The effect of elbow joint centre displacement on force generation and neural excitation.

Joint centre displacement may occur following total elbow replacement due to aseptic loosening or surgical misalignment, and has been linked to implant failure. In this study, the effects of joint centre displacement were examined using a neuromusculoskeletal model of the elbow joint. Isometric contractions were simulated at a range of joint angles during elbow flexion and extension. Displacement of the joint centre affected the force-generating capacity about the joint, due to changes in both muscle lengths and moment arms. The magnitude and direction of the maximum joint reaction force were also altered, potentially contributing to aseptic loosening and compromising joint stability. The relationship between force generated and the level of neural excitation to the elbow flexor and extensor muscles was also affected, suggesting that altered neural control patterns could be required following joint centre displacement.

Effect of elbow joint angle on force-EMG relationships in human elbow flexor and extensor muscles.

The purpose of this study was to examine the effect of joint angle on the relationship between force and electromyogram (EMG) amplitude and median frequency, in the biceps, brachioradialis and triceps muscles. Surface EMG were measured at eight elbow angles, during isometric flexion and extension at force levels from 10% to 100% of maximum voluntary contraction (MVC). Joint angle had a significant effect on MVC force, but not on MVC EMG amplitude in all of the muscles examined. The median frequency of the biceps and triceps EMG decreased with increasing muscle length, possibly due to relative changes in electrode position or a decrease in muscle fibre diameter. The relationship between EMG amplitude and force, normalised with respect to its maximum force at each angle, did not vary with joint angle in the biceps or brachioradialis muscles over all angles, or in the triceps between 45 degrees and 120 degrees of flexion. These results suggest that the neural excitation level to each muscle is determined by the required percentage of available force rather than the absolute force required. It is, therefore, recommended that when using surface EMG to estimate muscle excitation, force should be normalised with respect to its maximum value at each angle.

A neuromusculoskeletal model of the elbow joint for pre-clinical testing of total elbow replacement.

In this study, the effect of changing the geometry of the elbow joint was examined using a neuromusculoskeletal model. The position of the center of the joint was altered in order to simulate aseptic loosing or misalignment of the humeral component of a total elbow replacement. The effect of this change on model parameters, including the muscle moment arm and maximum wrist force, was examined. An isometric contraction with increasing voluntary drive was then simulated for a range of joint center positions, and the resulting joint reaction forces, and the direction of the force vectors were monitored. A change in the maximum force, measured at the wrist (17% - elbow flexion, 28% - extension), and in joint reaction force (up to 145N) was observed when the position of the joint center was altered. In addition, slight changes (up to 4.45 degrees ) in the direction of the joint reaction force vector were also observed.

Improved surface EMG electrode for measuring genioglossus muscle activity.

Activation of the genioglossus (GG) muscles is necessary to maintain the patency of the upper airway. In the condition of obstructive sleep apnea (OSA) this mechanism fails and the possible role of fatigue in its pathogenesis is still not fully understood. In this paper, a new electrode design for recording the genioglossus surface electromyogram (sEMG) is presented. The new design differs from a widely used GG surface electrode in both electrode configuration (unilateral rather than bilateral) and electrode material (Ag-AgCl rather than stainless steel (SS)). The separate effects of these factors were evaluated during force-varying and fatiguing contractions on normal human subjects and using GG sEMG model simulations. Unilateral sEMG was found to have lower amplitude, lower frequency content and a different rate of change of median frequency during fatiguing contractions. It was shown to overcome several disadvantages posed by the bilateral configuration and be more selective. Ag-AgCl has more favorable impedance characteristics and resulted in greater signal amplitudes. It was concluded that the new design is more suitable for detecting GG sEMG and allows more reliable interpretation of changes in sEMG due to physiological mechanisms, thus providing a new methodology for studying GG function and the role of fatigue in OSA.

Design of surface electrode array for measuring conduction velocity in the human genioglossus muscle.

A new appliance, incorporating linear arrays of pin electrodes for genioglossus (GG) surface electromyography measurement, is presented. This design enables the estimation of GG muscle fiber conduction velocity, which decreases with fatigue. The performance of the device was evaluated for ten healthy human subjects during fatiguing and force varying contractions.

Automatic detection of gait events using kinematic data.

The timing of heel strike (HS) and toe off (TO), the events that mark the transitions between stance and swing phase of gait, is essential when analysing gait. Force plate recordings are routinely used to identify these events. Additional instrumentation, such as force sensitive resistors, can also been used. These approaches, however, include restrictions on the number of steps that can be analyzed and further encumbrance of the subject. We developed an algorithm which automatically determines these times from kinematic data recorded by a motion capture system, which is routinely used in gait analysis laboratories. The foot velocity algorithm (FVA) uses data from the heel and toe markers and identifies features in the vertical velocity of the foot which correspond to the gait events. We verified the performance of the FVA using a large data set of 54 normal children that contained both force plate recordings and kinematic data and found errors of (mean+/-standard deviation) 16+/-15 ms for HS and 9+/-15 ms for TO. The algorithm also worked well when tested on a small number of children with spastic diplegia. We compared the performance of the FVA with another kinematic method previously described. Our foot velocity algorithm offered more accurate results and was easier to implement than the previously described one, and should be applicable in a variety of gait analysis settings.

Tracking motor unit action potentials in the tibialis anterior during fatigue.

