Cerebellar Transcranial Direct Current Stimulation Enhances Motor Learning in a Complex Overhand Throwing Task.

Cerebellar transcranial direct current stimulation (c-tDCS) enhances motor adaptation, skill acquisition, and learning in relatively simple motor tasks. The purpose was to examine the influence of c-tDCS on motor learning in a complex overhand throwing task. Forty-two young adults were randomized to a c-tDCS group or a SHAM group and completed a practice session and a retention session. The practice session involved an overhand throwing task to a small target (6 m away) in a pre-test block, 6 practice blocks, a post-test block, and a retention-test block (24 h later). c-tDCS or SHAM was applied during overhand throwing in the practice blocks. The decline in endpoint error was greater for the tDCS group compared to SHAM at the end of practice (P = 0.019) and at retention (P = 0.003). The findings indicate that a single application of c-tDCS enhances motor learning in a complex overhand throwing task.

Withdrawal reflexes in the upper limb adapt to arm posture and stimulus location.

INTRODUCTION: Withdrawal reflexes in the leg adapt in a context-appropriate manner to remove the limb from noxious stimuli, but the extent to which withdrawal reflexes adapt in the arm remains unknown. METHODS: We examined the adaptability of withdrawal reflexes in response to nociceptive stimuli applied in different arm postures and to different digits. Reflexes were elicited at rest, and kinetic and electromyographic responses were recorded under isometric conditions, thereby allowing motorneuron pool excitability to be controlled. RESULTS: Endpoint force changed from a posterior-lateral direction in a flexed posture to predominantly a posterior direction in a more extended posture [change in force angle (mean ± standard deviation) 35.6 ± 5.0°], and the force direction changed similarly with digit I stimulation compared with digit V (change = 22.9 ± 2.9°). CONCLUSIONS: The withdrawal reflex in the human upper limb adapts in a functionally relevant manner when elicited at rest.

Anodal M1 tDCS enhances online learning of rhythmic timing videogame skill.

Transcranial direct current stimulation (tDCS) has been shown to modify excitability of the primary motor cortex (M1) and influence online motor learning. However, research on the effects of tDCS on motor learning has focused predominantly on simplified motor tasks. The purpose of the present study was to investigate whether anodal stimulation of M1 over a single session of practice influences online learning of a relatively complex rhythmic timing video game. Fifty-eight healthy young adults were randomized to either a-tDCS or SHAM conditions and performed 2 familiarization blocks, a 20-minute 5 block practice period while receiving their assigned stimulation, and a post-test block with their non-dominant hand. To assess performance, a performance index was calculated that incorporated timing accuracy elements and incorrect key inputs. The results showed that M1 a-tDCS enhanced the learning of the video game based skill more than SHAM stimulation during practice, as well as overall learning at the post-test. These results provide evidence that M1 a-tDCS can enhance acquisition of skills where quality or success of performance depends on optimized timing between component motions of the skill, which could have implications for the application of tDCS in many real-world contexts.

Independent segmental inhibitory modulation of synaptic efficacy of the soleus H-reflex.

Synaptic efficacy associated with muscle spindle feedback is partly regulated via depression at the Ia-motorneuron synapse through paired reflex depression (PRD) and presynaptic inhibition (PI). The purpose of this study was to examine PRD and PI of the soleus H-reflex at rest and with a background voluntary muscle contraction. The experiment was conducted on 10 healthy males with no history of neurological deficits. Soleus H-reflex and M-wave curves were elicited in three conditions: unconditioned, PRD (two consecutive H-reflexes with 100 ms interval), and PI (1.2 × MT to tibialis anterior 100 ms prior to soleus H-reflex). Each condition was tested at rest and with a 10% soleus contraction. PRD and PI both produced a pronounced inhibition to the soleus motor pool at rest, with a significant difference observed between threshold values (78.9, 89.3, and 90.4% for unconditioned, PRD, and PI reflexes, respectively). During the voluntary contraction the threshold for both inhibitory mechanisms was significantly reduced, and were not different from the unconditioned H-reflex (74.5, 78.9, and 77.0% for unconditioned, PRD, and PI reflexes, respectively). The slope of PI and the PI Hmax/Mmax ratio were significantly altered during contraction whereas no differences were observed for PRD. The results suggest these inhibitory mechanisms depend on the interaction between background voluntary activation and stimulus intensity. This behavior of these inhibitory mechanisms underscores the specificity of spinal circuitry in the control of motor behaviors.

