Graeme A. Wood

On the use of spline functions for data smoothing

Abstract The appropriateness of various numerical procedures for obtaining valid time-derivative data recently reported in the literature (Zernicke et al. , 1976; McLaughlin et al. , 1977; Pezzack et al. , 1977) is discussed. A case for the use of quintic natural splines is presented, based on the smoothness of higher derivatives and flexibility in application.

The accuracy of DLT extrapolation in three-dimensional film analysis

An analysis of errors arising from the Direct Linear Transformation (DLT) approach to three-dimensional reconstructions from two-dimensional images has been undertaken, the principal factor studied being the number and distribution of control points used in the calibration procedure. Significantly increased error was found to be associated with extrapolation to unknown points outside the control point distribution space. Differences in accuracy between two camera position set-ups and 11 vs 12 DLT parameter solutions were also examined.

Kinematic and kinetic comparison of downhill and level walking

Kinematic and kinetic data were collected from 12 healthy subjects whilst they performed both downhill and level walking at a controlled cadence. A ramp of 6 m length and a gradient of -19% was used for downhill walking and this incorporated the same force platform that was used for level walking. Planar net joint moments and mechanical power at the ankle, knee, and hip joints were calculated for the sagittal view using force platform and video records based on standard inverse dynamics procedures. On the basis of differences in ankle, knee, and hip joint kinematics the ankle joint was seen to compensate for the gradient at push off and during the swing, the knee joint from early stance through until early swing phase, and the hip joint from early swing through until the early stance phase. The major differences in joint moments and muscle mechanical power were seen in the knee and ankle joint. Whereas peak moments and muscle power were much higher for downhill walking in the knee joint, these measures were significantly smaller at the ankle joint. Hip joint moments and muscle power estimates were only slightly larger for downhill walking. These data explain well the problems that patients with patellofemoral pathology and anterior cruciate ligament (ACL) deficiency encounter with downhill walking, and the muscle soreness experienced by mountain trekkers. RELEVANCE: Biomechanical estimates of musculoskeletal loadings in gait are invariably derived from laboratory studies of walking over a level surface. In this study comparisons were made between downhill and level walking in order to appreciate more fully the increased loadings on the lower extremity under more stressful but not atypical conditions. The data so derived provide the necessary basis for the prediction of loadings on specific muscle/joint structures and can serve as a foundation for exercise prescription with patients recovering from injury or orthopaedic surgery.

Stiffness of the hamstring muscles and its relationship to function in anterior cruciate ligament deficient individuals

The relationship between hamstring muscle stiffness and the functional ability level of 17 subjects with complete anterior cruciate ligament rupture confirmed at arthroscopy was examined. The hamstring muscles were modelled as a single degree of freedom mass spring system with a damping component. Using this model the stiffness of these muscles was examined at 30, 45, and 60% of a maximum voluntary isometric muscle action. The functional ability of the subjects, attained using the Noyes knee rating system, was then correlated to muscle stiffness measures. Positive correlations of 0.71, 0.72, and 0.62 at the three respective muscle loading levels were observed. These findings suggested that hamstring muscle stiffness may have an important role to play in the functional ability of subjects with anterior cruciate ligament deficiency. At this time there is no single effective treatment for all individuals with anterior cruciate ligament deficiency. Those individuals who undergo a conservative management programme are usually treated with hamstring muscle exercises for improving knee flexion strength. The current study provides evidence that hamstring exercises may alter other properties of muscles, such as their active stiffness, which in turn may influence the functional ability of the anterior cruciate ligament deficient individual.

The benefits of wearing a compression sleeve after ACL reconstruction.

