Shoulder Flexion Angle Research Articles

Relaxed hand postures

The objective of this study was to determine the relaxed hand postures in degree from a total of 14 finger joints including the thumb, index finger, middle finger, ring finger, and little finger without force exertions and constraints. Fifteen male students fully relaxed their right hands in 9 combinations of the hand positions of pronation, neutral, and supination at the shoulder flexion angles of 0 degree, 45 degrees, and 90 degrees. Vicon MX system was utilized to measure the joint angles with 8 cameras and 26 reflective markers. The hand position had more effects on the finger joint angles than the shoulder flexion angles. Overall, the fingers flexed with supination approximately 20% and 27% more than with neutral and pronation, respectively, due to the mass of the phalanges. For the same reason, the fingers were more flexed at the shoulder flexion angle of 90 degrees with the most hand positions except with the hand position of pronation because of the tensile force of the extensor muscles and connective tissues in the forearm.

Effects of a Proximity-Sensing Feedback Chair on Head, Shoulder, and Trunk Postures When Working at a Visual Display Terminal

This study was designed to identify the effects of feedback from a proximity-sensing chair on head, shoulder, and trunk postures when working at a visual display terminal (VDT). Twenty healthy adults were asked to perform VDT work, and their forward head, forward shoulder, and trunk flexion angles were analyzed using a 3-D motion analysis system. The statistical significance of differences between without and with an auditory feedback device was tested by paired t-tests, with the significance cutoff set at alpha=0.05. The forward head, forward shoulder, and trunk flexion angles significantly decreased during VDT work when using the proximity sensor with auditory feedback. We suggest that a feedback device promotes the adoption of beneficial postures, which may be effective in preventing VDT-work-related neck and upper-limb disorders.

Evaluation of a Redesigned Self-Checkout Station for Wheelchair Users

Self-checkout is the emergent/emerging retail technology wherein users (shoppers) check out their own items using an interactive kiosk. A dramatic growth is anticipated in the prevalence of self-checkout systems in retail environments. A study was conducted to develop and evaluate a redesign of a self-checkout system with a focus on issues related to physical accessibility for wheelchair users. Two checkout station prototypes were built: a full-scale model of an existing system and a full-scale model of a system with design modifications (e.g., inclusion of appropriate wheelchair clearance under the workstation, reduced vertical position of the credit card reader, etc.). Five wheelchair users and 10 non–wheelchair users performed simulated self-checkout activities using both workstations. The principal independent variable was workstation type (standard design vs. modified design). The dependent measures were productivity (time to complete a transaction), shoulder flexion angle, torso flexion angle, and the user's subjective assessment of the experience. The results of this study indicate that workstation type did not influence productivity levels in either participant group (wheelchair users or non–wheelchair users). Posture, on the other hand, was significantly improved: the peak shoulder angle was reduced by 64% in wheelchair users and by 69% in the non–wheelchair user group. Peak flexion angle of the torso was also reduced by 67% for wheelchair users. Subjective feedback from the wheelchair user group supported the postural data by showing an overall preference for the redesigned workstation, whereas the non–wheelchair group showed no preference between the two. These results indicate that design for populations with specific limitations does not need to come at a cost of reduced accessibility for persons without these limitations; universal design is achievable.

Maximum one-handed shoulder strength for overhead work as a function of shoulder posture in females

A laboratory study was conducted to determine the maximum voluntary strength (MVC) and maximum voluntary torque for one-handed lift and hold and hold only tasks as a function of shoulder posture. Twelve female volunteers of 21–33 years of age, claimed to be in good physical health and stated they had never had any musculoskeletal or cardiovascular problems participated in the study. One-handed maximum voluntary strength was defined as the maximum weight that a subject could lift and hold for 4 seconds (lift and hold) or hold the weight for 4 seconds without lifting from table height (hold only). MVC was measured in six different postures most commonly used in automobile assembly: 0/90 (shoulder flexion angle=0° and included elbow angle=90°), 30/90, 60/90, 90/120, 120/150, and 150/180. An analysis of variance showed that hold only MVCs were significantly higher than lift and hold MVCs by an average of 26%. Shoulder posture had significant effect on both types of MVCs. The strongest posture was 0/90 and the weakest posture was 90/120. There was large variability between subjects and some subjects had maximum shoulder strength as low as 4.3 kg. The findings of this study indicate that female have very low shoulder strength especially for jobs requiring overhead work.

