Biomedical subjects

J P Scholz

Publications and source records attributed to J P Scholz.

15 recordsLinked to original sources

PubMed2005

Learning a throwing task is associated with differential changes in the use of motor abundance.

This study sought to characterize changes in the synergy of joint motions related to learning a Frisbee throwing task and, in particular, how the use of abundant solutions to joint coordination changed during the course of learning for successful performance. The latter information was helpful in determining the relative importance of different performance-related variables (PVs) to performance improvement. Following a pre-test, the main experiment consisted of six subjects practicing a Frisbee throw to a laterally-placed target for five days, 150 throws per day, followed by a post-test. A subgroup of three subjects continued to practice for an extended period of extensive practice amounting to 1800-2700 additional throws each, followed by a second post-test. Motor abundance was addressed through the uncontrolled manifold approach (UCM), which was used to partition the variance of joint configurations into two components with respect to relevant PVs, one component leading to a consistent value of the PV across repetitions, and a reflection of motor abundance, and a second component resulting in unstable values of the relevant PV. The method was used to test hypotheses about the relative importance of controlling the PVs that have an impact on successful task performance: movement extent, movement direction, hand path velocity, and the hand's orientation to the target. In addition, the amount of self-motion, or apparently extraneous joint motion having no effect on the hand's motion, compared to joint motion that does affect the hand's motion, was determined. After a week of practice, all subjects showed improvement in terms of targeting accuracy. Hand movement variability also decreased with practice and this was associated with a decrease in overall joint configuration variance. This trend continued to a greater extent in the three subjects who participated in extended practice. Although the component of joint configuration variance that was consistent with a stable value of all PVs was typically substantially higher than variance leading to unstable values of those PVs, both components decreased with practice. However, the decrease in joint configuration variance reflecting motor abundance was less than the other variance component only in relation to control of movement direction and the hand's orientation to the target. These results indicate that improvement of throwing performance in this experiment was more related to improved stabilization of movement direction and to the hand's orientation to the target than to movement extent and hand velocity. Nonetheless, the relative values of the two joint variance components were such that the instantaneous value of both hand path velocity and movement extent were stabilized throughout the experiment and showed a consistent compensatory relationship at the time of Frisbee release, despite not changing with practice. Finally, the amount of self-motion increased significantly with practice, possibly reflecting better compensation for perturbations due to the limb's dynamics. The results are consistent with other studies, suggesting the need to reevaluate Bernstein's hypothesis of freeing and freezing DOFs with learning.

Arm↗

PubMed2001

Effects of varying task constraints on solutions to joint coordination in a sit-to-stand task.

The question of how multijoint movement is controlled can be studied by discovering how the variance of joint trajectories is structured in relation to important task-related variables. In a previous study of the sit-to-stand task, for instance, variations of body segment postures that leave the position of the body's center of mass (CM) unchanged were significantly greater than variations of body segment posture that varied the CM position. The present experiments tested the hypothesis that such structuring of joint configuration variability is accentuated when the mechanical or perceptual task demands are made more challenging. Six subjects performed the sit-to-stand task without vision (eyes closed), either on a normal or on a narrow support surface. An additional constraint on the postural task was introduced in a third condition by requiring subjects to maintain light touch (less than 1 N) with the fingertips while coming to a standing position on the narrow base of support. The joint configurations observed at each point in normalized time were analyzed with respect to trial-to-trial variability. The task variables CM and head position were used to define goal-equivalent sets of joint configurations ("uncontrolled manifolds," UCMs) within which variation of joint configuration leaves the task variables unchanged. The variability of joint configurations across trials was decomposed into components that did not affect (within the UCM) and that did affect (orthogonal to the UCM) the values of these task variables. Our results replicate the earlier finding of much larger variability in directions of joint space that leave the CM unchanged compared with directions that affect CM position. This effect was even more pronounced here than in the previous experiment, probably because of the more difficult perceptual conditions in the current study (eyes closed). When the mechanical difficulty of the task was increased, the difference between the two types of joint variability was further accentuated, primarily through increase in goal-equivalent variance. This provides evidence for the hypothesis that under challenging task constraints increased variability is selectively directed into task-irrelevant degrees of freedom. Because differential control along different directions of joint space requires coordination among joint angles, this observation supports the view that the CNS responds to increased task difficulty through enhanced coordination among degrees of freedom. The adaptive nature of this coordination is further illustrated by the similar enhanced use of goal-equivalent joint combinations to achieve a stable CM position when subjects stood up under the additional constraint of maintaining light touch with the fingertips. This was achieved by channeling goal-equivalent variability into different directions of joint configuration space.

