Electromyographic patterns of individuals suffering from lateral tennis elbow

Comparison of two different orthoses used in the treatment of lateral epicondylitis

BACKGROUND

Lateral epicondylitis (LE) is one of the most common causes of lateral elbow pain, and orthoses are often used in its conservative treatment. However, the number of studies examining the effectiveness of the orthoses used and their differences compared to each other is limited.

OBJECTIVE

To determine the effects of 2 orthoses types, wrist orthosis (WO) and counterforce brace (CB), commonly used in the treatment of lateral epicondylitis on muscle activation, grip strength, hand function, and satisfaction level in healthy individuals.

STUDY DESIGN

Quasi-experimental study.

METHODS

Thirty-seven healthy individuals were recruited. Muscle activation was evaluated by surface electromyography, and maximum grip strength was evaluated with a handheld dynamometer. Hand function and satisfaction level were evaluated with Jebsen Taylor Hand Function Test and Orthotics and Prosthetics Users Survey, respectively. All evaluations were conducted for each individual in 3 conditions (no orthosis, WO, and CB).

RESULTS

Extensor carpi radialis (ECR) muscle activation was greater when using CB compared with that when without orthosis (p = 0.036). There was no significant difference in extensor digitorum communis (EDC) muscle activation and maximum grip strength among the conditions (p > 0.05). Hand function was worse when using the WO compared with the other conditions (p < 0.001). In terms of orthosis satisfaction, individuals were found to be significantly more satisfied with the use of CB (p = 0.001).

CONCLUSIONS

In our study highlight the use of CB in terms of EDC muscle activity, grip strength, hand function, and satisfaction, while the use of WO is prominent in terms of keeping extensor carpi radialis muscle activity low, which has been shown to be important for LE. To obtain definitive results on the effectiveness of different orthoses in the treatment of LE, more research is needed to compare the muscular activation in the extensor carpi radialis brevis (ECRB) and EDC muscles with objective methods.

Surveillance of Injury Types, Locations, and Intensities in Male and Female Tennis Players: A Content Analysis of Online Newspaper Reports

The popularity of modern tennis has contributed to the increasing number of participants at both recreational and competitive levels. The influx of numerous tennis participants has resulted in a wave of injury occurrences of different types and magnitudes across both male and female players. Since tennis injury harms both players' economic and career development, a better understanding of its epidemiology could potentially curtail its prevalence and occurrences. We used online-based tennis-related injury reports to study the prevalence, location types, and injury intensities in both male and female tennis players for the past five years. It is demonstrated from the chi-square analysis that injury occurrences are significantly associated with a specific gender (χ2(18) = 50.773; p = 0.001), with male players having a higher risk of injury manifestation (68.10%) as compared with female players (31.90%). Nonetheless, knee, hip, ankle, and shoulder injuries are highly prevalent in both male and female players. Moreover, the injury intensities are distributed across gender (χ2(2) = 0.398; p = 0.820), with major injuries being dominant, followed by minor injuries, whilst a few cases of career-threatening injuries were also reported. It was similarly observed that male players recorded a higher degree of both major, minor, and career-threatening injuries than female players. In addition, male players sustained more elbow, hip, knee, shoulder, and thigh injuries than female players. Whereas, female players mostly suffered from Achilles and back injuries, ankle and hamstring injuries affected both genders. The usage of online newspaper reports is pivotal in characterizing the epidemiology of tennis-related injuries based on locations and gender to better understand the pattern and localization of injuries, which could be used to address the problem of modern tennis-related injuries.

Scapholunate interosseous ligament dysfunction as a source of elbow pain syndromes: Possible mechanisms and implications for hand surgeons and therapists

