Rapid Arm Movements Research Articles

Age-related changes in protective arm reaction kinematics, kinetics, and neuromuscular activation during evoked forward falls.

Fall related injuries in older adults are a major healthcare concern. During a fall, the hands and arms play an important role in minimizing trauma from ground impact. Although older adults are able to orient the hands and arms into a protective orientation after falling and prior to ground impact, an inability to avoid increased body impact occurs with age. Previous investigations have generally studied rapid arm movements in the pre-impact phase or absorbing energy in the post-impact phase. There are no known studies that have directly examined both the pre-impact and post-impact phase in sequence in a forward fall. The aim of this study was to identify age-related biomechanical and neuromuscular changes in evoked arm reactions in response to forward falls that may increase fall injury risk. Fourteen younger and 15 older adults participated. Falls were simulated while standing with torso and legs restrained via a moving pendulum system from 4 different initial lean angles. While there was not a significant age-related difference in the amount of energy absorbed post-impact (p=0.68), older adults exhibited an 11% smaller maximum vertical ground reaction force when normalized to body weight (p=0.031), and 8 degrees less elbow extension at impact (p=0.045). A significant interaction between age and initial lean angle (p=0.024), indicated that older adults required 54%, 54%, 41%, and 57% greater elbow angular displacement after impact at the low, medium, medium-high, and high initial lean angles compared to younger adults. These results suggested older adults may be at greater risk of increased body impact due to increased elbow flexion angular displacement after impact when the hands and arms are able to contact the ground first. Both groups exhibited robust modulation to the initial lean angle with no observed age-related differences in the initial onset timing or amplitude of muscle activation levels. There were no significant age-related differences in the EMG timing, amplitude or co-activation of muscle activation preceding impact or following impact indicating comparable neuromotor response patterns between older and younger adults. These results suggest that aging changes in muscular elements may be more implicated in the observed differences than changes in neuromuscular capacity. Future work is needed to test the efficacy of different modalities (e.g. instruction, strength, power, perturbation training, fall landing techniques) aimed at reducing fall injury risk.

Obstacle Avoidance and Dual-Tasking During Reaching While Standing in Patients With Mild Chronic Stroke

Background. Poststroke individuals use their paretic arms less often than expected in daily life situations, even when motor recovery is scored highly in clinical tests. Real-world environments are often unpredictable and require the ability to multitask and make decisions about rapid and accurate arm movement adjustments. Objective. To identify whether and to what extent cognitive–motor deficits in well-recovered individuals with stroke affect the ability to rapidly adapt reaching movements in changing cognitive and environmental conditions. Methods. Thirteen individuals with mild stroke and 11 healthy controls performed an obstacle avoidance task in a virtual environment while standing. Subjects reached for a virtual juice bottle with their hemiparetic arm as quickly as possible under single- and dual-task conditions. In the single-task condition, a sliding glass door partially obstructed the reaching path of the paretic arm. A successful trial was counted when the subject touched the bottle without the hand colliding with the door. In the dual-task condition, subjects repeated the same task while performing an auditory–verbal working memory task. Results. Individuals with stroke had significantly lower success rates than controls in avoiding the moving door in single-task (stroke: 51.8 ± 21.2%, control: 70.6 ± 12.7%; P = .018) and dual-task conditions (stroke: 40.0 ± 27.6%, control: 65.3 ± 20.0%; P = .015). Endpoint speed was lower in stroke subjects for successful trials in both conditions. Obstacle avoidance deficits were exacerbated by increased cognitive demands in both groups. Individuals reporting greater confidence using their hemiparetic arm had higher success rates. Conclusion. Clinically well-recovered individuals with stroke may have persistent deficits performing a complex reaching task.

Long-Term Application of Cerebellar Transcranial Direct Current Stimulation Does Not Improve Motor Learning in Parkinson's Disease.

