Movement Related EMGs Become More Variable During Learning of Fast Accurate Movements

Load-velocity relationships and predicted maximal strength: A systematic review of the validity and reliability of current methods

Maximal strength can be predicted from the load-velocity relationship (LVR), although it is important to understand methodological approaches which ensure the validity and reliability of these strength predictions. The aim of this systematic review was to determine factors which influence the validity of maximal strength predictions from the LVR, and secondarily to highlight the effects of these factors on the reliability of predictions. A search strategy was developed and implemented in PubMed, Scopus, Web of Science and CINAHL databases. Rayyan software was used to screen titles, abstracts, and full texts to determine their inclusion/eligibility. Eligible studies compared direct assessments of one-repetition maximum (1RM) with predictions performed using the LVR and reported prediction validity. Validity was extracted and represented graphically via effect size forest plots. Twenty-five eligible studies were included and comprised of a total of 842 participants, three different 1RM prediction methods, 16 different exercises, and 12 different velocity monitoring devices. Four primary factors appear relevant to the efficacy of predicting 1RM: the number of loads used, the exercise examined, the velocity metric used, and the velocity monitoring device. Additionally, the specific loads, provision of velocity feedback, use of lifting straps and regression model used may require further consideration.

Using a Load-Velocity Relationship to Predict One repetition maximum in Free-Weight Exercise: A Comparison of the Different Methods

Hughes, LJ, Banyard, HG, Dempsey, AR, and Scott, BR. Using a load-velocity relationship to predict one repetition maximum in free-weight exercise: a comparison of the different methods. J Strength Cond Res 33(9): 2409-2419, 2019-The purpose of this study was to investigate the reliability and validity of predicting 1 repetition maximum (1RM) in trained individuals using a load-velocity relationship. Twenty strength-trained men (age: 24.3 ± 2.9 years, height: 180.1 ± 5.9 cm, and body mass: 84.2 ± 10.5 kg) were recruited and visited the laboratory on 3 occasions. The load-velocity relationship was developed using the mean concentric velocity of repetitions performed at loads between 20 and 90% 1RM. Predicted 1RM was calculated using 3 different methods discussed in existing research: minimal velocity threshold 1RM (1RMMVT), load at zero velocity 1RM (1RMLD0), and force-velocity 1RM methods (1RMFV). The reliability of 1RM predictions was examined using intraclass correlation coefficient (ICC) and coefficient of variation (CV). 1RMMVT demonstrated the highest reliability (ICC = 0.92-0.96, CV = 3.6-5.0%), followed by 1RMLD0 (ICC = 0.78-0.82, CV = 8.2-8.6%) and 1RMFV (ICC = -0.28 to 0.00, CV = N/A). Both 1RMMVT and 1RMLD0 were very strongly correlated with measured 1RM (r = 0.91-0.95). The only method which was not significantly different to measured 1RM was the 1RMLD0 method. However, when analyzed on an individual basis (using Bland-Altman plots), all methods exhibited a high degree of variability. Overall, the results suggest that the 1RMMVT and 1RMLD0 predicted 1RM values could be used to monitor strength progress in trained individuals without the need for maximal testing. However, given the significant differences between 1RMMVT and measured 1RM, and the high variability associated with individual predictions performed using each method, they cannot be used interchangeably; therefore, it is recommended that predicted 1RM is not used to prescribe training loads as has been previously suggested.

Voluntary Control of Agonist Premotor Silence Preceding Limb Movements of Maximal Effort

The ability to gain voluntary control over agonist premotor silence through electromyographic (EMG) feedback was examined in healthy subjects performing maximal horizontal elbow flexions. Subjects exhibiting premotor silence on at least 50% of the pretest trials showed significantly greater peak angular velocity than subjects who produced the silent period on fewer than 20% of the trials during the pretest. The latter subjects acquired control of agonist premotor silence with practice and graphic feedback regarding their EMG patterns. The subjects who were the most successful in learning to produce the silent period increased their angular velocity to the level of the subjects who naturally exhibited the inhibition. The less successful subjects showed smaller increases in velocity. The data provide further evidence that premotor silence is primarily under central influence, that its control can be acquired, and that it may be functionally related to contractile rate.

Associations Between Practice-Related Changes in Motor Performance and Muscle Activity in Healthy Individuals: A Systematic Review

BACKGROUND

A well-learned motor skill is characterized by the efficient activation of muscles that are involved in movement execution. However, it is unclear if practice-related changes in motor performance correlate with those in quantitative markers of muscle activity and if so, whether the association is different with respect to the investigated muscle (i.e., agonist and antagonist) and quantitative myoelectric parameter. Thus, we conducted a systematic review and characterized associations between practice-related changes in motor performance and muscle activity in healthy individuals.

METHODS

A computerized systematic literature search was performed in the electronic databases PubMed, Web of Science, and SPORTDiscus up to September 2017 to capture all relevant articles. A systematic approach was applied to evaluate the 1670 articles identified for initial review. Studies were included only if they investigated healthy subjects aged 6 years and older and tested at least one measure of motor performance (e.g., error score, movement time) and quantitative muscle activity (i.e., amplitude domain: iEMG [integrated electromyography], RMS [root mean square]; time domain: duration of muscle activity, time to peak muscle activation). In total, 24 studies met the inclusionary criteria for review. The included studies were coded for the following criteria: age, learning task, practice modality, and investigated muscles (i.e., agonist and antagonist). Correlation coefficients for the relationship of motor performance changes with changes in electromyography (EMG) amplitude, and duration were extracted, transformed (i.e., Fisher's z-transformed r_z value), aggregated (i.e., weighted mean r_z value), and back-transformed to r values. To increase sample size, we additionally extracted pre and post practice data for motor performance and myoelectric variables and calculated percent change values as well as associations between both. Correlations were classified according to their magnitude (i.e., small r ≤ 0.69, medium r ≤ 0.89, large r ≥ 0.90).