Elbow pain syndromes are common upper extremity musculoskeletal disorders, and they are usually associated with repetitive occupational exposure. Ligaments are often one of the sources of musculoskeletal disorders because of their mechanical and neurological properties. The wrist ligaments are some of the ligaments most vulnerable to occupational exposure. Since most occupational tasks require wrist extension for handling tools and loading, the scapholunate interosseous ligament (SLIL) bears greater strain during loading, which results in creep deformation and hysteresis. Ligamentous creep may result in diminished ability to detect signal changes during joint movements, which impairs neuromuscular control established by ligamentomuscular reflex arcs elicited from mechanoreceptors in the ligaments. Changes in muscle activation patterns of forearm muscles due to diminished ligamentomuscular reflexes may initiate a positive feedback loop, leading to musculoskeletal pain syndromes. The relationship between elbow pain syndromes and SLIL injury will be presented through two hypotheses and relevant pain mechanisms: 1. Repetitive tasks may cause creep deformation of the SLIL, which then impairs ligamentomuscular reflexes, leading to elbow pain disorders. 2. Lateral epicondylalgia may increase the risk of SLIL injury through the compensation of the lower extensor carpi radialis muscle activity by higher extensor carpi ulnaris muscle activity, which may alter carpal kinematics, leading to SLIL degeneration over time. The differential diagnosis is usually complicated in musculoskeletal pain disorders. The failure of treatment methods is one of the issues of concern for many researchers. A key element in developing treatment strategies is to understand the source of the disorder and the nature of the injury. We proposed that the differential diagnosis include SLIL injuries when describing elbow pain syndromes, particularly, lateral epicondylalgia.

Multimodal approach to rehabilitation of the patients with lateral epicondylosis: a case series

Purpose

The objective of this study was to evaluate the effectiveness of mobilization with movement and kinesiotherapy in the treatment of patients with lateral epicondylosis.

Methods

This cases series included eight volunteers who had chronic lateral epicondylosis. The patients were treated with stretching, massage deep transverse at the lateral epicondyle and mobilization with movement associated with eccentric exercise. The mobilization with movement that consisted of a force of lateral glide of the proximal forearm. We performed twelve sessions, twice a week for 45 min/session. All patients underwent an evaluation with a visual analog scale and functional assessment through questionnaires patient-rated tennis elbow evaluation (PRTEE) and disabilities of the arm, shoulder and hand (DASH), before and after the treatment. Data were analyzed by student’s t test (p < 0.05).

Results

The results showed statistical differences in pain symptoms before and after treatment, in the analysis and functional assessment through both questionnaires comparing the pre and post treatment.

Conclusion

The data obtained in this study demonstrates improvement of the function and pain status of the sample investigated.

Comparison of muscle activity of wrist extensors and kinematics of wrist joint during wrist extension in automobile assembly line workers with and without lateral epicondylitis

BACKGROUND

Overuse of the extensor carpi radialis (ECR) may play a role in the development of lateral epicondylitis (LE). However, no studies have investigated the muscle activity ratio between the ECR and extensor carpi ulnaris (ECU) associated with the kinematics during wrist extension in workers with LE.

OBJECTIVE

We compared the ratio (ECR/ECU) of muscle activity between the ECR and ECU and the kinematics of the wrist during wrist extension between workers with and without LE.

METHODS

Fifteen automobile assembly line workers with LE and 15 workers without LE participated in this study. The ratio of muscle activity was measured using surface electromyography, and wrist kinematics were measured by a three-dimensional motion analysis system while the workers extended their wrists actively to the maximum range to which they did not feel uncomfortable.

RESULTS

Significantly greater ratios of muscle activity, ranges of radial deviation, and combined motion of radial deviation and extension (CMDE) were shown in workers with LE compared to those without LE. Also, the range of wrist extension was significantly lower in workers with LE than in those without LE.

CONCLUSIONS

Quantifying the ratio of muscle activity with altered kinematics of wrist extension may help researchers to understand why overuse of ECR is occurring and explain LE development in automobile assembly line workers.

Racquet string tension directly affects force experienced at the elbow: implications for the development of lateral epicondylitis in tennis players

BACKGROUND

Lateral epicondylitis (LE) occurs in almost half of all tennis players. Racket-string tension is considered to be an important factor influencing the development of LE. No literature yet exists that substantiates how string-tension affects force transmission to the elbow, as implicated in LE development. We establish a quantitative relationship between string-tension and elbow loading, analyzing tennis strokes using rackets with varying string-tensions.

METHODS

Twenty recreational tennis players simulated backhand tennis strokes using three rackets strung at tensions of 200 N, 222 N and 245 N. Accelerometers recorded accelerations at the elbow, wrist and racket handle. Average peak acceleration was determined to correlate string-tension with elbow loading.

RESULTS

Statistically significant differences (p < 0.05) were observed when average peak acceleration at the elbow at 200 N string-tension (acceleration of 5.58 m/s(2)) was compared with that at 222 N tension (acceleration of 6.83 m/s(2)) and 245 N tension (acceleration of 7.45 m/s(2)). The 200 N racket induced the least acceleration at the elbow.