PURPOSE It was the purpose of the present study to examine the possibility of increased muscle coordination after anterior cruciate ligament (ACL) reconstruction through the wearing of a compression sleeve. METHODS Thirty-six patients were studied who had undergone unilateral ACL reconstruction at least 12 months previously. All subjects were required to perform a 10-cm standing drop jump from an elevated platform onto a force plate, to land on one leg, and thereafter maintain a one-legged balance for 25 s. This task was repeated three times without and three times with an elastic compression sleeve worn on the reconstructed limb. For analysis, the task was partitioned into a landing phase (150 ms), an adjusting phase (10s), and a balancing phase (10s). The peak impact loadings were measured in each direction (Fx, Fy, and Fz) during landing, while force-time integrals (intFz, intFy, and intFz) and root mean square (RMS) error of these forces were calculated for the adjusting and balancing phases. The path length and RMS of the center of pressure coordinates (Ax and Ay) were obtained for the adjusting and balancing phases combined. RESULTS Drop landings with the bandage produced significantly larger (P < 0.001) peak ground reaction forces in the vertical and anteroposterior direction, suggesting increased subject confidence in their knee. Wearing the knee bandage also enabled the patients to reduce all measured parameters in the anteroposterior direction (rmsFx, intFx, rmsAx) during both the adjusting and balancing phases (P < 0.001 ). A significant reduction in the center of pressure path length further indicated an enhanced steadiness during the one-legged stance. CONCLUSIONS It was concluded that a compression sleeve improved the total integration of the balance control system and muscle coordination.

The entrainment of ventilation frequency to exercise rhythm

SummaryTo investigate whether ventilation frequency could be entrained to a sub-harmonic of the exercise rhythm, 19 experimentally naive male volunteers were tested during steady state bicycle ergometry and arm cranking under conditions of constant applied workload. Each exercise was performed at two separate ventilatory loads, one within the linear range and the other in the curvilinear range of ventilatory response to exercise. A preferred exercise rhythm was initially adopted (4 min.) followed by forced incremented and decremented rhythm changes each lasting 3 min during a 12 min exercise period. Ventilation, pedal pulse train and heart rate were sampled at 17 Hz on a PDP 11/23 computer. Ratios of limb frequency to dominant respiratory frequency were determined following Fourier analysis of these signals. Data that lay within ±0.05 of an integer and half-integer ratio were accepted as indices of entrainment, provided that the observed entrained scores were statistically significant. Ventilation frequency showed a clear, but intermittent tendency to entrain with limb frequency. This tendency was greater during bicycle ergometry, possibly as a consequence of task familiarisation, although both exercise entrainments were independent of workload. No difference between preferred versus varied exercise rhythm was evident, but more entrainment (p<0.01) was observed during a decremental change in exercise rhythm. These responses do not appear to support an appreciable role for limb-based afferents in the control of entrainment. The results of this study provide evidence that exercise rhythm has some regulatory role in the control of breathing during moderate rhythmical laboratory-based exercise ergometry.

Downhill walking: A stressful task for the anterior cruciate ligament?

Accelerated rehabilitation after anterior cruciate ligament (ACL) reconstruction has become increasingly popular. Methods employed include immediate extension of the knee and immediate full weight bearing despite the risks presented by a graft pull-out fixation strength of 200–500 N. The purpose of this study was to calculate the tibiofemoral shear forces and the dynamic stabilising factors at the knee joint for the reasonably demanding task of downhill walking, in order to determine whether or not this task presented a postoperative risk to the patient. Kinematic and kinetic data were collected on six male and six female healthy subjects during downhill walking on a ramp with a 19% gradient. Planer net joint moments and mechanical power at the knee joint were calculated for the sagittal view using a force platform and videographic records together with standard inverse dynamics procedures. A two-dimensional knee joint model was then utilised to calculate the tibiofemoral shear and compressive forces, based on the predictions of joint reaction force and net moment at the knee. Linear envelopes of the electromyographic (EMG) activity recorded from the rectus femoris, gastrocnemius and biceps femoris muscles were also obtained. The maximum tibiofemoral shear force occurred at 20% of stance phase and was, on average, 1.2 times body weight (BW) for male subjects and 1.7 times BW for female subjects. The tibiofemoral compressive force was 7 times BW for males and 8.5 times BW for females during downhill walking. The hamstring muscle showed almost continuous activity throughout the whole of the stance phase. The gastrocnemius muscle had its main activity at heelstrike, with a second brust during the late stance phase. Knee joint shear force predictions of approximately 1000 N for a 70-kg subject greatly exceed the strength of a typical ACL graft fixation and muscular stabilisation of the knee is therefore vital to joint integrity. The hamstring muscle shows almost continuous activity during the stance phase and thereby affords some stability, but the gastrocnemius is also seen to be an important stabiliser of the knee joint in the presence of increased shear forces during early stance. Associated stability to the knee joint is indicated by compressive loadings of 7–8 times BW across the tibiofemoral joint. Whereas under normal circumstances there is sufficient dynamic joint stabilisation during downhill walking, the muscular impairment often arising postoperatively from disturbed proprioception could endanger an ACL graft. Therefore downhill walking should be avoided during the postoperative phase in order to protect the reconstruction.

