularly for the assessment of spinal and supra spinal components. Advancing our understanding of age-related fatigability is likely to yield novel insights and approaches for improving mobility limitations in old adults.

Neural effects of sleep deprivation on inhibitory control and emotion processing.

Sleep deprivation is commonplace and impairs memory, inhibition, cognitive flexibility and attention. However, little is known about the neurophysiological impact of sleep deprivation in the context of go/no-go (GNG) task performance and emotion processing. To address this knowledge gap, 12 females performed two computerized GNG tasks (shapes; emotional facial expressions) and an object hit and avoid (OHA) task after a night of typical sleep and 24 h without sleep. Electroencephalographic (EEG) recordings were taken during a 3-minute eyes-open resting period as well as during GNG task performance. Resting EEG power in the theta band was 33% higher for the sleep-deprived than control condition (p < 0.05), whereas alpha activity was unchanged. When sleep deprived, participants had ~6% slower response times (go trials) and made ~7% more total errors during GNG tasks (p < 0.05). Reaction time and overall accuracy were ~25% and ~9% worse for the emotional compared to shape GNG task (p < 0.05), respectively, which suggests interference of emotion processing on task performance. Smaller differences in amplitude between go and no-go trials for the N2 and both the N2 and P3 event-related potential components were found during sleep deprivation for the emotional and shape GNG tasks, respectively (p < 0.05). No changes to the N170 component were found. Lastly, participants hit more distractors during the OHA task when sleep deprived (p < 0.05). Altogether, these results indicate sleep deprivation slows neural processing and impairs inhibitory task performance, possibly due to a more bottom-up, stimulus-driven approach to inhibiting motor responses.

The effect of increased cognitive processing on reactive balance control following perturbations to the upper limb.

Reactive balance control following hand perturbations is important for everyday living as humans constantly encounter perturbations to the upper limb while performing functional tasks while standing. When multiple tasks are performed simultaneously, cognitive processing is increased, and performance on at least one of the tasks is often disrupted, owing to attentional resources being divided. The purpose here was to assess the effects of increased cognitive processing on whole-body balance responses to perturbations of the hand during continuous voluntary reaching. Sixteen participants (8 females; 22.9 ± 4.5 years) stood and grasped the handle of a KINARM - a robotic-controlled manipulandum paired with an augmented visual display. Participants completed 10 total trials of 100 mediolateral arm movements at a consistent speed of one reach per second, and an auditory n-back task (cognitive task). Twenty anteroposterior hand perturbations were interspersed randomly throughout the reaching trials. The arm movements with random arm perturbations were either performed simultaneously with the cognitive task (combined task) or in isolation (arm perturbation task). Peak centre of pressure (COP) displacement and velocity, time to COP displacement onset and peak, as well as hand displacement and velocity following the hand perturbation were evaluated. N-back response times were 8% slower and 11% less accurate for the combined than the cognitive task. Peak COP displacement following posterior perturbations increased by 8% during the combined compared to the arm perturbation task alone, with no other differences detected. Hand peak displacement decreased by 5% during the combined compared to the arm perturbation task. The main findings indicate that with increased cognitive processing, attentional resources were allocated from the cognitive task towards upper limb movements, while attentional resources for balance seemed unaltered.

Age-related reduction in peak power and increased postural displacement variability are related to enhanced vestibular-evoked balance responses in females.