CONCLUSIONS

Although parameters determining force transmission to the elbow during a tennis stroke are complex, the present study was able to control these parameters, isolating the effect of string-tension. Lower string-tensions transmit less force to the elbow in backhand strokes. Reducing string-tension should be considered favourably with respect to reducing the risk of developing LE.

Evaluation of a Subject-Specific, Torque-Driven Computer Simulation Model of One-Handed Tennis Backhand Ground Strokes

A torque-driven, subject-specific 3-D computer simulation model of the impact phase of one-handed tennis backhand strokes was evaluated by comparing performance and simulation results. Backhand strokes of an elite subject were recorded on an artificial tennis court. Over the 50-ms period after impact, good agreement was found with an overall RMS difference of 3.3° between matching simulation and performance in terms of joint and racket angles. Consistent with previous experimental research, the evaluation process showed that grip tightness and ball impact location are important factors that affect postimpact racket and arm kinematics. Associated with these factors, the model can be used for a better understanding of the eccentric contraction of the wrist extensors during one-handed backhand ground strokes, a hypothesized mechanism of tennis elbow.

Forearm posture and grip effects during push and pull tasks

Direction of loading and performance of multiple tasks have been shown to elevate muscle activity in the upper extremity. The purpose of this study was to evaluate the effects of gripping on muscle activity and applied force during pushing and pulling tasks with three forearm postures. Twelve volunteers performed five hand-based tasks in supinated, neutral and pronated forearm postures with the elbow at 90 degrees and upper arm vertical. All tasks were performed with the right (dominant) hand and included hand grip alone, push and pull with and without hand grip. Surface EMG from eight upper extremity muscles, hand grip force, tri-axial push and pull forces and wrist angles were recorded during the 10 s trials. The addition of a pull force to hand grip elevated activity in all forearm muscles (all p < 0.017). During all push with grip tasks, forearm extensor muscle activity tended to increase when compared with grip only while flexor activity tended to decrease. Forearm extensor muscle activity was higher with the forearm pronated compared with neutral and supinated postures during most isolated grip tasks and push or pull with grip tasks (all p < 0.017). When the grip dynamometer was rotated so that the push and pull forces could act to assist in creating grip force, forearm muscle activity generally decreased. These results provide strategies for reducing forearm muscle loading in the workplace. STATEMENT OF RELEVANCE: Tools and tasks designed to take advantage of coupling grip with push or pull actions may be beneficial in reducing stress and injury in the muscles of the forearm. These factors should be considered in assessing the workplace in terms of acute and cumulative loading.

The effect of forearm posture on wrist flexion in computer workers with chronic upper extremity musculoskeletal disorders

Background

Occupational computer use has been associated with upper extremity musculoskeletal disorders (UEMSDs), but the etiology and pathophysiology of some of these disorders are poorly understood. Various theories attribute the symptoms to biomechanical and/or psychosocial stressors. The results of several clinical studies suggest that elevated antagonist muscle tension may be a biomechanical stress factor. Affected computer users often exhibit limited wrist range of motion, particularly wrist flexion, which has been attributed to increased extensor muscle tension, rather than to pain symptoms. Recreational or domestic activities requiring extremes of wrist flexion may produce injurious stress on the wrist joint and muscles, the symptoms of which are then exacerbated by computer use. As these activities may involve a variety of forearm postures, we examined whether changes in forearm posture have an effect on pain reports during wrist flexion, or whether pain would have a limiting effect on flexion angle.

Methods

We measured maximum active wrist flexion using a goniometer with the forearm supported in the prone, neutral, and supine postures. Data was obtained from 5 subjects with UEMSDs attributed to computer use and from 13 control subjects.

Results

The UEMSD group exhibited significantly restricted wrist flexion compared to the control group in both wrists at all forearm postures with the exception of the non-dominant wrist with the forearm prone. In both groups, maximum active wrist flexion decreased at the supine forearm posture compared to the prone posture. No UEMSD subjects reported an increase in pain symptoms during testing.

Conclusion

The UEMSD group exhibited reduced wrist flexion compared to controls that did not appear to be pain related. A supine forearm posture reduced wrist flexion in both groups, but the reduction was approximately 100% greater in the UEMSD group. The effect of a supine forearm posture on wrist flexion is consistent with known biomechanical changes in the distal extensor carpi ulnaris tendon that occur with forearm supination. We infer from these results that wrist extensor muscle passive tension may be elevated in UEMSD subjects compared to controls, particularly in the extensor carpi ulnaris muscle. Measuring wrist flexion at the supine forearm posture may highlight flexion restrictions that are not otherwise apparent.