Adult↗

PubMed2001

Dynamic stability in the anterior cruciate ligament deficient knee.

Some individuals can stabilize their knees following anterior cruciate ligament rupture even during activities involving cutting and pivoting (copers), others have instability with daily activities (non-copers). Movement and muscle activation patterns of 11 copers, ten non-copers and ten uninjured subjects were studied during walking and jogging. Results indicate that distinct gait adaptations appeared primarily in the non-copers. Copers used joint ranges of motion, moments and muscle activation patterns similar to uninjured subjects. Non-copers reduced their knee motion, and external knee flexion moments that correlated well with quadriceps strength. Non-copers also achieved peak hamstring activity later in the weight acceptance phase and used a strategy involving more generalized co-contraction. Both copers and non-copers had high levels of quadriceps femoris muscle activity. The reduced knee moment in the involved limbs of the non-copers did not represent "quadriceps avoidance" but rather represented a strategy of general co-contraction with a greater relative contribution from the hamstring muscles.

Adult↗

PubMed2000

Identifying the control structure of multijoint coordination during pistol shooting.

The question of degrees of freedom in the control of multijoint movement is posed as the problem of discovering how the motor control system constrains the many possible combinations of joint postures to stabilize task-dependent essential variables. Success at a task can be achieved, in principle, by always adopting a particular joint combination. In contrast, we propose a more selective control strategy: variations of the joint configuration that leave the values of essential task variables unchanged are predicted to be less controlled (i.e., stabilized to a lesser degree) than joint configuration changes that shift the values of the task variables. Our experimental task involved shooting with a laser pistol at a target under four conditions. The seven joint angles of the arm were obtained from the recorded positions of markers on the limb segments. The joint configurations observed at each point in normalized time were analyzed with respect to trial-to-trial variability. Different hypotheses about relevant task variables were used to define sets of joint configurations ("uncontrolled manifolds" or UCMs) that, if realized, would leave essential task variables unchanged. The variability of joint configurations was decomposed into components lying parallel to those sets and components lying in their complement. The orientation of the gun's barrel relative to a vector pointing from the gun to the target was the task variable most successful at showing a difference between the two components of joint variability. This variable determines success at the task. Throughout the movement, not only while the gun was pointing at the target, fluctuations of joint configuration that affected this variable were much reduced compared with fluctuations that did not affect this variable. The UCM principle applied to relative gun orientation thus captures the structure of the motor control system across different parts of joint configuration space as the movement evolves in time. This suggests a specific control strategy in which changes of joint configuration that are irrelevant to success at the task are selectively released from control. By contrast, constraints representing an invariant spatial position of the gun or of the arm's center of mass structured joint configuration variability in the early and mid-portion of the movement trajectory, but not at the time of shooting. This specific control strategy is not trivial, because a target can be hit successfully also by controlling irrelevant directions in joint space equally to relevant ones. The results indicate that the method can be successfully used to determine the structure of coordination in joint space that underlies the control of the essential variables for a given task.

Adult↗

PubMed1999

The uncontrolled manifold concept: identifying control variables for a functional task.