외부 부하가 부과된 어깨 자세의 심물리학적 불편도 평가

The purpose of this study is to quantitatively investigate perceived discomfort of complex shoulder postures with external loads and to propose a preliminary evaluation scheme of shoulder postures. Twelve healthy male adults participated in an experiment to rate their perceived discomfort of shoulder postures. The independent variables were shoulder flexion angle (45, 90 and 150°), adduction/abduction angle(-30, -10, 0, 30 and 60°), and external load(0, 1.5 and 3.0㎏). The results revealed that the flexion angle, external load and their interaction significantly affected the perceived discomfort(p<0.05) but the effect of adduction/abduction angle on the discomfort was not significant(p>0.05). The effect of external load was much larger than that of any other factor with explaining about 81% of the total variation of discomfort scores. Based on the experimental results a preliminary scheme was presented to evaluate the stress of shoulder postures with external loads.

単純X線画像による上腕骨後捻角撮影法の検討

The semi-axial-view method is used to measure humeral retroversion angles by a single x-ray exposure. We examined the clinical utility of this method using a humerus model and normal volunteers. In an experiment with a humerus model, humeral retroversion angles were measured at various positioning angles by the semi-axial-view method, and compared with the angles measured by the CT method, which is considered the standard, and the best position was determined. The upper arms of three volunteers were photographed by the semi-axial-view method in the most suitable position, and humeral retroversion angles were compared with those obtained by the CT method. As a result of examination with a humerus model, the best positioning angles were 20 degrees of shoulder abduction angle, 90 degrees of shoulder flexion angle, and 90 degrees of elbow flexion angle. There were no significant differences between the humeral retroversion angles of the CT and semi-axial-view methods with the best position determined in volunteers. In conclusion, the semi-axial-view method is a useful, easy method for measuring the humeral retroversion angle. This method will be a great asset in researching large numbers of samples.

A Predicted Optimal Performance of the Yurchenko Layout Vault in Women’s Artistic Gymnastics

The Yurchenko layout vault is the base vault from which more advanced forms of the Yurchenko family of vaults have evolved. The purpose of the study was to predict an individual’s optimal Yurchenko layout vault by modifying selected critical mechanical variables. The gymnast’s current performance characteristics were determined using the Peak-Motus video analysis system. Body segment parameters were determined using the elliptical zone mathematical modeling technique of Jensen (1978). A 5-segment computer simulation model was personalized for the gymnast comprising the hands, upper limbs, upper trunk, lower trunk, and lower limbs. Symmetry was assumed, as the motion was planar in nature. An objective function was identified which translated the subjective points-evaluation scheme of the Federation of International Gymnastics (FIG) Code of Points to an analytic expression that was mathematically tractable. The objective function was composed of performance variables that, if maximized, would result in minimal points being deducted and bonus points being allocated. A combined optimal control and optimal parameter selection approach was applied to the model to determine an optimum technique. The predicted optimal vault displayed greater postflight amplitude and angular momentum when compared with the gymnast’s best trial performance. Increased angular velocity, and consequently greater angular momentum at impact and greater shoulder flexion angle at impact with the horse, were related with this optimum technique. The impact phase therefore serves to increase the angular momentum during horse contact. Since the optimized parameters at impact with the horse were within the accepted physical capacity limits observed for the individual, the predicted vault is viable.

The effect of maximum voluntary contraction on endurance times for the shoulder girdle

