Visual and non-visual control of landing movements in humans.

1. The role of vision in controlling leg muscle activation in landing from a drop was investigated. Subjects (n = 8) performed 10 drops from four heights (0.2, 0.4, 0.6 and 0.8 m) with and without vision. Drop height was maintained constant throughout each block of trials to allow adaptation. The aim of the study was to assess the extent to which proprioceptive and vestibular information could substitute for the lack of vision in adapting landing movements to different heights. 2. At the final stages of the movement, subjects experienced similar peak centre of body mass (CM) displacements and joint rotations, regardless of the availability of vision. This implies that subjects were able to adapt the control of landing to different heights. The amplitude and timing of electromyographic signals from the leg muscles scaled to drop height in a similar fashion with and without vision. 3. However, variables measured throughout the execution of the movement indicated important differences. Without vision, landings were characterised by 10 % larger ground reaction forces, 10 % smaller knee joint rotations, different time lags between peak joint rotations, and more variable ground reaction forces and times to peak CM displacement. 4. We conclude that non-visual sensory information (a) could not fully compensate for the lack of continuous visual feedback and (b) this non-visual information was used to reorganise the motor output. These results suggest that vision is important for the very accurate timing of muscle activity onset and the kinematics of landing.

The biomechanics of an overarm throwing task: a simulation model examination of optimal timing of muscle activations.

A series of overarm throws, constrained to the parasagittal plane, were simulated using a muscle model actuated two-segment model representing the forearm and hand plus projectile. The parameters defining the modeled muscles and the anthropometry of the two-segment models were specific to the two young male subjects. All simulations commenced from a position of full elbow flexion and full wrist extension. The study was designed to elucidate the optimal inter-muscular coordination strategies for throwing projectiles to achieve maximum range, as well as maximum projectile kinetic energy for a variety of projectile masses. A proximal to distal (PD) sequence of muscle activations was seen in many of the simulated throws but not all. Under certain conditions moment reversal produced a longer throw and greater projectile energy, and deactivation of the muscles resulted in increased projectile energy. Therefore, simple timing of muscle activation does not fully describe the patterns of muscle recruitment which can produce optimal throws. The models of the two subjects required different timings of muscle activations, and for some of the tasks used different coordination patterns. Optimal strategies were found to vary with the mass of the projectile, the anthropometry and the muscle characteristics of the subjects modeled. The tasks examined were relatively simple, but basic rules for coordinating these tasks were not evident.

Estimation of the finite center of rotation in planar movements.

The finite center of rotation (FCR) is often used to assess joint function. It was the purpose of this study to compare the accuracy of the procedure of Crisco et al. [4] for estimating the FCR with a procedure which uses least-squares principles. The procedures were evaluated using noisy data rotated about a known FCR. Both procedures demonstrated increasing accuracy of FCR estimation with increasing rotation angle. As the centroid of a pair of markers was moved further from the FCR, accuracy of its location decreased. Noise levels had a strong influence on FCR estimation accuracy, with the least-squares procedure being better able to cope with noise. Increasing the number of landmarks increased FCR estimation accuracy. The accuracy of the procedure of Crisco et al. [4] increased when multiple estimates of the FCR were averaged. On all of the evaluations performed, the least-squares procedure gave small improvements in the accuracy of estimating the FCR, but was not able to circumvent the inaccuracies which arise when landmarks are not appropriately positioned, numerous, or if the rotation angle is small.

The role of the heel pad and shank soft tissue during impacts: a further resolution of a paradox.

