Biceps Brachii Muscle Synergy and Target Reaching in a Virtual Environment.

A muscular synergy is a theory suggesting that the central nervous system uses few commands to activate a group of muscles to produce a given movement. Here, we investigate how a muscle synergy extracted from a single muscle can be at the origin of different signals which could facilitate the control of modern upper limb myoelectric prostheses with many degrees of freedom. Five pairs of surface electrodes were positioned across the biceps of 12 normal subjects and electromyographic (EMG) signals were collected while their upper limbs were in eight different static postures. Those signals were used to move, within a virtual cube, a small red sphere toward different targets. With three muscular synergies extracted from the five EMG signals, a classifier was trained to identify which synergy pattern was associated with a given static posture. Later, when a posture was recognized, the result was a displacement of a red sphere toward a corner of a virtual cube presented on a computer screen. The axes of the cube were assigned to the shoulder, elbow and wrist joint while each of its the corners was associated with a static posture. The goal for subjects was to reach, one at a time, the four targets positioned at different locations and heights in the virtual cube with different sequences of postures. The results of 12 normal subjects indicate that with the muscular synergies of the biceps brachii, it was possible, but not easy for an untrained person, to reach a target on each trial. Thus, as a proof of concept, we show that features of the biceps muscular synergy have the potential to facilitate the control of upper limb myoelectric prostheses. To our knowledge, this has never been shown before.

Static hand posture classification based on the biceps brachii muscle synergy features.

With modern upper limb myoelectric prostheses, many movements can be produced when many control signals are available. Some of them could originate from the multifunctional biceps brachii which appears to be composed of up to six individually innervated compartments. Given its compartmental nature, this muscle behaves as if composed of many individual muscles acting in synergy to achieve a given motor task. Through an appropriate synergy model, its EMG signals could thus be used to produce some of the many control signals required with modern upper limb myoelectric prostheses. Exploring that possibility, muscular synergy which is usually applied to a group of different muscles, was tested on the biceps brachii only. A non-negative matrix factorization method was applied on pre-recorded data consisting of 8 surface electromyographic signals collected across the biceps of 10 normal subjects who, in Seat or Stand posture, held their hand in 3 different postures. We found that muscular synergies can be extracted from the biceps brachii. With a learning process and a classifier, it was also possible, between a pair of static hand postures, to identify which one was used when a given record was made. The mean score of correctly detected hand posture was >80% for our subjects.

Use of a decision support system improves the management of hemodynamic and respiratory events in orthopedic patients under propofol sedation and spinal analgesia: a randomized trial.

Decision support systems (DSSs) have been successfully implemented into clinical practice offering clinical suggestions and treatment options with excellent results in various clinical settings. Although their results appeared promising, showing that DSSs can increase anesthesiologists' vigilance and patient safety during surgery, DSSs have never been used before to help anesthesiologists in identifying critical events in patients under spinal analgesia with sedation. We have developed and clinically evaluated a DSS for this specific task. The DSS was developed with the ability to indicate respiratory and hemodynamic critical events via audio-visual alarms and give decisional aid. Critical respiratory events were defined as SpO2 <92 % and/or respiratory rate <8/min. Critical hemodynamic events were defined as mean arterial pressure (MAP) <60 mmHg and/or heart rate <40 bpm. The objective of this trial was to determine the duration to detect and treat these critical events with the help of the DSS (DSS Group) compared with a standard Control Group where the system was not in place. One hundred and fifty orthopedic patients undergoing spinal analgesia with propofol sedation were enrolled in this randomized control trial, 75 each group. All respiratory and hemodynamic critical events were detected in the DSS Group, while in the Control Group 26 % of the events were not detected.The delay to detect and treat critical events was significantly shorter (P < 0.0001) in the DSS Group at 9.1 ± 3.6 s, whereas 27.5 ± 18.9 s were necessary to identify them in the Control Group. There were no significant differences in physiological parameters in the two groups during surgery. The number of critical events/h occurring and the duration of surgery were similar in both groups. The number of hypoxemia episodes was significantly less (P = 0.036) in the DSS group (0.7 ± 1.0 vs. 1.4 ± 2.2 for the Control Group). The DSS tested in this trial could help the clinician to detect and treat critical events more efficiently and in a shorter length of time.

