Electromyographic patterns of individuals suffering from lateral tennis elbow

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Abstract

This study investigated the applicability of using surface electromyography (EMG) as a tool for differentiating between persons suffering from lateral tennis elbow and the healthy age-matched adults. Temporal muscle activation patterns of the tennis elbow group were evaluated to determine if they varied between subject groups and if noted variations might be interpreted as arresting or exacerbating the injury. Sixteen subjects (Healthy Controls, n=6; Tennis Elbow, n=10) were tested under simulated tennis playing conditions. All subjects were males (Healthy group (CON) 38.8±13.1, Injured group (INJ) 40.8±10.8 yrs). EMG response data, temporal and spatial muscle activities, of the forearm extensors (Ext), the forearm flexors (Flex) and the triceps (Tri) were recorded for each subject during a single test session using all combinations of three different velocities on three different racket head impact locations. Data were collected at a frequency of 1000 Hz. Statistical analysis was performed using a 2×3×3 (Health status×Impact velocity×Impact location) ANOVA with repeated measures. Results indicated statistically significant differences (p<0.05) between the CON and INJ subject groups for the response variables associated with forearm extensor muscle activation. During simulated play, the INJ group employed an earlier, longer, and greater activation of Ext than the CON group, such changes may be considered detrimental to the healing process. These results support the use of surface EMG to quantify differences in muscle activation strategies employed by individuals suffering from soft tissue muscle microtrauma injuries and healthy controls.

Introduction

Diagnosis and treatment of soft tissue damage are often complicated by reliance on self-report to determine the extent of injury. Recently, the clinician's ability to more accurately view the damage through quantitative means, such as MRIs, has been greatly improved. However, such evaluation is expensive and may be deemed unnecessary for many non-life-threatening conditions. This study was conducted to determine if simple surface electromyographic analysis of muscles associated with lateral tennis elbow could be used to assist in accurately diagnosing this common soft tissue injury.

While rest is the most frequently prescribed treatment for most soft tissue injury, for athletes and industrial workers, it may not be possible to cease or severely limit the activity that caused the condition to appear. Nirschl [12]has proposed using the term tendonosis due to the lack of inflammation accompanying common lateral tennis elbow. With an injury such as lateral epicondyle tendonosis, because the absence of tissue inflammation accompanying common tennis elbow (TE), the pain associated with the injury may actually encourage counter productive muscle activation [12]. It is therefore important to determine if: (1) the patient is, in fact, suffering from the condition, i.e. exhibiting signs different from the normal population, and (2) possible detrimental muscle activation patterns have been developed in response to the injury. Electromyographic (EMG) responses showing increases in muscular activation, frequency, duration, and earlier onset related to specific events would contradict Nirschl's [12]assertion that rest is the most effective initial form of treatment.

Assuming that a person sought medical attention for his/her tennis elbow, a simple elbow exam may be performed to determine the location of the pain. Self-report of pain is usually sufficient for a clinician to make a recommendation of treatment, which typically includes rest, strengthening and re-evaluation of activities believed to have caused the injury 11, 21, 14, 12. However, absent from these evaluations is the ability to determine if muscle usage patterns have been altered by the injury. EMG data from the extensor carpi radialis brevis (ECRB) showing increased activity during tennis specific movements would indicate that muscular strain on the damaged soft tissue, usually the hyaline region of the ECRB [20], would also increase. Such data, as seen in Fig. 3, would support our theory that individuals suffering from TE employ muscle activation strategies that actually increase their likelihood of further soft tissue damage or at the very least slow the healing process.

Although much has changed since the early days of tennis, unfortunately, one aspect has remained remarkably consistent: those individuals who play the game often will be physically injured by their participation in the sport [16]. The most common of the upper body injuries is “tennis elbow” or more correctly, lateral epicondyle tendonosis [16]. Morris [10]documented this injury just 10 years after Major Wingfield first introduced the game. Morris [10]named the condition “lawn tennis arm”. While the medical community has known about the injury for over one hundred years, there still seems to be little that physicians or physical therapists can do to prevent the injury. Recent studies have also provided a more sobering evaluation; although pain temporarily disappears the tendon injury is irreversible [18].