The degrees of freedom problem is often posed by asking which of the many possible degrees of freedom does the nervous system control? By implication, other degrees of freedom are not controlled. We give an operational meaning to "controlled" and "uncontrolled" and describe a method of analysis through which hypotheses about controlled and uncontrolled degrees of freedom can be tested. In this conception, control refers to stabilization, so that lack of control implies reduced stability. The method was used to analyze an experiment on the sit-to-stand transition. By testing different hypotheses about the controlled variables, we systematically approximated the structure of control in joint space. We found that, for the task of sit-to-stand, the position of the center of mass in the sagittal plane was controlled. The horizontal head position and the position of the hand were controlled less stably, while vertical head position appears to be no more controlled than joint motions.

Biomechanical Phenomena↗

PubMed1995

Neuromuscular coordination of squat lifting, I: Effect of load magnitude.

BACKGROUND AND PURPOSE: In this study, we examined changes in kinematic and electromyographic (EMG) measurements of the coordination (ie, the relative timing of joint movements and muscle activity) of a squat-lifting task in response to lifting increasing loads. SUBJECTS: Fifteen male industrial workers served as a sample of convenience. METHODS: Subjects lifted a weighted crate containing 15% to 75% of their maximum lifting capacity using a symmetrical squat-lift technique. Movement kinematics were obtained with videography. The relative phase between joint motions was derived. The EMG activity of the vastus lateralis muscle (VL) and the erector spinae muscle (ES) was recorded, and the relative timing of their onsets and peaks was estimated. RESULTS: The relative phase of movement between joints such as the knee and lumbar spine changed in a quasi-linear fashion with increasing load during lifting but not during lowering. The relative time of onset of ES EMG activity and its peak activity changed in a manner consistent with the interjoint relative phase results. The timing of VL events were not affected by increasing the load. CONCLUSION AND DISCUSSION: Relatively continuous changes in interlimb coordination occur when increasing the load lifted from an initial squatting posture. Changes in EMG relative timing partially corroborate the kinematic evidence for changes in coordination with load scaling. The results indicate the need for further study to determine whether the observed changes in coordination are beneficial or detrimental to the musculoskeletal system. Clinicians should evaluate performance of this task under a range of task conditions.

Adult↗

PubMed1995

Neuromuscular coordination of squat lifting, II: Individual differences.

BACKGROUND AND PURPOSE: This article reports individual differences in the coordination (ie, the relative timing of joint movements and muscle activity) of squat lifting identified by extended analysis of data reported in the authors' companion article in this issue. SUBJECTS: Two post hoc groups of 6 subjects each were identified from the original sample of 15 subjects based on qualitative differences in knee-lumbar spine relative motion plots during load acceleration. METHODS: Subjects lifted a crate containing 15% to 75% of their maximum lifting capacity using a symmetrical squat-lift technique. Movement kinematic data were obtained with videography, and the electromyographic (EMG) activity of the vastus lateralis and erector spinae muscles was recorded with surface EMG. Measurements of coordination derived both kinematically and via EMG and the kinematic data were examined for group differences. RESULTS: Subjects in group 2 limited lumbar spine motion during load acceleration for all loads lifted, whereas those in group 1 limited lumbar spine motion more when lifting the heaviest loads. These differences were obvious both qualitatively, via knee-lumbar spine relative motion plots, and quantitatively, via measures of the relative timing of joint motions early in the lift. The effect of load on the coordination of these joints was the same for both post hoc groups after initial load acceleration. Significant differences in other kinematic measurements were also found between these groups. CONCLUSION AND DISCUSSION: Despite specific instructions about how to lift the load, individual subjects coordinated their joints differently during the initial, accelerative phase of squat lifting. Individual differences in coordination in response to load increases could be categorized into two patterns, although the data of 2 subjects were difficult to categorize and thus not included in these analyses. Whether the two dominant patterns have consequences for stress to the joints during lifting remains to be determined.

Acceleration↗

PubMed1993

The effect of imposed leg length difference on pelvic bone symmetry.