A technical description of a novel pharmacological anesthesia robot.

To control the three components of general anesthesia (hypnosis, analgesia, and neuromuscular blockade), an automated closed-loop, anesthesia-drug delivery system (McSleepy) was developed. Bispectral index was used as the control variable for hypnosis, the analgoscore for analgesia, and phonomyography for neuromuscular blockade. McSleepy can be used to control the induction, maintenance and emergence from general anesthesia. To do so, a large touch screen is used to provide a user friendly interface, permitting bidirectional communication: the user giving information about the different stages of anesthesia, and the system prompting the anesthesiologist to perform certain actions such as mask ventilation, intubation or waking-up the patient using audio clips with voice commands. Several safety features were implemented to provide a secure and reliable anesthesia. Preliminary results of 15 patients are presented in this paper. Evaluation of McSleepy was done through an assessment of its clinical performance and using Varvel's performance indices. The system was found to be clinically useful by providing good precision in drug administration and reliable results for the duration of a general anesthesia.

Time-dependence between upper arm muscles activity during rapid movements: observation of the proportional effects predicted by the kinematic theory.

Rapid human movements can be assimilated to the output of a neuromuscular system with an impulse response modeled by a Delta-Lognormal equation. In such a model, the main assumption concerns the cumulative time delays of the response as it propagates toward the effector following a command. To verify the validity of this assumption, delays between bursts in electromyographic (EMG) signals of agonist and antagonist muscles activated during a rapid hand movement were investigated. Delays were measured between the surface EMG signals of six muscles of the upper limb during single rapid handwriting strokes. From EMG envelopes, regressions were obtained between the timing of the burst of activity produced by each monitored muscle. High correlation coefficients were obtained supporting the proportionality of the cumulative time delays, the basic hypothesis of the Delta-Lognormal model. A paradigm governing the sequence of muscle activities in a rapid movement could, in the long run, be useful for applications dealing with the analysis and synthesis of human movements.

Relaxofon: a neuromuscular blockade monitor for patients under general anesthesia.

Anesthesia consists of three components: unconsciousness, analgesia and neuromuscular blockade (NMB). A specific drug is administered by the anesthesiologist to control these different components. In this paper we propose a new system for monitoring the neuromuscular blockade in anesthesized patients during surgery. Neuromuscular blockade drugs are used routinely by clinicians to induce muscle relaxation in patients. However, the use of these drugs has some risks, so an adequate monitoring of the effects of these drugs is essential. This paper describes the Relaxofon, a NMB monitoring device based on phonomyography. The Relaxofon is composed of a hardware subsystem that records muscle sounds using microphones and a special circuit to filter out the noise and amplify the signal, and a software subsystem that analyses the acquired signal. We tested the ability of the system to record phonomyographic signals from the adductor pollicis and the corrugator supercilii muscles. We then performed the Bland-Altman test to compare the manual Train-of-Four ratio (a measure of the depth of muscle relaxation) calculation against the one performed by the Relaxofon. Finally, we calculated the Pearson correlation coefficient to measure the linear dependence between the two methods. Automatic Train-of-Four ratio calculations using this system showed very good agreement with manual calculations. Results from this work may ultimately lead to integration of NMB monitoring to an automated closed-loop anesthesia system.

A randomized controlled trial demonstrates that a novel closed-loop propofol system performs better hypnosis control than manual administration.