Pre- and post-impact muscle activation in the tennis volley: effects of ball speed, ball size and side of the body

AIM

To examine the pre- and post-impact activation of five upper extremity muscles in the tennis volley across conditions of ball speed, ball type and side of the body.

METHODS

A repeated measures design in a biomechanics laboratory setting was used. A total of 24 recreational tennis players (mean (SD) age 24 (5) years, height 176 (10) cm, mass 76 (13) kg) were recruited from a university. Participants performed tennis volleys under 18 ball conditions: three ball speeds (slow, medium and fast), with three ball types (two oversize and one regular size) each from two sides (forehand and backhand). Average normalised electromyographic levels of the flexor carpi radialis, extensor carpi radialis, triceps brachii, anterior/middle deltoid and posterior/middle deltoid of the hitting arm during pre- and post-impact phases (200 ms before and after ball-racquet impact, respectively) were assessed.

RESULTS

For the pre-impact phase, a significant muscle and side interaction (p<0.001) and significant main effects for speed (p = 0.002) and muscle (p<0.001) were observed. For the post-impact phase, significant interactions were observed for ball type and side (p = 0.002), ball speed and side (p = 0.011) and muscle and side (p = 0.001), as well significant main effects for muscle (p<0.001), speed (p = 0.035) and side (p<0.001).

CONCLUSION

Oversize tennis balls do not significantly increase upper extremity muscle activation compared to regular size balls during a tennis volley. The highest post-impact activation was observed in the ECR indicating a vigorous wrist stabilisation role that could irritate players with lateral epicondylalgia.

Upper limb muscle imbalance in tennis elbow: a functional and electromyographic assessment

The purpose of this study was to investigate strength, fatigability, and activity of upper limb musculature to elucidate the role of muscular imbalance in the pathophysiology of tennis elbow. Sixteen patients clinically diagnosed with tennis elbow, recruited from a university hospital upper limb orthopedic clinic, were compared with 16 control subjects with no history of upper limb musculoskeletal problem, recruited from university students and staff. Muscle strength was measured for grip, metacarpophalangeal, wrist, and shoulder on both sides. Electromyographic activity (RMS amplitude) and fatigue characteristics (median frequency slope) of five forearm and two shoulder muscles were measured during isometric contraction at 50% maximum voluntary contraction. All strength measurements showed dominance difference in C, but none in TE. In tennis elbow compared to controls, hand/wrist and shoulder strength and extensor carpi radialis (ECR) activity were reduced (p < 0.05), while fatigue was normal. A global upper limb weakness exists in tennis elbow. This may be due to disuse and deconditioning syndrome caused by fear avoidance, and needs to be addressed in prevention and treatment. Activation imbalance among forearm muscles (reduced extensor carpi radialis activity) in tennis elbow, probably due to protective pain-related inhibition, could lead to a widespread upper limb muscle imbalance.

Poorer elbow proprioception in patients with lateral epicondylitis than in healthy controls: a cross-sectional study

Two groups of women, 15 patients with lateral epicondylitis and 21 healthy controls, were studied to compare proprioception in the elbows and knees between the groups. Outcome measures were absolute error and variable error for joint position sense and for threshold to detection of a passive movement. Both absolute error and variable error of threshold to detection of a passive movement were greater in the lateral epicondylitis-diagnosed elbows than in the controls' elbows (lateral epicondylitis, 1.8 degrees vs controls 1.1 degrees, P = .026; lateral epicondylitis, 0.8 degrees vs controls 0.3 degrees, P = .015), and there was a tendency toward a greater absolute error of joint position sense compared with the control elbows (lateral epicondylitis, 8.2 degrees vs controls, 5.6 degrees; P = .078). Absolute error of joint position sense was greater in the elbows than in the knees of the lateral epicondylitis patients, but no group differences were found for knees. Proprioception seems, therefore, to be poorer in elbows with lateral epicondylitis elbows than in the controls' elbows. This needs to be taken into consideration in the management of lateral epicondylitis.

