Horizontal Flexion Research Articles

Transformation of the kinematic characteristics of a precise movement after a change in a spatial task

The central mechanism of motor programming was studied using a model of precise horizontal flexion of the arm at the elbow joint. Training was performed in the dark to ensure that formation of the motor program was based predominantly on the use of proprioceptive afferentation. The target was not demonstrated before training: subjects determined the angle of arm flexion during training, the moment at which the target position was reached being identified by a brief LED flash. Subjects had to perform the movement as quickly and accurately as possible. The amplitude, speed, and accuracy of the movement were measured in real time. The ten subjects were divided into two groups for initial training to precise movements, using two different protocols: flexion of the elbow to 70 degrees and to 55 degrees . At the second stage of the experiment, each subject's initial target position was suddenly changed (from 70 degrees to 55 degrees and vice versa). Training was continued until a stable accuracy in the new conditions was achieved (with errors of no more than 5% of the specified amplitude). The nature of the transformation in the kinematics of the precise movement in response to the change in the single task parameter illuminated the fundamental principle of organization of the supraspinal motor command for movements of this type. For both specified flexion amplitudes, the ratio between the acceleration and deceleration phases of the movement were identical during the period of skill fixation. On average, 70% of the total amplitude of flexion was accounted for by the acceleration phase and 30% by the deceleration phase. Adaptation of the precise movement to changes in the specified horizontal elbow flexion angle (i.e., re-achievement of the required movement accuracy in the changed conditions) during rearrangement was completed by inversion of these values. According to the results of previous studies, the most informative measure for analysis of the dynamics of the controlling central command was the acceleration of the movement. In terms of current concepts of the mechanism of motor control, the acceleration plateau can be regarded as mirroring long-term depression--the voltage plateau in Purkinje cells and motoneurons. Data processing involved calculation of the integral acceleration in both phases of the movement in relation to the angle of flexion (phase plots). These data underlie our understanding of the mechanism of transformation of movement kinematics responsible for the formation of a new central command.

Kinesiotherapy associated with laser GaAs application in shoulder-rotator cuff syndrome

We verified the effects of kinesiotherapy associated with the application of a gallium arsenate AsGa laser (904 nm) to pain, joint amplitude, and hidroxiproline excretion (HP) in subjects with shoulder rotator cuff syndrome (SRCS). Sixty-nine heterogeneous individuals of both sexes from Vicosa, MG, aged between 40 and 65, diagnosed with shoulder rotator cuff syndrome and referred to physiotherapy participated voluntarily in this study. Pain was measured on the Borg CR10 scale; HP was analyzed by the colorimetric protocol and joint amplitude by the goniometric LABIFIE protocol. Descriptive statistics were used, with the mean values and standard deviation and a probability interval of 95% minimum and maximum of the mean. In the inferential, 2 × 2 ANOVA for repeated measures was used to evaluate the HP and joint amplitude, followed by the Tukey post hoc test to identify the significant differences. For evaluating pain, the Friedman test was used and followed by the Wilcoxon test to identify the significant differences. The significance level adopted for all the tests was 0.05, and all the tests were performed by the software Premir by Biostatistics, version 4.0. The results showed that there was a significant improvement in the pain variables, hidroxiproline, and in the joint amplitudes of the flexion, horizontal flexion, abduction, shoulder medial rotation, lateral rotation, and horizontal rotation movements in the group submitted to kinesiotherapy associated with an AsGa laser.