This study investigated endurance times as percentages of maximum voluntary contractions (MVCs) in 12 healthy females (mean 25.8±4.3 years) in 5 postures and at 7% MVCs. The shoulder postures utilized were 30/90 (shoulder forward flexion=30° and included elbow angle=90°), 60/90, 90/120, 120/150, and 150/180. The %MVCs were 5%, 15%, 30%, 45%, 60%, 75%, and 90% of MVC at each of these postures. Outcome measures included: endurance times, ratings of perceived exertion, fatigue ratings, pain ratings, and surface electromyography (trapezius and mid-deltoid). As expected the endurance time decreased non-linearly with an increase in %MVC. However, the relationship between endurance time and %MVC differed significantly from Rohmert's curve and suggests it considerably overestimates endurance times for %MVCs <45% and it underestimates endurance times for %MVCs >45%. This study's curve did not become asymptotic even at 5% MVC. Shoulder posture (shoulder forward flexion angle) had a significant effect on endurance time. In general, endurance time decreased with an increase in shoulder flexion angle up to 120° and then it increased. Thus, overhead work (hands above the head) would appear to be better designed with a greater degree of forward flexion, rather than in front of the worker at lower degrees of forward flexion. Relevance to industry Published data on endurance times for the shoulder girdle are scant, yet shoulder injuries are neither rare nor inexpensive. This study completely redefines the relationship between endurance time and %MVC for the shoulder girdle. It will allow for a better definition of worker capabilities for the design of overhead work.

Effects of gender and reach distance on risks of musculoskeletal injuries in an assembly task

This study investigated differences in elbow and shoulder flexion angles in an assembly task. The experiment involved ten subjects on a simulated assembly task that consisted of seventeen task elements. The locations of the components were at three distances from the subjects. Confidence intervals (90%) were estimated and large differences in risk levels were found when data were pooled from both males and females. Between gender comparisons of joint angles revealed that the male elbow angles were smaller than the females, but the male shoulder angles were greater than the females on average. A within-gender analysis found greater change in angles for the female group with an increase in task distance from the body. This was not the case for the males. This was explained with reference to a previous study that related anthropometrics to differences in joint angles. The shoulder was identified as a joint sensitive to small physical changes in the workplace layout that may make a task more awkward to reach. This was not the case for the elbow. Finally, it was noted that both directions of movement and initial hand location affected final elbow and shoulder joint angles for task elements. Relevance to industry Data is available in the form of anthropometric tables, reach range distances and proposed workstation heights for industry so that differences both between and within genders can be best accommodated for good ergonomic design of workstations. There is a need to supplement this data with information on the variability of induced upper limb joint angles for repetitive assembly tasks within normal reach so as to assist the optimum design of workstations and reduce the likelihood of injuries.

Technical Note: The use of mirrors during an assembly task: a study of ergonomics and productivity

Industrial assembly tasks often require awkward, sustained neck and/or shoulder postures that can lead to increased musculoskeletal discomfort and reduced productivity. The aim of this study was to investigate the effects of mirror and periscope visual aids as ergonomic interventions designed to eliminate awkward postures of the cervicobrachial region during assembly tasks. Participants simulated a simple assembly task by using a cordless screwdriver to drive screws into a pre-tapped aluminium block. Trials of 15 min were run for each of four distinct assembly workstation configurations: industry standard (in-line screwdriver, work at elbow height, no visual aid); pistol grip (pistol grip screwdriver, work at shoulder height, no visual aid); mirror (in-line screwdriver, work at elbow height, single mirror visual aid); and periscope (in-line screwdriver, work at elbow height, two-mirror visual aid system). Muscular activity, discomfort, body posture, productivity and operator subjective assessment were recorded to determine the effects of the visual aid interventions. The results show that when comparing the interventions to the industry standard condition, there was a 45% reduction in average cervical erector spinae activity, a 90% reduction in average neck flexion angle and a 72% reduction in neck discomfort with the interventions. When comparing these interventions to the pistol grip condition there was an 80% reduction in activity of the dominant side deltoid, a 92% reduction in shoulder flexion angle and an 81% decrease in shoulder discomfort with the interventions. Productivity was greatest in the industry standard configuration followed by the pistol grip (9% lower), the periscope (13% lower) and the mirror (23% lower) configurations. A follow-up study that compared the productivity of the periscope configuration with that of the industry standard configuration showed that within a 4-h work period this productivity differential decreased by over 33%.

Endurance time as a Function of the Percentage of Maximum Voluntary Contraction and Upper Extremity Posture in Healthy Females

Endurance times were measured on 12 female subjects for seven different %MVCs and five different shoulder postures. Endurance time decreased nonlinearly with an increase in %MVC. It decresed with an increase in shoulder flexion angle up to 120° and then it increased. This study does not support the recommendation that weights corresponding to 5% of MVC, much less 20% of MVC as suggested by Rohmert, can be held indefinitely.