Tennis injuries: occurrence, aetiology, and prevention

A systematic search of published reports was carried out in three electronic databases from 1966 on to identify relevant articles relating to tennis injuries. There were 39 case reports, 49 laboratory studies, 28 descriptive epidemiological studies, and three analytical epidemiological studies. The principal findings of the review were: first, there is a great variation in the reported incidence of tennis injuries; second, most injuries occur in the lower extremities, followed by the upper extremities and then the trunk; third, there have been very few longitudinal cohort studies that investigated the association between risk factors and the occurrence of tennis injuries (odds ratios, risk ratios, hazard ratios); and fourth, there were no randomised controlled trials investigating injury prevention measures in tennis. More methodologically sound studies are needed for a better understanding of risk factors, in order to design useful strategies to prevent tennis injuries.

Comparison of shock transmission and forearm electromyography between experienced and recreational tennis players during backhand strokes

OBJECTIVE

To test the hypothesis that recreational tennis players transmit more shock impact from the racket to the elbow joint than experienced tennis players during the backhand stroke. Also, to test whether recreational tennis players used higher electromyographic (EMG) activities in common wrist extensor and flexor around epicondylar region at follow-through phase.

DESIGN

A repeated-measure, cross-sectional study.

SETTING

National College of Physical Education and Sports at Taipei, Taiwan.

PARTICIPANTS

Twenty-four male tennis players with no abnormal forearm musculoskeletal injury participated in the study. According to performance level, subjects were categorized into 2 groups: experienced and recreational.

MAIN OUTCOME MEASUREMENTS

Impact transmission and wrist extensor-flexor EMG for backhand acceleration, impact, and follow-through phases were recorded for each player. An independent t test with a significance level of 0.05 was used to examine mean differences of shock impact and EMG between the 2 test groups. One-way ANOVA associated with Tukey multiple comparisons was used to identify differences among different impact locations and EMG phases.

RESULTS

Experienced athletes reduced the racket impact to the elbow joint by 89.2%, but recreational players reduced it by only 61.8%. The largest EMG differences were found in the follow-through phase (P<0.05). Experienced athletes showed that their extensor and flexor EMGs were at submaximal level for follow-through phase, whereas recreational players maintained their flexor and extensor EMGs at either supramaximal or maximal level.

CONCLUSIONS

Our results support the hypothesis that recreational players transmit more shock impact from the racket to the elbow joint and use larger wrist flexor and extensor EMG activities at follow-through phase of the backhand stroke. Follow-through control is proposed as a critical factor for reduction of shock transmission. Clinicians or trainers should instruct beginners to quickly release their grip tightness after ball-to-racket impact to reduce shock impact transmission to the wrist and elbow.

A dynamic extensor brace reduces electromyographic activity of wrist extensor muscles in patients with lateral epicondylalgia

STUDY DESIGN

Semiexperimental study.

OBJECTIVE

To investigate the effect of an external wrist extension force on extensor muscle activity during hand gripping in patients with lateral epicondylalgia.

BACKGROUND

Lateral epicondylalgia or "tennis elbow" is a common, often disabling ailment affecting millions of people. An optimal treatment strategy remains to be identified. The use of an external wrist extension force may reduce the extensor muscle activity during gripping in these patients.

METHODS

Muscle activity of the extensor carpi radialis brevis (ECRB), extensor digitorum communis (EDC), and extensor carpi radialis longus (ECRL) was measured using surface EMG. Subjects gripped at an intensity of 10%, 20%, and 30% of the maximum voluntary contraction (MVC) force with and without the dynamic extensor brace and with and without an applied external wrist extension force of 1%, 2%, and 3% of MVC.

RESULTS

At all levels of MVC gripping, the EMG signal of the ECRB and EDC were significantly lower for gripping with than without brace. An extension force of 3% of the MVC force significantly reduced the EMG signal of all muscles in almost all measurement conditions.

CONCLUSIONS

The results of this study indicate that the dynamic extensor brace as well as the external extension force significantly reduced the EMG signal of the wrist extensor muscles during gripping in patients with lateral epicondylalgia. Based on these results, the dynamic extensor brace could be a promising new intervention for lateral epicondylalgia.