Cerebellar transcranial direct current stimulation (c-tDCS) enhances motor skill acquisition and motor learning in young and old adults. Since the cerebellum is involved in the pathophysiology of Parkinson’s disease (PD), c-tDCS may represent an intervention with potential to improve motor learning in PD. The primary purpose was to determine the influence of long-term application of c-tDCS on motor learning in PD. The secondary purpose was to examine the influence of long-term application of c-tDCS on transfer of motor learning in PD. The study was a randomized, double-blind, SHAM-controlled, between-subjects design. Twenty-one participants with PD were allocated to either a tDCS group or a SHAM stimulation group. Participants completed 9 practice sessions over a 2-week period that involved extensive practice of an isometric pinch grip task (PGT) and a rapid arm movement task (AMT). These practice tasks were performed over a 25-min period concurrent with either anodal c-tDCS or SHAM stimulation. A set of transfer tasks that included clinical rating scales, manual dexterity tests, and lower extremity assessments were quantified in Test sessions at Baseline, 1, 14, and 28 days after the end of practice (EOP). There were no significant differences between the c-tDCS and SHAM groups as indicated by performance changes in the practice and transfer tasks from Baseline to the 3 EOP Tests. The findings indicate that long-term application of c-tDCS does not improve motor learning or transfer of motor learning to a greater extent than practice alone in PD.

Intermittent short-arm centrifugation is a partially effective countermeasure against upright balance deterioration following 60-day head-down tilt bed rest.

This study investigated whether artificial gravity (AG), induced by short-radius centrifugation, mitigated deterioration in standing balance and anticipatory postural adjustments (APAs) of trunk muscles following 60-day head-down tilt bed rest. Twenty-four participants were allocated to one of three groups: control group (n = 8); 30-min continuous AG daily (n = 8); and intermittent 6 × 5 min AG daily (n = 8). Before and immediately after bed rest, standing balance was assessed in four conditions: eyes open and closed on both stable and foam surfaces. Measures including sway path, root mean square, and peak sway velocity, sway area, sway frequency power, and sway density curve were extracted from the center of pressure displacement. APAs were assessed during rapid arm movements using intramuscular or surface electromyography electrodes of the rectus abdominis; obliquus externus and internus abdominis; transversus abdominis; erector spinae at L1, L2, L3, and L4 vertebral levels; and deep lumbar multifidus muscles. The relative latency between the EMG onset of the deltoid and each of the trunk muscles was calculated. All three groups had poorer balance performance in most of the parameters (all P < 0.05) and delayed APAs of the trunk muscles following bed rest (all P < 0.05). Sway path and sway velocity were deteriorated, and sway frequency power was less in those who received intermittent AG than in the control group (all P < 0.05), particularly in conditions with reduced proprioceptive feedback. These data highlight the potential of intermittent AG to mitigate deterioration of some aspects of postural control induced by gravitational unloading, but no protective effects on trunk muscle responses were observed.NEW & NOTEWORTHY This study presents novel insights into the effect of artificial gravity (AG) on the deterioration of standing balance and anticipatory postural adjustments (APAs) of trunk muscles induced by 60-day strict head-down bed rest. The results indicated severe balance dysfunction and delayed APAs during rapid arm movement. AG partially mitigated the deterioration in standing balance and may thus be considered as a potential countermeasure for future planetary surface explorations. Optimization of AG protocols might enhance effects.