Repeatable anatomical calibration in in vivo movement analysis

Motion analysis systems deliver quantitative information, e.g. on the progress of rehabilitation programs aimed at improving range of motion. Markerless systems are of interest for clinical application because they are low-cost and easy to use. The first generation of the Kinect™ sensor showed promising results in validity assessment compared to an established marker-based system. However, no literature is available on the validity of the new ‘Kinect™ for Xbox one’ (KinectOne) in tracking upper body motion. Consequently, this study was conducted to analyze the accuracy and reliability of the KinectOne in tracking upper body motion.Twenty subjects performed shoulder abduction in frontal and scapula plane, flexion, external rotation and horizontal flexion in two conditions (sitting and standing). Arm and trunk motion were analyzed using the KinectOne and compared to a marker-based system. Comparisons were made using Bland Altman statistics and Coefficient of Multiple Correlation.On average, differences between systems of 3.9 ± 4.0° and 0.1 ± 3.8° were found for arm and trunk motion, respectively. Correlation was higher for the arm than for the trunk motion.Based on the observed bias, the accuracy of the KinectOne was found to be adequate to measure arm motion in a clinical setting. Although trunk motion showed a very low absolute bias between the two systems, the KinectOne was not able to track small changes over time. Before the KinectOne can find clinical application, further research is required analyzing whether validity can be improved using a customized tracking algorithm or other sensor placement, and to analyze test–retest reliability.

Rotation sequence as an important factor in shoulder kinematics

The International Society of Biomechanics has proposed a standardization recommendation for motion recordings of the upper extremity defining the set of bony landmarks, local coordinate systems and joint coordinate systems. The aim of our study was to verify the clinical interpretation of the proposed rotation sequence for the glenohumeral joint and to compare it with other sequences. Fifteen glenohumeral movements in their maximal ranges were tested on five healthy subjects. The movements were separated into five groups (flexion, extension, abduction, horizontal flexion and circumduction) with three humeral rotation positions (full external, full internal and neutral). Four glenohumeral rotation sequences were constructed using YXY, YXZ, ZXY and XZY orders and angle amplitudes were examined in terms of gimbal lock and amplitude coherence. The results of the gimbal lock incidence and amplitude coherence should be taken into account together. Therefore, the suitable rotation sequences for all rotation variations of abduction and extension were found and no tested rotation sequence was found to be clinically interpretable for all tested movements. Before glenohumeral three-dimension experiments the choice of the rotation sequence should be made in agreement with the no-gimbal lock incidence and amplitude interpretability of the performed movements.

Activation characteristics of shoulder muscles during maximal and submaximal efforts

Determination of individual maximum voluntary contraction (MVC) force is used as the gold standard for normalising surface EMG (SEMG) data. Assuming a linear amplitude-force relationship, individual strain levels are defined according to percentage rates of the measured MVC levels. The purpose of the study was to investigate if the assumed force-strain relationship can be applied without qualification. Therefore, healthy volunteers (nine men, ten women) were investigated during isometric exercises of shoulder muscles at MVC and 50% levels. Tasks were performed at four different angular positions in frontal, sagittal and horizontal planes. In each plane, both possible force directions were investigated. SEMG was taken simultaneously from 13 muscles of the shoulder and upper arms from both sides of the body. At a force level of 50% MVC, SEMG amplitude levels were compared to the expected 50% level. Differences in muscular co-ordination patterns were also determined. During retroversion and horizontal flexion, amplitude levels significantly remained at levels below 50%. This was seen for all the muscles investigated, independent of relative contribution to force production. During horizontal extension and abduction, the main force-producing muscles showed amplitude levels that significantly exceeded the expected 50% level. Co-ordination patterns differed significantly between MVC and submaximal conditions for anteversion, retroversion and horizontal extension. Specifically, four shoulder muscles showed higher proportions at the 50% level compared to MVC. Therefore, certain percentage rates of MVC force levels exhibit quite different strain rates, as identified by SEMG. Depending on force direction, differences in co-ordination patterns exist between MVC and submaximal test conditions. Both findings have implications for therapeutic and training applications.

