Motor Output Variability Research Articles

Nonuniform activation of the agonist muscle does not covary with index finger acceleration in old adults

This study examined the patterns of activation in the superficial and deep parts of the first dorsal interosseus muscle and in the antagonist muscle, second palmar interosseus, during postural tasks (position holding) and slow movements (position tracking) of the index finger performed by young and old adults. The position-tracking task involved the index finger lifting light loads (2.5, 10, and 35% of maximum) with shortening and lengthening contractions as steadily as possible. Steadiness was quantified in both tasks as the standard deviation of index finger acceleration. The fluctuations in acceleration during the two tasks were greater for the old subjects (62-72 yr) compared with young subjects (19-27 yr), especially with the lightest loads. The two groups of subjects activated the superficial and deep parts of first dorsal interosseus at similar intensities during the position-holding task, whereas the deep part was more active during the shortening and lengthening contractions of the position-tracking task. The nonuniform activation of first dorsal interosseus, therefore, was not associated with the difference in the standard deviation of acceleration between the young and old subjects. Furthermore, there was no association between the average level of coactivation by the antagonist muscle and the standard deviation of acceleration for either group of subjects across these tasks. Thus the greater variability in motor output exhibited by the older adults could not be explained by either the nonuniform activation of the agonist muscle or the average level of coactivation by the antagonist muscle.

Old adults exhibit greater motor output variability than young adults only during rapid discrete isometric contractions.

The purpose of this study was to examine the ability of young and elderly individuals to control submaximum levels of force (5-90%) during continuous and rapid discrete isometric contractions of the quadriceps femoris. Participants were 24 young (25.3 +/- 2.8 years) and 24 elderly individuals (73.3 +/- 5.5 years) that were healthy and active. The strength of elderly participants was approximately 40% lower than young participants. The standard deviation and coefficient of variation (CV) of force were greater during discrete contractions than during continuous isometric contractions. During continuous isometric contractions, young and elderly participants exhibited similar CVs of force. During discrete contractions, however, elderly participants exhibited greater CVs for peak force and impulse and greater standard deviations and CVs for temporal characteristics than young participants. Results suggest that the control of force in active elderly people declines only during rapid discrete contractions and that this decline may be associated with declines in temporal characteristics of the force production.

Intermittency in the visual control of force in Parkinson's disease.

Studies on the variability of motor output in Parkinson's disease have found contrasting results depending on the speed-accuracy constraints of the task. The first goal of this study was to determine if Parkinson's disease subjects are more variable than control subjects. The second goal of the study was to examine the limitations on visual and motor processing that contribute to the changes in force variability in Parkinson's disease. Eight mild to moderate Parkinson's disease (age: 68-80 years) and eight matched control (age: 68-80 years) subjects maintained a constant level of force at 25% of their maximum voluntary contraction with their index finger and thumb (grip precision task) for 20 s while online visual feedback of the total force was viewed on a computer monitor. During the force task, subjects received visual feedback at varying frequencies. The sampled visual feedback levels were presented at intervals as slow as every 5 s to as fast as every 0.04 s (0.2, 0.4, 0.8, 1.6, 3.2, 6.4, 12.8, 25.6 Hz). Force variability decreased over sampled visual feedback according to hyperbolic decay functions. The minimal visual processing time for both the Parkinson's disease and control subjects was approximately 160 ms. Motor output corrections were generated in both groups at a frequency of 1 Hz over a wide range of sampled visual feedback levels. However, the amplitude of the 1-2 Hz visuo-motor corrective process was amplified in Parkinson's disease, and this related to increases in force-output variability. The findings suggest that the basal ganglia are important for adjusting the amplitude of motor output at 1-2 Hz during visuo-motor feedback control.

Speed/accuracy relations: The kinetic–kinematic link and predictions for rapid timing tasks

Recent accounts of the speed/accuracy relation for motor tasks have focused on the concept of motor output variability. We outline the advantages of this approach and the limitation of Plamondon's model in explaining movement error. We also examine and present complimentary data for rapid timing tasks. While these tasks do not meet the presented assumptions, the data still fit the model predictions.

Manual asymmetries: feedback processing, output variability, and spatial complexity-resolving some inconsistencies.

There is considerable evidence to document the case that the preferred hand is demonstrably superior for a number of manual tasks, although the mechanisms underlying this effect are less clear. Two perspectives have been dominant; one that emphasizes differential efficiency of feedback processing and one that suggests that asymmetries are a function of increased variability of output for the nonpreferred hand. This review considers the mediating effect of the complexity of the visual space in which aimed movements occur. Some inconsistencies may be resolved by noting the superiority of the right cerebral hemisphere for manipulation of spatial relationships. A multilevel, transactional perspective, which must then be adopted, may accommodate both feedback processing and motor output variability.

Relations between movement size and movement duration in 5 to 9 year-old children

Previous research into the handwriting of adult subjects has highlighted the possible roles of ‘relative timing’ (Viviani and Terzuolo 1980) and simple oscillations (Hollerbach 1981) in controlling movement. Both of these factors provide interesting perspectives with which to view the development of fine motor control in children. The spatio-temporal structure of writing movements was examined between children with refined handwriting skills and those exhibiting unusually poor writing for their age. The analysis concentrated on three sites of temporal variability: absolute timing, temporal patterning, and rhythmic content of the writing movements. The results supported the notion that the relative timing of segments within a movement sequence acts as a major constraint on motor output variability. It is suggested, however, that previous simplistic views of ‘relative timing’ be expanded, and that such timing may be dictated by optimisation strategies. Differences in the stability of the graphic product between groups could not be accounted for by differences in the rhythmic content of the movements. In the absolute timing of writing tasks, the results suggest that variability in writing time from trial to trial, and duration of intra-task pauses, are better indicators of writing difficulties than total writing time or the number of pauses.

