Shoulder Muscle Activity Dampens Arm Swing Motion When Altering Upper Limb Mass Characteristics During Locomotion

Weighting the arms during locomotion results in decreased swing motion and increased shoulder muscle activity. To determine the functional relevance of this activity, participants walked on a treadmill with the arms unweighted, or weighted unilaterally or bilaterally. Similar to past work, the weighted arms decreased in swing amplitude and increased their shoulder muscle activity. A close examination of shoulder muscle activities in specific regions of the arm swing cycle suggested these muscles primarily acted eccentrically for all weighting conditions. These findings suggest that the increased shoulder muscle activities when weighting the arms act to dampen the arms when the inertial characteristics of the arms are altered, as opposed to assisting in driving swing of the heavier arms.     

Grasp-Based Functional Coupling Between Reach- and Grasp-Related Components of Forelimb Muscle Activity

How are appropriate combinations of forelimb muscles selected during reach-to-grasp movements in the presence of neuromotor redundancy and important task-related constraints? The authors tested whether grasp type or target location preferentially influence the selection and synergistic coupling between forelimb muscles during reach-to-grasp movements. Factor analysis applied to 14–20 forelimb electromyograms recorded from monkeys performing reach-to-grasp tasks revealed 4–6 muscle components that showed transport/preshape- or grasp-related features. Weighting coefficients of transport/preshape-related components demonstrated strongest similarities for reaches that shared the same grasp type rather than the same target location. Scaling coefficients of transport/preshape- and grasp-related components showed invariant temporal coupling. Thus, grasp type influenced strongly both transport/preshape- and grasp-related muscle components, giving rise to grasp-based functional coupling between forelimb muscles.     

Muscle Strength Training Alters Muscle Activation of the Lower Extremity during Side-Step Cutting in Females

Abstract

The objective of this study was to examine the effects of muscle strength training on knee kinematics/kinetics and muscle activation patterns during anticipated side-step cutting. Three-dimensional knee kinematics/kinetics data and muscle activation of selected lower extremity muscles were measured while performing cutting before and after completing 10-week circuit strength training mixed typical resistance training and power training (intervention) or no training (control) from 25 female subjects. The muscle strength of quadriceps and hamstrings were measured before and after training using isokinetic dynamometer. No statistically significant differences were observed in quadriceps and hamstrings muscle strength, all kinematic/kinetic variables, and muscle activation for the control group. Both quadriceps (p?=?0.005) and hamstrings (p?=?0.030) muscle strength were increased after training. An increased biceps femoris (p?=?0.003) and H:Q ratio of activation (p?=?0.016), as well as decreased gastrocnemius muscle activation (p?=?0.012) during pre-activation phase in intervention group were found. No significant differences were found in knee kinematics and kinetics both at the time frame of the initial contact and the peak tibial anterior shear force after training. In conclusion, muscle strength training altered some muscle activations of lower extremity muscles, which might affect the risk of ACL injury, but it did not change the kinematic/kinetic parameters.     

Influence of Strategy on Muscle Activity During Impact Movements

Participants (N = 10) made flexions or extensions about the elbow. Movements either were pointing (i.e., self-terminated) or terminated by impact on a barrier. The author examined how the trajectory and the electromyographic (EMG) patterns varied according to the distance moved, the instruction provided concerning speed, or the type of termination. Variations in kinematics induced by changes in the target distance or the instruction regarding speed were the same for impact and pointing movements. In comparison with a pointing movement of similar distance and speed instruction, an impact movement (a) accelerated longer and reached a higher velocity, (b) had a longer agonist EMG burst, and (c) had a low level of contraction that started slightly after the agonist burst and continued throughout the movement but had little or no antagonist burst. Because the different types of movements required different forces from the muscles, there were systematic, task-specific differences in EMG patterns that reflected task-specific differences in central control. The results of this experiment demonstrate that impact movements share some of the rules used in the control of other tasks, such as pointing and reversing movements. The sharing is not imposed by mechanical or physiological constraints but, rather, represents the imposition of internal constraints.     