Adult aging is associated with reductions in muscle function and standing balance control. However, whether sensorimotor function adapts to maintain upright posture in the presence of age-related muscle weakness is unclear. The purpose was to determine whether vestibular control of balance is altered in older compared to younger females and whether vestibular-evoked balance responses are related to muscle power. Eight young (22.6 ± 1.8 years) and eight older (69.7 ± 6.7 years) females stood quietly on a force plate, while subjected to random, continuous electrical vestibular stimulation (EVS; 0-20 Hz, root mean square amplitude: 1.13 mA). Medial gastrocnemius (MG) and tibialis anterior (TA) surface electromyography (EMG) and force plate anterior-posterior (AP) forces were sampled and associated with the EVS signal in the frequency and time domains. Knee extensor function was evaluated using a Biodex multi-joint dynamometer. The weaker, less powerful older females exhibited a 99 and 42% greater medium-latency peak amplitude for the TA and AP force (p < 0.05), respectively, but no other differences were detected for short- and medium-latency peak amplitudes. The TA (<10 Hz) and MG (<4 Hz) EVS-EMG coherence and EVS-AP force coherence (<2 Hz) was greater in older females than young. A strong correlation was detected for AP force medium-latency peak amplitude with center of pressure displacement variability (r = 0.75; p < 0.05) and TA medium-latency peak amplitude (r = 0.86; p < 0.05). Power was negatively correlated with AP force medium-latency peak amplitude (r = -0.47; p < 0.05). Taken together, an increased vestibular control of balance may compensate for an age-related reduction in power and accompanies greater postural instability in older females than young.

Power attenuation from restricting range of motion is minimized in subjects with fast RTD and following isometric training.

Time-dependent measures consisting of rate of torque development (RTD), rate of velocity development (RVD), and rate of neuromuscular activation can be used to evaluate explosive muscular performance, which becomes critical when performing movements throughout limited ranges of motion (ROM). In this study, we investigated how restricting ROM influences power production while also exploring the relationship with time-dependent measures before and after isometric resistance training. Using a HUMAC NORM dynamometer, seven males (27 ± 7 yr) and six females (22 ± 3 yr) underwent 8 wk of maximal isometric dorsiflexion training 3 days/wk. One leg was trained at 0° [short-muscle tendon unit (MTU) length] and the other at 40° of plantar flexion (long-MTU length). RTD and rate of neuromuscular activation were evaluated during "fast" maximal isometric contractions. Power, RVD, and rate of neuromuscular activation were assessed during maximal isotonic contractions in four conditions [small (40°-30° of plantar flexion) ROM at 10% and 50% MVC; large (40°-0° of plantar flexion) ROM at 10% and 50% MVC] for both legs, pre- and posttraining. Despite no change in rate of neuromuscular activation following training, peak power, RTD, and RVD increased at both MTU lengths (P < 0.05). Strong relationships (R2 = 0.73) were observed between RTD and peak power in the small ROM, indicating that fast time-dependent measures are critical for optimal performance when ROM is constrained. Meanwhile, strong relationships (R2 = 0.90) between RVD and power were observed at the 50% load, indicating that RVD is critical when limited by load and ROM is not confined. Maximal isometric dorsiflexion training can be used to improve time-dependent measures (RTD, RVD) to minimize power attenuation when ROM is restricted.NEW & NOTEWORTHY Power output was greater in the unrestricted than restricted ROM, and there were strong relationships between rate of torque development (RTD) and velocity development (RVD) with peak power. RTD and RVD had the strongest relationships with power when ROM was restricted and unrestricted, respectively. Following 8 wk of isometric training, discrepancies in power between restricted and unrestricted ROM were reduced. Increasing RTD through isometric training increased power in dynamic movements, especially when ROM was restricted.

Neuroprotective effects of exercise on the aging human neuromuscular system.

