Biomedical subjects

John H Challis

Publications and source records attributed to John H Challis.

14 recordsLinked to original sources

PubMed2007

The influence of squat depth on maximal vertical jump performance.

An increase in the period over which a muscle generates force can lead to the generation of greater force and, therefore, for example in jumping, to greater jump height. The aim of this study was to examine the effect of squat depth on maximum vertical jump performance. We hypothesized that jump height would increase with increasing depth of squat due to the greater time available for the generation of muscular force. Ten participants performed jumps from preferred and deep squat positions. A computer model simulated jumps from the different starting postures. The participants showed no difference in jump height in jumps from deep and preferred positions. Simulated jumps produced similar kinematics to the participants' jumps. The optimal squat depth for the simulated jumps was the lowest position the model was able to jump from. Because jumping from a deep squat is rarely practised, it is unlikely that these jumps were optimally coordinated by the participants. Differences in experimental vertical ground reaction force patterns also suggest that jumps from a deep squat are not optimally coordinated. These results suggest there is the potential for athletes to increase jump performance by exploiting a greater range of motion.

Adult↗

PubMed2006

Speed influences on the scaling behavior of gait cycle fluctuations during treadmill running.

The current study examined the temporal structure of gait cycle fluctuations in running. Participants ran at 80%, 90%, 100%, 110% and 120% of preferred running speed for 8min trials. Kinematic and kinetic gait cycle variables were generated from ground reaction force data. Mean, SD and CV of the kinematic and kinetic variables changed linearly with speed, whereas U-shaped functions were found for the scaling exponent alpha in 5 of the 8 variables investigated. Our findings reveal that long range correlations are present in both kinetic and kinematic variables of the gait cycle. The dependent structure of the stride interval is reduced at preferred running speed and this is hypothesized to be related to the enhanced stability and flexibility of this gait speed.

Adult↗

PubMed2006

Walking speed influences on gait cycle variability.

The purpose of this study was to investigate the influence of walking speed on the amount and structure of the stride-to-stride fluctuations of the gait cycle. Based on previous findings for both walking [Hausdorff JM, Purdon PL, Peng CK, Ladin Z, Wei JY, Goldberger AL. Fractal dynamics of human gait: stability of long-range correlations in stride interval fluctuations. J Appl Physiol 1996;80:1448-57], and running [Jordan K, Challis JH, Newell KM. Long range correlations in the stride interval of running. Gait Posture 2006;24:120-5] it was hypothesized that the fractal nature of human locomotion is a reflection of the attractor dynamics of human locomotion. Female participants walked for 12min trials at 80%, 90%, 100%, 110% and 120% of their preferred walking speed. Eight gait cycle variables were investigated: stride interval and length, step interval and length, and from the vertical ground reaction force profile the impulse, first and second peak forces, and the trough force. Detrended fluctuation analysis (DFA) revealed the presence of long range correlations in all gait cycle variables investigated. Speed related U-shaped functions occurred in five of the eight variables, with the minima of these curves falling between 100% and 110% of the preferred walking speed. These findings are consistent with those previously shown in running studies and support the hypothesis that reduced strength of long range correlations at preferred locomotion speeds is reflective of enhanced stability and adaptability at these speeds.

Adult↗

PubMed2006

Haptic tracking permits bimanual independence.

This study shows that in a novel task-bimanual haptic tracking-neurologically normal human adults can move their 2 hands independently for extended periods of time with little or no training. Participants lightly touched buttons whose positions were moved either quasi-randomly in the horizontal plane by 1 or 2 human drivers (Experiment 1), in circle and square patterns in the vertical plane by 2 human drivers (Experiment 2), or at different frequencies in the horizontal plane by 2 human drivers (Experiment 3). Bimanual contact was maintained equally well in all conditions even though in Experiment 1 the left- and right-hand motions were uncorrelated (in the 2-driver condition), in Experiment 2 the left- and right-hand motions were spatially incongruous when circles and squares were tracked at the same time, and in Experiment 3 the left- and right-hand motions maintained different frequency ratios. Because haptic tracking has revealed that humans can in fact move their 2 hands independently, it may have potential as a new behavioral tool for revealing other perceptual-motor capabilities.

Form Perception↗

PubMed2006

Time-to-boundary measures of postural control during single leg quiet standing.