The Influence of Transcranial Alternating Current Stimulation on Fatigue Resistance.

Previous research has shown that some forms of non-invasive brain stimulation can increase fatigue resistance. The purpose of this study is to determine the influence of transcranial alternating current stimulation (tACS) on the time to task failure (TTF) of a precision grip task. The study utilized a randomized, double-blind, SHAM-controlled, within-subjects design. Twenty-six young adults completed two experimental sessions (tACS and SHAM) with a 7-day washout period between sessions. Each session involved a fatiguing isometric contraction of the right hand with a precision grip with either a tACS or SHAM stimulation applied to the primary motor cortex (M1) simultaneously. For the fatiguing contraction, the participants matched an isometric target force of 20% of the maximum voluntary contraction (MVC) force until task failure. Pre- and post-MVCs were performed to quantify the force decline due to fatigue. Accordingly, the dependent variables were the TTF and MVC force decline as well as the average EMG activity, force error, and standard deviation (SD) of force during the fatiguing contractions. The results indicate that there were no significant differences in any of the dependent variables between the tACS and SHAM conditions (p value range: 0.256-0.820). These findings suggest that tACS does not increase the TTF during fatiguing contractions in young adults.

Transcranial Direct Current Stimulation of Primary Motor Cortex over Multiple Days Improves Motor Learning of a Complex Overhand Throwing Task.

Transcranial direct current stimulation (tDCS) applied to the primary motor cortex (M1) improves motor learning in relatively simple motor tasks performed with the hand and arm. However, it is unknown if tDCS can improve motor learning in complex motor tasks involving whole-body coordination with significant endpoint accuracy requirements. The primary purpose was to determine the influence of tDCS on motor learning over multiple days in a complex over-hand throwing task. This study utilized a double-blind, randomized, SHAM-controlled, between-subjects experimental design. Forty-six young adults were allocated to either a tDCS group or a SHAM group and completed three experimental sessions on three consecutive days at the same time of day. Each experimental session was identical and consisted of overhand throwing trials to a target in a pre-test block, five practice blocks performed simultaneously with 20 min of tDCS, and a post-test block. Overhand throwing performance was quantified as the endpoint error. Transcranial magnetic stimulation was used to obtain motor-evoked potentials (MEPs) from the first dorsal interosseus muscle to quantify changes in M1 excitability due to tDCS. Endpoint error significantly decreased over the three days of practice in the tDCS group but not in the SHAM group. MEP amplitude significantly increased in the tDCS group, but the MEP increases were not associated with increases in motor learning. These findings indicate that tDCS applied over multiple days can improve motor learning in a complex motor tasks in healthy young adults.

The Influence of Different Inter-Trial Intervals on the Quantification of Intracortical Facilitation in the Primary Motor Cortex.

Intracortical facilitation (ICF) is a paired-pulse transcranial magnetic stimulation (TMS) measurement used to quantify interneuron activity in the primary motor cortex (M1) in healthy populations and motor disorders. Due to the prevalence of the technique, most of the stimulation parameters to optimize ICF quantification have been established. However, the underappreciated methodological issue of the time between ICF trials (inter-trial interval; ITI) has been unstandardized, and different ITIs have never been compared in a paired-pulse TMS study. This is important because single-pulse TMS studies have found motor evoked potential (MEP) amplitude reductions over time during TMS trial blocks for short, but not long ITIs. The primary purpose was to determine the influence of different ITIs on the measurement of ICF. Twenty adults completed one experimental session that involved 4 separate ICF trial blocks with each utilizing a different ITI (4, 6, 8, and 10 s). Two-way ANOVAs indicated no significant ITI main effects for test MEP amplitudes, condition-test MEP amplitudes, and therefore ICF. Accordingly, all ITIs studied provided nearly identical ICF values when averaged over entire trial blocks. Therefore, it is recommended that ITIs of 4-6 s be utilized for ICF quantification to optimize participant comfort and experiment time efficiency.