Human biological aging from maturity to senescence is associated with a gradual loss of muscle mass and neuromuscular function. It is not until very old age (>80 years) however, that these changes often manifest into functional impairments. A driving factor underlying the age-related loss of muscle mass and function is the reduction in the number and quality of motor units (MUs). A MU consists of a single motoneuron, located either in the spinal cord or the brain stem, and all of the muscle fibres it innervates via its peripheral axon. Throughout the adult lifespan, MUs are slowly, but progressively lost. The compensatory process of collateral reinnervation attempts to recapture orphaned muscle fibres following the death of a motoneuron. Whereas this process helps mitigate loss of muscle mass during the latter decades of adult aging, the neuromuscular system has fewer and larger MUs, which have lower quality connections between the axon terminal and innervated muscle fibres. Whether this process of MU death and degradation can be attenuated with habitual physical activity has been a challenging question of great interest. This review focuses on age-related alterations of the human neuromuscular system, with an emphasis on the MU, and presents findings on the potential protective effects of lifelong physical activity. Although there is some discrepancy across studies of masters athletes, if one considers all experimental limitations as well as the available literature in animals, there is compelling evidence of a protective effect of chronic physical training on human MUs. Our tenet is that high-levels of physical activity can mitigate the natural trajectory of loss of quantity and quality of MUs in old age.

The torque-frequency relationship is impaired similarly following two bouts of eccentric exercise: No evidence of a protective repeated bout effect.

High-intensity eccentric exercise can lead to muscle damage and weakness. The 'repeated bout effect' (RBE) can attenuate these impairments when performing a subsequent bout. The influence of eccentric exercise-induced muscle damage on low-frequency force production is well-characterized; however, it is unclear how eccentric exercise and the RBE affect torque production across a range of stimulation frequencies (i.e., the torque-frequency relationship). We investigated the influence of an initial (Bout 1) and repeated bout (Bout 2) of eccentric exercise on the elbow flexor torque-frequency relationship. Eleven males completed two bouts of high-intensity eccentric elbow flexions, 4 weeks apart. Torque-frequency relationships were constructed at baseline and 0.5, 24, 48, 72, 96, and 168 h following both bouts via percutaneous stimulation at 1, 6, 10, 20, 30, 40, 50, and 100 Hz. Serum creatine kinase activity, self-reported muscle soreness, and isometric maximum voluntary contraction torque indirectly inferred the presence of muscle damage following Bout 1, and attenuation of muscle damage following Bout 2. Torque amplitude at all stimulation frequencies was impaired 30 min following eccentric exercise, however, torque at lower (1-10 Hz) and higher frequencies (40-100 Hz) recovered within 24 h while torque across the middle frequency range (20-30 Hz) recovered by 48 h. No between-bout differences were detected in absolute or normalized torque at any stimulation frequency, indicating no protective RBE on the elbow flexor torque-frequency relationship.

Hypoxia and standing balance.

PURPOSE: Standing balance control is important for everyday function and often goes unnoticed until impairments appear. Presently, more than 200 million people live at altitudes > 2500 m above sea level, and many others work at or travel to these elevations. Thus, it is important to understand how hypoxia alters balance owing to implications for occupations and travelers. Herein, the influence of normobaric and hypobaric hypoxia on standing balance control is reviewed and summarized. As postural control relies on the integration of sensorimotor signals, the potential hypoxic-sensitive neurophysiological factors that contribute to balance impairments are also reviewed. Specifically, we examine how hypoxia impairs visual, vestibular, and proprioceptive cues, and their integration within subcortical or cortical areas. METHODS: This systematic review included a literature search conducted via multiple databases with keywords related to postural balance, hypoxia, and altitude. Articles (n = 13) were included if they met distinct criteria. RESULTS: Compared to normoxia, normobaric hypoxia worsened parameters of standing balance by 2-10% and up to 83 and 240% in hypobaric hypoxia (high-altitude and lab-based, respectively). Although balance was only disrupted during normobaric hypoxia at FIO2 < ~ 0.15, impairments consistently occurred during hypobaric hypoxia at altitudes > 1524 m (~ FIO2 < 0.18). CONCLUSION: Hypoxia, especially hypobaric, impairs standing balance. The mechanisms underpinning postural decrements likely involve alterations to processing and integration of sensorimotor signals within subcortical or cortical structures involving visual, vestibular, and proprioceptive pathways and subsequent motor commands that direct postural adjustments. Future studies are required to determine the sensorimotor factors that may influence balance control in hypoxia.