Motor Output Variability Research Articles

Grasping time does not influence the early adherence of aperture shaping to Weber's law

The “just noticeable difference” (JND) represents the minimum amount by which a stimulus must change to produce a noticeable variation in one's perceptual experience (i.e., Weber's law). Recent work has shown that within-participant standard deviations of grip aperture (i.e., JNDs) increase linearly with increasing object size during the early, but not the late, stages of goal-directed grasping. A visually based explanation for this finding is that the early and late stages of grasping are respectively mediated by relative and absolute visual information and therefore render a time-dependent adherence to Weber's law. Alternatively, a motor-based explanation contends that the larger aperture shaping impulses required for larger objects gives rise to a stochastic increase in the variability of motor output (i.e., impulse-variability hypothesis). To test the second explanation, we had participants grasp differently sized objects in grasping time criteria of 400 and 800 ms. Thus, the 400 ms condition required larger aperture shaping impulses than the 800 ms condition. In line with previous work, JNDs during early aperture shaping (i.e., at the time of peak aperture acceleration and peak aperture velocity) for both the 400 and 800 ms conditions scaled linearly with object size, whereas JNDs later in the response (i.e., at the time of peak grip aperture) did not. Moreover, the 400 and 800 ms conditions produced comparable slopes relating JNDs to object size. In other words, larger aperture shaping impulses did not give rise to a stochastic increase in aperture variability at each object size. As such, the theoretical tenets of the impulse-variability hypothesis do not provide a viable framework for the time-dependent scaling of JNDs to object size. Instead, we propose that a dynamic interplay between relative and absolute visual information gives rise to grasp trajectories that exhibit an early adherence and late violation to Weber's law.

Age-Associated Differences in Positional Variability Are Greater With the Lower Limb

ABSTRACT The authors’ purpose was to determine the interaction of age and limb used on positional variability at different loads. Eleven young adults and 10 older adults were asked to accurately match and maintain a horizontal target line with 5° abduction of their index finger and 5° dorsiflexion of their ankle for 20 s at loads ranging from 2 to 50% of the maximal load that could be lifted with each limb. The visual gain was kept constant at 1° (visual angle). Positional variability was greater in older adults for both limbs, nonetheless age-associated differences were greater for the ankle dorsiflexion task compared with the abduction of the index finger task. In addition, we found that, independent of age, motor output variability was greater with the lower limb. These results provide novel evidence that older adults may exhibit greater impairments in motor control with the foot compared with the finger. Furthermore, these findings support the idea, using a different task than previous literature, that the lower limb has greater motor output variability than the upper limb.

Older Adults exhibit Greater Positional Variability during Ankle Dorsiflexion than Index Finger Abduction

The impaired ability of older adults to control force and position during low intensity contractions has been extensively demonstrated. Nonetheless, it is not clear whether these age-associated differences get influenced by the limb and load lifted during positional tasks. PURPOSE: To determine the interaction of age and limb on motor output variability during positional tasks. METHODS: Eleven young adults (25.6 ± 4.9 yrs, 7 women and 4 men) and ten older adults (76.7 ± 5.6 yrs, 6 women and 4 men) were asked to accurately match and maintain a horizontal target line with 5° abduction of their index finger and 5° dorsiflexion of their ankle. The loads were 2% (5% for the leg), 15%, 30%, and 50% of the maximal load (1 RM) that could be lifted with each limb. Each trial lasted 20 s and was performed 3 times at each condition. The visual gain was kept constant at 1° (visual angle) for all trials. Positional variability was quantified as the standard deviation (SD) from the detrended position trace of each limb. The neural activation of the involved agonist muscles was quantified as the EMG amplitude and modulation of the first dorsal interosseus (index finger abduction) and tibialis anterior (dorsiflexion) muscles. RESULTS: The heaviest load (50% 1 RM) significantly increased (P = 0.019) positional variability for both limbs compared with the lighter loads (2-30% MVC). Although positional variability was greater in older adults for both limbs (P = 0.004), age differences were significantly greater for the dorsiflexion task (young: 0.064 ° ± 0.011 vs. older: 0.143 ° ± 0.013) compared with the abduction of the index finger (young: 0.048 ° ± 0.003 vs. older: 0.082 ° ± 0.009) task (age × limb interaction; P = 0.012). The amplified positional variability in older adults was related to differential agonist modulation. Furthermore, the exacerbated positional variability during dorsiflexion in older adults was related to their inability to modulate power from 30-60 Hz in the tibialis anterior muscle. CONCLUSIONS: These results provide novel evidence that older adults may exhibit greater impairments in motor control with the leg compared with the hand because of their inability to modulate agonist muscle activity from 30-60 Hz. Supported by NIH Grant R01 AG031769 to EA Christou.