This study was designed to examine the effect of varying degrees of imposed leg length difference on symmetry of the innominate bones in healthy college women with relatively equal leg lengths. Subjects' leg length was determined by clinical and roentgenographic procedures. Position of the innominate bones with and without lifts under one foot was measured with respect to the transverse plane using the Waterloo Spatial Motion and Recording Technique (WATSMART; Northern Digital, Inc., Waterloo, Ontario, Canada), which allows very high spatial resolution. Our results show that posterior innominate bone rotation occurs on the side of the lengthened limb, and anterior rotation occurs on the shorter limb. The amount of pelvic obliquity increased in an approximately linear fashion as the leg length difference was increased from 2/8 to 7/8 inch. Some individual differences were evident.

Adult↗

PubMed1993

Accuracy and precision of the PEAK Performance Technologies Motion Measurement System.

This study evaluated the accuracy and precision of the PEAK Performance Technologies, lnc.'s motion analysis system for three-dimensional angle reconstruction. Pendular motion of a bar, on which 18 retroreflective markers were mounted, was videotaped at three different orientations (parallel, and rotated 30 degrees right and left) to a plane at which two standard video cameras were aimed. The videotaped motion was digitized off-line, and 32 angles between the 18 markers were calculated. intraclass correlation coefficients (ICCs) were calculated between trials within each pendulum orientation and across orientations to determine system precision, and between randomly selected trials and actual angles to determine accuracy. lCCs were in all cases greater than.99. Within-trial standard deviations ranged between 0.05-0.8 degrees for the different angles. Deviations from the actual angle averaged 0.0-0.8 degrees across all angles and orientations. The results indicate that accurate and reliable angular measurements can be made with this motion analysis system.

Journal Article↗

PubMed1990

Intentional switching between patterns of bimanual coordination depends on the intrinsic dynamics of the patterns.

Two predictions arising from previous theoretical and empirical work which demonstrated that spontaneous changes of bimanual coordination patterns result from a loss of pattern stability (i.e., a nonequilibrium phase transition) were tested: (a) that the time it takes to intentionally switch from one pattern to another depends on the differential stability of the patterns themselves; and (b) that an intention, defined as an intended behavioral pattern, can change the dynamical characteristics, e.g., the overall stability of the behavioral patterns. Subjects moved both index fingers rhythmically at one of six movement frequencies while performing either an in-phase or antiphase pattern of finger coordination. On cue from an auditory signal, subjects switched from the ongoing pattern to the other pattern. The relative phase of movement between the two fingers was used to characterize the ongoing coordinative pattern. The time taken to switch between patterns, or switching time, and relative phase fluctuations were used to evaluate the modified pattern dynamics resulting from a subject's intention to change patterns. Switching from the in-phase to the antiphase pattern was significantly slower than switching in the opposite direction for all subjects. Both the mean and distribution of switching times in each direction were found to be in agreement with model predictions. movement frequency had little effect on switching time, a finding that is also consistent with the model. Relative phase fluctuations were significantly larger when moving in the antiphase pattern at the highest movement frequencies studied. The results show that, although intentional influences act to modify a coordinative pattern's intrinsic dynamics, the influence of these dynamics on the resulting behavior is always present and is particularly strong at high movement frequencies.

Journal Article↗

PubMed1990

Dynamic pattern theory--some implications for therapeutics.

This article introduces the Dynamic Pattern Theory of movement coordination and discusses possible implications of the theory for therapy. Basic constructs such as order parameters and control parameters, fluctuations, time-scale relations, and self-organization are discussed. Emphasis is given to their potential use to assess the coordination of functional motor acts. The theory's predictions about how motor acts are organized and the implications of its tenets for motor learning and the recovery of motor function are described briefly. A number of implications derived from the theory may support treatment strategies already in place and provide new insights for the future development of such strategies.

Exercise Therapy↗

PubMed1989

A quantitative approach to understanding the formation and change of coordinated movement patterns.