Maximum Voluntary Contraction as a Function of Upper Extremity Posture in Females

Background: Little has been published on the maximum strength of the shoulder girdle as a function of varying postures. Such information may help aid in job design by better assessing the capabilities of workers. Methods: Female subjects (n=12) were required to hold a weight in place for 4 seconds in one of 6 different functional postures: 0/90 (shoulder flexion angle = 0° and included elbow angle = 90°), 30/90, 60/90, 90/120, 120/150 and 150/180. Postures and weights were chosen at random and measurements were repeated. Final weight adjustments were in increments of 0.1 lb. At least 3 minutes of rest between lifts was provided. Maximum voluntary contractions (MVCs) were required to be reproduced within 2 lbs. Dynamic lifting of the weight and holding in place MVCs (“lift and hold”), as well as only holding the weight in place MVCs (“hold only”) were assessed separately. Maximum voluntary torques were also computed and adjusted for arm weight. Ratings of perceived exertion (Borg CR-10) and pain ratings (0–10 scale) were obtained. Results: Maximum voluntary contractions varied greatly, with the strongest to weakest measurements approximating 2:1 ratios. For the 90/120 posture, lift and hold MVC was 14.6±3.0 lb. (range 9.5–19.0). The hold only MVC for 90/120 was 18.1±2.9 lb. (range 13.5–25.3). Similar variability occurred in all postures. Adjusted for arm weight, MVC for lift and hold at 90/120 was 18.1±3.3 lb, while hold only was 21.7±3.2 lb. Across all postures, hold only MVCs were greater than lift and hold MVCs by 4.3 lb. (26%). Lift and hold MVC torque at 120/150 was 217.2±39.3 in-lb. (range 157.5–266.8), while hold only was 280.6±38.0 in-lb. Across all postures, hold only maximum voluntary torques were greater than lift and hold torques by 25%. Except for the 0/90 and 150/180 postures, posture had little effect on hold only maximum voluntary torques. Borg CR-10 ratings averaged 6.6 for lift and hold and 6.9 for hold only (p&gt;0.05). Posture had no effect on pain ratings, which averaged 0.8. Conclusions: Variability in female shoulder girdle lifting strength is substantial. Removing the lifting portion of job tasks likely substantially increases the percentage of workers capable of performing the tasks. Weakest postures are 90/120 and 120/150 and the female lift and hold MVC for those postures is 15 lb. (hold only 19 lb.). 30/90, 60/90 and 150/180 postures are equally strong; MVCs for lift and hold in those postures are 18 lb. (hold only 23 lb.). When overhead work is required, 150/180 posture appears preferable to 90/120 or 120/150. Some screening or selection process may be necessary on some jobs due to the degree of variability in strength.

The coordination of reaching and grasping in spastic hemiparesis

The kinematics and intrinsic dynamics of reaching and grasping movements in six subjects with spastic hemiparesis were studied. Movements were performed with both hands together as well as with each hand in isolation, and two target widths were used. As expected, large manual asymmetries existed in unimanual task performance. These asymmetries were more pronounced for grasping than for reaching, which was a consequence of the fact that in grasping the impaired hand stayed longer at zero velocity before lifting the object. This implies difficulties coordinating the more distal movement component (hand and fingers). In addition, the impaired hand attained the movement goal with a (1) decreased angular range of motion of the joints and (2) an increased trunk involvement. With respect to the intrinsic dynamics, intra- and inter-limb coupling was studied by evaluating cross-correlations of position–time functions joint pairs. In effector space the displacement of the wrist and ipsilateral shoulder were correlated, and in joint-angle space the elbow extension–shoulder flexion angle pair and the elbow extension–shoulder elevation angle pair were analysed. It was found that despite tight couplings between the different pairs, the couping strengths for the impaired limb were consistently lower. This was caused by fragmentation of the movement, mainly due to a large shoulder involvement at the start of the movement concurrent with little or no elbow extension. It is concluded that fragmentation of the movement, operationalised by the value of the cross-correlation between joint pairs is an essential variable by which the level of recovery of function can be captured. It was further shown that the unimpaired hand `mimicked' the impaired hand on a number of movement characteristics under bimanual responding. The impaired hand did not change its style of movement organisation among uni- and bimanual movement responding. It is argued that the apparent stability of the impaired hand can only be sustained at the cost of a decreased flexibility. Finally, inter-limb coupling, assessed by means of cross-correlations of the tangential velocity profiles of both hands, was shown to be high during the course of movement.