Morphology of hamstring torque‐time curves following acl injury and reconstruction: mechanisms and implications

The purposes of this study were (i) to examine the effects of anterior cruciate ligament (ACL) status on hamstring force steadiness, peak hamstring strength, quadriceps (antagonist) activation, and physical performance, and (ii) to evaluate the associations of physical performance with hamstring steadiness and hamstring strength. Thirteen subjects with unilateral deficiency of the ACL (ACLD), 39 matched subjects with unilateral reconstructed ACL (ACLR; n = 25 with bone-patella tendon-bone (ACLR-PT) graft and n = 14 with combined semitendinosus and gracilis tendon (ACLR-STGT) graft) and 33 control subjects participated. Each subject performed maximal-effort isokinetic knee flexion repetitions at 180° s(-1) with electromyography (EMG) electrodes attached to their medial and lateral quadriceps muscles. Physical performance was assessed using the single-limb long hop for distance. Wavelet-derived mean instantaneous frequency (Mif) of flexor torque-time curves was significantly (p < 0.05) higher (i.e., less smooth) in ACLR-STGT subjects compared to the ACLD, ACLR-PT and control subjects. No significant differences existed for peak hamstrings strength (i.e., peak torque produced) or quadriceps antagonist EMG activity. Positive correlations were identified between hamstrings force steadiness and quadriceps antagonist activity for ACLD (r = 0.797), ACLR-PT (r = 0.467), and ACLR-STGT (r = 0.628) subjects. For ACLR-STGT subjects, reduced hamstrings force steadiness associated with poorer long-hop performance (r = -0.695). Reduced steadiness amongst ACLR-STGT subjects may reflect motor output variability of the antagonist (i.e., quadriceps dyskinesia) and/or agonist musculature-a maladaptive feature which potentially contributes to poorer single-limb hop performance. Measures of hamstring force steadiness in combination with traditional measures of peak hamstring strength provide valuable clinical information regarding knee joint function following ACL injury/ACLR.

The motor neuron recruitment strategy and muscle anatomical properties determine the influence of synaptic noise on the variability of motor output

The motor neuron recruitment strategy and muscle anatomical properties determine the influence of synaptic noise on the variability of motor output

Human Theta Burst Stimulation Enhances Subsequent Motor Learning and Increases Performance Variability

Intermittent theta burst stimulation (iTBS) transiently increases motor cortex excitability in healthy humans by a process thought to involve synaptic long-term potentiation (LTP), and this is enhanced by nicotine. Acquisition of a ballistic motor task is likewise accompanied by increased excitability and presumed intracortical LTP. Here, we test how iTBS and nicotine influences subsequent motor learning. Ten healthy subjects participated in a double-blinded placebo-controlled trial testing the effects of iTBS and nicotine. iTBS alone increased the rate of learning but this increase was blocked by nicotine. We then investigated factors other than synaptic strengthening that may play a role. Behavioral analysis and modeling suggested that iTBS increased performance variability, which correlated with learning outcome. A control experiment confirmed the increase in motor output variability by showing that iTBS increased the dispersion of involuntary transcranial magnetic stimulation-evoked thumb movements. We suggest that in addition to the effect on synaptic plasticity, iTBS may have facilitated performance by increasing motor output variability; nicotine negated this effect on variability perhaps via increasing the signal-to-noise ratio in cerebral cortex.

Activity in a Cortical-Basal Ganglia Circuit for Song Is Required for Social Context-Dependent Vocal Variability