Postural adjustments impairments in elderly people with chronic low back pain

Chronic low back pain (CLBP) is associated with postural control impairments and is highly prevalent in elderly people. The objective of this study is to verify whether anticipatory postural adjustments (APAs) and compensatory postural adjustments (CPAs) are affected by CLBP in elderly people by assessing their postural control during a self-initiated perturbation paradigm induced by rapid upper arm movement when pointing to a target. The participants’ lower limb muscle onset and center of pressure (COP) displacements were assessed prior to perturbation and throughout the entire movement. T0 moment (i.e., the beginning of the movement) was defined as the anterior deltoid (DEL) onset, and all parameters were calculated with respect to it. The rectus femoris (RT), semitendinosus (ST), and soleous (SOL) showed delayed onset in the CLBP group compared with the control group: RF (control: − 0.094 ± 0.017 s; CLBP: − 0.026 ± 0.012 s, t = 12, p < 0.0001); ST (control: − 0.093 ± 0.013 s; CLBP: − 0.018 ± 0.019 s, t = 12, p < 0.0001); and SOL (control: − 0.086 ± 0.018 s; CLBP: − 0.029 ± 0.015 s, t = 8.98, p < 0.0001). In addition, COP displacement was delayed in the CLBP group (control: − 0.035 ± 0.021 s; CLBP: − 0.015 ± 0.009 s, t = 3; p = 0.003) and presented a smaller amplitude during APA COPAPA [control: 0.444 cm (0.187; 0.648); CLBP: 0.228 cm (0.096; 0.310), U = 53, p = 0.012]. The CLBP group required a longer time to reach the maximum displacement after the perturbation (control: 0.211 ± 0.047 s; CLBP 0.296 ± 0.078 s, t = 3.582, p = 0.0013). This indicates that CLBP elderly patients have impairments to recover their postural control and less efficient anticipatory adjustments. Our results suggest that people with CLBP have altered feedforward hip and ankle muscle control, as shown from the SOL, ST, and RT muscle onset. This study is the first study in the field of aging that investigates the postural adjustments of an elderly population with CLBP. Clinical assessment of this population should consider postural stability as part of a rehabilitation program.

Adaptation of reach action to a novel force-field is not predicted by acuity of dynamic proprioception in either older or younger adults

Healthy ageing involves degeneration of the neuromuscular system which impacts movement control and proprioception. Yet the relationship between these sensory and motor deficits in upper limb reaching has not been examined in detail. Recently, we reported that age-related proprioceptive deficits were unrelated to accuracy in rapid arm movements, but whether this applied in motor tasks more heavily dependent on proprioceptive feedback was not clear. To address this, we have tested groups of younger and older adults on a force-field adaptation task under either full or limited visual feedback conditions and examined how performance was related to dynamic proprioceptive acuity. Adaptive performance was similar between the age groups, regardless of visual feedback condition, although older adults showed increased after-effects. Physically inactive individuals made larger systematic (but not variable) proprioceptive errors, irrespective of age. However, dynamic proprioceptive acuity was unrelated to adaptation and there was no consistent evidence of proprioceptive recalibration with adaptation to the force-field for any group. Finally, in spite of clear age-dependent loss of spatial working memory capacity, we found no relationship between memory capacity and adaptive performance or proprioceptive acuity. Thus, non-clinical levels of deficit in dynamic proprioception, due to age or physical inactivity, do not affect force-field adaptation, even under conditions of limited visual feedback that might require greater proprioceptive control.

An Acute Application of Cerebellar Transcranial Direct Current Stimulation Does Not Improve Motor Performance in Parkinson's Disease.

Transcranial direct current stimulation of the cerebellum (c-tDCS) improves motor performance in young and old adults. Based on the cerebellar involvement in Parkinson’s disease (PD), c-tDCS could have potential to improve motor function in PD. The purpose was to determine the effects of c-tDCS on motor performance in PD while participants were on medications. The study was a randomized, double-blind, SHAM-controlled, between-subjects design. Twenty-two participants with PD were allocated to either a c-tDCS group or a SHAM group. All participants completed one experimental session and performed two motor tasks with their most affected hand in a Baseline condition (no stimulation) and an Experimental condition. The motor tasks were a visuomotor isometric precision grip task (PGT) and a rapid arm movement task (AMT). The primary dependent variables were force error and endpoint error in the PGT and AMT, respectively. There were no significant differences in force error or endpoint error in the Experimental condition between the c-tDCS and SHAM groups. These results indicate that an acute application of c-tDCS does not enhance motor performance in hand and arm tasks in PD. Longer-term c-tDCS application over multiple days may be needed to enhance motor function in PD.

Physical or Cognitive Exertion Does Not Influence Cortical Movement Preparation for Rapid Arm Movements.