Temporal depression of the soleus H-reflex during passive stretch.

Synaptic efficacy associated with muscle spindle feedback is regulated via depression at the Ia-motoneurone synapse. The inhibitory effects of repetitive Ia afferent discharge on target motoneurones of different sizes were investigated during a passive stretch of ankle extensors in humans. H-reflex recruitment curves were collected from the soleus muscle for two conditions in ten subjects. H-reflexes were elicited during passive stretch at latencies of 50, 100, 300, and 500 ms after a slow (20°/s) dorsiflexion about the right ankle (from 100 to 90°). Control H-reflexes were recorded at corresponding static (without movement) ankle angles of 99, 98, 94, and 90° of flexion. The slope of the H-reflex recruitment curves (Hslp) was then calculated for both conditions. H-reflex values were similar for the static and passive stretch conditions prior to 50-100 ms, not showing the early facilitation typical of increased muscle spindle discharge rates. However, the H-reflex was significantly depressed by 300 ms and persisted through 500 ms. Furthermore, less than 300 ms into the stretch, there was significantly greater H-reflex depression with a lower stimulus intensity (20 % Mmax) versus a higher stimulus intensity (Hmax), though the effects begin to converge at later latencies (>300 ms). This suggests there is a distinct two-stage temporal process in the depression observed in the Ia afferent pathway for all motoneurones during a passive stretch. Additionally, there is not a single mechanism responsible for the depression, but rather both heterosynaptic presynaptic inhibition and homosynaptic post-activation depression are independently influencing the Ia-motoneurone pathway temporally during movement.

Modulation of Motor Evoked Potentials via the Cutaneous Silent Period within Proximal-Distal Muscles in the Upper-Limb.

A single pulse of high intensity electrical current delivered to the digits of the hand during voluntary contractions produces a period of decreased electromyographic (EMG) activity, known as a cutaneous silent period (CSP) (Caccia and Violini, 1973; Inghilleri et al., 1997; Uncini et al., 1991). Pairing transcranial magnetic stimulation (TMS) with digit stimulation results in motor evoked potentials (MEPs) with reduced amplitudes in a thenar muscle (Kofler, 2008). It is not known if similar behavior can be observed in more proximal upper-limb muscles. The current study investigated the CSP on several muscles throughout the upper-limb. Fourteen subjects performed isometric contractions with the following muscles: abductor pollicis brevis (APB), flexor carpi radialis (FCR), extensor carpi radialis (ECR), biceps brachii (BIC), triceps brachii (TRI), anterior deltoid (AD), and posterior deltoid (PD). During the isometric contractions, subjects experienced three different stimulation conditions: electrical stimulation (10x perceptual threshold) of digit II only (CSP), transcranial magnetic stimulation only (TMS), and a pairing of digit II stimulation and TMS (TMS+). The TMS evoked MEP was significantly greater than the TMS+ MEP for APB (p < 0.001), FCR (p = 0.006), and BIC (p < 0.049) muscles. The opposite relationship was seen within the PD (p < 0.047) muscle. An ANOVA test of normalized MEP values (TMS+/TMS) showed significant differences in APB vs TRI (p = 0.004) and PD (p = 0.003), and in FCR vs TRI (p = 0.046) and PD (p = 0.037) muscles. The results suggest that the CSP modulates descending drive differentially across upper-limb muscles.

Kinesiophobia Predicts Physical Function and Physical Activity Levels in Chronic Pain-Free Older Adults.