RESULTS

Five studies reported correlation coefficients for the association between practice-related alterations in motor performance and EMG activity. We found small associations (range r = 0.015-0.50) of practice-related changes in motor performance with measures of agonist and antagonist EMG amplitude and duration. A secondary analysis (17 studies) that was based on the calculation of percent change values also revealed small correlations for changes in motor performance with agonist (r = - 0.25, 11 studies) and antagonist (r = - 0.24, 7 studies) EMG amplitude as well as agonist (r = 0.46, 8 studies) and antagonist (r = 0.29, 5 studies) EMG duration.

CONCLUSIONS

Our systematic review showed small-sized correlations between practice-related changes in motor performance and agonist and antagonist EMG amplitude and duration in healthy individuals. These findings indicate that practice-related changes can only partly be explained by quantitative myoelectric measures. Thus, future studies investigating biomechanical mechanisms of practice-related changes in motor performance should additionally include qualitative measures of muscle activity (e.g., timing of muscle activity, level of coactivation) and other biomechanical variables (i.e., kinetics, kinematics).

Motor learning in a complex balance task and associated neuroplasticity: a comparison between endurance athletes and nonathletes

Studies suggested that motor expertise is associated with functional and structural brain alterations, which positively affect sensorimotor performance and learning capabilities. The purpose of the present study was to unravel differences in motor skill learning and associated functional neuroplasticity between endurance athletes (EA) and nonathletes (NA). For this purpose, participants had to perform a multimodal balance task (MBT) training on 2 sessions, which were separated by 1 wk. Before and after MBT training, a static balance task (SBT) had to be performed. MBT-induced functional neuroplasticity and neuromuscular alterations were assessed by means of functional near-infrared spectroscopy (fNIRS) and electromyography (EMG) during SBT performance. We hypothesized that EA would showed superior initial SBT performance and stronger MBT-induced improvements in SBT learning rates compared with NA. On a cortical level, we hypothesized that MBT training would lead to differential learning-dependent functional changes in motor-related brain regions [such as primary motor cortex (M1)] during SBT performance. In fact, EA showed superior initial SBT performance, whereas learning rates did not differ between groups. On a cortical level, fNIRS recordings (time × group interaction) revealed a stronger MBT-induced decrease in left M1 and inferior parietal lobe (IPL) for deoxygenated hemoglobin in EA. Even more interesting, learning rates were correlated with fNIRS changes in right M1/IPL. On the basis of these findings, we provide novel evidence for superior MBT training-induced functional neuroplasticity in highly trained athletes. Future studies should investigate these effects in different sports disciplines to strengthen previous work on experience-dependent neuroplasticity.NEW & NOTEWORTHY Motor expertise is associated with functional/structural brain plasticity. How such neuroplastic reorganization translates into altered motor learning processes remains elusive. We investigated endurance athletes (EA) and nonathletes (NA) in a multimodal balance task (MBT). EA showed superior static balance performance (SBT), whereas MBT-induced SBT improvements did not differ between groups. Functional near-infrared spectroscopy recordings revealed a differential MBT training-induced decrease of deoxygenated hemoglobin in left primary motor cortex and inferior parietal lobe between groups.

A review on the basketball jump shot

The ability to shoot an effective jump shot in the sport of basketball is critical to a player's success. In an attempt to better understand the aspects related to expert performance, researchers have investigated successful free throws and jump shots of various basketball players and identified movement variables that contribute to their success. The purpose of this study was to complete a systematic review of the scientific literature on the basketball free throw and jump shot for the purpose of revealing the critical components of shooting that coaches, teachers, and players should focus on when teaching, learning, practising, and performing a jump shot. The results of this review are presented in three sections: (a) variables that affect ball trajectory, (b) phases of the jump shot, and

Force steadiness during a co-contraction task can be improved with practice, but only by young adults and not by middle-aged or old adults

NEW FINDINGS

What is the central question of this study? Does the capacity to modulate afferent input to spinal motor neurons during steady submaximal contractions change with advancing age? What is the main finding and its importance? After practising a co-contraction task involving lower leg muscles, young subjects improved force steadiness by reducing the amount of Ia presynaptic inhibition as indexed by D1 inhibition. Middle-aged and old adults both found the task challenging, and force steadiness even worsened for old adults after practising the co-contraction task. Despite similar muscle strength for young and middle-aged adults, the capacity to modulate a spinal reflex pathway was reduced in middle-aged adults. This study compared the changes in steadiness and the modulation of presynaptic inhibition of soleus Ia afferents in young, middle-aged and old adults before and after a single session of practising a task that involved concurrent contraction of dorsiflexor and plantarflexor muscles. The hypothesis was that young subjects would be able to improve steadiness with practice by modulating Ia afferent feedback as indicated by changes in a measure of presynaptic inhibition (D1 inhibition), but that middle-aged and older subjects would exhibit a lesser ability to augment steadiness. There were no differences in steadiness between groups during an initial co-contraction trial (P = 0.713). Maximal voluntary contraction force for the plantarflexors was not significantly different between young and middle-aged subjects (P > 0.05), but it was significantly less in old subjects (P < 0.05). The main finding of the study was that young adults were able to improve steadiness by ∼19% (P < 0.001) during a co-contraction task after 50 min of practice, whereas there was no change for the middle-aged adults, and old adults became less steady by ∼15% (P < 0.05). The improvement in steadiness by young adults was accompanied by a significant reduction in the amount of Ia presynaptic inhibition as indexed by D1 inhibition (P < 0.01). Conversely, neither of the other two groups exhibited any change in D1 inhibition after practising the co-contraction task. In contrast to young subjects, middle-aged and old adults found the co-contraction task challenging and were not able to improve steadiness after practising the low-force isometric contraction.