The contribution of central factors to movement preparation (e.g.,the contingent negative variation [CNV]) and the influence of fatigue on such factors are still unclear, even though executive cognitive functions are regarded as key elements in motor control. Therefore, this study examined CNV amplitude with electroencephalography in 22 healthy humans during a rapid arm movement task prior to and following three experimental conditions: (a)a no exertion/control condition, (b)a physical exertion, and (c)a cognitive exertion. CNV amplitude was affected neither by a single bout of physical/cognitive exertion nor by the control condition. Furthermore, no time-on-task effects of the rapid arm movement task on the CNV were found. Exertion did not affect cortical movement preparation, which is in contrast to previous findings regarding time-on-task effects of exertion on CNV. Based on the current findings, the rapid arm movement task is deemed suitable to measure cortical movement preparation, without being affected by learning effects and physical/cognitive exertion.

Potential of IMU-Based Systems in Measuring Single Rapid Movement Variables in Females with Different Training Backgrounds and Specialization.

The aim of this paper is to determine the discriminative potential of the IMU-based system for the measurement of rapid hand movement properties, i.e., relevant kinematic variables in relation to different groups of participants. The measurement of the kinematics of the rapid hand movement was performed using a standard hand tapping test. The sample in this research included a total of 70 female participants and was divided into 3 subsamples. The discriminant analysis has identified two functions, DF1 and DF2, that explain 91.1 and 8.1% of the variance, respectively. The differences between the examined subsamples originate from the variables grouped in DF1, which were statistically significant (p ≤ 0.000). In relation to this function, the national volleyball team centroid position was shifted with -1.108 and -1.968 standard deviation values from the control group and youth volleyball team, respectively. The difference between control and Voll_Youth groups was -0.860 standard deviation value. The factors with the greatest discriminative potential among the groups represent the temporal characteristics of the rapid hand movement, i.e., the time elapsed between the onset of the movement and the first and second tap, as defined by the variables t1 and t2, respectively. The established findings clearly indicate that IMU sensors are practically applicable in relation to the sensitive measurement of rapid arm movement capability of female athletes.

Measurement latency significantly contributes to reduced heart rate measurement accuracy in wearable devices

The purpose of this study was to determine the extent to which measurement latency has significance for the accuracy of heart rate (HR) measured by wearable devices during physical activities. Thirty participants (15 female) aged 18–50 years (mean 24.8, 6.3 SD years) with weight ranging from 53–87 kg (mean 70.5, 9.4 SD kg) participated in this cross-sectional study. Participants completed three submaximal exercise conditions; cycling, treadmill walking/running and rapid arm movements. During the exercises, HR was concurrently measured using the Polar H10 (PL), the Apple Watch 2 (AW) and the Garmin Forerunner 235 (GF). Measurement latency significantly affected the discrepancy in HR measures and measurement latency accounted for 15% (GF) and 2% (AW) of the explained discrepancy in HR between the criterion measure PL and the GF and AW. However, the findings also showed that the magnitude of HR measurement latency differed notably among the two wearable devices and measurement latency was greatly influenced not only by type but also by the intensity of exercise. These findings contribute to our understanding of wearable device HR measures discrepancy and this knowledge may be of assistance when electing to use wearable devices to monitor physical exertion in different contexts.

Does experimentally induced pain-related fear influence central and peripheral movement preparation in healthy people and patients with low back pain?

Nonspecific chronic low back pain (CLBP) is a multifactorial disorder. Pain-related fear and altered movement preparation are considered to be key factors in the chronification process. Interactions between both have been hypothesized, but studies examining the influence of situational fear on movement preparation in low back pain (LBP) are wanting, as well as studies differentiating between recurrent LBP (RLBP) and CLBP. Therefore, this study examined whether experimentally induced pain-related fear influences movement preparation. In healthy controls (n = 32), RLBP (n = 31) and CLBP (n = 30) patients central and peripheral measures of movement preparation were assessed by concurrently measuring trunk muscle anticipatory postural adjustments (APA) with electromyography and contingent negative variation with EEG during performance of rapid arm movements. Two conditions were compared, one without (no fear) and one with (fear) possibility of painful stimulation to the back during rapid arm movements. Visual analogue scales were used to assess pain-related expectations/fear in both conditions. The experimentally induced fear of pain during movement performance led to an increase in contingent negative variation amplitude, which was similar in all 3 groups. Concerning APAs, no effects of fear were found, but group differences with generally delayed APAs in CLBP compared with controls and RLBP patients were evident. These results suggest that with fear, an attentional redirection towards more conscious central movement preparation strategies occurs. Furthermore, differences in movement preparation in patients with RLBP and CLBP exist, which could explain why patients with RLBP have more recovery capabilities than patients with CLBP.