Physiological coxa varus?genu valgus influences internal knee and ankle joint moments in females during crossover cutting

This study evaluated the ankle and knee electromyographic, kinematic, and kinetic differences of 20 nonimpaired females with either neutral (group 1) or coxa varus-genu valgus (group 2) alignment during crossover cutting stance phase. Two-way mixed model ANOVA (group, session) assessed mean differences ( p<0.05) and correlation analysis further delineated relationships. During impact absorption, group 2 displayed earlier peak horizontal braking (anterior-posterior) ground reaction force timing, decreased and earlier peak internal knee extension moments (eccentric function), and earlier peak internal ankle dorsiflexion moment timing (eccentric function). During the pivot phase, group 2 displayed later and eccentrically-biased peak ankle plantar flexion moments, increased peak internal knee flexion moments (eccentric function), and later peak knee internal rotation timing. Correlation analysis revealed that during impact absorption, subjects with coxa varus-genu valgus alignment (group 2) displayed a stronger relationship between knee internal rotation velocity and peak internal ankle dorsiflexion moment onset timing ( r= -0.64 vs r = -0.26) and between peak horizontal braking ground reaction forces and peak internal ankle dorsiflexion moment onset timing ( r= 0.61 vs r= 0.24). During the pivot phase these subjects displayed a stronger relationship between peak horizontal braking ground reaction forces and peak internal ankle plantar flexion moment onset timing ( r= -0.63 vs r= -0.09) and between peak horizontal braking forces and peak internal ankle plantar flexion moments ( r= -0.72 vs r= -0.26). Group differences suggest that subjects with coxa varus-genu valgus frontal-plane alignment have an increased dependence on both ankle dorsiflexor and plantar flexor muscle group function during crossover cutting. Greater dependence on ankle muscle group function during the performance of a task that requires considerable 3D dynamic knee joint control suggests a greater need for frontal and transverse plane weight bearing tasks that facilitate eccentric ankle muscle group function to optimize injury prevention conditioning and post-surgical rehabilitation programs.

Unintentional forces developed during isometric test of the shoulder

Objective. To study unintentional, spontaneous forces developed during isometric shoulder strength testing. Design. An experimental study. Background. During torque measurements, subjects may apply forces in directions other than requested, thereby obscuring the interpretation of the results. Methods. A shoulder strength test was performed with a strength test device permitting measurements of force in the requested direction as well as perpendicular to it. Nine fishermen and nine welders performed isometric abduction and elevation with the arm held in various angles. Moreover four rotational postures were investigated. Results. Spontaneous force deviations from requested direction were found in almost all posture tested, with errors––defined as 100 times the ratio of the magnitude of intentional torque to the magnitude of unintentional (erroneous) torque generated perpendicular to it––of more than 30% found for some postures. Abduction in 45° horizontal flexion was better controlled than abduction in 90° horizontal flexion. Detailed analysis indicated some groupwise differences between fishermen and welders. Also pairwise comparison of handiness indicated some differences. Conclusions. Considerable deviations from requested direction of action can arise during strength tests. Thus, one must either monitor these deviations or minimize them by a proper design of the examination to get relevant data. As the error can differ between certain subject categories depending on posture this indicate a new tool for discriminating between musculo-skeletal problems. Relevance Muscle strength devices must be used with care as their measurement setups may mask the true muscle action of the patients.

Anticipatory Modulation of Precision Grip Force With Variations in Limb Velocity of a Curvilinear Movement

The authors examined the relationship between peak velocity of a discrete horizontal elbow flexion movement in which the hand path was curvilinear and premovement modulation of precision grip force. The velocity of the movements of 7 participants was varied from maximal velocity to a velocity that required several seconds to reach a target. An object instrumented with force transducers for the forefinger and thumb measured precision grip force. There was a positively accelerating quadratic relationship between grip force change before movement and peak velocity of the ensuing limb movement. On some low-velocity trials, premovement grip force modulation reflected a net decrease. In contrast, high-velocity trials were preceded by net increases in grip force. Using cluster analysis, the authors classified grip forces in low-velocity movements as an empirically distinct set of entities from grip forces in high-velocity movements. The cluster of high-value grip forces suggested an anticipatory strategy that allowed participants a large safety margin in grip force to avoid object slip on movement initiation. The cluster of low-value grip forces at movement initiation suggested a second anticipatory strategy in which participants changed grip force very little, perhaps to increase the ability of proprioceptors in the hand to sense force changes. Those findings suggest that modulation of grip force before initiation of movements in which the hand path is curvilinear may be governed by two distinct velocity-dependent anticipatory strategies.