Quantifying the familiarization period for maximal resistive exercise

Resistive exercise is used in the assessment of musculoskeletal health, performance, training interventions, and population differences (i.e., gender, age, training status). There is a need to determine the amount of familiarization required to stabilize performance prior to testing. Fifteen males completed a familiarization session consisting of 3 blocks of 5 maximal isometric dorsiflexion contractions, followed by a retention test (an additional block of 5 contractions) performed 3 days later. Mean force and surface electromyography (sEMG) from both the agonist and antagonist muscles were collected. A variance ratio, representing the stability between trials, was calculated for each of the 4 blocks of 5 contractions for both force and sEMG. The variance ratio for both force and agonist sEMG decreased significantly within the first 10 trials and remained stable during the retention test. The variance ratio for antagonist sEMG was stable across the 3 blocks of familiarization and significantly decreased during the retention test. The magnitude variables all remained stable across the 3 familiarization blocks. However, an 11% increase in mean force was seen during the retention test while both agonist and antagonist sEMG remained stable. Although slight changes occurred in the magnitude variables during the retention test, the stabilization of the force and agonist sEMG variance ratios suggest that familiarization to the task was achieved within the first 10 contractions and was sustained over a 3-day period.

Wingbeat kinematics and motor control of yaw turns in Anna's hummingbirds (Calypte anna)

The biomechanical and neuromuscular mechanisms used by different animals to generate turns in flight are highly variable. Body size and body plan exert some influence, e.g. birds typically roll their body to orient forces generated by the wings whereas insects are capable of turning via left-right wingbeat asymmetries. Turns are also relatively brief and have low repeatability, with almost every wingbeat serving a different function throughout the change in heading. Here we present an analysis of Anna's hummingbirds (Calypte anna) as they fed continuously from an artificial feeder revolving around the outside of the animal. This setup allowed for examination of sustained changes in yaw without requiring any corresponding changes in pitch, roll or body position. Hummingbirds sustained yaw turns by expanding the wing stroke amplitude of the outer wing during the downstroke and by altering the deviation of the wingtip path during both downstroke and upstroke. The latter led to a shift in the inner-outer stroke plane angle during the upstroke and shifts in the elevation of the stroke plane and in the deviation of the wingtip path during both strokes. These features are generally more similar to how insects, as opposed to birds, turn. However, time series analysis also revealed considerable stroke-to-stroke variation. Changes in the stroke amplitude and the wingtip velocity were highly cross-correlated, as were changes in the stroke deviation and the elevation of the stroke plane. As was the case for wingbeat kinematics, electromyogram recordings from pectoral and wing muscles were highly variable, but no correlations were found between these two features of motor control. The high variability of both kinematic and muscle activation features indicates a high level of wingbeat-to-wingbeat adjustments during sustained yaw. The activation timing of the muscles was more repeatable than the activation intensity, which suggests that the former may be constrained by harmonic motion and that the latter may play a large role in kinematic adjustments. Comparing the revolution frequency of the feeder with measurements of free flight yaws reveals that feeder tracking, even at one revolution every 2 s, is well below the maximum yaw capacity of the hummingbirds.

Smart mug to measure hand's geometrical mechanical impedance

A novel device, which looks like a mug, has been proposed for measuring the impedance of human hand. The device is designed to have convenient size and light weight similar to an ordinary coffee mug. It contains a 2-axis inertia sensor to monitor vibration and a small motor to carry an eccentric mass (m=100 gr, r=2 cm, rpm=600). The centrifugal force due to the rotating mass applies a dynamic force to the hand that holds the mug. Correlation of the acceleration signals with the perturbing force gives the geometrical mechanical impedance. Experimental results on a healthy subject shows that impedance is posture dependant while it changes with the direction of the applied perturbing force. For nine postures the geometrical impedance is obtained all of which have elliptical shapes. The method can be used for assessment of spasticity and monitoring stability in patients with stroke or similar problems.

Rise rate and timing variability of surface electromyographic activity during rhythmic drumming movements in the world's fastest drummer

The winner of an international contest to find the world's fastest drummer (WFD) can perform repetitive wrist tapping movements with one hand using a handheld drumstick at 10Hz, much faster than the maximum tapping frequency of 5-7Hz in the general population. The muscle activity facilitating this improved performance, however, has only recently been explored. The present study investigated the rise rate and timing variability of surface electromyographic (EMG) activity of wrist flexor/extensor muscles in the WFD, and compared them with those in two control groups: non-drummers (NDs) and ordinary drummers (ODs). The WFD showed more rapid EMG amplitude rise, earlier decline of EMG activity, and more stable muscle activation time than the NDs and ODs. In addition, there was a significant correlation between the EMG rise rate and the duration of drum training in the group of drummers (i.e., ODs and WFD). These results indicate that the 10-Hz performance of the WFD was achieved by a 'sharper' and 'less noisy' burst pattern of wrist muscles, and that drum training would have the effect to increase the speed of development of muscle tension.