The aim of this study was to test the hypothesis that the motion of the soft tissue of the lower leg contributes significantly to the attenuation of the forces during heel impacts. To examine this, a two-dimensional model of the shank and heel pad was developed using DADS. The model contained a heel pad element and a rigid skeleton to which was connected soft tissue which could move relative to the bone. Simulations permitted estimation of heel pad properties directly from heel pad deformations, and from the kinematics of an impacting pendulum. These two approaches paralleled those used in vitro and in vivo, respectively. Measurements from the pendulum indicated that heel pad properties changed from those found in vitro to those found in vivo as relative motion of the bone and soft tissue was allowed. This would indicate that pendulum measures of the in vivo heel pad properties are also measuring the properties of the whole lower leg. The ability of the wobbling mass of the shank to dissipate energy during an impact was found to be significant. These results demonstrate the important role of both the heel pad and soft tissue of the shank to the dissipation of mechanical energy during impacts. These results provide a further clarification of the paradox between the measurements of heel pad properties made in vivo and in vitro.

Postural coordination patterns as a function of dynamics of the support surface.

The present study investigated the compensatory postural coordination patterns that emerge in the face of dynamic changes in the surface of support. Adult subjects stood on a moving platform that was sinusoidally translated in the anterior-posterior direction. The frequency and amplitude of the support surface translation were manipulated over a wide range of parameter values. The results revealed that as the frequency of platform motion increased, the postural system systematically exploited the available joint-space degrees of freedom and generated four distinct postural coordination modes (a rigid mode --> ankle mode --> ankle-hip mode --> ankle-hip-knee mode). It appears that upright standing posture has a small set of coordination patterns that are particular to the dynamics of the surface of support.

Timing accuracy in human throwing.

This study examines the precision required in the timing of muscle activations and projectile release to hit a target of 20 cm in diameter oriented horizontally either 6 or 8 m away. Over-arm throws, constrained to the sagittal plane, were simulated using a muscle-actuated, two-segment model representing the forearm and hand plus projectile. The parameters defining the modeled muscles and the anthropometry were specific to two male subjects. An objective function specified that throws must be both fast and accurate. Once an optimal solution had been found, the sensitivity of these timings was investigated. The times of activation or release were changed and the simulation model re-run with the new timings, and it was determined whether the projectile would still have struck the target. For one set of simulations, to hit the target at 8 m, the optimal throw was achieved with a time delay between the onset of wrist activation and elbow extensor activation [Proximal-distal (PD) delay] of 49 ms and a release time of 83.4 ms. At this optimal point in the solution space, the launch window was 1.2 ms (assuming the original PD delay). The launch window was the time available within which the projectile must be released and still strike the target. The window during which the wrist flexors could be activated was 10. 41 ms (assuming the projectile was released at the pre-planned optimal time). The control scheme which required the least timing precision had a PD delay of 56 ms and a release time of 89.4 ms. Errors in timing could occur in activation and release simultaneously under this scheme, the timing windows were 4 ms in PD delay and 2.4 ms in release. Similar results were found for a second set of simulations. These simulations revealed the precise timings required in muscle activations and release required for fast accurate throws.

EEG correlates of finger movements with different inertial load conditions as revealed by averaging techniques.

OBJECTIVE: The present study was aimed to further address the general empirical question regarding the sensitivity of EEG correlates toward specific kinematic and/or kinetic movement parameters. In particular, we examined whether adding different inertial loads to the index finger, while a subject produced various amplitudes of discrete finger movements, influenced the movement-related potentials (MRP). METHODS: Our experimental design systematically controlled the angular displacement, velocity and acceleration (kinematic) profiles of finger movement while torque (kinetics) was varied by adding different external loads opposing finger flexion movement. We applied time-domain averaging of EEG single trials in order to extract three movement-related potentials (BP-600 to -500 BP-100 to 0 and N0 to 100) preceding and accompanying 25, 50 and 75 degrees unilateral finger movements with no inertial load, small (100 g) and large (200 g) loading. RESULTS: It was shown that both inertial load and the degree of angular displacement of index finger flexion increased the amplitude of late components of MRP (BP-100 to 0 and N0 to 100) over frontal and precentral areas. In contrast, the external load and movement amplitude manipulations did not influence the earlier component of the MRP (BP- 600 to -500). CONCLUSIONS: Overall, the data demonstrate that adding inertial load to the finger with larger angular displacements involves systematic increase in activation across frontal and precentral areas that are related to movement initiation as reflected in BP-100 to 0 and N0 to 100.