Suprascapular neuropathy in volleyball players

Background—Suprascapular nerve entrapment with isolated paralysis of the infraspinatus muscle is uncommon. However, this pathology has been reported in volleyball players. Despite a lack of scientific evidence, excessive strain on...

A clinical test for superior glenoid labral or 'SLAP' lesions.

To describe a clinical test associated with unstable lesions of the superior glenoid labrum-long head biceps tendon origin, or SLAP (superior labrum anterior to posterior). Description of a newly discovered clinical sign that correlated with SLAP pathology. Retrospective review of 66 consecutive arthroscopically confirmed SLAP lesions to determine the sensitivity of the SLAPprehension test. Orthopedic sports medicine clinics with an emphasis on shoulder problems. Patients with shoulder pain and arthroscopically verified lesions of the superior glenoid labrum and conjoined long head biceps tendon. Shoulder arthroscopy and in some cases arthroscopic SLAP lesion repair. Nonapplicable. The SLAPprehension test involves cross chest adduction (horizontal flexion) of the affected shoulder with the elbow extended and forearm pronated. A positive maneuver produces either apprehension, pain referable to the bicipital groove, and an audible or palpable click. The test is repeated with the forearm supinated, which must cause diminution of the pain. Mechanically, elbow extension and forearm pronation places traction on the long head biceps tendon. When anterior scapular protraction is limited by the clavicle, further adduction entraps the unstable biceps tendon and superior glenoid labrum between the glenoid fossa and humeral head. Forearm supination decreases traction on the long head biceps tendon and allows for reduction of the unstable labrum complex with lessening of the pain. A retrospective chart review of 66 consecutive arthroscopically verified shoulders with SLAP lesions revealed the SLAPprehension test to be 87.5% sensitive for unstable SLAP lesions. The SLAPprehension test is helpful in the clinical evaluation of patients with unstable superior glenoid labrum lesions whose symptoms are often confused and overlap with those of shoulder impingement or acromioclavicular arthrosis.

Kinesthetic perceptions of intrinsic anterior-posterior axes.

The purpose of this research was to investigate whether kinesthetic and proprioceptive perceptions of "straight ahead" were defined by a head- or trunk-fixed axis. Subjects were instructed to align the forearm with the head or trunk anterior-posterior (a/p) axis by elbow flexion or extension in the horizontal plane in five different conditions. In each condition the experimenter varied initial elbow and shoulder horizontal flexion or extension angles and head and/or trunk orientation (by rotation about a vertical axis) on each trial before the subject moved the forearm to align it with the head or trunk axis. The upper limb motion was voluntarily constrained to the horizontal plane through the shoulder. Variable errors were significantly lower when subjects aligned the forearm to the trunk-fixed a/p axis. Furthermore, the perceptual errors showed a greater dependence on body segment orientations when the forearm was aligned to the head axis than to the trunk axis. We conclude that the trunk a/p axis is preferred to the head a/p axis for specifying upper limb segment orientations in the horizontal plane at the kinesthetic perceptual level.

The relationship between elbow flexor volume and angular stiffness at the elbow

Objective. The purpose of this experiment was to determine if a correlation exists between the volume of the elbow flexors and angular stiffness at the elbow, and to determine the contribution of the biceps brachii and the brachialis muscles to angular stiffness. Design. This study is a descriptive, correlational study and presents a graphical model of the passive properties of muscle. Background. The correlation between arm volume and angular stiffness has been shown, but the measurement of arm volume was not specific to the structures being strained. Pre-positioning a bi-articular muscle by stretching over one joint decreases the range of motion at the other joint and may affect the stiffness. Methods. Angular stiffness at the elbow of 14 female and 15 male volunteers was measured, and the volume of the elbow flexors was calculated from compounded ultra-sound imaging. Initial biceps length was set by pre-positioning the shoulder in two different positions. Results. A significant linear relationship was observed between the slope of phase 1 of the stiffness curve and volume of the elbow flexors in both horizontal flexion ( r = 0.92) and horizontal extension ( r = 0.79) of the shoulder. Phase 2 of the stiffness curve showed no linear relationship to muscle volume in either shoulder position (flexion, r = 0.22; extension r = 0.33). The slopes of phases 1 and 2 were significantly greater with the shoulder in horizontal extension than in horizontal flexion. Conclusion. The volume of the elbow flexor muscles is a good predictor of angular stiffness in phase 1 of the curve. A model of the additive contribution of the biceps and brachialis muscles is presented to account for the increased stiffness in the shoulder extended position.