Advanced aging is associated with a general decline in physical function and physical activity. The current evidence suggests that pain-related fear of movement (i.e., kinesiophobia) is increased in the general older adult population and impacts physical activity levels in patients with chronic pain. However, whether kinesiophobia could impact physical activity and function in relatively healthy, chronic pain-free older adults remain unclear. Thus, the purpose of this study was to examine whether fear of movement due to pain predicted self-reported and objective levels of physical function and physical activity in healthy older adults without chronic pain. Fifty-two older adults were enrolled in this study. The participants completed the International Physical Activity Questionnaire (IPAQ) and wore an accelerometer on the hip for 7 days to measure physical activity. Measures of sedentary time, light physical activity, and moderate to vigorous physical activity were obtained from the accelerometer. Measures of physical function included the Physical Functioning subscale of the Short Form-36, Short Physical Performance Battery (SPPB), the 30-s Chair Stand test, and a maximal isometric hand-grip. The Tampa Scale of Kinesiophobia (TSK) was used to measure fear of movement or re-injury associated with pain. Potential covariates included self-reported activity-related pain and demographics. Hierarchical linear regressions were conducted to determine the relationship of kinesiophobia with levels of physical activity and physical function while controlling for activity-related pain and demographics. TSK scores did not predict self-reported physical activity on the IPAQ. However, TSK scores predicted self-reported physical function (Beta = -0.291, p = 0.015), 30-s Chair Stand test scores (Beta = -0.447, p = 0.001), measures from the SPPB (Gait speed time: Beta = 0.486, p < 0.001; Chair stand time: Beta = 0.423, p = 0.003), percentage of time spent in sedentary time (Beta = 0.420, p = 0.002) and light physical activity (Beta = -0.350, p = 0.008), and moderate to vigorous physical activity (Beta = -0.271, p = 0.044), even after controlling for significant covariates. These results suggest that greater pain-related fear of movement/re-injury is associated with lower levels of light and moderate to vigorous physical activity, greater sedentary behavior, and worse physical function in healthy, chronic pain-free older adults. These findings elucidate the potential negative impact of kinesiophobia in older adults who don't report chronic pain.

The Influence of Transcranial Direct Current Stimulation on Shooting Performance in Elite Deaflympic Athletes: A Case Series.

Transcranial direct current stimulation (tDCS) has been shown to improve motor learning in numerous studies. However, only a few of these studies have been conducted on elite-level performers or in complex motor tasks that have been practiced extensively. The purpose was to determine the influence of tDCS applied to the dorsolateral prefrontal cortex (DLPFC) on motor learning over multiple days on 10-m air rifle shooting performance in elite Deaflympic athletes. Two male and two female elite Deaflympic athletes (World, European, and National medalists) participated in this case series. The study utilized a randomized, double-blind, SHAM-controlled, cross-over design. Anodal tDCS or SHAM stimulation was applied to the left DLPFC for 25 min with a current strength of 2 mA concurrent with three days of standard shooting practice sessions. Shooting performance was quantified as the points and the endpoint error. Separate 2 Condition (DLPFC-tDCS, SHAM) × 3 Day (1,2,3) within-subjects ANOVAs revealed no significant main effects or interactions for either points or endpoint error. These results indicate that DLPFC-tDCS applied over multiple days does not improve shooting performance in elite athletes. Different stimulation parameters or very long-term (weeks/months) application of tDCS may be needed to improve motor learning in elite athletes.

Neuroenhancement of a dexterous motor task with anodal tDCS.