EFFECT OF RESISTANCE TRAINING ON POSTURAL STABILITY AND A SIT-TO-STAND TASK642

This study was performed to assess changes in postural stability and performance of a sit-to-stand task in elderly (68±5 yrs; mean±S.D.) men and women after 12 weeks of resistance training. Leg strength was assessed with a 1-RM using a Cybex II™ isokinetic dynamometer and a Keiser™ isotonic machine. Postural stability was assessed during quiet standing and one-legged standing on a force platform. Three trials of a sit-to-stand task from a 38 cm high bench were filmed from a sagittal view. Subjects were randomly divided into training (TR; n=10) or control (CON; n=9) groups. The CON group was asked not to change their lifestyle over the 12 weeks of the study. Training consisted of 10 exercises. designed to isolate the major muscle groups of upper and lower body. Subjects performed 3 sets of 8-12 repetitions at 80-85% of 1-RM, 3x/wk. The increase in isotonic leg strength during the 12 wks was greater (p<0.05) in TR(380±41 N vs. 534±55 N) than CON (443±47 N vs. 471±47 N). Both TR (81±9 vs. 105±11 Nm) and CON(86±8 vs. 102±9 NM) groups significantly (p<0.05) increased isokinetic leg strength at 180°/s. No significant differences were found in stability measures. Total migration of the center of pressure (CP) during quiet standing decreased for both TR (35±13 vs. 28±17 cm. effect size (ES)=0.4) and CON (44±25 vs. 34±16, ES=0.4) after training. Kinematic analysis of the sit-to-stand task for 6 subjects indicated that TR achieved a greater peak hip flexion angle after training (116±8 vs. 126±3°, ES=3.0) than CON (115±11 vs. 119±11°, ES= 0.7). This adjustment was accompanied by a greater increase in peak shoulder flexion angle for TR (25±19 vs. 41±31°, ES=1.3) than CON(29±23 vs. 39±28°, ES=0.8). It was concluded that this program of resistance training had a limited effect on improving postural stability. However, greater contributions from the upper body to the sit-to-stand task were observed after training.

Predicting peak pinch strength: Artificial neural networks vs. regression

Cumulative trauma disorders (CTDs) of the upper extremities are one of the major ergonomics areas of research. Pinching is a common risk factor associated with the development of hand/wrist CTDs. The capacity standards of peak pinch strength for various postures are needed to design the tasks in harmony with the workers. This paper describes the formulation, building and comparison of pinch strength prediction models that were obtained using two approaches: Statistical and artificial neural networks (ANN). Statistical and ANN models were developed to predict the peak chuck pinch strength as a function of different combinations of five elbow and seven shoulder flexion angles, and several anthropometric and physiological variables. The two modeling approaches were compared. The results indicate ANN models to provide more accurate predictions over the standard statistical models.

Can visual perception be used to estimate body part angles?

For two decades, observational methods have been used to measure postural loads in industry. These methods depend on the analyst's judgment to characterize correctly the working postures. The errors associated with estimating the body part's angular deviation from the neutral position are not known. Recently, videotaping of a work situation, followed by examination of the tape to assess postural position has become more commonly practised. This study was conducted to determine the error which accompanies the judging (without tools) of postural angles. In the present study, a method was used whereby 20 subjects were asked to report the perceived angles which closely matched the actual shoulder flexion angle displayed on the TV screen. The results showed no significant differences among the three angular ranges ('low', 1-60 degrees; 'medium', 61-120 degrees; and 'high', 121-180 degrees) of shoulder flexion, in terms of algebraic and absolute errors. Subjects tended to slightly overestimate the true angle in the low range and to slightly underestimate the true angle in the medium and high angle ranges. This study suggests that visual perception can be used to estimate shoulder flexion angles displayed on a VDT.