Contributions of Upper Limb Segment Rotations during the Power Serve in Tennis

In the high-velocity tennis serve, the contributions that the upper limb segments' anatomical rotations make to racket head speed at impact depend on both their angular velocity and the instantaneous position of the racket with respect to the segments' axes of rotation. Eleven high-performance tennis players were filmed at a nominal rate of 200 Hz by three Photosonics cameras while hitting a high-velocity serve. The three-dimensional (3-D) displacement histories of 11 selected landmarks were then calculated using the direct linear transformation approach, and 3-D individual segment rotations for the upper limb were calculated using vector equations (Sprigings, Marshall, Elliott, &amp; Jennings, 1994). The major contributors to the mean linear velocity of the center of the racket head of 31.0 m · s-1 at impact were internal rotation of the upper arm (54.2%), flexion of the hand (31.0%), horizontal flexion and abduction of the upper arm (12.9%), and racket shoulder linear velocity (9.7%). Forearm extension at the elbow joint played a negative role (-14.4%) and reduced the forward velocity of the center of the racket at impact.

Muscular synergy in the shoulder during a fatiguing static contraction

The synergic operation of shoulder muscles during a fatiguing submaximal arm flexion task was studied using both a biomechanical 3-dimensional shoulder model and electromyographic recordings. A new optimization scheme aiming to maximize the task endurance time by constantly regulating the force output of each muscle is utilized in the model. The method can be used to simulate the muscle rotation phenomenon, which has been proposed to occur during an endurance type contraction. The model predictions on the fatigue order of the shoulder muscles were compared to results derived using the median frequencies of the electromyographic signals from nine muscles or muscle parts. In the test performed 10 men held until exhaustion a weight (4 kg) suspended on the wrist with the arm in horizontal flexion. The deltoid, infraspinatus, and supraspinatus muscles were the first to show electromyographic signs of fatigue. The times for detecting electromyographic changes in the trapezius muscle were longer than those in the muscles first showing electromyographic signs of fatigue. The biomechanical model used predicted the upper and lower infraspinatus and the anterior part of the deltoid to be the first to show signs of fatigue during the flexion task. The predictions of the biomechanical model on the order of fatigue of the nine shoulder muscles monitored also using electromyographic recordings corresponded to the electromyographic results. However, the accuracy of this comparison is limited by the fact that the electromyographic recordings did not cover all the muscles used in the model. No clear order was found for the development of electromyographic signs of muscle fatigue.

Capsular restraints to anterior-posterior motion of the abducted shoulder: A biomechanical study

Twenty-three fresh-frozen cadaver shoulders free of degenerative arthritis or rotator cuff disease were tested biomechanically to quantitate the contribution of specific capsular structures to restricting anterior-posterior translation of the abducted shoulder. With the glenohumeral joint in 90 degrees of abduction on a servohydraulic control testing system, translation was measured in 30 degrees of forward flexion (with regard to the coronal plane of the scapula), 0 degree, and 30 degrees of extension while a 25 N anterior-posterior load was applied. Measurements were taken both in the intact (vented) shoulder and after selective cutting of different capsuloligamentous structures was performed. In the intact shoulder the largest anterior-posterior translation occurred in 0 degree of horizontal flexion and extension with regard to the scapular plane, with equal amounts of anterior and posterior translation noted. The primary anterior-posterior stabilizer of the abducted shoulder is the inferior glenohumeral ligament complex. The anterior band is the primary stabilizer in 30 degrees of horizontal extension and at 0 degree (neutral). The posterior band is the primary stabilizer in 30 degrees of horizontal flexion. This study quantifies for the first time the normal amount of anterior-posterior translation in the intact cadaveric shoulder model. In addition, it demonstrates the relative role of the anterior and posterior band of the inferior glenohumeral ligament complex in stabilizing the glenohumeral joint at 90 degrees of abduction, where most clinical instability of the shoulder occurs.