Variability in adult motor output is important for enabling animals to respond to changing external conditions. Songbirds are useful for studying variability because they alter the amount of variation in their song depending on social context. When an adult zebra finch male sings to a female ("directed"), his song is highly stereotyped, but when he sings alone ("undirected"), his song varies across renditions. Lesions of the lateral magnocellular nucleus of the anterior nidopallium (LMAN), the output nucleus of a cortical-basal ganglia circuit for song, reduce song variability to that of the stereotyped "performance" state. However, such lesions not only eliminate LMAN's synaptic input to its targets, but can also cause structural or physiological changes in connected brain regions, and thus cannot assess whether the acute activity of LMAN is important for social modulation of adult song variability. To evaluate the effects of ongoing LMAN activity, we reversibly silenced LMAN in singing zebra finches by bilateral reverse microdialysis of the GABA(A) receptor agonist muscimol. We found that LMAN inactivation acutely reduced undirected song variability, both across and even within syllable renditions, to the level of directed song variability in all birds examined. Song variability returned to pre-muscimol inactivation levels after drug washout. However, unlike LMAN lesions, LMAN inactivation did not eliminate social context effects on song tempo in adult birds. These results indicate that the activity of LMAN neurons acutely and actively generates social context-dependent increases in adult song variability but that social regulation of tempo is more complex.

Changes in muscle fascicles of tibialis anterior during anisometric contractions are not associated with motor-output variability of the ankle dorsiflexors in young and old adults

This study examined the associations between the fluctuations of foot acceleration during shortening and lengthening contractions with the electromyographic (EMG) activity of lower leg muscles and ultrasound measures of tibialis anterior fascicle length and pennation angle. Young (24.9 ± 4.17 years) and old (74.8 ± 3.31 years) adults lifted and lowered a submaximal load with the foot at different speeds (3°/s-50°/s). The standard deviation (SD) of foot acceleration normalized to the load lifted was similar for young (12.2 ± 7.22 cm s(-2)/kg) and old adults (14.3 ± 8.03 cm s(-2)/kg; P = 0.093). The changes in tibialis anterior muscle fascicle length and pennation angle were similar for young and old adults (P ≥ 0.233), but greater for shortening (fascicle length: 0.937 ± 0.633 cm, pennation angle: 1.61 ± 0.918º) than for lengthening contractions (fascicle length: 0.806 ± 0.521 cm, pennation angle: 0.966 ± 0.632º; P ≤ 0.014). The changes in fascicle length and pennation angle were not associated with the SD of foot acceleration (r(2) ≤ 0.031; P ≥ 0.092). The surface EMG of tibialis anterior was greater for the shortening contractions than for the lengthening contractions (P < 0.001), but triceps surae EMG was similar for the two types of contractions (P = 0.304). The results suggested that the influence of movement speed on variability in performance was similar for shortening and lengthening contractions with the dorsiflexor muscles; furthermore, old adults were able to match the performance of young adults.

Variability in spinal and supraspinal respiratory motor output in a mouse model of Pompe Disease

Pompe disease is a lysosomal storage disorder reflecting systemic acid α‐glucosidase (Gaa) deficiency. We recently reported that a murine Pompe model (the Gaa−/− mouse) shows glycogen accumulation in phrenic motoneurons, reduced breathing frequency and an apparent reduction in phrenic burst amplitude (DeRuisseau et al. PNAS, 2009). We hypothesized that alterations in the neural control of breathing in Gaa−/− mice would result in increased breath‐to‐breath variability in the timing and amplitude of efferent spinal (phrenic) and brainstem (hypoglossal, XII) signals. Neurograms recorded in anesthetized, vagotomized, and ventilated mice showed that the coefficient of variation (CV) of inspiratory duration was larger in Gaa−/− vs. B6/129 control mice (P&lt;0.05). The burst amplitude CV for Gaa−/− mice tended to be greater for phrenic (P=0.07) and was greater for XII output (P&lt;0.05). Additional experiments are testing the hypothesis that intraspinal delivery of the Gaa gene via adeno‐associated virus (AAV5‐GAA) mitigates glycogen accumulation and respiratory control deficits in Gaa−/− mice. Immunhistological data obtained 4–8 wks post AAV5‐GAA injection (4–6 0.5 μL injections, 4.5×1013 vg/ml) indicate Gaa expression in cervical spinal motoneurons and interneurons. We are presently evaluating the impact of the spinal injection on glycogen accumulation and ventilation.

Aging, time scales, and sensorimotor variability.

It is well established that there is an increased amount of intraindividual variability with aging in a variety of behavioral contexts. Here, we elaborate from a self-organization and dynamic systems framework to investigate the relevant time scales of variability as a function of aging and their relation to the changes in the amount and structure (frequency and time domains) of movement and postural variability. In particular, we examine evidence for the general hypotheses that (a) there is a reduction or even loss of shorter time scales in the control of movement with aging and (b) the shorter the time scale in motor output variability, the more sensitive the measure is as a biomarker to revealing the onset and early influence of aging and disease. The dynamic analysis of the time scales of variability distinguishes the distinctive roles of stability and noise in the increased amount of intraindividual variability with aging.