This study shows how the identification of relevant collective variables for behavioral patterns and evaluation of their stability may lead to a better understanding of processes underlying pattern change in coordinated human motor behavior. Based upon observed transitions in behavioral patterns, relative phase was identified as an appropriate collective variable for characterizing different patterns of bimanual coordination. The stability of this collective variable was determined through perturbation techniques. Subjects performed one of two patterns of hand coordination (in phase and out of phase) as the frequency of movement was systematically increased. Torque perturbations were administered in order to disturb the ongoing coordinative pattern, and the time required to return to the same pattern (the relaxation time) was measured. A transition from an out-of-phase to an in-phase pattern of coordination occurred at a critical frequency in all subjects. Moreover, when subjects began a trial in the out-of-phase pattern, a strong and statistically significant increase in relaxation time was observed as the frequency increased prior to the transition, indicating a loss of pattern stability. No transitions occurred when subjects began in the in-phase pattern of coordination; nor was there a change in the measured relaxation time as a function of frequency. Together with earlier results on critical fluctuations, these findings indicate that instabilities play an important role in effecting change in patterns of motor coordination. The potential relevance of these concepts and quantitative tools for other motor activities and other levels of description, for example, neural patterns, is discussed.

Journal Article↗

PubMed1989

Reliability and validity of the WATSMART three-dimensional optoelectric motion analysis system.

Reliability and validity of the WATSMART (Waterloo Spatial Motion Analysis Recording Technique) system was evaluated under static and dynamic conditions. In experiment 1, infrared light-emitting diodes (IREDs) were placed at the axis and along the arms of a clinical goniometer. Twelve angles in 5-degree increments were each recorded 10 times at each of three spatial locations. Reliability was assessed using intraclass correlation coefficients (ICCs) and analysis-of-variance procedures to determine within-trial variability. The ICCs for all spatial locations exceeded .99. The 95% confidence interval for each angle was less than 0.5 degree in all cases. Criterion-referenced instrument validity was assessed with regression analysis. Slopes of the regression of reconstructed angle on reference angle were close to unity for each spatial location. A systematic error, however, that increased as the goniometer was rotated 45 degrees away from the cameras was evident. In experiment 2, a robotic arm was fitted with four IREDs and made to repeat a defined movement trajectory 10 times at each of three spatial locations. The ICCs for portions of each trajectory ranged from .20 to .99. The results show that reliable and valid results can be obtained from this motion analysis system if adequate precautions are taken to reduce unwanted light reflections. Reliability and validity decreased somewhat as the object was rotated further away from the plane in which the cameras were mounted.

Humans↗

PubMed1988

Infants at risk for sudden infant death syndrome (SIDS): differential prediction for three siblings of SIDS infants.

The outcome for three siblings of SIDS (SSIDS) infants was predicted, using as a risk model the sleep and respiratory characteristics of a SIDS victim studied extensively during the neonatal period. The SIDS infant had shown unstable state organization and deviant respiration patterns, including a deficit of brief apneic pauses. Like the SIDS infant, the SSIDS infants and a group of 16 normal infants were observed in the home for 7-hr periods when they were 2, 3, 4, and 5 weeks old. Two of the infants showed normal sleep and respiratory characteristics, and they were predicted to develop without respiratory dysfunction. In contrast, the third infant showed a pattern of deviancies similar to the SIDS infant; and at 4 months, she had prolonged apneic episodes, requiring resuscitation on two occasions. The findings are consistent with the notion of subtle central nervous system (CNS) dysfunction in SIDS risk infants from the time of birth.

Arousal↗

PubMed1980

Muscle spindles and the regulation of movement.

This paper presents the complexities of muscle spindle structure and function. Experimental evidence indicates that the contribution of muscle spindles to motor control depends upon the cooperation of two distinct systems, one dynamic and one static. The traditional idea that load compensation depends on the segmental stretch reflex action of muscle spindle afferents on alpha motoneurons is questioned and evidence is provided to support the direct dependence of load compensation upon supraspinal centers. Information from muscle spindle afferents allows the nervous system 1) to compare the actual muscle length and velocity of contraction to the desired length and velocity for a particular movement and 2) to compensate for any discrepancies that develop. Segmental pathways from muscle spindle afferents appear to function with Golgi tendon organs to regulate the stiffness or compliance of a muscle.

Afferent Pathways↗