At L. Hof

Assessment of spatio-temporal gait parameters from trunk accelerations during human walking

This paper studies the feasibility of an analysis of spatio-temporal gait parameters based upon accelerometry. To this purpose, acceleration patterns of the trunk and their relationships with spatio-temporal gait parameters were analysed in healthy subjects. Based on model predictions of the bodys centre of mass trajectory during walking, algorithms were developed to determine spatio-temporal gait parameters from trunk acceleration data. In a first experiment, predicted gait parameters were compared with gait parameters determined from ground reaction forces measured by a treadmill. In a second experiment, spatio-temporal gait parameters were determined during overground walking. From the results of these experiments, it is concluded that, in healthy subjects, the duration of subsequent stride cycles and left/right steps, and estimations of step length and walking speed can be obtained from lower trunk accelerations. The possibility to identify subsequent stride cycles can be the basis for an analysis of other signals (e.g. kinematic or muscle activity) within the stride cycle.

Speed dependence of averaged EMG profiles in walking

Electromyogram (EMG) profiles strongly depend on walking speed and, in pathological gait, patients do not usually walk at normal speeds. EMG data was collected from 14 muscles in two groups of healthy young subjects who walked at five different speeds ranging from 0.75 to 1.75 ms(-1). We found that average EMG profiles varied in a predictable way with speed. The average EMG profile for each muscle at any speed could be estimated in a simple way from two functions, one constant and one proportionally increasing with walking speed. By taking into account the similarity among profiles within functional groups, the number of basic functions could be reduced further. Any average EMG profile among the 14 leg muscles studied at all speeds in the measured range could be predicted from six constant and ten speed-dependent basic patterns. These results can be interpreted in terms of a central pattern generator for human walking.

EMG to force processing I: An electrical analogue of the Hill muscle model.

Abstract A processing method is presented by which the surface electromyogram can be processed to the muscle force. The smoothed rectified EMG and the joint angle (proportional to the muscle length) are the inputs of an electrical analogue of the Hill muscle model. The output of the analogue is the torque around the joint due to the muscle force and, by multiplication with the joint rotation, the muscle work. Both can be obtained instantaneously, irrespective of the type of contraction (isometric, isotonic, auxotonic, etc.). The processor is described and some potentialities are shown. Methods by which to find the parameter values of the muscle model and methods for the evaluation of the processor performance are discussed. The relevant experiments, on the human calf muscles (M. triceps surae), will be described in the subsequent parts of this series of papers.

Linearity between the weighted sum of the EMGs of the human triceps surae and the total torque

Abstract The relation between mean rectified EMG and muscle torque was investigated on the muscles of the human triceps surae for slowly varying isometric contractions. It was found that the mean rectified EMG of any of the muscles involved (soleus, gastrocnemius medial head and lateral head) is linearly proportional to the torque it develops. As a consequence the sum of the individual musle torques as derived from their EMGs was equal to the mechanically measured total torque, though the ratio of the contributing individual torques was far from constant. A method is given for the determination of the proportionality factor for each muscle. This method is based on the fact that it is possible to prevent the gastrocnemius from developing torque by shortening it strongly. This was brought about by bending the knee, which changes gastrocnemius length without altering soleus length. The influence of fatigue was negligible during the experiments.

The Importance of Sensory-Motor Control in Providing Core Stability Implications for Measurement and Training

Although the hip musculature is found to be very important in connecting the core to the lower extremities and in transferring forces from and to the core, it is proposed to leave the hip musculature out of consideration when talking about the concept of core stability. A low level of co-contraction of the trunk muscles is important for core stability. It provides a level of stiffness, which gives sufficient stability against minor perturbations. Next to this stiffness, direction-specific muscle reflex responses are also important in providing core stability, particularly when encountering sudden perturbations.It appears that most trunk muscles, both the local and global stabilization system, must work coherently to achieve core stability. The contributions of the various trunk muscles depend on the task being performed. In the search for a precise balance between the amount of stability and mobility, the role of sensory-motor control is much more important than the role of strength or endurance of the trunk muscles. The CNS creates a stable foundation for movement of the extremities through co-contraction of particular muscles. Appropriate muscle recruitment and timing is extremely important in providing core stability.No clear evidence has been found for a positive relationship between core stability and physical performance and more research in this area is needed. On the other hand, with respect to the relationship between core stability and injury, several studies have found an association between a decreased stability and a higher risk of sustaining a low back or knee injury. Subjects with such injuries have been shown to demonstrate impaired postural control, delayed muscle reflex responses following sudden trunk unloading and abnormal trunk muscle recruitment patterns. In addition, various relationships have been demonstrated between core stability, balance performance and activation characteristics of the trunk muscles. Most importantly, a significant correlation was found between poor balance performance in a sitting balance task and delayed firing of the trunk muscles during sudden perturbation. It was suggested that both phenomena are caused by proprioceptive deficits.The importance of sensory-motor control has implications for the development of measurement and training protocols. It has been shown that challenging propriocepsis during training activities, for example, by making use of unstable surfaces, leads to increased demands on trunk muscles, thereby improving core stability and balance. Various tests to directly or indirectly measure neuromuscular control and coordination have been developed and are discussed in the present article. Sitting balance performance and trunk muscle response times may be good indicators of core stability. In light of this, it would be interesting to quantify core stability using a sitting balance task, for example by making use of accelerometry. Further research is required to develop training programmes and evaluation methods that are suitable for various target groups.