EMG Activity as a Function of the Performer's Focus of Attention

In previous studies of attentional focus effects, investigators have measured performance outcome. Here, however, the authors used electromyography (EMG) to determine whether differences between external and internal foci would also be manifested at the neuromuscular level. In 2 experiments, participants (N = 11, Experiment 1; N = 12, Experiment 2) performed biceps curls while focusing on the movements of the curl bar (external focus) or on their arms (internal focus). In Experiment 1, movements were performed faster under external than under internal focus conditions. Also, integrated EMG (iEMG) activity was reduced when performers adopted an external focus. In Experiment 2, movement time was controlled through the use of a metronome, and iEMG activity was again reduced under external focus conditions. Those findings are in line with the constrained action hypothesis (G. Wulf, N. McNevin, & C. H. Shea, 2001), according to which an external focus promotes the use of more automatic control processes.     

Medial Gastrocnemius Muscle Activity during Single-Leg Hopping to Exhaustion

Abstract

This study described changes in leg muscle activation characteristics during exhaustive single-leg hopping. Twenty-seven healthy men performed trials (132 hops/min) to exhaustion, without a target height, to a target height with visual feedback and target height with tactile feedback. Mean muscle activation amplitude of the medial gastrocnemius (MG) decreased during the anticipatory period while duration of MG activity was maintained when hopping to a target height and contrasted the changes during hopping without a target height. Changes to MG activity were specific to whether the hopping height had been maintained or not. Changes during the anticipatory period of MG activity, indicative of adaptation in descending motor pathways, implicate utility of a motor learning strategy to allow completion of an exhaustive task.     

Force Control Characteristics for Generation and Relaxation in the Lower Limb

We investigated the characteristics for force generation and relaxation using graded isometric contractions of the knee extensors. Participants performed the following tasks as quickly and accurately as possible. For the force generation task, force was increased from 0% to 20%, 40% and 60% of the maximal voluntary force (MVF). For the force relaxation task, force was decreased from 60% to 40%, 20% and 0%. The following parameters of the recorded force were calculated: error, time, and rate of force development. The error was consistently greater for force relaxation than generation. Reaction and adjustment times were independent of the tasks. The control strategy was markedly different for force relaxation and generation, this tendency was particularly evident for the lower limb compared to the upper limb.     

Submovement Organization,Pen Pressure,and Muscle Activity Are Modulated to Precision Demands in 2D Tracking

The authors investigated how tracking performance, submovement organization, pen pressure and muscle activity in forearm and shoulder muscles were affected by target size in a 2D tracking task performed with a pen on a digitizer tablet. Twenty-six subjects took part in an experiment, in which either a small dot or a large dot was tracked, while it moved quasirandomly across a computer screen at a constant velocity of 2 cm/s. The manipulation of precision level was successful, because mean distance to target and the standard deviation of this distance were significantly smaller with the small target than with the large target. With a small target, subjects trailed more behind the center of target and used submovements with larger amplitudes and of shorter duration, resulting in higher tracking accuracy. This change in submovement organization was accompanied by higher pen pressure, while at the same time muscle activity in the forearm extensors and flexors was increased, indicating higher endpoint stability. In conclusion, increased precision demands were accommodated by both a different organization of submovements and higher endpoint stability in a 2D tracking task performed with a pen on a digitizer tablet.     

Using Corticomuscular and Intermuscular Coherence to Assess Cortical Contribution to Ankle Plantar Flexor Activity During Gait

The present study used coherence and directionality analyses to explore whether the motor cortex contributes to plantar flexor muscle activity during the stance phase and push-off phase during gait. Subjects walked on a treadmill, while EEG over the leg motorcortex area and EMG from the medial gastrocnemius and soleus muscles was recorded. Corticomuscular and intermuscular coherence were calculated from pair-wise recordings. Significant EEG–EMG and EMG–EMG coherence in the beta and gamma frequency bands was found throughout the stance phase with the largest coherence towards push-off. Analysis of directionality revealed that EEG activity preceded EMG activity throughout the stance phase until the time of push-off. These findings suggest that the motor cortex contributes to ankle plantar flexor muscle activity and forward propulsion during gait.     

Single-Joint Movements in Parkinson's Disease: A Pulse-Width and Pulse-Height Theory Review

The process by which the brain controls single-joint movements (SJM) is still not well understood. Some studies have defined rules describing the duration and magnitude of the agonist and antagonist muscles. Therefore, the purpose of this study was to analyze scientific publications about the electromyographic characteristics of SJM performed by patients with Parkinson's disease. A bibliographical review of the years 1989–2015 was performed using keywords such as electromyography, upper limb, and Parkinson's disease. After applying the inclusion criteria, 8 articles were included for analysis. The literature indicates that despite the lack of studies, it is possible to assume that considering the SJM, those with Parkinson's disease only control the magnitude of electromyography activation, being consistent only with the pulse-height theory control.     