Perturbation-based measurement of real and imaginary parts of human arm's mechanical impedance

Mechanical impedance is a complex number and a system's property. Impedance of human arm is the control variable when the central nervous system coordinates a motion. This research proposes a new method for measuring the mechanical impedance as a complex number. Impedance is measured at the hand point while sinusoidal perturbation is applied. That helps extracting the real and imaginary part of the impedance. Simulations reveal how spring, mass, and damper contribute to the mechanical impedance. Despite the simulation results, our experiment shows that damping which is the real part of impedance is not frequency independent and imaginary part of mechanical impedance decreased with increasing frequency that in turn suggests the stiffness is increasing.

Time-Referenced Effects of an Internal vs. External Focus of Attention on Muscular Activity and Compensatory Variability

The paralysis-by-analysis phenomenon, i.e., attending to the execution of one's movement impairs performance, has gathered a lot of attention over recent years (see Wulf, 2007, for a review). Explanations of this phenomenon, e.g., the hypotheses of constrained action (Wulf et al., 2001) or of step-by-step execution (Masters, 1992; Beilock et al., 2002), however, do not refer to the level of underlying mechanisms on the level of sensorimotor control. For this purpose, a "nodal-point hypothesis" is presented here with the core assumption that skilled motor behavior is internally based on sensorimotor chains of nodal points, that attending to intermediate nodal points leads to a muscular re-freezing of the motor system at exactly and exclusively these points in time, and that this re-freezing is accompanied by the disruption of compensatory processes, resulting in an overall decrease of motor performance. Two experiments, on lever sequencing and basketball free throws, respectively, are reported that successfully tested these time-referenced predictions, i.e., showing that muscular activity is selectively increased and compensatory variability selectively decreased at movement-related nodal points if these points are in the focus of attention.

Deconstructing neurobiological coordination: the role of the biomechanics-motor control nexus

Inherent indeterminacy of neurobiological systems has been revealed by research on coordination of multiarticular actions. We consider three important issues that these investigations raise for biomechanical measurement and performance modeling. These issues highlight the role of dynamic systems theory as a platform for integration of motor control and biomechanics in exercise and sports science.

Principles for learning horizontal-planar arm movements with reversal

PURPOSE

This study tested the hypothesis that muscle and interaction torques can be altered independently in order to improve in specific kinematics performance observed following practice. We also tested the hypothesis that a simple set of rules of EMG-control and kinetic-control models could explain the EMG and kinetic changes due to practice of movements with reversal.

SCOPE

Kinematics of the upper arm with reversal, performed over three distances, was reconstructed using motion analysis. The muscle and interaction torques were calculated using inverse-dynamics. EMG activities of the major arm muscles were also recorded. The results demonstrate that improved performance is facilitated by an increase in muscle torque (and therefore acceleration) at the proximal joint (shoulder) and by an increase in the interaction torque at the distal joint (elbow). No changes were observed in the amount of muscle activity underlying these kinetic modifications, except for a decrease in the shoulder antagonist latency.

CONCLUSION

The results confirm Bernstein's idea that the central nervous system takes advantage of the passive-interactive properties of the moving system. Also the modulation of the EMG patterns should be explained taking in account the reactive forces and the dual functions (maintenance of posture and generation of movement) of the muscles.

Different neural adjustments improve endpoint accuracy with practice in young and old adults

The purpose of the study was to determine the practice-induced adjustments in the motor-output variability and the agonist-antagonist activity that accompanied improvements in endpoint accuracy of goal-directed isometric contractions in young and old adults. Young and old adults performed 100 trials that involved accurately matching the peak of a force trajectory (25% maximum) to a target force in 150 ms. Endpoint accuracy was quantified as the absolute difference between the target and the peak force and time-to-peak force. Motor-output variability was expressed as the SDs of the force trajectory, peak force, and time-to-peak force. The force and time errors differed between the two groups initially, but after 35 practice trials the errors were similar for the two groups. Reductions in force endpoint error were predicted by decreases in the variability of the force trajectory for both groups, adaptations in the agonist (first dorsal interosseus) and antagonist (second palmar interosseus) EMG for young adults, and adaptations only for the agonist EMG for old adults. Reductions in time endpoint error were predicted by increases in the SD of time-to-peak force and a longer delay to the peak EMG of the antagonist muscle for young adults, but by decreases in the SDs of time-to-peak force and force trajectory and a shorter delay to the peak EMG of the antagonist muscle for the old adults. The findings indicate that the neural adjustments underlying the improvement in endpoint accuracy with practice differed for young and old adults.

Information entropy analysis of discrete aiming movements

Information entropy and mutual information were investigated in discrete movement aiming tasks over a wide range of spatial (20-160 mm) and temporal (250-1250 ms) constraints. Information entropy was calculated using two distinct analyses: (1) with no assumption on the nature of the data distribution; and (2) assuming the data have a normal distribution. The two analyses showed different results in the estimate of entropy that also changed as a function of task goals, indicating that the movement trajectory data were not from a normal distribution. It was also found that the information entropy of the discrete aiming movements was lower than the task defined indices of difficulty (ID) that were selected for the congruence with Fitts' law. Mutual information between time points of the trajectory was strongly influenced by the average movement velocity and the acceleration/deceleration segments of the movement. The entropy analysis revealed structure to the variability of the movement trajectory and outcome that has been masked by the traditional distributional analyses of discrete aiming movements.