New surface electromyogram (SEMG) techniques offer the potential to advance knowledge of healthy and diseased motor units. Conduction velocity (CV) estimates, obtained from indwelling electrodes, may provide diagnostic information, but the standard method of CV estimation from SEMG may be of only limited value. We developed a motor unit (MU) tracking algorithm to extract motor unit conduction velocity (MUCV) and motor unit action potential (MUAP) amplitude estimates from SEMG. The technique is designed to provide a noninvasive means of accessing fatigue and recruitment behavior of individual MUs. We have applied this MU tracking algorithm to SEMG data recorded during isometric fatiguing contractions of the tibialis anterior (TA) muscle in nine healthy subjects, at 30%-40% maximum voluntary contraction (MVC). The results reveal that MUCVs and MUAP amplitudes of individual MUs can be estimated and tracked across time. Time-related changes in the MU population may also be monitored. Thus, the SEMG technique employed provides insight into the behavior of the underlying muscle at the MU level by noninvasive means.

A gait nomogram used with fuzzy clustering to monitor functional status of children and young adults with cerebral palsy.

We have developed a gait nomogram based on dynamic similarity to characterize and compare neuromuscular function. We used temporal-distance data based on 669 normal participants (age range 5 to 98 years), and 78 children and young adults with spastic diplegia (43 males, 35 females; mean age 10 y 8 mo, standard deviation 3 y 11 mo, range 5 to 20 y), all of whom were independent ambulators. A new statistical algorithm known as fuzzy clustering was implemented and five cluster centres were identified, each representing distinct walking strategies adopted by children with cerebral palsy. Using just three easily obtained parameters--leg length in metres, stride length in metres, and cadence in steps per minute--our program calculates a child's dimensionless step length and step frequency, generates the individual's membership values for each of the five clusters, and plots the gait nomogram. The clinical utility of our approach has been demonstrated for two test participants with spastic diplegia, using pre- and postoperative data (one neurosurgical and one orthopaedic), where changes in membership of the five clusters provide objective measures of improvement in their neuromuscular function.

A technique to track individual motor unit action potentials in surface EMG by monitoring their conduction velocities and amplitudes.

The speed of propagation of an action potential along a muscle fiber, its conduction velocity (CV), can be used as an indication of the physiological or pathological state of the muscle fiber membrane. The motor unit action potential (MUAP), the waveform resulting from the spatial and temporal summation of the individual muscle fiber action potentials of that motor unit (MU), propagates with a speed referred to as the motor unit conduction velocity (MUCV). This paper introduces a new algorithm, the MU tracking algorithm, which estimates MUCVs and MUAP amplitudes for individual MUs in a localized MU population using SEMG signals. By tracking these values across time, the electrical activity of the localized MU pool can be monitored. An assessment of the performance of the algorithm has been achieved using simulated SEMG signals. It is concluded that this analysis technique enhances the suitability of SEMG for clinical applications and points toward a future of noninvasive diagnosis and assessment of neuromuscular disorders.

Froude and the contribution of naval architecture to our understanding of bipedal locomotion.

It is fascinating to think that the ideas of two 19th century naval architects could offer useful insights for 21st century scientists contemplating the exploration of our planetary system or monitoring the long-term effects of a neurosurgical procedure on gait. The Froude number, defined as Fr = v2/gL, where v is velocity, g is gravitational acceleration and L is a characteristic linear dimension (such as leg length), has found widespread application in the biomechanics of bipedal locomotion. This review of two parameters, Fr and dimensionless velocity beta = (Fr)1/2, that have served as the criterion for dynamic similarity, has been arranged in two parts: (I) historical development, including the contributions by William Froude and his son Edmund, two ship designers who lived more than 130 years ago, the classic insights of D'Arcy Wentworth Thompson who, in his magnum opus On Growth and Form, espoused the connection between mathematics and biology, and the pioneering efforts of Robert McNeill Alexander, who popularised the application of Fr to animal locomotion; and (II) selected applications, including a comparison of walking for people of different heights, exploring the effects of different gravitational fields on human locomotion, establishing the impact of pathology and the benefits of treatment, and understanding the walking patterns of bipedal robots. Although not all applications of Fr to locomotion have been covered, the review offers an important historical context for all researchers of bipedal gait, and extends the idea of dimensionless scaling of gait parameters.

Neuromaturation of human locomotion revealed by non-dimensional scaling.

When two fundamental gait parameters--step length and step frequency--are scaled non-dimensionally, thereby accounting for increases in a child's physical size, ontogenetic changes in the locomotor control strategy are revealed. We believe dimensionless velocity beta, the product of dimensionless step length and frequency, serves as a measure of neural development. It increases from the age of 18 months and reaches a plateau between 50 and 90 months, attaining the adult value of 0.45. Based on a study of 200 children, our findings lend support to a theory that posits a neuromaturation growth curve with the form: beta (t)=0.45 (1-e(-0.05t)) where t is the age in months and 0.05 is the growth coefficient.

Analysis and simulation of changes in EMG amplitude during high-level fatiguing contractions.

Changes in surface electromyographic (EMG) amplitude during sustained, fatiguing contractions are commonly attributed to variations in muscle fiber conduction velocity (MFCV), motor unit firing rates, transmembrane action potentials and the synchronization or recruitment of motor units. However, the relative contribution of each factor remains unclear. Analytical relationships relating changes in MFCV and mean motor unit firing rates to the root mean square (RMS) and average rectified (AR) value of the surface EMG signal are derived. The relationships are then confirmed using model simulation. The simulations and analysis illustrate the different behaviors of the surface EMG RMS and AR value with changing MFCV and firing rate, as the level of motor unit superposition varies. Levels of firing rate modulation and short-term synchronization that, combined with variations in MFCV, could cause changes in EMG amplitude similar to those observed during sustained isometric contraction of the brachioradialis at 80% of maximum voluntary contraction were estimated. While it is not possible to draw conclusions about changes in neural control without further information about the underlying motor unit activation patterns, the examples presented illustrate how a combined analytical and simulation approach may provide insight into the manner in which different factors affect EMG amplitude during sustained isometric contractions.