Motor skill learning can cause structural and functional changes in the primary motor cortex (M1) leading to cortical plasticity that can be associated with the performance change during the motor skill that is practiced. Similarly, anodal transcranial direct current stimulation (a-tDCS) has been shown to facilitate and enhance plasticity in M1, causing even greater motor skill improvement. By using a fine motor task (O'Connor Tweezer Dexterity Task) in combination with a-tDCS we theorized that a-tDCS could increase the speed of skill acquisition. Forty subjects were recruited and randomized into either a-tDCS or SHAM groups. Subjects completed a single session performing the O'Connor Tweezer Dexterity Task with their non-dominant hand while receiving either a-tDCS stimulation or SHAM stimulation of the hand region of M1. The time it took to place 50- pins was assessed before and after 20 min of practice with a-tDCS or SHAM. We found that both groups had similar pre-test performance (P = 0.94) and they both had a similar amount of practice pins placed (P = 0.69). However, the a-tDCS group had a greater improvement than the SHAM group (p = 0.028) for overall learning from pretest to posttest. These results suggest that a-tDCS improved the rate of motor learning and fine motor task performance. These results are in line with previous research and demonstrate that a-tDCS applied to M1 can increase manual precision and steadiness needed for delicate tasks and could have implications in the advancement of surgical training as well as in athletic, military, and other occupational settings.

Subject factors influencing blood flow restriction in the arm at low cuff pressures.

BACKGROUND: Limb circumference predicts the pressure needed for complete occlusion. However, that relationship is inconsistent at moderate pressures typical of effective blood flow restriction (BFR) training. The purpose of this study was to investigate the influence of subject factors on BFR at low restriction pressures in the arm. METHODS: Fifty subjects had arm anthropometrics assessed by peripheral quantitative computed tomography (pQCT), sum of skinfold thickness (sumSKF) and Gulick tape (Gulick tape circumference [Gulick Circ.]) at cuff level. Blood flow (BF) was measured with ultrasound at baseline and five restrictive pressures (20, 30, 40, 50 and 60 mmHg). Relationships between subject characteristics and BFR were assessed using Pearson's correlations and hierarchical regression. RESULTS: BF decreased (p < 0.05) at each incremental pressure. Regression models including percent muscle composition (%Muscle), pQCT circumference and systolic blood pressure (SBP), were significant at all five pressures (R2 = 0.18-0.49). %Muscle explained the most variance at each pressure. Regression models including sumSKF, Gulick Circ. and SBP, were significant at 30-60 mmHg (R2 = 0.28-0.49). SumSKF explained the most variance at each pressure. CONCLUSIONS: At low pressures (20-60 mmHg), there is considerable variability in the magnitude of BFR across individuals. Arm composition factors (muscle and fat) explained the greatest variance at each cuff pressure and may be the most important consideration when using BFR protocols.

Cortical Representation and Excitability Increases for a Thenar Muscle Mediate Improvement in Short-Term Cellular Phone Text Messaging Ability.

Cortical representations expand during skilled motor learning. We studied a unique model of motor learning with cellular phone texting, where the thumbs are used exclusively to interact with the device and the prominence of use can be seen where 3200 text messages are exchanged a month in the 18-24 age demographic. The purpose of the present study was to examine the motor cortex representation and input-output (IO) recruitment curves of the abductor pollicis brevis (APB) muscle of the thumb and the ADM muscle with transcranial magnetic stimulation (TMS), relative to individuals' texting abilities and short-term texting practice. Eighteen individuals performed a functional texting task (FTT) where we scored their texting speed and accuracy. TMS was then used to examine the cortical volumes and areas of activity in the two muscles and IO curves were constructed to measure excitability. Subjects also performed a 10-min practice texting task, after which we repeated the cortical measures. There were no associations between the cortical measures and the FTT scores before practice. However, after practice the APB cortical map expanded and excitability increased, whereas the ADM map constricted. The increase in the active cortical areas in APB correlated with the improvement in the FTT score. Based on the homogenous group of subjects that were already good at texting, we conclude that the cortical representations and excitability for the thumb muscle were already enlarged and more receptive to changes with short-term practice, as noted by the increase in FTT performance after 10-min of practice.

Anodal tDCS accelerates on-line learning of dart throwing.