PURPOSE: The purpose of this randomized control trial was to determine the performance of a novel rule-based adaptive closed-loop system for propofol administration using the bispectral index (BIS(R)) and to compare the system's performance with manual administration. The effectiveness of the closed-loop system to maintain BIS close to a target of 45 was determined and compared with manual administration. METHODS: After Institutional Review Board approval and written consent, 40 patients undergoing major surgery in a tertiary university hospital were allocated to two groups using computer-generated block randomization. In the Closed-loop group (n = 20), closed-loop control was used to maintain anesthesia at a target BIS of 45, and in the Control group (n = 20), propofol was administered manually to maintain the same BIS target. To evaluate each technique's performance in maintaining a steady level of hypnosis, the BIS values obtained during the surgical procedure were stratified into four clinical performance categories relative to the target BIS: < or = 10%, 11-20%, 21-30%, or > 30% defined as excellent, good, poor, or inadequate control of hypnosis, respectively. The controller performance was compared using Varvel's controller performance indices. Data were compared using Fisher's exact test and the Mann-Whitney U test, P < 0.05 showing statistical significance. RESULTS: In the Closed-loop group, four females and 16 males (aged 54 +/- 20 yr; weight 79 +/- 7 kg) underwent anesthesia lasting 143 +/- 57 min. During 55%, 29%, 9%, and 7% of the total anesthesia time, the system showed excellent, good, poor, and inadequate control, respectively. In the Control group, five females and 15 males (aged 59 +/- 16 yr; weight 75 +/- 13 kg) underwent anesthesia lasting 157 +/- 81 min. Excellent, good, poor, and inadequate control were noted during 33%, 33%, 15%, and 19% of the total anesthesia time, respectively. In the Closed-loop group, excellent control of anesthesia occurred significantly more often (P < 0.0001), and poor and inadequate control occurred less often than in the Control group (P < 0.01). The median performance error and the median absolute performance error were significantly lower in the Closed-loop group compared with the Control group (-1.1 +/- 5.3% vs -10.7 +/- 13.1%; P = 0.004 and 9.1 +/- 1.9% vs 15.7 +/- 7.4%; P < 0.0001, respectively). CONCLUSION: The closed-loop system for propofol administration showed better clinical and control system performance than manual administration of propofol. (Clinical Trials gov. NCT 01019746).

Magnetic resonance imaging of the erector spinae muscles in Duchenne muscular dystrophy: implication for scoliotic deformities.

BACKGROUND: In Duchenne muscular dystrophy (DMD), the muscular degeneration often leads to the development of scoliosis. Our objective was to investigate how anatomical changes in back muscles can lead to scoliosis. Muscular volume and the level of fat infiltration in those muscles were thus evaluated, in non-scoliotic, pre-scoliotic and scoliotic patients. The overlying skin thickness over the apex level of scoliotic deformations was also measured to facilitate the interpretation of electromyographic signals when recorded on the skin surface. METHODS: In 8 DMD patients and two healthy controls with no known muscular deficiencies, magnetic resonance imaging (MRI) was used to measure continuously at 3 mm intervals the distribution of the erector spinae (ES) muscle in the T8-L4 region as well as fat infiltration in the muscle and overlying skin thickness: four patients were non-scoliotic (NS), two were pre-scoliotic (PS, Cobb angle < 15 degrees ) and two were scoliotic (S, Cobb angle >/= 15 degrees ). For each subject, 63 images 3 mm thick of the ES muscle were obtained in the T8-L4 region on both sides of the spine. The pixel dimension was 0.39 x 0.39 mm. With a commercial software, on each 12 bits image, the ES contour on the left and on the right sides of the spine were manually determined as well as those of its constituents i.e., the iliocostalis (IL), the longissimus (LO) and the spinalis (SP) muscles. Following this segmentation, the surfaces within the contours were determined, the muscles volume were obtained, the amount of fat infiltration inside each muscle was evaluated and the overlying skin thickness measured. FINDINGS: The volume of the ES muscle of our S and PS patients was found smaller on the convex side relative to the concave one by 5.3 +/- 0.7% and 2.8 +/- 0.2% respectively. For the 4 NS patients, the volume difference of this muscle between right and left sides was 2.1 +/- 1.5% and for the 2 controls, it was 1.4 +/- 1.2%. Fat infiltration for the S and the PS patients was larger on the convex side than on the concave one (4.4 +/- 1.6% and 4.5 +/- 0.7% respectively) and the difference was more important near the apex. Infiltration was more important in the lateral IL muscle than in the medial SP and it was always larger near L2 than at any other spinal level. Fat infiltration was much more important in the ES for the DMD patients (49.9% +/- 1.6%) than for the two controls (2.6 +/- 0.8%). As for the overlying skin thickness measured near the deformity of the patients, it was larger on the concave than on the convex side: 14.8 +/- 6.1 vs 13.5 +/- 5.7 mm for the S and 10.3 +/- 6.3 vs 9.8 +/- 5.6 mm for the PS. INTERPRETATION: In DMD patients, our results indicate that a larger replacement of muscles fibers by fat infiltration on one side of the spine is a factor that can lead to the development of scoliosis. Efforts to slow such an infiltration on the most affected side of the spine could thus be beneficial to those patients by delaying the apparition of the scoliotic deformation. In addition to anatomical considerations, results obtained from the same patients but in experiments dealing with electromyography recordings, point to differences in the muscular contraction mechanisms and/or of the neural input to back muscles. This is similar to the adolescent idiopathic scoliosis (AIS) where a role of the nervous system in the development of the deformation has also been suggested.