The Effect of Size and Fabric Weight of Protective Coveralls on Range of Gross Body Motions

This study evaluated the effects of garment size and fabric weight on range-of-motion (ROM). Ten male subjects performed a series of twelve gross body movements while wearing each of nine similarly styled coveralls. The coveralls were undersized, appropriately sized, and oversized, and were constructed from three different weights of poly/cotton fabric. A balanced 3 x 3 repeated measures experimental design was used, along with a seminude control condition. ROM was measured with a two-arm manual goniometer. Garment size significantly affected (p < .05) ROM for all movements except shoulder extension and trunk lateral flexion. Compared to seminude ROM, undersized garments reduced the mean ROM by as much as 24% in the case of hip flexion. Fabric weights on ROM were significant for shoulder extension and elbow, hip, knee, and shoulder horizontal flexion. Fabric weight affected ROM less than garment size. Interaction effects between fabric weight and size generally were not significant. These results demonstrate that undersized garments can measurably reduce the wearer's movement capability. Providers of protective clothing should ensure that garments are not undersized and should consider the benefits of oversizing against possible safety and wearer acceptance problems.

An electromyographic analysis of functional differentiation in human pectoralis major muscle

The aim of this investigation was to determine whether functional differentiation was present within the radiate muscle, pectoralis major, by determining whether motor units within individual muscular segments were independently controlled by the central nervous system (CNS) during a series of isometric muscle actions. Six miniature surface electrode pairs were placed along the origin of the pectoralis major of eight male subjects. Electromyographic (EMG) potentials were then recorded during isometric shoulder actions in various planes and at two levels of contraction intensity. The EMG waveforms were integrated (IEMG) and then normalized. The normalized IEMG waveforms (nIEMG) were then statistically compared to determine whether motor units within the detection area of each electrode pair had varied their contribution to the total activity of the muscle from one isometric action to another. An alteration in the contribution of each electrode site (muscular segment) to the total activity of the pectoralis major was taken to be indicative of functional differentiation. The results of this study indicated that the muscular segments of pectoralis major were controlled independently during performance of the isometric tasks. Isometric shoulder adduction saw moderate to high levels of activation expand from the inferior segments of the muscle to all segments, excluding the clavicular head, as the position of the upper limb changed from 90° to 0° of abduction. The most inferior muscular segments of pectoralis major were utilized preferentially when the subjects performed shoulder extension motions from a flexed shoulder position. Conversely, shoulder flexion motions utilized the superior segments of the muscle. Finally, horizontal flexion of the shoulder joint elicited activity from the middle segments of the muscle regardless of the degree of shoulder flexion. The results of this study support the notion that the CNS ‘fine tunes’ the activation patterns of muscular segments within pectoralis major to maximize the muscle's potential to contribute efficiently to isometric actions of the shoulder.

Isokinetic and isometric measurement of strength of external rotation and abduction of the shoulder.