Transcranial direct current stimulation (tDCS) has been shown to enhance or block online learning of motor skills, depending on the current direction. However, most research on the use of tDCS has been limited to the study of relatively simple motor tasks. The purpose of the present study was to examine the influence of anodal (a-tDCS) and cathodal (c-tDCS) direct current stimulation on the online learning during a single session of dart throwing. Fifty-eight young adults were randomized to a-tDCS, c-tDCS, or SHAM groups and completed a pre-test block of dart throws, a 20-minute practice block of throws while receiving their stimulation condition, and a post-test block of dart throws. The results showed that a-tDCS accelerated the skill learning of dart throwing more than SHAM and c-tDCS conditions. The SHAM and c-tDCS conditions were not different. We conclude that a-tDCS may have a positive effect in a single training session which would be ideal in a recreational game environment where repeated practice is not common.

An acute application of transcranial random noise stimulation does not enhance motor skill acquisition or retention in a golf putting task.

Transcranial random noise stimulation (tRNS) is a brain stimulation technique that has been shown to increase motor performance in simple motor tasks. The purpose was to determine the influence of tRNS on motor skill acquisition and retention in a complex golf putting task. Thirty-four young adults were randomly assigned to a tRNS group or a SHAM stimulation group. Each subject completed a practice session followed by a retention session. In the practice session, subjects performed golf putting trials in a baseline test block, four practice blocks, and a post test block. Twenty-four hours later subjects completed the retention test block. The golf putting task involved performing putts to a small target located 3 m away. tRNS or SHAM was applied during the practice blocks concurrently with the golf putting task. tRNS was applied over the first dorsal interosseus muscle representation area of the motor cortex for 20 min at a current strength of 2 mA. Endpoint error and endpoint variance were reduced across the both the practice blocks and the test blocks, but these reductions were not different between groups. These findings suggest that an acute application of tRNS failed to enhance skill acquisition or retention in a golf putting task.

Motor unit recruitment in human biceps brachii during sustained voluntary contractions.

The purpose of the study was to examine the influence of the difference between the recruitment threshold of a motor unit and the target force of the sustained contraction on the discharge of the motor unit at recruitment. The discharge characteristics of 53 motor units in biceps brachii were recorded after being recruited during a sustained contraction. Some motor units (n = 22) discharged action potentials tonically after being recruited, whereas others (n = 31) discharged intermittent trains of action potentials. The two groups of motor units were distinguished by the difference between the recruitment threshold of the motor unit and the target force for the sustained contraction: tonic, 5.9 +/- 2.5%; intermittent, 10.7 +/- 2.9%. Discharge rate for the tonic units decreased progressively (13.9 +/- 2.7 to 11.7 +/- 2.6 pulses s(-1); P = 0.04) during the 99 +/- 111 s contraction. Train rate, train duration and average discharge rate for the intermittent motor units did not change across 211 +/- 153 s of intermittent discharge. The initial discharge rate at recruitment during the sustained contraction was lower for the intermittent motor units (11.0 +/- 3.3 pulses s(-1)) than the tonic motor units (13.7 +/- 3.3 pulses s(-1); P = 0.005), and the coefficient of variation for interspike interval was higher for the intermittent motor units (34.6 +/- 12.3%) than the tonic motor units (21.2 +/- 9.4%) at recruitment (P = 0.001) and remained elevated for discharge duration (34.6 +/- 9.2% versus 19.1 +/- 11.7%, P < 0.001). In an additional experiment, 12 motor units were recorded at two different target forces below recruitment threshold (5.7 +/- 1.9% and 10.5 +/- 2.4%). Each motor unit exhibited the two discharge patterns (tonic and intermittent) as observed for the 53 motor units. The results suggest that newly recruited motor units with recruitment thresholds closer to the target force experienced less synaptic noise at the time of recruitment that resulted in them discharging action potentials at more regular and greater rates than motor units with recruitment thresholds further from the target force.

A spinal pathway between synergists can modulate activity in human elbow flexor muscles.