Changes in the variability of movement trajectories with practice

We studied variability in movement phase plane trajectories (velocity-position relation) during movement. Human subjects performed 10 degrees and 30 degrees elbow flexion and extension movements in a visual step tracking paradigm. The area of ellipses with radii equal to one standard deviation in position and velocity was taken as a measure of trajectory variability. Trajectory variability was determined at 10-ms intervals throughout movements. Trajectory variability in both the acceleration and deceleration phases of movement decreased with practice. The average trajectory variability during deceleration was greater than that during acceleration even after extended practice (1000 trials). During practice, subjects usually increased movement speed while maintaining end-position accuracy. Trajectory variability was also related to movement speed when equal amounts of practice were given. Short duration (fast) movements had greater trajectory variability than long duration movements. Thus there is a tradeoff between movement speed and trajectory variability similar to the classical speed-accuracy tradeoff. Trajectory variability increased rapidly during the acceleratory phase of movement. The rate of increase was positively related to both movement amplitude and speed. Thus, the forces producing limb acceleration were variable and this variability was more marked in faster and larger movements. In contrast, trajectory variability increased more slowly or actually decreased during the deceleratory phase of movements. Forces involved in limb deceleration thus appeared to compensate to a greater or lesser degree for the variability in accelerative forces. The experiments indicate that the entire trajectory of simple limb movements is controlled by the central nervous system. Variations in accelerative forces may be compensated for by associated variations in decelerative forces. The linkage between accelerative and decelerative forces is progressively refined with practice resulting in decreased variability of the movement trajectory.

Optimal Impedance Control for Task Achievement in the Presence of Signal-Dependent Noise

There is an infinity of impedance parameter values, and thus different co-contraction levels, that can produce similar movement kinematics from which the CNS must select one. Although signal-dependent noise (SDN) predicts larger motor-command variability during higher co-contraction, the relationship between impedance and task performance is not theoretically obvious and thus was examined here. Subjects made goal-directed, single-joint elbow movements to either move naturally to different target sizes or voluntarily co-contract at different levels. Stiffness was estimated as the weighted summation of rectified EMG signals through the index of muscle co-contraction around the joint (IMCJ) proposed previously. When subjects made movements to targets of different sizes, IMCJ increased with the accuracy requirements, leading to reduced endpoint deviations. Therefore without the need for great accuracy, subjects accepted worse performance with lower co-contraction. When subjects were asked to increase co-contraction, the variability of EMG and torque both increased, suggesting that noise in the neuromotor command increased with muscle activation. In contrast, the final positional error was smallest for the highest IMCJ level. Although co-contraction increases the motor-command noise, the effect of this noise on the task performance is reduced. Subjects were able to regulate their impedance and control endpoint variance as the task requirements changed, and they did not voluntarily select the high impedance that generated the minimum endpoint error. These data contradict predictions of the SDN-based theory, which postulates minimization of only endpoint variance and thus require its revision.

Decomposition of variability in the execution of goal-oriented tasks: three components of skill improvement

A central ability of the motor system is to achieve goals with great reliability, although never with zero variability. It is argued that variability is reduced with practice by 3 separate means: reduction of stochastic noise (N), exploitation of task tolerance (T), and covariation (C) between central variables. A method is presented that decomposes variability into these components in relation to task space that is defined by the execution variables. Successful variable combinations form the solution manifold. In a virtual skittles task, it is demonstrated that participants' improvement over repetitions, indicated by increasing accuracy, is accounted for by N, T, and, to a lesser degree, C. The relative contribution of these components changes over the course of practice and task variations.

Adaptations in physiology and propulsion techniques during the initial phase of learning manual wheelchair propulsion

OBJECTIVE

The purpose of this study was to analyze adaptations in gross mechanical efficiency and wheelchair propulsion technique in novice able-bodied subjects during the initial phase of learning hand-rim wheelchair propulsion.

DESIGN

Nine able-bodied subjects performed three 4-min practice blocks on a wheelchair ergometer. The external power output and velocity of all blocks was, respectively, 0.25 W/kg and 1.11 m/sec. Gross mechanical efficiency, force application, timing, and intercycle variability were measured.

RESULTS

No change in gross mechanical efficiency was found. However, a decrease in push frequency occurred, which was accompanied by an increase in work per cycle and a decrease in percentage push time. The increase in work per cycle was associated with a higher peak torque. No changes in intercycle variability were visible over time.

CONCLUSIONS

The timing variables had already changed during the initial phase of learning manual wheelchair propulsion. However, for other variables, such as force production, gross mechanical efficiency, and intercycle variability, a longer practice period might be necessary to induce a change. The effective force direction seemed to be optimized from the start of the learning process onward.