Effects of fatigue on intermuscular coordination during repetitive hammering.

Fatigue affects the capacity of muscles to generate forces and is associated with characteristic changes in EMG signals. It may also influence interjoint and intermuscular coordination. To understand better the global effects of fatigue on multijoint movement, we studied movement kinematics and EMG changes in healthy volunteers asked to hammer repetitively. Movement kinematics and the activity of 20 muscles of the arm, trunk, and leg were recorded before and after subjects became fatigued (as measured using a Borg scale). When fatigue was reached, maximal grip strength and elbow range of motion decreased while the EMG amplitude of the contralateral external oblique muscle was increased. Fatigue did not affect shoulder and wrist kinematics or movement frequency. Results suggest that fatigue influences motion at both local and global levels. Specifically, interjoint and intermuscular coordination adapt to compensate for local effects of fatigue and to maintain key movement characteristics, such as the trajectory of the end effector and the movement frequency. Nonlocal compensations may be a focus of future studies of how fatigue affects complex movements such as those typically performed in the workplace.

A genetic algorithm for the resolution of superimposed motor unit action potentials.

This paper presents a novel method, which aims at resolving difficult superimpositions of motor unit action potentials (MUAPs) obtained from single-channel intramuscular electromyographic recordings. Resolution is achieved by means of a genetic algorithm (GA) combined with a gradient descent method. This dual optimization scheme has been tested by means of simulations of isolated superimpositions involving two to six MUAPs, along with simulated extended signals of 10-s duration where the density reached 300 MUAPs/s. Of the hundreds of isolated superimpositions tested, more than 90% of the MUAPs were positively identified. With extended signals, identification rates of better than 85% were obtained. The GA alone accounted for up to an 8% improvement over the decomposition conducted using only template matching.

[Tridimensional evaluation and optimization of the orthotic treatment of adolescent idiopathic scoliosis]. / Evaluation tridimensionnelle et optimisation du traitement orthopédique de la scoliose idiopathique adolescente.