A novel approach to quantifying postural stability in single leg stance is assessment of time-to-boundary (TTB) of center of pressure (COP) excursions. TTB measures estimate the time required for the COP to reach the boundary of the base of support if it were to continue on its instantaneous trajectory and velocity, thus quantifying the spatiotemporal characteristics of postural control. Our purposes were to examine: (a) the intrasession reliability of TTB and traditional COP-based measures of postural control, and (b) the correlations between these measures. Twenty-four young women completed three 10-second trials of single-limb quiet standing on each limb. Traditional measures included mean velocity, standard deviation, and range of mediolateral (ML) and anterior-posterior (AP) COP excursions. TTB variables were the absolute minimum, mean of minimum samples, and standard deviation of minimum samples in the ML and AP directions. The intrasession reliability of TTB measures was comparable to traditional COP based measures. Correlations between TTB and traditional COP based measures were weaker than those within each category of measures, indicating that TTB measures capture different aspects of postural control than traditional measures. TTB measures provide a unique method of assessing spatiotemporal characteristics of postural control during single limb stance.

Adult↗

PubMed2005

Long range correlations in the stride interval of running.

Fluctuations in the stride interval time series of unconstrained walking are not random but exhibit long range correlations that decay as a power law [Hausdorff JM, Peng CK, Ladin Z, Wei JY, Goldberger AL. Is walking a random walk? Evidence for long range correlations in stride interval of human gait. J Appl Physiol 1995;78:349-58]. Here, we examine whether the long range correlations are present in the stride interval time series of running. Recreational female runners ran 8min trials at their preferred running speed as well as 10% and 20% slower and faster than their preferred speed. Both the average time and the amount of variability of the stride interval decreased with increasing speed. Detrended fluctuation analysis (DFA) showed that there were long range correlations present in the stride interval time series and these correlations followed a quasi U-shaped function, with the minimum at the preferred running speed. These results are consistent with the hypothesis that the preferred running speed, falling as it does between the upper and lower limits of possible running speeds, is the speed at which the most dynamical degrees of freedom are available for adaptive control of locomotion.

Adult↗

PubMed2005

Aging, regularity and variability in maximum isometric moments.

This study examined the variability and regularity of maximum isometric moment production of the plantar flexors in young and old subjects. It was hypothesized that in the development of maximum isometric moments there would be greater regularity in the moment profiles for older subjects compared with young subjects, due to the reduced number of motor units present in elderly muscle. Two groups of subjects produced three maximal isometric plantar flexions (young: n=11, mean age 23.8+/-2.8 years, mean mass 81.2+/-10.4 kg, mean height 1.78+/-0.05 m; elderly: n=13, mean age 74.0+/-3.3 years, mean mass 78.5+/-3.4 kg, mean height 1.73+/-0.05 m). The plateau of the moment-time curve was analyzed for each trial. A repeat measures analysis of variance showed the young subjects produced statistically greater peak plantar flexion moments than the elderly subjects, but similar coefficients of variation. Signal regularity was determined by computing the signal's approximate entropy, which demonstrated that the older group had greater regularity in their generation of moment profiles. The hypothesis was accepted, with a potential explanation for this increased regularity in old age being the reduced number of motor units to coordinate.

Adult↗

PubMed2004

The influence of soft tissue movement on ground reaction forces, joint torques and joint reaction forces in drop landings.

The aim of this study was to determine the effects that soft tissue motion has on ground reaction forces, joint torques and joint reaction forces in drop landings. To this end a four body-segment wobbling mass model was developed to reproduce the vertical ground reaction force curve for the first 100 ms of landing. Particular attention was paid to the passive impact phase, while selecting most model parameters a priori, thus permitting examination of the rigid body assumption on system kinetics. A two-dimensional wobbling mass model was developed in DADS (version 9.00, CADSI) to simulate landing from a drop of 43 cm. Subject-specific inertia parameters were calculated for both the rigid links and the wobbling masses. The magnitude and frequency response of the soft tissue of the subject to impulsive loading was measured and used as a criterion for assessing the wobbling mass motion. The model successfully reproduced the vertical ground reaction force for the first 100 ms of the landing with a peak vertical ground reaction force error of 1.2% and root mean square errors of 5% for the first 15 ms and 12% for the first 40 ms. The resultant joint forces and torques were lower for the wobbling mass model compared with a rigid body model, up to nearly 50% lower, indicating the important contribution of the wobbling masses on reducing system loading.

Adult↗

PubMed2004

Influences of variation in force application on tibial displacement and strain in the anterior cruciate ligament during the Lachman test.

OBJECTIVE: The purpose of this study was to examine the influence of Lachman test performance technique on tibial displacement and strain in the anterior cruciate ligament. DESIGN: Model simulation of experimental Lachman test performance by trained clinicians. BACKGROUND: Differences in clinician hand placement during Lachman test performance have been observed. METHODS: A two-dimensional computer sagittal plane model of the knee was designed to simulate experimentally observed Lachman test performance, and determine anterior cruciate ligament strain and tibial translation that occurred during variation in clinician hand placement and force magnitude. RESULTS: Anterior cruciate ligament strain and tibial translation were greater under conditions mimicking clinician hand placement utilizing a more proximal force application on the tibia. CONCLUSIONS: Tibial translation and strain behavior of the anterior cruciate ligament during the Lachman test appear to be influenced by clinician hand position used in the application of force to the tibia.