Short-term adaptations in co-ordination during the initial phase of learning manual wheelchair propulsion

The purpose of this study was to analyse adaptations in kinematics and muscle activity/co-contraction in novice able-bodied subjects during the initial phase of learning hand rim wheelchair propulsion. Nine able-bodied subjects performed three 4-min practice blocks on a wheelchair ergometer. The external power output and velocity were constant for all blocks, respectively 0.25 W x kg(-1) and 1.11 m x s(-1). Electromyography of 16 arm, shoulder, back and chest muscles and kinematics were measured. Some small changes in the segmental movement pattern were seen during the practice period. Moreover, an increase in muscle activity and co-contraction of several muscles was found over time. The hypothesis that subjects instinctively search for an optimum frequency, in which the recovery phase is related to the eigenfrequency of the arms and, therefore, the least muscle activity, could not be supported. Since co-contraction of antagonist pairs remained the same or even increased, the hypothesis that there would be a decrease in muscle co-contraction as a result of practice, was not confirmed. This study was probably too short for the novice subjects to explore this new task of wheelchair propulsion completely and reach an optimum in terms of cycle frequency and muscle activity/co-contraction.

Changes in kinematic and EMG variability while practicing a maximal performance task

This paper examines changes in the variability of electromyographic (EMG) activity and kinematics as a result of practicing a maximal performance task. Eight subjects performed rapid elbow flexion to a target in the horizontal plane. Four hundred trials were distributed equally over four practice sessions. A potentiometer at the elbow axis of rotation of a manipulandum recorded the angular displacement. The EMG activity of the biceps and the triceps brachii was monitored using Beckman surface electrodes. Limb speed increased while both target error and trajectory (velocity versus position) variability decreased. There was an increase in the absolute measure of total EMG variability (the first standard deviation at each point of the biceps and triceps waveform multiplied together). However, the coefficient of variation (the first standard deviation divided by the mean and the result multiplied by 100) of the mean amplitude value of the individual EMG bursts decreased. The variability of triceps motor time also decreased while the variability biceps motor time remained unchanged. The results demonstrated a clear relationship between kinematic and EMG variability. The EMG and the trajectory data suggest that practice resulted in greater central nervous system control over both the spatial-temporal aspects of movement and the magnitude of the biceps and triceps muscle force-impulses.

Wheelchair propulsion technique and mechanical efficiency after 3 wk of practice

PURPOSE

Differences in gross mechanical efficiency between experienced and inexperienced wheelchair users may be brought about by differences in propulsion technique. The purpose of this experiment was to study changes in propulsion technique (defined by force application, left-right symmetry, intercycle variability, and timing) and gross mechanical efficiency during a 3-wk wheelchair practice period in a group of novice able-bodied nonwheelchair users.

METHODS

Subjects were randomly divided over an experimental group (N = 10) and a control group (N = 10). The experimental group received a 3-wk wheelchair practice period (3.wk-1, i.e., 9 practice trials) on a computer-controlled wheelchair ergometer, whereas the control group only participated in trials 1 and 9. During all nine practice trials, propulsion technique variables and mechanical efficiency were measured.

RESULTS

No significant differences between the groups were found for force application, left-right symmetry, and intercycle variability. The push frequency and negative power deflection at the start of the push phase diminished significantly in the experimental group in contrast to the control group (P < 0.05). Work per cycle, push time, cycle time, and mechanical efficiency increased.

CONCLUSION

The practice period had a favorable effect on some technique variables and mechanical efficiency, which may indicate a positive effect of improved technique on mechanical efficiency. Although muscle activation and kinematic segment characteristics were not measured in the present study, they may also impact mechanical efficiency. No changes occurred over time in most force application parameters, left-right symmetry, and intercycle variability during the 3-wk practice period; however, these variables may change on another time scale.

Compensatory coordination of release parameters in a throwing task

The consistency and coordination of release parameters in ball-throwing movements were investigated. The authors used a newly developed index of coordination for release parameters (ICRP) that quantifies the degree of improvement of performance consistency caused by compensatory relationships among parameters (i.e., not caused by consistency of parameters). Eight participants practiced for 150 trials, with the nondominant hand, a ball-throwing task aimed at a stationary target. The magnitude of the ball-release velocity vector, among release parameters, as well as the performance was found to become consistent with practice. The ICRP score suggested that the release parameters were complementarily coordinated with one another, and that the coordination improved with practice. Those results indicate that compensatory relationships among varying release parameters contribute to reducing the variability of performance in a ball-throwing task whose goal is accuracy.

Practicing a maximal performance task: a cooperative strategy for muscle activity

The effect of practice on predicting elbow flexion movement time was studied. Participants (N = 18) performed 400 elbow flexion trials to a target in the horizontal plane. The trials were distributed equally over four sessions. The goal was to decrease the movement time (MT) for the same degree of accuracy. The electromyographic (EMG) activity of the biceps and triceps brachii was monitored with standard Beckman Ag/AgCl surface electrodes. The EMG measures formed two variable sets within one prediction equation. One variable set was composed of the onset of muscle activity relative to the start of movement (motor time) and the duration of muscle activity. The other variable set consisted of the mean amplitude value of the entire burst and of the first 30 ms (Q30) of activity. As the maximal speed of limb movement increased, the duration of muscle activity (motor time and EMG duration) decreased, and the magnitude of muscle activity (MAV and Q30) increased. Most of the change in the duration of muscle activity occurred in Session 1, while the magnitude of muscle activity continued to increase until Session 3. Multiple regression analysis revealed a cooperative strategy between the magnitude and duration of muscle activity. Early in learning, participants adjusted the magnitude of muscle activity to increase limb movement speed. As practice continued, alterations in the duration of muscle activity became more important, while the magnitude changes were less involved. Late in learning, both dimensions of muscle activity were used to decrease MT. We suggest that the interplay between the magnitude and duration of muscle activity may be due to: (a) cognitive factors related to the division of attention in a motor skill, (b) an increase in the frequency of motor unit firing that affects both dimensions of muscle activity, or (c) some combination of (a) and (b).