Steadiness During Plantar Flexion Is Associated With Postural Sway In Young But Not Elderly Adults

Fluctuations in motor output during steady isometric contractions with a muscle or muscle group are often greater in elderly than young adults, but the functional significance of force fluctuations during isolated muscle contractions is not determined. PURPOSE: The present study examined the association between force fluctuations during isolated contractions with plantar flexor muscles and postural sway during quiet standing in young and elderly adults. METHODS: Twenty young (28.1 ± 4.0 yrs) and 20 elderly (69.7 ± 2.8 yrs) subjects performed a force-matching task with unilateral isometric plantar flexion (PF) at 2.5%, 5%, 10%, 15%, and 20% of subjects‘ maximal voluntary contraction (MVC) for 30 s. The knee joint was fully extended and the ankle joint was in the neutral position during PF. They also maintained quiet standing (QS) on a force platform for 30 s as a separate task. To quantify the variability of motor output, the coefficient of variation (CV) of force during PF and the CV of center of pressure displacement during QS were calculated. Surface electromyogram (EMG) was recorded from the soleus, medial gastrocnemius and lateral gastrocnemius muscles during PF and QS. Power spectrum density of the rectified EMG was calculated to characterize the muscle activation strategies. RESULTS: In young subjects, a significant (P < 0.05) positive correlation was found in the CV between QS and PF at lower contraction intensities (2.5% and 5% of MVC). There was no correlation between QS and PF at any contraction intensity in elderly adults. During PF, the EMG power around 10 Hz was significantly (P < 0.05) greater in the medial gastrocnemius in elderly than young adults at low contraction intensities (2.5% and 5% of MVC). There was no difference in the EMG power distribution in other muscles between young and elderly adults during PF. During QS, the EMG power around 10 Hz was significantly (P < 0.05) greater in all the muscles in elderly than young adults. CONCLUSION: The variability in postural sway during quiet standing was correlated to the force fluctuations during isolated plantar flexion at low contraction intensities in young but not elderly adults. The presence or absence of association between postural sway and force fluctuations may be influenced by the muscle activation strategy around 10 Hz. Supported by NIH Grant NS052480

Dissociating Variability and Effort as Determinants of Coordination

When coordinating movements, the nervous system often has to decide how to distribute work across a number of redundant effectors. Here, we show that humans solve this problem by trying to minimize both the variability of motor output and the effort involved. In previous studies that investigated the temporal shape of movements, these two selective pressures, despite having very different theoretical implications, could not be distinguished; because noise in the motor system increases with the motor commands, minimization of effort or variability leads to very similar predictions. When multiple effectors with different noise and effort characteristics have to be combined, however, these two cost terms can be dissociated. Here, we measure the importance of variability and effort in coordination by studying how humans share force production between two fingers. To capture variability, we identified the coefficient of variation of the index and little fingers. For effort, we used the sum of squared forces and the sum of squared forces normalized by the maximum strength of each effector. These terms were then used to predict the optimal force distribution for a task in which participants had to produce a target total force of 4–16 N, by pressing onto two isometric transducers using different combinations of fingers. By comparing the predicted distribution across fingers to the actual distribution chosen by participants, we were able to estimate the relative importance of variability and effort of 1∶7, with the unnormalized effort being most important. Our results indicate that the nervous system uses multi-effector redundancy to minimize both the variability of the produced output and effort, although effort costs clearly outweighed variability costs.

Origins of submovements during pointing movements

Submovements that are frequently observed in the final portion of pointing movements have traditionally been viewed as pointing accuracy adjustments. Here we re-examine this long-lasting interpretation by developing evidence that many of submovements may be non-corrective fluctuations arising from various sources of motor output variability. In particular, non-corrective submovements may emerge during motion termination and during motion of low speed. The contribution of these factors and the factor of accuracy regulation in submovement production is investigated here by manipulating movement mode (discrete, reciprocal, and passing) and target size (small and large). The three modes provided different temporal combinations of accuracy regulation and motion termination, thus allowing us to disentangle submovements associated with each factor. The target size manipulations further emphasized the role of accuracy regulation and provided variations in movement speed. Gross and fine submovements were distinguished based on the degree of perturbation of smooth motion. It was found that gross submovements were predominantly related to motion termination and not to pointing accuracy regulation. Although fine submovements were more frequent during movements to small than to large targets, other results show that they may also be not corrective submovements but rather motion fluctuations attributed to decreases in movement speed accompanying decreases in target size. Together, the findings challenge the traditional interpretation, suggesting that the majority of submovements are fluctuations emerging from mechanical and neural sources of motion variability. The implications of the findings for the mechanisms responsible for accurate target achievement are discussed.