The Impact of Deep Muscle Training on the Quality of Posture and Breathing

Postural control and breathing are mechanically and neuromuscularly interdependent. Both systems– of spinal stability and respiration– involve the diaphragm, transversus abdominis, intercostal muscles, internal oblique muscles and pelvic floor muscles. The aim of the study was to evaluate the effect of exercises activating deep stabilizer muscles on postural control and quality of breathing movements. Eighteen volunteers (25,7 ± 3,5) were recruited from the general population. All the subjects implemented an exercise program activating deep muscles. Head, pelvic and trunk positions in the sagittal and frontal planes were assessed with the photogrammetric method. Breathing movements were estimated with the respiratory inductive plethysmography. The results indicate that the use of deep muscle training contributed to a significant change in the position of the body in the sagittal plane (p = 0.008) and the increase in the amplitude of breathing (p = 0.001).     

Limb Dominance and Its Effects on the Benefits of Intralimb Transfer of Learning: A Visuomotor Aiming Task

Research has attempted to address what characteristics benefit from transfer of learning; however, it is still unclear which characteristics are effector dependent or independent. Furthermore, it is not clear if intralimb transfer shows, similarly to interlimb transfer, an asymmetry of benefits between the upper limbs. The purpose of the current study is to examine if effector independence effects emerge, as observed in interlimb transfer studies, when transfer to new effector group within the same limb occurs, and whether the pattern of intralimb transfer benefits differ between the limbs. Our results suggest that a visuomotor task transfers within both limbs, even though the transfer benefits within the limbs seem to differ. This was supported by more transfer occurring in the dominant limb than the nondominant limb. Potential control mechanisms used for intralimb transfer are discussed.     

Response Complexity and the Latency of Different Movement Patterns

Two series of experiments are described in which RT of responses of varying complexity is investigated. Complexity of arm movements was manipulated in terms of (a) the amount of force exerted in resisted movements, (b) the distance moved by the limb and the direction of movement, using reversing or continuing movements, and (c) the combining of the arms in reversing and continuing movements. Simple and two-choice RT situations were used. As response complexity increased in terms of the number of sets of response units that have to be organized into a spatio-temporal sequence, the latency correspondingly increased. However latency was not affected by changes in the amount of force exerted, or in simple reversal movements. The implications of these results are discussed.     

Development of Force Adaptation During Childhood

Humans learn to make reaching movements in novel dynamic environments by acquiring an internal motor model of their limb dynamics. Here, the authors investigated how 4- to 11-year-old children (N = 39) and adults (N = 7) adapted to changes in arm dynamics, and they examined whether those data support the view that the human brain acquires inverse dynamics models (IDM) during development. While external damping forces were applied, the children learned to perform goal-directed forearm flexion movements. After changes in damping, all children showed kinematic aftereffects indicative of a neural controller that still attempted to compensate the no longer existing damping force. With increasing age, the number of trials toward complete adaptation decreased. When damping was present, forearm paths were most perturbed and most variable in the youngest children but were improved in the older children. The findings indicate that the neural representations of limb dynamics are less precise in children and less stable in time than those of adults. Such controller instability might be a primary cause of the high kinematic variability observed in many motor tasks during childhood. Finally, the young children were not able to update those models at the same rate as the older children, who, in turn, adapted more slowly than adults. In conclusion, the ability to adapt to unknown forces is a developmental achievement. The present results are consistent with the view that the acquisition and modification of internal models of the limb dynamics form the basis of that adaptive process.     

Neuromuscular Control of Posture in the Infant and Child

This study explored the effects of vision and maturation on the characteristics of neuromuscular responses underlying balance control in both seated and standing children of five age groups (3½–5 months, 8–14 months, 2–3 years, 4–6 years, and 7–10 years). A platform was used to unexpectedly disturb the child’s balance in the anterior or posterior direction. Responses of the leg, trunk, and neck muscles were recorded using surface electromyograms.