Performance objectives in human movement: A review and application to the stance phase of normal walking ☆

Abstract That some form of optimisation takes place in skilled human movements may be inferred from metabolic, electromyographic and perceptual experimental results. However, the parameter(s) or function(s) minimised in activities such as walking has not been satisfactorily identified. This work used non-linear optimal control and multiple-segment simulation software to evaluate the predictive ability of seven performance objective functions. The results indicated that minimisation of the time integrals of the sums of the joint torques, the segmental angular accelerations, or the segmental mechanical energies produced the best simulations of the single stance phase of walking. Strong support was also provided for the existence of simultaneous multiple performance objectives in complex movements.

A general Newtonian simulation of an N-segment open chain model

This paper presents a set of general Newtonian equations which govern the simulation of movement of a body represented by n open chain links. The input for the simulation consisted of the joint moment of force histories, lengths, masses and moments of inertia, the initial absolute angular displacements and velocities and, for the fixed or constrained axis of the nth segment, the acceleration history. Angular accelerations were then determined by solving n linear equations simultaneously, and angular velocities and displacements determined by integrating forwards. The final output was in the form of a graphical display of the linked figure. Applications of the simulation were demonstrated using three-segment representations of movements of the upper and lower extremities and a five-segment representation of a jump. Good agreement was achieved between the displayed angular displacements for the original and simulated movements. The potential for varying the input data has been examined and the implications of anticipating the effects of changed torques, inertial characteristics including attached prosthetic or sports implements and/or the initial conditions for a movement are discussed.

The anterior cruciate ligament-deficient knee: compensatory mechanisms during downhill walking

Abstract This investigation sought to identify neuromechanical mechanisms by which a subject with an anterior cruciate ligament (ACL)-deficient knee might cope with the potentially destabilizing joint stresses during both level and downhill walking. Kinematic, kinetic and electromyographic data were collected from 21 subjects with arthroscopically verified ACL-deficient knees and Lysholm scores of 55–100 ( X = 82 ), as well as from 12 healthy control subjects. Electromyographic data were recorded from the skin surface overlying rectus femoris, biceps femoris and gastrocnemius muscles. A dismountable slope of 6 m length and a gradient of 19% was constructed for downhill walking. Sagittal plane net joint moments and muscle mechanical power at the knee joint were calculated from force platform and videographic records using the inverse dynamics approach. During level walking there were no kinematic nor kinetic differences seen between ACL-deficient subjects and normals. The typical profile of muscle power at the knee contained three peaks during stance: an eccentric peak during early stance (K1); a concentric peak at mid stance (K2); and a second smaller eccentric peak (K3) during late stance. During downhill walking ACL-deficient subjects displayed a significantly smaller K1 compared to normals and their K1:K3 ratio was significantly less than that of normal subjects. Whereas normal subjects showed no hamstring activity during stance in level walking there was continuous activity throughout the stance phase displayed by the ACL-deficient and normal subjects. During downhill walking both the ACL-deficient and normal subjects showed continuous hamstring activity. However, the ACL-deficient subjects showed a significant delay in peak hamstring activity during late swing. Both groups on average displayed gastrocnemius peak activity just on heel strike during downhill walking but the linear envelopes of the ACL-deficient subjects were much more tightly time-locked to this critical event.