Displacement of the pelvis during human walking: experimental data and model predictions

Abstract Displacements of the pelvis during treadmill walking were studied in dependence of walking speed, stride frequency and stride length. Displacement curves per stride cycle were described by means of harmonic analysis. Simple mechanical, or geometrical models of the bodys center of mass (COM) tajectory during walking were used to predict amplitude and timing of pelvic displacements. As predicted by inverted pendulum models, the amplitude of pelvic displacement in vertical direction depended on stride length. Anterior-posterior displacements were predicted by assuming equal maxima of potential and forward kinetic energy of the body. Predictions for left-right displacements were based upon a model that assumed a constant stride width and a sinusoidal movement pattern. In agreement with this model, the amplitude of pelvic displacement in left-right direction depended on stride frequency. The presented models give insight into mechanical mechanisms which determine the pelvic trajectory during walking. The presented data may be useful in the clinical evaluation of gait disturbances.

Balance responses to lateral perturbations in human treadmill walking

SUMMARY During walking on a treadmill 10 human subjects (mean age 20 years) were perturbed by 100 ms pushes or pulls to the left or the right, of various magnitudes and in various phases of the gait cycle. Balance was maintained by (1) a stepping strategy (synergy), in which the foot at the next step is positioned a fixed distance outward of the ‘extrapolated centre of mass’, and (2) a lateral ankle strategy, which comprises a medial or lateral movement of the centre of pressure under the foot sole. The extrapolated centre of mass is defined as the centre of mass position plus the centre of mass velocity multiplied by a parameter related to the subjects leg length. The ankle strategy is the fastest, with a mechanical delay of about 200 ms (20% of a stride), but it can displace the centre of pressure no more than 2 cm. The stepping strategy needs at least 300 ms (30% of a stride) before foot placement, but has a range of 20 cm. When reaction time is sufficient, the magnitude of the total response is in good agreement with our hypothesis: mean centre of pressure (foot) position is a constant distance outward of the extrapolated centre of mass. If the reaction time falls short, a further correction is applied in the next step. In the healthy subjects studied here, no further corrections were necessary, so balance was recovered within two steps (one stride).

EMG AND MUSCLE FORCE - AN INTRODUCTION

Abstract This paper is intended as an introduction to those methods of processing and presentation of the electromyogram (EMG) that give information about the muscle force or work in human movement. The physiological origin of the EMG is treated briefly, followed by a discussion of EMG recording and preprocessing: electrodes, preamplifier, rectifier, smoothing filter. Special attention is given to the prevention and suppression of interferences. The relation between EMG, muscle length and muscle force is explained on the basis of a three-component Hill muscle model. With this background, some possibilities on how to obtain quantitative information about muscle force, work and energy consumption in various categories of movement are summed up and discussed. Some possible applications in human movement studies are suggested.

EMG to force processing II: Estimation of parameters of the Hill muscle model for the human triceps surae by means of a calfergometer

On a calfergometer isotonic contractions were performed by the calf muscles of human subjects. The measured torque Mm was compared to the torque M processed from the EMG and the joint angle by means of an electronic processor based on the Hill muscle model. By means of these experiments the model parameters were determined for the torque-angle relation, the torque-angular velocity relation and the parallel elastic component (PEC). Experiments were done (a) for the soleus only (4 subjects) and (b) for the combination of soleus and gastrocnemius (8 subjects). It turned out that: • — the torque-angle parameters varied among subjects; • — the torque-angular velocity parameters for soleus only varied also among subjects; • — for the combined calf muscle group the interindividual differences in the torque-angular velocity parameters could be neglected; • — one of the parameters of the PEC was the same for all subjects, while the other was variable among the subjects. The obtained parameter values are discussed and compared with available literature data. Values for the r.m.s. and the peak error in the isotonic phases of the contraction are given and compared with values predicted from the stochastic properties of the EMG signal.

Abnormal landing strategies after ACL reconstruction

The objective was to analyze muscle activity and movement patterns during landing of a single leg hop for distance after anterior cruciate ligament (ACL) reconstruction. Nine (six males, three females) ACL‐reconstructed patients 6 months after surgery and 11 (eight males, three females) healthy control subjects performed the hop task. Electromyographic signals from lower limb muscles were analyzed to determine onset time before landing. Biomechanical data were collected using an Optotrak Motion Analysis System and force plate. Matlab was used to calculate kinetics and joint kinematics. Side‐to‐side differences in ACL‐reconstructed patients and healthy subjects as well as differences between the patients and control group were analyzed. In ACL‐reconstructed limbs, significantly earlier onset times were found for all muscles, except vastus medialis, compared with the uninvolved side. The involved limbs had significantly reduced knee flexion during the take‐off and increased plantarflexion at initial contact. The knee extension moment was significantly lower in the involved limb. In the control group, significantly earlier onset times were found for the semitendinosus, vastus lateralis and medial gastrocnemius of the non‐dominant side compared with the dominant side. Muscle onset times are earlier and movement patterns are altered in the involved limb 6 months after ACL reconstruction.