Vision was not required for the activation of these responses in any of the age groups tested. However, comparison of muscle response latencies of standing children to posterior platform translation in the two visual conditions showed a significant reduction in latency for neck flexors in the 2- to 3-year-olds with vision removed and an increase in the total number of monosynaptic reflexes. No reduction in latency was found in the older age groups. The hypothesis of a shift from an early long latency visual dominance to a shorter latency proprioceptive one during childhood is discussed.

Postural control showed a cephalo-caudal developmental gradient with postural responses appearing first in the neck, then trunk, and finally, legs, as children developed from 3 to 14 months of age. A wide variety of response patterns was seen in the 3- to 5-month-olds, indicating that postural responses are not functional prior to experience with stabilizing the center of mass.     

Neuromuscular control in individuals with chronic ankle instability: A comparison of unexpected and expected ankle inversion perturbations during a single leg drop-landing

While neuromuscular control deficits during inversion perturbations in chronic ankle instability (CAI) cohorts are well documented in the literature, anticipatory motor control strategies to inversion perturbations in CAI are largely unknown. The purpose of this study was to examine neuromuscular control and ankle kinematics in individuals with CAI (n = 15) and matched controls (n = 15) during unexpected and expected single leg drop-landings onto a tilted surface rotated 20° in the frontal plane. Muscle activity from 200 ms pre- to post-landing was recorded from the tibialis anterior (TA), medial gastrocnemius (MG), peroneus longus (PL) and peroneus brevis (PB). Mean muscle activity, co-contraction index (CCI), and peroneal latency was analyzed. Ankle inversion angle at initial contact, time to maximum inversion angle, maximum inversion angle and velocity were also assessed. Significantly longer PL latency, less time to maximum inversion and greater maximum inversion angle was found in CAI compared to controls. Regarding landing condition, significantly greater maximum inversion angle, less inversion at initial contact, longer PB latency, less TA activity and frontal plane CCI during the post-landing phase was found during the unexpected perturbation. Prolonged PL latency and altered ankle kinematics suggests reduced frontal plane ankle stabilization in CAI. However, similar motor control strategies were utilized in both groups during the ankle inversion perturbations.     

Relationship Between Muscle Fiber Type and Reactive Balance: A Preliminary Study

The authors’ objective was to quantify the relationship between type II muscle fiber composition and recovery latency from controlled perturbations. Thirty healthy adults performed a Thorstensson test to indirectly determine muscle fiber composition of the knee extensors. On a subsequent day, reactive balance was measured by assessing latency time required to accommodate three controlled perturbations via computerized dynamic posturography. The proportion of type II fibers had a negative correlation with latency caused by a large backward perturbation (r = –.62, p = .001). These data may indicate a higher percentage of type II fibers in the knee extensors are associated with better reactive balance. Further investigation is warranted, in particular as it pertains to populations at risk for balance impairment.     

The Effect of Interpolated Motor Activity on the Short-Term Retention of Movement Distance and End-Location

The effect of an interpolated movement on the recall of a blind, lever-positioning movement was assessed separately for the reproduction cues of distance and end-location. Both cues showed the context effect of assimilation, that is, the end-location of the interpolated movement caused directional biasing of the algebraic or constant error (CE) for location reproduction while the length of the interpolated movement caused similar directional biasing for distance reproduction. Variability about algebraic error (VE) was used to assess forgetting. There was no change in VE for location reproduction, and an increase in VE for distance reproduction was limited to the relatively large movements. Thus, an interpolated movement that does not have to be remembered does not cause interference with the retention of location information, but does cause some interference with the retention of distance information. The findings are discussed in relation to a model of motor short-term memory which incorporates both CE and VE.     

Interindividual Variability in Use-Dependent Plasticity Following Visuomotor Learning: The Effect of Handedness and Muscle Trained

Motor learning has been linked with increases in corticospinal excitability (CSE). However, the robustness of this link is unclear. In this study, changes in CSE associated with learning a visuomotor tracking task were mapped using transcranial magnetic stimulation (TMS). TMS maps were obtained before and after training with the first dorsal interosseous (FDI) of the dominant and nondominant hand, and for a distal (FDI) and proximal (biceps brachii) muscle. Tracking performance improved following 20 min of visuomotor training, while map area was unaffected. Large individual differences were observed with 18%–36% of the participants revealing an increase in TMS map area. This result highlights the complex relationship between motor learning and use-dependent plasticity of the motor cortex.