Sensory and motor effects of experimental muscle pain in patients with lateral epicondylalgia and controls with delayed onset muscle soreness

This study compares the effect of experimental muscle pain on deep tissue sensitivity and force attenuation in the wrist extensors of patients with lateral epicondylalgia (n=20), and healthy controls (n=20) with experimentally induced sensori-motor characteristics simulating lateral epicondylalgia. Delayed onset muscle soreness (DOMS) in wrist extensors of healthy controls was induced by eccentric exercise in one arm 24h prior to injection (Day 0). Saline-induced pain intensity (visual analogue scale, VAS), distribution, and quality were assessed quantitatively in both arms for both groups. Pressure pain thresholds (PPT) were assessed at three different sites in the wrist extensors. Maximal grip force and wrist extension force were recorded. In response to saline-induced pain in the extensor carpi radialis brevis, regardless of arm, the patient group demonstrated a significantly quicker pain onset (P<0.01), mapped larger pain areas and more referred pain areas, compared to healthy controls (P<0.03). Pain persisted significantly longer in the sore arm of the patient group, compared with all other arms (P<0.02). Patients demonstrated significant bilateral hyperalgesia at extensor carpi radialis brevis during and post saline-induced pain compared to pre-injection and healthy controls (P<0.04). The sore arm in patients and the DOMS arms in healthy subjects showed significantly reduced maximal force (P<0.0001), at all Day 1 times compared with the control arms. In patients, the bilateral increase in deep tissue sensitivity and enlarged referred pain areas during saline-induced pain might suggest involvement of central sensitisation.

Experimental deep tissue pain in wrist extensors--a model of lateral epicondylalgia

The aim of this experimental study was to develop an in vivo model demonstrating sensory and motor interactions comparable to those seen in patients presenting with lateral epicondylalgia (i.e., deep tissue pain and hyperalgesia localised to specific sites in the wrist extensors, attenuation of wrist extension force). The effect of saline-induced deep pain combined with delayed onset muscle soreness (DOMS) on deep tissue sensitivity and motor function in wrist extensors was examined. Muscle pain intensity (visual analogue scale: VAS), distribution, and quality were assessed in 12 subjects. Pressure pain thresholds (PPTs) were recorded at five different sites around the elbow. Maximal wrist extension force was recorded. In the absence of DOMS, hypertonic saline administrated into different parts of the wrist extensors (extensor carpi radialis brevis, supinator, common extensor origin) induced significantly (P<0.05) higher VAS scores and larger pain areas compared with a control injection of isotonic saline. The typical quality of saline-induced pain was described as "drilling", "taut", "nagging" and "intense". In non-exercised wrist extensors, hyperalgesia to pressure was not detected during saline-induced pain but maximal wrist extensor force decreased significantly (P<0.05) compared with pre-pain recordings and recordings post isotonic saline. DOMS induced by eccentric wrist extension contractions generated moderate levels of soreness but no resting pain up to 24h post exercise. PPTs and maximal wrist extension force were significantly decreased (P<0.05) during DOMS compared with baseline and 7 days post exercise (P<0.05). VAS scores to injection of hypertonic saline into the DOMS arm were significantly increased (P<0.05) compared with injections into the unexercised arm. This is another manifestation of muscle hyperalgesia. Saline-induced pain combined with DOMS further decreased maximal wrist extension force (P<0.05). The simultaneous deep tissue pain and hyperalgesia linked with force attenuation support the use of the saline-induced deep tissue pain combined with DOMS as an experimental model simulating the clinical sensorimotor correlates of lateral epicondylalgia.

Aspects of shoulder function in relation to exposure demands and fatigue - a mini review

OBJECTIVE

To discuss the literature on control and function of multiple muscle systems with special focus on shoulder and upper extremity under submaximal conditions.

DESIGN

The paper is a mini review based on 31 studies.

BACKGROUND

Control mechanisms underlying the recruitment and gradation of muscle activity in complex multiple muscle systems during various voluntary exertions is still not fully understood. Load sharing principles may be influenced by several factors like work demand, fatigability, metabolic factors etc.

METHODS

Several methods, e.g., electromyography, intramuscular pressure, and tissue oxygenation are used. The definition of fatigue is discussed.

RESULTS AND CONCLUSION

A relatively fixed load sharing of the shoulder muscles has been found at low load levels, submaximal speeds and with a limited range of movements of the arms for healthy subjects. However, the load sharing of the shoulder muscles can be changed to some extent in response to fatigue, mental demands, visual feedback and in patients suffering from disorders in the upper extremities. It is hypothesized that lack of ability to redistribute muscle activation pattern in the shoulder region or the upper extremity increases the risk of development of work related symptoms.