Role of Proprioception in Slow and Rapid Movements.

This study aimed to compare the contributions of sources of proprioception to the reproduction accuracy of relatively slower and more rapid arm movements. We recruited 34 volunteers and gave them dart throwing tasks under two different durations followed by joint position sense (JPS) tests and force sense (FS) tests at the elbow and the wrist. We found moderately positive correlations between slow movement performance and proprioceptive acuity with FS (wrist) and JPS (elbow), accounting for 52% of the absolute errors ( p &lt; .001), and, with FS (wrist), accounting for 50% of the variable error ( p &lt; .001). Moreover, we observed a smaller correlation between rapid movement performance and proprioceptive acuity, accounting for 17% of absolute errors with JPS (elbow; p = .008) and 11% of variable error ( p = .033). These results suggest that relatively slow movement performance is partly determined by performers’ proprioceptive acuity of the movement-related limbs. Relatively rapid movement performance is also affected by correctional proprioceptive feedback, though to a lesser degree.

Is postural control during unstable sitting a proxy measure for determinants associated with lumbar stability?

The aim of this study was to test whether determinants associated with lumbar stability can predict performance during unstable sitting (trunk postural control - TPC). If confirmed, unstable sitting could be viewed as a proxy measure for these determinants.Wobbling chair motion was measured in 58 subjects with an inertial sensor, and six outcomes were computed (mean frequency and velocity, frequency dispersion, two variables from the sway density analysis and Lyapunov exponent - short interval) to represent TPC performance. Subjects also performed five other trunk neuromuscular tests to quantify the thickness of back and abdominal muscles and connective tissues, lumbar proprioception, lumbar stiffness, feedforward and feedback control mechanisms, and trunk/muscle coordination.Four to five predictors explained between 36 and 47% of TPC outcomes variance, as determined with multivariate analyses. These predictors were mainly related to (1) angular kinematic parameters of the pelvis or lumbar spine following rapid arm movement, (2) lumbar intrinsic stiffness, (3) thickness of perimuscular connective tissues surrounding specific abdominal muscles, (4) activation onsets of specific trunk muscles (IO/TrA and iliocostalis lumborum) before rapid arm movement, and (5) percent thickness change of internal oblique (IO) and transversus abdominis (TrA) muscles. Lumbar proprioception and reflex responses were not predictive, possibly due to the lack of appropriate measurements.These findings support the use of TPC in unstable sitting as a proxy measure for determinants associated with lumbar stability. This might be useful in research and clinical settings, considering time and equipment constraints associated with measuring these determinants individually.

Physical and cognitive exertion do not influence feedforward activation of the trunk muscles: a randomized crossover trial.

Fatigue arises during everyday activities, diminishes movement performance, and increases injury risk. Physical (PE) and cognitive exertion (CE) can induce similar feelings of fatigue, but it is not clear whether these also similarly affect movement performance. Therefore, this study examined the influence of PE and CE on anticipatory postural adjustments (APAs) of trunk muscles, which are feedforward mechanisms that contribute to motor control and controlled movement. Rapid arm movement tasks (RAM) were used to induce APAs of the trunk muscles prior and following three experimental conditions in 20 healthy adults: seated rest without exertion (NE), a combined isometric modified Biering-Sörensen and static abdominal curl to induce PE, and a modified incongruent Stroop colour-word task to induce CE. Fatigue was assessed using self-reported measures, and APA onset latencies of the trunk muscles with surface electromyography. Statistical analyses revealed that neither PE nor CE influence APAs of the trunk. Therefore, it is hypothesized that the influence of fatigue on movement performance might not be through altered motor control, but rather by reduced motivation. However, the possibility that fatigue might influence other mechanisms which contribute to trunk motor control, such as APA amplitude and variability, cannot be excluded and need further examination.