Adaptation of stuttering frequency during repeated readings: associated changes in acoustic parameters of perceptually fluent speech

This study is part of a series investigating the hypothesis that stuttering adaptation is a result of motor learning. Previous investigations indicate that nonspeech motor learning typically is associated with an increase in speed of performance. Previous investigations of stuttering, on the other hand, indicate that improvements in fluency during most fluency-enhancing conditions or after stuttering treatment tend to be associated with decreased speech rate, increased duration of specific acoustic segments, and decreased vowel duration variability. The present acoustic findings, obtained from 8 individuals who stutter, reveal that speech adjustments occurring during adaptation differ from those reported for other fluency-enhancing conditions or stuttering treatment. Instead, the observed changes are consistent with those occurring during skill improvements for nonspeech motor tasks and, thus, with a motor learning hypothesis of stuttering adaptation. During the last of 6 repeated readings, a statistically significant increase in articulation rate was observed, together with a decrease in word duration, vowel duration, and consonant-vowel (CV) transition extent. Other adjustments showing relatively consistent trends across individual subjects included decreased CV transition rate and duration, and increased variability of both CV transition extent and vowel duration.

Practice effects on the timing and magnitude of antagonist activity during ballistic elbow flexion to a target

This study examined changes in antagonist timing and magnitude in response to ballistic elbow flexion practice. Seventeen men performed 400 ballistic elbow flexion trials to a target in the horizontal plane over 4 days of testing. A potentiometer and microswitch system at the elbow axis of rotation of a manipulandum recorded angular displacement and movement onset. Surface electrodes (Beckman Ag/AgCl) monitored the triceps brachii lateral head, and the electromyographic (EMG) signals were bandpassed between 20 and 300 Hz. The antagonist EMG burst was divided in two: early low-level activity (ANT1), and the large portion of the burst which occurs near target achievement (ANT2). Movement time decreased from 178 ms on the first test day to 136 ms on the last session. As practice improved the speed of limb movement, onset of the first component (ANT1) remained unchanged, while the second component (ANT2) started earlier. The magnitude of both portions of the antagonist burst increased from the first to last test day, but the change in ANT2 relative to ANT1 was more pronounced. These findings are used to explain discrepant observations in the literature for the temporal measure.

Effects of practice with and without knowledge of results on jitter and shimmer levels in normally speaking women

The effects of practice on jitter and shimmer were assessed in two groups of normally speaking women. Subjects in both groups sustained trials of /a/ as steadily as possible during a baseline session, two practice sessions, and a transfer session. Subjects in one group received visual and verbal feedback during the practice sessions. Subjects in the other group received no feedback. Shimmer means remained essentially stable over the four sessions for both groups, and no differences were apparent between the groups. Jitter values were significantly different between sessions for both groups, and between the two groups for the practice sessions. These results are consistent with findings from manual performance and retention tasks. The present findings also support a recently developed neurologic model of jitter.

Variations in several mechanical parameters associated with elbow flexion during practice under different load criteria

The purpose of this study was to analyze variations in the accuracy of the moment produced by the elbow flexors during feedback-assisted acquisition of a motor skill. The task consisted of minimizing the error around three criterional levels: 20%, 30%, or 50% of the maximal isokinetic concentric moment of these muscles measured at 90 degrees/sec. Healthy women, aged 22 to 30 years, were divided into three groups (nA = 6, nB = 6, nC = 4) corresponding to the above criteria. They were asked to perform 10 sets of 10 right-elbow flexions per day over a period of three consecutive days. The results indicated a significant difference among the groups mainly in terms of overshooting (Group A) or undershooting (Group C) the criterion. On the other hand, Group B subjects performed optimally as indicated both by a significant convergence to the criterion (30%) and a comparatively small number of repetitions required for achievements. These findings demonstrate the existence of an optimal performance point which is located at about 30% of the maximal isokinetic concentric moment.

Changes in inter-joint relationships of muscle moments and powers accompanying the acquisition of a multi-articular kicking task

When the nervous system produces a multi-joint movement it must select a unique set of motor patterns for movement production from a variety of appropriate motor pattern solutions: this is the so-called degrees-of-freedom problem. One strategy which the nervous system could use to simplify the selection of a unique set of motor patterns (i.e. net muscle moments) for movement production would be to constrain the relationships between the net muscle moments of the various joints. Therefore, we addressed the following question: Do the inter-joint relationships of muscle moments and powers become more phase-locked during the acquisition of a novel multi-joint movement. Subjects performed 16 blocks (16 trials per block) of a discrete kicking movement, which involved motion of the hip, knee, and ankle--a weight (1.674 kg) attached to the subject's foot increased movement novelty. Subjects attempted to perform all movements as close as possible to a goal movement time of 400 ms, while maintaining spatial accuracy. An optoelectric imaging system was used to record movement kinematics for the hip, knee, ankle, and toe of the kicking leg, and inverse dynamics were then used to obtain net-muscle-moment profiles. Following learning, inter-joint moments became more closely phase-locked, with hip-flexor and knee-extensor moments being produced simultaneously, as were hip-extensor and knee-flexor moments. This increased phase-locking of inter-joint moment relationships with learning resulted in increased phase-locking of inter-joint power relationships.(ABSTRACT TRUNCATED AT 250 WORDS)