Endpoint accuracy for a small and a large hand muscle in young and old adults during rapid, goal-directed isometric contractions

The minimum variance theory proposes that stronger (larger) muscles produce less variable trajectories compared with weaker (smaller) muscles and thus can accomplish more accurate contractions. The purpose of the study was to determine the influence of muscle size and trajectory variability on the endpoint accuracy of goal-directed isometric contractions. Twelve young (25 +/- 5 years) and 12 old adults (76 +/- 6 years) performed 100 trials with each of two muscles in both hands. Subjects were instructed to match the peak of a force trajectory to a target force by controlling either the abduction (first dorsal interosseus muscle; FDI) or adduction force (second palmar interosseus muscle; SPI) exerted by the index finger of each hand. The time to peak force was 150 ms and the peak force required was 25% of the maximal force that could be achieved in 150 ms. Endpoint accuracy and variability in force and time along with intramuscular EMG activity of the agonist muscle (FDI and SPI) involved in each task were quantified for each set of 100 trials. The MVC force was less for the SPI muscle, and the force endpoint error and variance were greater in the SPI muscle compared with the FDI muscle. Conversely, endpoint measures that included timing were similar for the two muscles. Trajectory variability was greater for the FDI muscle, but did not influence endpoint error for either muscle. The young and old adults had similar strength values, but the old adults were less accurate and more variable than the young subjects. Nonetheless, the accuracy and variability displayed by the old adults for the two muscles was the same as that observed for the young adults. The force accuracy and variability findings are consistent with the predictions of the minimum variance theory that motor-output variability is inversely related to muscle size, strength, and motor unit number.

Speed, variability, and timing of motor output in ADHD: which measures are useful for endophenotypic research?

Attention-Deficit/Hyperactivity Disorder (ADHD) shares a genetic basis with motor coordination problems and probably motor timing problems. In line with this, comparable problems in motor timing should be observed in first degree relatives and might, therefore, form a suitable endophenotypic candidate. This hypothesis was investigated in 238 ADHD-families (545 children) and 147 control-families (271 children). A motor timing task was administered, in which children had to produce a 1,000 ms interval. In addition to this task, two basic motor tasks were administered to examine speed and variability of motor output, when no timing component was required. Results indicated that variability in motor timing is a useful endophenotypic candidate: It was clearly associated with ADHD, it was also present in non-affected siblings, and it correlated within families. Accuracy (under- versus over-production) in motor timing appeared less useful: Even though accuracy was associated with ADHD (probands and affected siblings had a tendency to under-produce the 1,000 ms interval compared to controls), non-affected siblings did not differ from controls and sibling correlations were only marginally significant. Slow and variable motor output without timing component also appears present in ADHD, but not in non-affected siblings, suggesting these deficits not to be related to a familial vulnerability for ADHD. Deficits in motor timing could not be explained by deficits already present in basic motor output without a timing component. This suggests abnormalities in motor timing were predominantly related to deficient motor timing processes and not to general deficient motor functioning. The finding that deficits in motor timing run in ADHD-families suggests this to be a fruitful domain for further exploration in relation to the genetic underpinnings of ADHD.

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.

The amplitude of force variability is correlated in the knee extensor and elbow flexor muscles

The purpose of this study was to determine motor output variability for different muscles in the same individuals. Ten young (21.7 +/- 3.4 years) and ten elderly (72.1 +/- 3.9 years) men underwent assessment of maximal isometric (MVC) and dynamic (1-RM) strength, and performed constant-force (2-50% MVC), constant-load (5-50% 1-RM load), and unloaded postural contractions as steadily as possible with the first dorsal interosseus (FDI), elbow flexors (EF), and knee extensors (KE). The coefficient of variation (CV) of force for isometric contractions and the standard deviation (SD) of acceleration for concentric, eccentric, and postural contractions were calculated. The 1-RM load, the CV of force for four of five isometric target forces, and the SD of acceleration during postural contractions were correlated between the EF and KE muscles. MVC force, 1-RM load, and SD of acceleration during postural contractions were not correlated between the FDI/EF or FDI/KE. The CV of force was correlated between the FDI/EF and FDI/KE for two of five isometric target forces. The SD of acceleration during concentric and eccentric contractions was not correlated between muscles. The normalized fluctuations during isometric contractions were greater for the FDI compared with the EF and KE. Elderly adults displayed greater fluctuations only for the FDI during low-force isometric and postural contractions. The dominant frequency of fluctuations was similar for the EF and KE muscles. The correlated fluctuations for the EF and KE muscles, within subjects, suggests that the two motor neuron pools transform the various neural inputs similarly.