Adult↗

PubMed2004

Examination of the scaling of human jumping.

On the basis of the principles of geometric scaling, maximum vertical-jump height should decrease in an approximately linear fashion with increasing mass. To test this prediction, a group of 10 male subjects performed maximum vertical jumps with masses up to 22.7 kg strapped to their trunks. The results from these jumps indicated that jump height did scale on an individual basis in a linear fashion. A computer simulation model of jumping was developed that permitted the examination of a greater range of masses than was possible experimentally. The simulations also support the trend of linear scaling, but do replicate the decrement expected based on geometric scaling principles. Experimental and simulation model results provide evidence for a linear decrement in subject maximum vertical-jump height with increasing mass, which is relevant information for athletes aiming to increase their body mass or performing jump training while carrying additional mass.

Adult↗

PubMed2003

Learning to coordinate redundant degrees of freedom in a dynamic balance task.

The present study investigated Bernstein's [The co-ordination and regulation of movements, 1967] proposal regarding the three stages of learning in the changing coordination and control of redundant joint-space degrees of freedom. Six participants practiced maintaining balance on a moving platform that was sinusoidally translated in the anterior-posterior direction for 30 trials on day 1 and 10 trials on day 2. At the beginning of practice, the motion of the torso and limb segments was less coherent in the attempt to compensate for the movement of the support surface in retaining a balanced posture. However, with practice, the organization of a compensatory postural coordination mode became highly coherent and also progressively utilized the passive, inertial forces generated by the movement of the support surface. The findings support the propositions that: (a) the pathway of change over time in the coordination pattern of the torso and joint motions depends on the task goal and constraints to action and (b) the changes in limb and torso motion are in support of the learning of a global body center of mass/platform dynamic.

Adult↗

PubMed2003

A simple method to determine body segment masses in vivo: reliability, accuracy and sensitivity analysis.

OBJECTIVE: To show that force plates can be used to quickly acquire subject-specific segment mass data. DESIGN: In vivo measurements were performed on subjects belonging to three populations: female varsity swimmers, female varsity volleyball players, and male college students. Segmental masses were measured using a force plate technique, and were compared with published data. BACKGROUND: Patients from populations for which data from the literature are not applicable (e.g. pathological, aging females, obese, children, etc.) would benefit from a direct measure of inertial parameters for accurate joint moment calculations. METHODS: Eight female varsity volleyball players, 17 female varsity swimmers, and 10 male college students were measured anthropometrically. They then lay on a board placed on a force plate and the center of pressure was recorded while the subjects adopted various prescribed limb positions. Their limb masses were subsequently calculated from the center of pressure data given estimated center of mass locations. RESULTS: The method was highly reproducible with an average reliability coefficient of 0.83 and yielded results similar to those of published methods. Significantly different mass distributions were found between the two female populations tested (P<0.025). CONCLUSIONS: The method can quickly provide subject-specific limb segment mass information. RELEVANCE: Measuring subjects' segment masses individualizes clinical assessments and may be necessary for those from special populations to avoid erroneous biomechanical conclusions.

Adult↗

PubMed2003

Organization of compensatory postural coordination patterns.

The authors investigated whether compensatory postural coordination patterns are organized according to the same dynamical principles as are nonequilibrium phase transitions. Eight participants were asked to maintain upright balance on a moving platform that was sinusoidally translated in the anterior-posterior direction and was systematically increased and decreased 0.19 Hz as a step function every 10 platform cycles through the frequency range 0.19-1.46 Hz. At low platform frequencies, all participants exhibited small joint angular motions with high variability, and the relative phase between the joint motions exhibited drifting patterns and large fluctuations. As platform frequency increased, the amplitude of joint motion increased systematically and joint-specific oscillatory patterns emerged. The findings provided no evidence for a Hopf bifurcation or hysteresis in the transitions of postural coordination modes, however, or, more generally, a basis for distinguishing the relevance of linear versus nonlinear models of postural control.

Adult↗

PubMed2002

Further evidence on the dynamics of self-injurious behaviors: impact forces and limb motions.

The dynamics of self-injurious behaviors (SIBs) were examined in 8 adults with mental retardation. The trajectories of the arm movements and the impact forces of the head blows were determined from a dynamic analysis of videotapes of discrete bouts of self-injury. The results revealed a high degree of cycle-to-cycle consistency in the qualitative dynamics of the limb motions, indicating that the motions involved in SIB are often stereotyped in nature. The resultant individual peak impact forces ranged from 50 to 1560 N. The impact forces of SIB as a percentage of body mass are either near or at the low end of forces generated in boxing blows and karate hits.

Adult↗