Producing physiologically realistic individual muscle force estimations by imposing constraints when using optimization techniques.

Static optimization techniques have been used to estimate individual muscle forces in order to assess joint loads and muscle function. This study examined the validity of such techniques. Forces in the individual muscles, causing elbow flexion, were estimated using four different objective functions, minimizing the sum of the muscle stress either squared or cubed, and minimizing the sum of the relative muscle forces either squared or cubed. Constraints were placed on the maximum muscle forces based on physiological considerations. The resulting force estimates were compared with those from a validated muscle model that took account of the physiological properties of the muscles. The objective functions produced physiologically unrealistic muscle force estimations, unless the maximum muscle forces were constrained. By imposing constraints, individual muscle force predictions were restricted to those that were within physiologically realistic bounds. Using this procedure for sub-maximal activity resulted in some muscle activity being equal to the constraint, which, whilst possible, is still unrealistic. Therefore, by imposing constraints, the muscle forces can be kept within physiological boundaries, but the inferred recruitment is not necessarily the solution that the 'body' selects, but reflects a set of muscle forces that meet the solution to the optimization problem.

Quantification of the uncertainties in resultant joint moments computed in a dynamic activity.

Resultant joint moments are an important variable with which to examine human movement, but the uncertainty with which resultant joint moments are calculated is often ignored. This paper presents a procedure for examining the uncertainty with which resultant joint moments are calculated. The uncertainty was calculated by changing the parameters and variables required to compute the resultant joint moments, by amounts relating to their estimated uncertainties, and then quantifying the resulting change in the resultant joint moments. The procedure was applied to the elbow joint during loaded elbow flexion executed at maximum volitional speed. For this activity, the estimated moments were most sensitive to uncertainties in the derivatives of the position data. A number of other sources of error and uncertainty were identified which warrant further investigation. The protocols outlined in this study are applicable to other activities.

A procedure for determining rigid body transformation parameters.

For many biomechanical applications it is necessary to determine the parameters which describe the transformation of a rigid body from one reference frame to another. These parameters are a scaling factor, an attitude matrix, and a translation vector. The paper presents a new procedure for the determination of these parameters incorporating the work of Arun et al. [IEEE Trans. Pattern Anal. Machine Intell, 9, 698-700 (1987)] but expanding their analysis to allow for the determination of a scale factor, the scalar weighting of the least-squares problem, and the problem of obtaining the incorrect determinant when determining the attitude matrix. The procedure, which requires the coordinates of three or more non-collinear points, is based around the singular value decomposition, and provides a least-squares estimate of the rigid body transformation parameters. Examples are presented of the use of this procedure for determining the attitude of a rigid body, and for osteometric scaling. When used for osteometric scaling mirror transformations are possible, therefore a right-hand specimen can be scaled to the left-hand side of another specimen.

An examination of procedures for determining body segment attitude and position from noisy biomechanical data.

For the kinematic analysis of movement it is necessary to determine the position and attitude of rigid bodies in some specified reference frame. In biomechanics, these rigid bodies are usually segments of the human body, and the position of a distal segment is normally defined relative to a proximal segment. It was the purpose of this study to compare the accuracy of three approaches for the determination of the position and attitude of rigid bodies under four different noise conditions which were designed to model the conditions found in biomechanics studies. One technique investigated assumes the data are error free; another uses matrix algebra and employs matrix perturbation theory; and the third is a least-squares procedure. The evaluation was performed using a computer simulation which attempted to model 'typical' experimental conditions found in biomechanical studies. The attitude of the distal rigid body was defined using helical angles, with these angles being generated using a random number generator. All three techniques were assessed by their ability to predict a set of known helical angles and position vectors under different noise conditions. The study demonstrated that the least-squares technique was the most accurate for determining the attitude matrix and position vector for the cases investigated. None of the techniques investigated could allow for anisotropic noise conditions, yet anisotropic noise conditions are often obtained when using measurement procedures common in biomechanics. The study also highlighted the need to low-pass filter data prior to the computation of the position and attitude of rigid bodies.