Electrical stimulation of the brachioradialis branch of the radial nerve has been shown to inhibit the discharge of voluntarily activated motor units in biceps brachii during weak contractions with the elbow flexor muscles. The purpose of the present study was to characterise the inhibitory reflex by comparing its strength in the short and long heads of the biceps brachii and examining the influence of forearm position on the strength of the reflex. Spike-triggered stimulation was used to assess the influence of radial nerve stimulation on the discharge of single motor units in the biceps brachii of 15 subjects. Stimulation of the radial nerve prolonged the interspike interval (P < 0.001) of motor units in the long (n = 31, 4.8 +/- 5.6 ms) and short heads (n = 26, 8.1 +/- 12.3 ms) of biceps brachii with no difference between the two heads (P = 0.11). The strength of inhibition varied with forearm position for motor units in both heads (n = 18, P < 0.05). The amount of inhibition was greatest in pronation (7.9 +/- 8.9 ms), intermediate in neutral (5.8 +/- 7.1 ms), and least in supination (2.8 +/- 3.4 ms). These findings indicate that the inhibition evoked by afferent feedback from brachioradialis to low-threshold motor units (mean force 3-5% MVC) in biceps brachii varied with forearm posture yet was similar for the two heads of biceps brachii. This reflex pathway provides a mechanism to adjust the activation of biceps brachii with changes in forearm position, and represents a spinal basis for a muscle synergy in humans.

Differential processing of nociceptive input within upper limb muscles.

The cutaneous silent period is an inhibitory evoked response that demonstrates a wide variety of responses in muscles of the human upper limb. Classically, the cutaneous silent period results in a characteristic muscle pattern of extensor inhibition and flexor facilitation within the upper limb, in the presence of nociceptive input. The aims of the current study were: 1) to primarily investigate the presence and characteristics of the cutaneous silent period response across multiple extensor and flexor muscles of the upper limb, and 2) to secondarily investigate the influence of stimulation site on this nociceptive reflex response. It was hypothesized that the cutaneous silent period would be present in all muscles, regardless of role (flexion/extension) or the stimulation site. Twenty-two healthy, university-age adults (14 males; 8 females; 23 ± 5 yrs) participated in the study. Testing consisted of three different stimulation sites (Digit II, V, and II+III nociceptive stimulation) during a low intensity, sustained muscle contraction, in which, 7 upper limb muscles were monitored via surface EMG recording electrodes. Distal muscles of the upper limb presented with the earliest reflex onset times, longest reflex duration, and lowest level of EMG suppression when compared to the more proximal muscles, regardless of extensor/flexor role. Additionally, the greatest overall inhibitory influence was expressed within the distal muscles. In conclusion, the present study provides a new level of refinement within the current understanding of the spinal organization associated with nociceptive input processing and the associated motor control of the upper limb. Subsequently, these results have further implications on the impact of nociception on supraspinal processing.

Reflex responsiveness of a human hand muscle when controlling isometric force and joint position.

OBJECTIVE: This study compared reflex responsiveness of the first dorsal interosseus muscle during two tasks that employ different strategies to stabilize the finger while exerting the same net muscle torque. METHODS: Healthy human subjects performed two motor tasks that involved either pushing up against a rigid restraint to exert a constant isometric force equal to 20% of maximum or maintaining a constant angle at the metacarpophalangeal joint while supporting an equivalent inertial load. Each task consisted of six 40-s contractions during which electrical and mechanical stimuli were delivered. RESULTS: The amplitude of short and long latency reflex responses to mechanical stretch did not differ significantly between tasks. In contrast, reflexes evoked by electrical stimulation were significantly greater when supporting the inertial load. CONCLUSIONS: Agonist motor neurons exhibited heightened reflex responsiveness to synaptic input from heteronymous afferents when controlling the position of an inertial load. Task differences in the reflex response to electrical stimulation were not reflected in the response to mechanical perturbation, indicating a difference in the efficacy of the pathways that mediate these effects. SIGNIFICANCE: Results from this study suggest that modulation of spinal reflex pathways may contribute to differences in the control of force and position during isometric contractions of the first dorsal interosseus muscle.