Adolescent idiopathic scoliosis involves complex tridimensional deformities of the spine, rib cage and pelvis. Moderate curves generally are treated using an orthosis. This paper presents different studies performed over the last fifteen years related to the biomechanical evaluation and optimization of the orthopedic treatment of scoliotic deformities. Patient specific 3D models of the spine, pelvis and rib cage are computed from calibrated radiographs, and are used to calculate 2D and 3D clinical indices. The torso shape is acquired using surface topography. With such internal and external 3D models, the efficacy of the most frequently used orthoses can be analyzed and new treatments can be developed. Pressures generated by a brace on the patient's trunk were measured using a flexible matrix of pressure sensors and displayed over the patient's internal geometry in order to analyze the brace efficacy. Patient specific finite element models have been developed, including the osseo-ligamentous structures as well as the muscles, the neuro-control, trunk growth and its adaptation to the stress. These models were used to analyze the effects of the Boston brace. The electro-myographic activity also was measured to analyze the << active >> correction mechanisms. Adjustment techniques and software are used to help the orthotists with real time feedback when the brace is being fabricated and adjusted to the patient. Residual growth potential is also being added to the computer model to simulate the long term effect of a brace. The improvement of the orthotic treatments of scoliotic deformities is very encouraging. The exploitation of such tools is expected to allow reaching optimal treatment personalized to each patient. double dagger.

Relationship between stretch reflex thresholds and voluntary arm muscle activation in patients with spasticity.

Previous studies have shown that deficits in agonist-antagonist muscle activation in the single-joint elbow system in patients with spastic hemiparesis are directly related to limitations in the range of regulation of the thresholds of muscle activation. We extended these findings to the double-joint, shoulder-elbow system in these patients. Ten non-disabled individuals and 11 stroke survivors with spasticity in upper limb muscles participated. Stroke survivors had sustained a single unilateral stroke 6-36 months previously, had full pain-free passive range of motion of the affected shoulder and elbow and had some voluntary control of the arm. EMG activity from four elbow and two shoulder muscles was recorded during quasi-static (<5 degrees /s) stretching of elbow flexors/extensors and during slow voluntary elbow flexion/extension movement through full range. Stretches and active movements were initiated from full elbow flexion or extension with the shoulder in three different initial positions (60 degrees , 90 degrees , 145 degrees horizontal abduction). SRTs were defined as the elbow angle at which EMG signals began to exceed 2SD of background noise. SRT angles obtained by passive muscle stretch were compared with the angles at which the respective muscles became activated during voluntary elbow movements. SRTs in elbow flexors were correlated with clinical spasticity scores. SRTs of elbow flexors and extensors were within the biomechanical range of the joint and varied with changes in the shoulder angle in all subjects with hemiparesis but could not be reached in this range in all healthy subjects when muscles were initially relaxed. In patients, limitations in the regulation of SRTs resulted in a subdivision of all-possible shoulder-elbow arm configurations into two areas, one in which spasticity was present ("spatial spasticity zone") and another in which it was absent. Spatial spasticity zones were different for different muscles in different patients but, taken together, for all elbow muscles, the zones occupied a large part of elbow-shoulder joint space in each patient. The shape of the boundary between the spasticity and no-spasticity zones depended on the state of reflex inter-joint interaction. SRTs in single- and double-joint flexor muscles correlated with the positions at which muscles were activated during voluntary movements, for all shoulder angles, and this effect was greater in elbow flexor muscles (brachioradialis, biceps brachii). Flexor SRTs correlated with clinical spasticity in elbow flexors only when elbow muscles were at mid-length (90 degrees ). These findings support the notion that motor impairments after CNS damage are related to deficits in the specification and regulation of SRTs, resulting in the occurrence of spasticity zones in the space of elbow-shoulder configurations. It is suggested that the presence of spatial spasticity zones might be a major cause of motor impairments in general and deficits in inter-joint coordination in particular in patients with spasticity.

Back muscles biometry in adolescent idiopathic scoliosis.