Deformations of abdominal muscles under experimentally induced low back pain

PurposeLow back pain (LBP) is associated with altered motor control and muscle activity; however, it remains unknown whether these changes predispose humans to injury and pain or are the consequence of ongoing nociceptive processes. In this experimental study, we aimed to use a novel ultrasound imaging technique for the measurement of lateral abdominal wall muscle activity: the tissue deformation index (TDI).MethodsForty-two healthy subjects (22.30 ± 1.49 years of age) were exposed to postural perturbation induced by rapid arm movement. Activity of three muscles, the transversus abdominis (TrA) and the internal and external oblique (EO), was recorded by ultrasound imaging (M-mode) with and without induction of LBP. Pain was induced by electrical stimulation applied bilaterally to the lumbar spine.ResultsNo significant differences in the TDI between right and left sides of the body (p > 0.05) were found. Generally, muscles deformed slower with pain compared to non-painful conditions; however, only EO muscle displayed a statistically significant reduction in deformation velocity (p ≤ 0.00001). TDI for EO decreased from 0.065% per ms ( ± 0.038, 95% CI 0.057–0.074) to 0.053% per ms ( ± 0.035, 95% CI 0.046–0.061). Furthermore, characteristic inter-muscular TDI gradients were observed, directed from inner towards outer muscular layers, with TrA showing the lowest TDI and EO the highest.ConclusionExperimentally induced LBP suppresses deformation of the superficial abdominal muscles and increases the variability of local/deep muscles. Further research is required to confirm these observations.Graphical abstractThese slides can be retrieved under Electronic Supplementary Material.

Accuracy of the wearable activity tracker Garmin Forerunner 235 for the assessment of heart rate during rest and activity

ABSTRACTAccurate measures of heart rate (HR) during rehabilitation and sporting activities are important for precise exercise prescription to maintain or increase capacity. Wrist-worn activity monitors utilizing photoplethysmography technology (PPG) to configure HR show discrepant findings regarding validity depending on the type and intensity of exercises measured, and no previous study has yet investigated the accuracy during running at speeds exceeding 9.6 km/h. The purpose of the study was to assess the accuracy of the Garmin Forerunner 235 (GF), at different exercises at various intensities. Twenty-nine participants participated in the study. HR was measured with the (GF) during rest and three submaximal exercise conditions; cycling, treadmill walking, running and rapid arm movements. The GF had high agreement with the PL during rest (r = 0.997) cycling at 150 W (Rho = 0.889), treadmill running at 8.7 km/h (r = 0.906) and 12.1 km/h (r = 0.845) and rapid arm movements (r = 0.928, r = 0.745) but a low agreement during cycling at 50 W (Rho = 0.269) and 100W (Rho = 0.462) and treadmill walking at 4.8 km/h (r = 0.481). The results varied across exercise conditions and intensities and although the GF provided accurate measurements of HR during rest, cycling at 150W, treadmill running, and rapid arm movement measurement latency may potentially affect application.

Functional behaviour of spinal muscles after training with an exercise device developed to recruit and train postural muscles

This study investigated the effects of a single exercise session using a device developed for postural muscle training on the function of postural muscles in healthy, pain free individuals. During standardised rapid arm movements, timing of onset of electromyography (EMG) was measured using intramuscular and surface recordings of the transversus abdominis (TrA), obliquus internus abdominis (OI), obliquus externus abdominis (OE), lumbar multifidus (LM) and lumbar erector spinae (LES) muscles. A single exercise session with the device led to significantly (main effect of time: P = 0.03) earlier LES EMG onset in advance of the postural perturbation induced by rapid forward arm movements from -1 ms (SD: 32 ms) at baseline to -11 ms (SD: 27 ms) post-exercise and -16 ms (SD: 22 ms) at 10-min Wash-Out after the FRED exercise bout. The timing of EMG onset of the other trunk muscles was not affected by the single bout of exercise. A significant correlation was found between background activity and the EMG onset times of of TrA (r = 0.6; P < 0.001), OI (r = 0.59; P < 0.001), LES (r = 0.32; P = 0.046) and LMs (r = 0.77; P < 0.001). Higher levels of trunk muscle background activity were associated with later onset times. The present findings suggest that a single exposure to the postural training device can induce small changes in spinal muscle function in healthy pain free individuals.