Space-time invariance in handwriting: Contrasts between primary school children displaying advanced or retarded handwriting acquisition

Previous research into the handwriting of adult subjects has highlighted the possible roles of ‘relative timing’ (Viviani and Terzuolo 1980) and simple oscillations (Hollerbach 1981) in controlling movement. Both of these factors provide interesting perspectives with which to view the development of fine motor control in children. The spatio-temporal structure of writing movements was examined between children with refined handwriting skills and those exhibiting unusually poor writing for their age. The analysis concentrated on three sites of temporal variability: absolute timing, temporal patterning, and rhythmic content of the writing movements. The results supported the notion that the relative timing of segments within a movement sequence acts as a major constraint on motor output variability. It is suggested, however, that previous simplistic views of ‘relative timing’ be expanded, and that such timing may be dictated by optimisation strategies. Differences in the stability of the graphic product between groups could not be accounted for by differences in the rhythmic content of the movements. In the absolute timing of writing tasks, the results suggest that variability in writing time from trial to trial, and duration of intra-task pauses, are better indicators of writing difficulties than total writing time or the number of pauses.

Variability of motor output during flight steering in locusts

1. A flight balance has been used to demonstrate the generation of torques by a flying locust,Locusta migratoria, in response to roll movements of an artificial horizon. 2. The movements of the horizon can be perceived by either the compound eyes or the ocelli alone and lead to roll responses from the animal. 3. Amputation experiments show that either fore- or hindwings alone are able to produce a roll response of appropriate sign. 4. EMGs have been recorded from the flight muscles. Shifts in the relative latency between homologous pairs of depressor and elevator muscles are frequently correlated with roll responses. The directions of these shifts are opposite in the fore- and hindwings. The relative latencies of both elevators and depressors of a given wing shift in parallel. 5. No systematic changes in burst length correlated with roll responses are seen. This may be a consequence of the experimental conditions. 6. Roll torques can be developed without changes in the relative firing times of basalar (pronator) and subalar (supinator) muscles. 7. Changes in wingbeat amplitude are not always systematically correlated with roll responses. 8. Both fore- and hindwings beat out of phase during roll responses, a wing being advanced or retarded in its cycle in comparison to its contralateral homologue. As seen for latency shifts, the direction of these phase shifts is reversed between fore- and hindwings. The phase shift of the wing is probably the main result of the latency shifts of the flight muscles. 9. The motor output alterations which occur during roll responses of flying locusts are variable. The magnitude of the latency shifts can vary, changes in burst length are not indispensable, and neither rudderlike movements of the abdomen and the hindlegs nor changes in the wingbeat amplitude are necessarily correlated with torques. The flying locust is in every case producing a roll response.

Distinctions in variability of motor output between learning disabled and normal children.

This study measured the differences in motor output between groups of 8- and 10-yr.-old learning disabled and normal boys. Variability of motor output was investigated by having children learn an alternating tapping task to a criterion of a specified number of taps per minute and then having them transfer to more difficult tapping tasks with the same rhythmic beat but requiring more distance to move or more accuracy. Results suggest variability of motor output distinguishes normal and learning disabled boys.

The Relationship Between Force and Force Variability in Minimal and Near-Maximal Static and Dynamic Contractions

The critical assumption of linearity between force and force variability for rapid movements made by Schmidt, Zelaznik, and Frank (1978) was studied in four experiments in the present study. The first three experiments extended earlier work showing linearity between force and force variability for submaximal force levels in static and dynamic contractions. However, at near-maximal force levels, when force was increased, force variability leveled off and then decreased in both static and dynamic conditions. A fourth experiment using the rapid-timing paradigm showed that increased submaximal loads on the movement produced slight decreases in VE. But when the loads were larger, where force and force variability were no longer linearly related, increased load led to larger decreases in VE. These observations led to the hypothesis that VE is linearly related to the ratio of force variability to force; data from two experiments are presented that support this idea. The motor-output variability theory seems to hold for a wide range of sub-maximal force values, but needs modification for those conditions where near-maximal forces are required.

Motor-output variability: A theory for the accuracy of rapid motor acts.

Theoretical accounts of the speed-accuracy trade-off in rapid movement have usually focused on within-moveme nt error detection and correction, and have consistently ignored the possibility that motor-output variability might be predictably related to movement amplitude and movement time. This article presents a theory of motor-output variability that accounts for the relationship among the movement amplitude, movement time, the mass to be moved, and the resulting movement error. Predictions are derived from physical principles; empirical evidence supporting the principles is presented for three movement paradigms (single-aiming responses, reciprocal movements, and rapid-timing tasks); and the theory and data are discussed in terms of past theoretical accounts and future research directions. Examining the current level of understanding about the production and control of motor responses, many would no doubt be tempted to say that we have not come very far since the early writings of Woodworth (1899) and Hollingworth (1909). These writers were concerned with the basic laws of limb movements (analogous, perhaps to the basic laws of motion that were the cornerstone of physics) that denned the relationship between the simplest aspects of motor

Motor-output variability: A theory for the accuracy of rapid motor acts.

Motor-output variability: A theory for the accuracy of rapid motor acts.