The strength of active external rotation and of abduction of the shoulder when the humerus was in the plane of the scapula (30 degrees of horizontal flexion anterior to the coronal plane) was measured isokinetically and isometrically in thirty-nine normal volunteers, who were stratified by age and sex. The angles at which peak torque was produced were similar when tested isokinetically and isometrically; these angles were similar for external rotation (at 60 and 30 degrees of internal rotation) and for abduction (at 30 and 60 degrees of abduction). Isometric peak torque was greater than slow-speed (90 degrees per second) isokinetic peak torque, which in turn was greater than fast-speed (210 degrees per second) isokinetic peak torque. There were highly significant differences in strength, measured isokinetically and isometrically, between younger and older men and between older men and older women. The variability of normal values for torque was similar in each group. Repeat testing demonstrated a high reliability of isokinetic measurements and of isometric measurements at angles within the range of the production of peak torque. Complete testing was performed in four normal volunteers before and after a block of the suprascapular nerve. The supraspinatus and infraspinatus components of the rotator cuff contributed a variable proportion to the total strength of abduction (25 to 50 per cent) and external rotation (50 to 75 per cent) throughout the range of motion. This study demonstrated that both isokinetic and isometric testing in the scapular plane are valid methods for measurement of the strength of external rotation and abduction of the shoulder. The data support standardization of the positions for testing the strength of motions of the shoulder: isometric strength of external rotation should be measured in the scapular plane with the shoulder in 45 degrees of abduction and 45 degrees of internal rotation; isometric strength of abduction, in the scapular plane with the shoulder in 45 degrees of abduction; and isokinetic strength of external rotation and abduction, in the scapular plane at 90 degrees per second.

Glenohumeral translation after total shoulder arthroplasty

To define the amount and direction of anteroposterior translation of the glenohumeral joint after total shoulder arthroplasty, 13 patients who had previously undergone nonconstrained total shoulder arthroplasty were studied roentgenographically. Patients were placed on the x-ray table in a supine position, and axillary roentgenograms of the shoulder were made with the arm at 90° abduction and in four positions within the horizontal plane of motion: - 30° (horizontal extension), 0° (neutral), 30°, and 60° (horizontal flexion). For all positions the shoulder was in neutral rotation. The center of the prosthetic humeral head was identified, and the distance between it and a perpendicular line bisecting the glenoid component was measured. At follow-up the patients had statistically significant improvements in pain, motion, and function. No patients had any clinical evidence of instability. The mean total translation was 4 mm posteriorly (range 0 to 12 mm). Most translation occurred between - 30° and 30°. Anteroposterior translation appears to occur in a posterior direction. For this to occur the prosthetic humeral head slides onto the polyethylene rim of the glenoid component, subjecting it to eccentric loading and the possibilities of wear, polyethylene debris formation, induction of osteolysis, and subsequent component loosening.

Isokinetic and isometric measurement of strength of external rotation and abduction of the shoulder.

The strength of active external rotation and of abduction of the shoulder when the humerus was in the plane of the scapula (30 degrees of horizontal flexion anterior to the coronal plane) was measured isokinetically and isometrically in thirty-nine normal volunteers, who were stratified by age and sex. The angles at which peak torque was produced were similar when tested isokinetically and isometrically; these angles were similar for external rotation (at 60 and 30 degrees of internal rotation) and for abduction (at 30 and 60 degrees of abduction). Isometric peak torque was greater than slow-speed (90 degrees per second) isokinetic peak torque, which in turn was greater than fast-speed (210 degrees per second) isokinetic peak torque. There were highly significant differences in strength, measured isokinetically and isometrically, between younger and older men and between older men and older women. The variability of normal values for torque was similar in each group. Repeat testing demonstrated a high reliability of isokinetic measurements and of isometric measurements at angles within the range of the production of peak torque. Complete testing was performed in four normal volunteers before and after a block of the suprascapular nerve. The supraspinatus and infraspinatus components of the rotator cuff contributed a variable proportion to the total strength of abduction (25 to 50 per cent) and external rotation (50 to 75 per cent) throughout the range of motion. This study demonstrated that both isokinetic and isometric testing in the scapular plane are valid methods for measurement of the strength of external rotation and abduction of the shoulder. The data support standardization of the positions for testing the strength of motions of the shoulder: isometric strength of external rotation should be measured in the scapular plane with the shoulder in 45 degrees of abduction and 45 degrees of internal rotation; isometric strength of abduction, in the scapular plane with the shoulder in 45 degrees of abduction; and isokinetic strength of external rotation and abduction, in the scapular plane at 90 degrees per second.