Stability and motor adaptation in human arm movements

In control, stability captures the reproducibility of motions and the robustness to environmental and internal perturbations. This paper examines how stability can be evaluated in human movements, and possible mechanisms by which humans ensure stability. First, a measure of stability is introduced, which is simple to apply to human movements and corresponds to Lyapunov exponents. Its application to real data shows that it is able to distinguish effectively between stable and unstable dynamics. A computational model is then used to investigate stability in human arm movements, which takes into account motor output variability and computes the force to perform a task according to an inverse dynamics model. Simulation results suggest that even a large time delay does not affect movement stability as long as the reflex feedback is small relative to muscle elasticity. Simulations are also used to demonstrate that existing learning schemes, using a monotonic antisymmetric update law, cannot compensate for unstable dynamics. An impedance compensation algorithm is introduced to learn unstable dynamics, which produces similar adaptation responses to those found in experiments.

Yoga Training Does Not Alter The Steadiness Of Muscle Contractions In Young Adults

The variability of motor output often changes after exercise interventions such as strength training and Tai-Chi. PURPOSE To determine the effect of Bikram yoga training on the strength and steadiness of the elbow flexor (EF) and knee extensor (KE) muscles. METHODS Healthy adults were assigned to yoga training (24 sessions in eight weeks, N=10; 29 ± 6 yrs) or a control group (N=8; 26 ± 7 yrs). Yoga sessions consisted of 1.5 hours of supervised and highly standardized postures. Assessments before and after training included 1) maximum voluntary contractions (MVC), 2) steadiness during submaximal constant-force isometric contractions of the EF and KE muscles and slow unloaded shortening and lengthening contractions of the KE muscles, 3) sit-and-reach flexibility, 4) standing balance test, 5) isometric dead lift, 6) maximal treadmill test, and 7) body composition (DEXA). The isometric, shortening, and lengthening contractions were performed as steadily as possible with and without visual feedback. DC drift in the force (< 0.05Hz) was removed before measuring the fluctuations. Measurements included coefficient of variation (CV) of force and standard deviation (SD) of acceleration during constant-force and constant-velocity muscle contractions, peak force (N) during strength tasks, time to balance on one leg, sit-and-reach score, resting and maximal heart rate, blood pressure, VO2 max, and total body percent fat. RESULTS MVC force increased 5.1% for elbow flexors, 10.1% for knee extensors, and 11.5% for the dead lift after yoga training. Sit-and-reach and balance performance improved by 26% (+10.3cm, +4.9s). Total body percent fat decreased from 28.4 to 27.3% after training. Resting and maximal heart rate, resting blood pressure, and V02 max were unchanged after training. Reductions in the CV of force during constant-force contractions and SD of acceleration during shortening and lengthening contractions were not different from control. CONCLUSION: This highly standardized yoga program resulted in improved muscle strength, balance, flexibility, and modestly reduced body fat, but did not alter cardiovascular fitness. Although this training requires precise control of muscle force, the neural factors that underlie variability of the output of the motor unit pool of upper and lower body muscles did not demonstrate plasticity in healthy young adults. Supported by AG19171 to BLT.

Drift in blind reciprocal aiming movements.

There is anecdotal evidence of drift in various reciprocal motor tasks, but as far as is known, no investigations into this phenomenon have been reported. Yet, systematic drift can potentially explain a significant proportion of the total variability in motor output. Three experiments were conducted to ascertain the nature of drift in reciprocal aiming tasks and to develop methods and measures to isolate and quantify drift for analyses. We also evaluated a computational posture-based model of reaching movements with respect to the findings of the experiments. Drift was observed in all three experiments, generally toward the middle of the joint motility range. Simulations based on the model produced drift to the middle of the task movement range rather than middle of the joint movement range. Adding noise to the model could increase its power for simulating the underlying principles of movement control as reflected in performance features such as drift.