The future of performance-related sports biomechanics research.

An overview of performance-related research in sports biomechanics is presented describing the relevant techniques of data analysis and data processing together with the methods used in experimental and theoretical studies. Advances in data collection and processing techniques which are necessary for the future development of sports biomechanics research are identified. The difficulties associated with experimental studies in sports biomechanics are described with examples of the different approaches that have been used. The strengths and weaknesses of theoretical studies are discussed with examples drawn from a number of sports. It is concluded that progress in performance-related research will result from the application of a suitable combination of theoretical and experimental approaches to those sports in which technique is the primary requirement for success.

An analytical examination of muscle force estimations using optimization techniques.

Muscle forces are often estimated during human movement using optimization procedures. The optimization procedures involve the minimization of an objective function relating to the muscle forces. In this study 15 different objective functions were evaluated by examining the analytical solutions to the objective functions and by comparing their force predictions with the forces estimated using a validated muscle model. The muscle forces estimated by the objective functions were shown to give poor correspondence with the muscle model predicted muscle forces. The objective function estimates were criticized for not taking sufficient account of the physiological properties of the muscles. As a consequence of the analysis of the objective functions an alternative, simpler function was presented with which to estimate muscle forces in vivo. This function required that to satisfy a given joint moment, the force exerted by each of the muscles divided by the maximum force possible by the muscle was constant for all muscles. For this function the maximum muscle force was determined using a muscle model assuming maximal activation.

Accuracy assessment and control point configuration when using the DLT for photogrammetry.

The direct linear transformation (DLT) is a common technique used to calibrate cameras and subsequently reconstruct points filmed with two or more cameras in a three-dimensional object space. The assessment of the accuracy of this technique, and of the influence of the distribution of control points on accuracy were examined. It was concluded that to obtain a true estimation of reconstruction accuracy, an independent assessment criterion is required, and that the use of control points distributed around the outside, rather than within the space to be calibrated, is preferred.

Calculating upper limb inertial parameters.

In the kinematic and kinetic analysis of human movement, an accurate assessment of the inertial properties of the body segments under investigation is often essential. Statistical and geometric models of the segments of the upper limb were examined to determine their ability to predict the moments of inertia of these segments. The cadaver data of Chandler et al. (1975) were used as the criteria with which computed values were compared. The regression equations of Hinrichs (1985) gave the most accurate results; the geometric models appeared to offer the greatest flexibility.

Sepsis rates in hip replacement surgery with special reference to the use of ultra clean air.

A study is reported of sepsis rates in 836 total hip replacements carried out in various physical environments including ultra-clean air. In a further sutdy the bacteriology of the air around the wound and of the wound itself was investigated during the course of 32 joint replacement operations with and without ultra-clean air. Bacteriological contamination of the air was markedly reduced by the use of ultra-clean air but the wounds themselves were no cleaner than with conventional air conditioning provided that unsterile personel were kept at least Imetri away from the sterile area. The sepsis rate was high at the outset of the study (when a physically apalling theatre was in use, the operative technique was novel and prophylactic antibiotics were not employed). Thereafter it fell and remained unchanged in spite of the introduction of ultra-clean air. We conclude that the efficacy of techniques amiced at sterilizing the air around the wound may now be unprovable in view of the low sepsis rates which can be obtained in a conventional environment. Our results do however show that unsterile personel should not be allowed to approach within Imetri of the sterile area, whatever venitlation is employed.

Strangulated lumbar hernia and volvulus following removal of iliac crest bone graft.

A case of a woman aged 88 years with a volvulus and strangulated small bowel in a hernia, following removal of iliac crest for bone grafting, is described. Emphasis is laid on prevention of an incisional hernia when obtaining the bone graft.