Anodal Transcranial Direct Current Stimulation Over the Supplementary Motor Area Improves Anticipatory Postural Adjustments in Older Adults.

We examined the influence of anodal transcranial direct current stimulation (tDCS) over the supplementary motor area (SMA) on anticipatory postural adjustments (APAs) and center of pressure (COP) sway in older adults. The study enrolled 12 healthy older adult volunteers. Subjects received anodal tDCS (2 mA) or sham stimulation over the SMA for 15 min and performed a self-paced rapid upward arm movement task on a force plate before, immediately after, and 15 min after the stimulation condition. APAs were measured as the temporal difference between activation onset in the deltoid anterior (AD) and biceps femoris (BF) muscles. The root mean square (RMS) area of COP sway, sway path length, medio-lateral mean velocity, and antero-posterior mean velocity of standing posture were also measured before and after the stimulation condition during the task. Anodal tDCS of the SMA extended APAs and decreased COP sway path length immediately after and 15 min after stimulation compared to baseline. These findings suggest that anodal tDCS over the SMA enhanced APAs function and improved postural sway during rapid upward arm movement in older adults.

Modulation of Corticospinal Excitability of Trunk Muscles in Preparation of Rapid Arm Movement

Many studies have described the dynamic modulation of corticospinal excitability of the prime movers during motor preparation. However although anticipatory postural adjustments (APA) are an inherent part of most voluntary movements, investigation of trunk muscle corticospinal excitability during motor preparation has been neglected in the literature. In the present study, the corticospinal excitability of the superficial multifidus (sMF) and rectus abdominis (RA) muscle has been assessed during the preparation of rapid arm flexions and extensions in fifteen participants. A Warning signal informed participants to prepare to move prior to a Go signal. Transcranial magnetic stimulation was applied during baseline and at 6 time intervals before (Delay period) or after (Motor execution period) the Go signal. Results revealed a significant inhibition of the amplitude of sMF motor-evoked potentials in both flexion and extension movements within the Delay period compared to baseline, while no significant modulation was observed for RA. During the Motor Execution period for arm extension, sMF displayed even more inhibition, along with a large and significant facilitation of RA. During the Motor execution period for arm flexion, sMF presented a trend toward larger motor-evoked potential amplitude compared to Delay period. These results suggest the existence of two concurrent mechanisms underlying motor preparation for APA: (i) before the Go signal, a nonspecific inhibitory mechanism for sMF, likely to preclude motor program release; (ii) after the Go signal, a task-specific modulation of corticospinal excitability consistent with the EMG pattern during the early phase of movement.

Multi-body dynamic coupling mechanism for generating throwing arm velocity during baseball pitching

The purpose of this study was to identify the detailed mechanism how the maximum throwing arm endpoint velocity is determined by the muscular torques and non-muscular interactive torques from the perspective of the dynamic coupling among the trunk, thorax and throwing and non-throwing arm segments. The pitching movements of ten male collegiate baseball pitchers were measured by a three-dimensional motion capture system. Using the induced-segmental velocity analysis (IVA) developed in this study, the maximum fingertip velocity of the throwing arm (MFV) was decomposed into each contribution of the muscular torques, passive motion-dependent torques due to gyroscopic moment, Coriolis force and centrifugal force, and other interactive torque components. The results showed that MFV (31.6±1.7m/s) was mainly attributed to two different mechanisms. The first is the passive motion-dependent effect on increasing the angular velocities of three joints (thorax rotation, elbow extension and wrist flexion). The second is the muscular torque effect of the shoulder internal rotation (IR) torque on generating IR angular velocity. In particular, the centrifugal force-induced elbow extension motion, which was the greatest contributor among individual joint contributions, was caused primarily by the angular velocity-dependent forces associated with the humerus, thorax, and trunk rotations. Our study also found that a compensatory mechanism was achieved by the negative and positive contributions of the muscular torque components. The current IVA is helpful to understand how the rapid throwing arm movement is determined by the dynamic coupling mechanism.