BACKGROUND CONTEXT: Many studies have been devoted to the role of back muscle activity in the development of scoliosis. While an imbalance in the electromyographic (EMG) activity has often been detected at the skin surface, very little information is available on the mechanisms by which such an imbalance could take place. To gain insight into those mechanisms, an important step could be the collection of anatomical data on the volume of the erector spinae muscle on both sides of the spine as well as on the skin and subcutaneous fat (skinfold) thickness separating those muscles from the body surface. For this purpose, the use of magnetic resonance (MR) imaging is appropriate. PURPOSE: To collect anatomical information on the erector spinae muscles and skinfold thickness along the spinal deviations of scoliotic patients. STUDY DESIGN: In an observational retrospective study, MR images of scoliotic patients treated in a pediatric hospital in the last 5 years were analyzed. PATIENT SAMPLE: Images were obtained from adolescent idiopathic scoliosis patients. METHODS: For 15 patients (Group I), three clinical acquisition protocols were used. Five investigators were asked to grade the contrast of the images obtained with each protocol. All the assessments were carried on the same monitor without any change in its settings. For the MR sequence providing the best contrast, 25 fully imaged scoliotic deviations were obtained from 17 patients (Group II). A manual segmentation with an image processing software package was done on the erector spinae muscle on both sides of the spine on each of the available images in order to determine their volume. Skinfold was also measured; first at regular intervals from C7 to L3 over the erector spinae muscle and then at sites centered over the apex of each curve. RESULTS: For Group I, the spin echo (SE-T1) was found to provide the best contrast to identify the contour of individual muscle. With this sequence, the analysis of the fully imaged scoliotic curves (Group II) revealed that back muscle volume was found larger 14 times on the concave side and 11 times on the convex one. When the length of each curve was normalized and then divided into three equal regions, muscle volume was larger 11 times at the apex (6 times on concave side), 7 times above and 7 times below (4 times on the concave side for both positions). From C7 to L3, the mean skinfold thickness of each patient ranged from 7.3 mm to 16.3 mm. On average, this thickness was <10 mm between T3 and T12 but became larger at L3 level. At the apex of each scoliotic deviation, skinfold thickness was always larger on the concave side, and the difference decreased progressively as the distance from the apex increased. CONCLUSION: A larger back muscle volume in adolescent idiopathic scoliosis patients was slightly more frequent on the concave than on the convex side. The differences were more frequent at the apex of the curve. Skinfold thickness was always greater on the concave side at the apex region.

Automated decomposition of intramuscular electromyographic signals.

We present a novel method for extracting and classifying motor unit action potentials (MUAPs) from one-channel electromyographic recordings. The extraction of MUAP templates is carried out using a symbolic representation of waveforms, a common technique in signature verification applications. The assignment of MUAPs to their specific trains is achieved by means of repeated template matching passes using pseudocorrelation, a new matched-filter-based similarity measure. Identified MUAPs are peeled off and the residual signal is analyzed using shortened templates to facilitate the resolution of superimpositions. The program was tested with simulated data and with experimental signals obtained using fine-wire electrodes in the biceps brachii during isometric contractions ranging from 5% to 30% of the maximum voluntary contraction. Analyzed signals were made of up to 14 MUAP trains. Most templates were extracted automatically, but complex signals sometimes required the adjustment of 2 parameters to account for all the MUAP trains present. Classification accuracy rates for simulations ranged from an average of 96.3% +/- 0.9% (4 trains) to 75.6% +/- 11.0% (12 trains). The classification portion of the program never required user intervention. Decomposition of most 10-s-long signals required less than 10 s using a conventional desktop computer, thus showing capabilities for real-time applications.

Differences in multi-joint kinematic patterns of repetitive hammering in healthy, fatigued and shoulder-injured individuals.