Principles for learning single-joint movements. I. Enhanced performance by practice

This study investigated changes in myoelectric and mechanical variables for movements made "as fast as possible" as a function of practice in the context of the dual-strategy hypothesis of motor control (Gottlieb et al. 1989b). Five male subjects made 1400 rapid elbow flexion movements in ten blocks of 20 trials over seven experimental sessions. Improved performance was defined as increased peak movement velocity, decreased peak velocity variability, increased acceleration and deceleration, a proportionately greater increase in peak deceleration than peak acceleration, and greater consistency in terminal location. The changes observed over experimental sessions were very similar to (but larger and more consistent than) those seen for the first experimental session, with the partial exception of the timing of the antagonist electromyogram (EMG). In general, the increases in the values of the measured mechanical variables covary with myoelectric measures in the same way as when subjects are asked to intentionally change speed in accordance with the rules of the speed-sensitive strategy (Corcos et al. 1989). However, there are differences between subjects in the extent to which speed changes can be attributable to the agonist muscle, the antagonist muscle, or in the timing between the muscles. In one of the five subjects, the latency of the antagonist EMG decreased over blocks on the 1st day but increased over experimental sessions and was consequently activated proportionately later in the movement. This suggests that extended practice can give at least some subjects flexibility in modifying the motor programs that underlie movement.

A model for the generation of movements requiring endpoint precision

A model is proposed in which movement accuracy is regulated by means of corrective actions taken at discrete intervals throughout the course of a movement. A movement, as represented by its tangential velocity profile, cna be decomposed into a series of one or more submovements. Each submovement consists of a prototype velocity profile which can be scaled in magnitude and duration. For planar two-joint movements, we demonstrate that these submovements can be mathematically represented either in terms of velocity profiles or in terms of the underlying joint torque profiles. In either case, the submovements superimpose linearly to produce the composite movement. The model provides a very good fit to tangential velocity profiles recorded from human subjects during three-dimensional arm movements with constraints on accuracy and speed. The model assumes that when a submovement is present, its onset is associated with a change in the direction of the hand path and/or a zero crossing or inflection in at least one of the components of the velocity vector. The model is consistent with a strategy in which precision is achieved by periodic discrete actions which redirect the moving arm in order to bring the hand closer to the target. Since submovements were also observed in slow movements where accuracy constraints had been relaxed, we hypothesize that the strategy of superimposing a series of submovements to make one composite movement may be a general one. We suggest that it would be particularly appropriate for the process of learning a new motor skill.

Strategy and learning effects on perturbed movements: an electromyographic and kinematic study

The effects of practice and movement strategy were studied in a goal-directed movement with different levels of perturbation applied in four independent groups (0%, 20%, 50% or 100%). The phase-plane trajectory data revealed that for all subjects there was a decrease in the mean trajectory variability with learning and that variability was affected by the level of uncertainty confronting the subjects. Both the average electromyographic (EMG) profiles and the mean subject variability for agonist and antagonist muscles decreased with learning. In each experimental group a specific interaction developed between reflex responses and voluntary activity to create a balanced level of EMG activity to improve the performance with learning.

Voluntary control of agonist premotor silence preceding limb movements of maximal effort

The ability to gain voluntary control over agonist premotor silence through electromyographic (EMG) feedback was examined in healthy subjects performing maximal horizontal elbow flexions. Subjects exhibiting premotor silence on at least 50% of the pretest trials showed significantly greater peak angular velocity than subjects who produced the silent period on fewer than 20% of the trials during the pretest. The latter subjects acquired control of agonist premotor silence with practice and graphic feedback regarding their EMG patterns. The subjects who were the most successful in learning to produce the silent period increased their angular velocity to the level of the subjects who naturally exhibited the inhibition. The less successful subjects showed smaller increases in velocity. The data provide further evidence that premotor silence is primarily under central influence, that its control can be acquired, and that it may be functionally related to contractile rate.

Responses of Ankle Musculature of Healthy Subjects and Hemiplegic Patients to Sinusoidal Anterior-Posterior Movements of the Base of Support

The responses of the medical gastrocnemius and tibialis anterior muscles to continuous sinusoidal perturbation of the base of support were studied in two groups of subjects, forty-two volunteers aged 20 to 84 years and 17 hemiplegic patients aged 52 to 81 years. EMGs were recorded while subjects stood on a platform oscillating in the anterior-posterior direction. Movement amplitude was adjusted to the maximum the subject could sustain without assisted support within the limits of the instrument. In healthy subjects, two basic activation modes were indicated: (a) a reciprocal contraction pattern in which activity of the medial gastrocnemius and tibialis anterior muscles was concentrated in the anterior and posterior half of the oscillation, respectively. This pattern prevailed in the younger subjects whose maximal movement amplitude reached the highest limit of the instrument; (b) a less discrete pattern, characterized by tonic contraction and periods of coactivation of the two muscles, was seen in older subjects who had difficulties in maintaining stance on the moving platform. The disruption of the discrete reciprocal response mode in this group of patients was related to decline in function of postural mechanisms with aging and demanded additional stiffening of the ankle join, presumably a manifestation of reduced automatism in favor of closer CNS modulation. The typical response mode in the sound leg of the hemiplegic patients was variable coactivation of the two muscles. In the afflicted leg, the establishment of a motor set was impaired: Low tone was associated with negligible muscular activity, although with more elevated tone, low level uniform cocontraction was evident.