BACKGROUND: Work-related musculo-skeletal disorders have been previously related to movement repetition, inadequate postures, non-ergonomic environments, muscular imbalance and fatigue. However, no direct link between fatigue and injury has been experimentally shown. To address this problem, we compared the effects of fatigue and injury on the kinematics of repetitive hammering. METHODS: Healthy subjects (n=30) hammered repetitively both before and after fatigue. Fatigue was induced by a combination of static and dynamic procedures. Shoulder-injured subjects (n=15) hammered for 30s without fatigue. Kinematics of motion was recorded. FINDINGS: The movement time and shoulder range of motion during hammering were not affected by either fatigue or shoulder injury. When fatigued, the healthy subjects displayed decreased range of joint motion, peak velocity and peak acceleration of elbow motion during hammering as well as reduced grip strength. Shoulder-injured individuals had a smaller hammer trajectory amplitude than healthy controls with or without fatigue. They also had lower wrist range of motion, elbow peak velocity, and peak wrist and elbow acceleration compared to healthy subjects hammering without fatigue but only lower wrist peak acceleration compared to healthy subjects hammering with fatigue. INTERPRETATION: Results showed that fatigue affects elbow motion while shoulder injury affects both wrist and elbow motions during hammering. However, shoulder kinematics were not changed by either fatigue or shoulder injury. These changes at the wrist and elbow may reflect strategies used by individuals with shoulder injury to maintain constant movement duration and shoulder kinematics during movement.

Electromyography of scoliotic patients treated with a brace.

When a brace is used to correct spinal deviation, patients may seek to ease the discomfort from the pressure exerted by the orthosis by actively recruiting specific trunk muscles. The effect of bracing on trunk electromyography (EMG) has been reported in only one study where a limited number of electrodes were placed mainly in the thoracic region. Our hypothesis was that a multi-electrode mapping of the activity of the thoracic, lumbar, and abdominal trunk muscles would provide a more representative picture of the muscular reaction in response to bracing. With a larger number of EMG measuring sites, the presence of any brace-induced trunk muscle activity should be detected. Therefore, EMG signals of 11 adolescent idiopathic scoliosis patients who had been undergoing Boston brace treatment for 0.7-3 years were collected during four isometric tasks to evaluate the response of trunk muscles in the minutes following the application of the brace. Twenty-two pairs of bipolar electrodes were used to measure the EMG signals of the main superficial trunk muscles during four isometric tasks. EMG signals of trunk muscles were compared in braced and unbraced conditions. Brace-induced increases in EMG activity were significant in 43% of the individual measurements and in three of the four tasks for the group mean values. Increases were greater in the lumbar area, especially on the convex side of the secondary (lumbar) curve. These results thus suggest that immediately following the application of the brace, significant muscular responses can be observed in some patients.

Continuous curve registration as an intertrial gait variability reduction technique.

Timing in peak values shifts slightly between gait trials. When gait data are averaged, part of the standard deviation could be associated with this intertrial variability unless normalization is carried out beforehand. The objective of this study was to determine how continuous curve registration, an alignment technique, can reduce intersubject variability in gait data without altering the original curve characteristics. Gait data were obtained by means of a four-camera high-speed video system synchronized to a force plate. The data for 60 gait trials were collected from 20 young, healthy subjects. Curve registration was applied to hip angular displacement, net moment, and power curves generated in the sagittal plane. Following registration, the peak values increased by an average of 1.2% (0.11 +/- 0.26 degrees) for angular displacement, and by 11.2% (0.11 +/- 0.09 W/kg) for power, while there were no changes for moments. First and second derivatives of the unregistered and registered curves did not display significant differences, and the harmonics were barely affected. Continuous curve registration would thus be an appropriate technique for application prior to any statistical analysis using able-bodied gait patterns.

Movement reorganization to compensate for fatigue during sawing.

Peripheral (muscle) aspects of fatigue are well documented. However, little is known about the central aspects of fatigue that could influence, in particular, multijoint coordination. To investigate the central aspects of fatigue, we compared the multijoint kinematics of non-fatigued and fatigued individuals while sawing. Muscle fatigue was associated with decreases in sawing force and movement amplitude at the elbow whereas the basic characteristics of the saw trajectory, including the movement direction, extent and duration, remained invariant. This invariance was maintained by increasing the movement amplitude at the wrist, shoulder and trunk. The system thus takes advantage of the redundancy of the motor apparatus to maintain the endpoint trajectory despite fatigue.