When a player strikes a tennis ball with a racket, the whole arm complex i.e., hand, wrist, forearm, upper arm, and shoulder is mechanically stressed. Renstrom and Johnson [19]calculated the magnitude of force stressing the arm for a ball–racket impact at 13.9 m/s, the equivalent of jerking up a 25-kg weight with one arm. The lower arm, consisting of the hand, wrist and forearm, sustains most of the impact-generated stress. The soft tissue of the lower arm is poorly suited to withstand this repeated stress of hitting a ball.

While the exact frequency and level of mechanical stress necessary to cause tennis elbow have not been documented, it is now generally agreed that tennis elbow is an overuse injury 13, 6, 21whose origin can often be traced to the backhand stroke in tennis. Even Morris [10]stated as much over one hundred years ago in his first reference to the injury when he noted that during the execution of hitting a backhand the extensor muscles are most stressed and most likely to be injured. Since the back of the racket hand is parallel to the racket face and on the leading side of the racket grip during the impact portion of a typical one-handed backhand stroke, only the fingers can provide bony support for the racket. The extensor muscles are activated eccentrically to stabilize the hand–wrist during the momentum transfer that occurs during impact. It is well known that eccentric muscle actions generate forces much greater than those seen during normal concentric muscle actions [3]. These large, eccentric forces put a great deal of stress on the tendon ligament complex of the extensor muscles and usually can be safely absorbed if the tendon insertion is made over a relatively large area of bone. However, the tendon most often identified as the site of tennis elbow pain is the Extensor Carpi Radialis Brevis (ECRB), a small muscle of the forearm, which has a distinctly poor biomechanical design for withstanding high loads [20]. The ECRB inserts into the lateral epicondyle of the humerus, an extremely small insertion area that Groppel [5]hypothesized as the major reason for injury occurring regularly at this point. None of the other extensor muscles active during the backhand stroke have such a biomechanical disadvantage.

Chop [2]has estimated that only five percent of tennis elbow sufferers are tennis players. The injury may affect carpenters, dental technicians, or computer operators. Furthermore, it is believed that between 40 and 50% of people who play tennis will develop tennis elbow during their playing career 1, 6, 15, 16. For those who seek medical advice about the injury, the most recommended treatment is rest. Even if an athlete agrees to stop playing tennis, there is still no assurance this action will prevent further aggravation of the injury. People suffering from tennis elbow want to continue the activity that is causing the pain because it is part of their occupation or a sport which they enjoy playing. During a tennis stroke they attempt pain alleviation by trying different techniques to limit the amount of wrist flexion during a swing. To decrease pain temporarily and to stabilize the wrist, a player flexes and rotates the wrist to grip the handle tighter. This grip may decrease their pain but increases the tension on the muscle and increases the likelihood of injury at the lateral epicondlye of the humerus. Although these techniques may temporarily reduce pain they lead to greater structural damage.

While the medical and tennis playing communities have known about tennis elbow for many years, a definitive etiology has yet to be determined. Furthermore, the criteria used in assessing treatment success are (1) the patient's diminished perceived level of pain and (2) his/her ability to participate in activities previously prohibited by the pain. Currently there are no quantitative methods used to determine if treatments allow the injured tendon to fully heal or to assess if the injury results from muscle activity modification, possibly inhibiting the healing of the damaged tissue.

The purpose of this study was to determine under simulated tennis conditions if (1) surface EMG could be used to accurately delineate between healthy subjects and subjects suffering from tennis elbow and, (2) if activation patterns are different, to assess whether the differences should be considered helpful or detrimental to the healing process.

Section snippets

Methods

This investigation attempted to determine the ability of surface EMG to accurately distinguish the health status of individuals suffering from a soft tissue injury (tennis elbow) from that of a healthy control group based on observed responses to forced wrist flexion similar to that experienced during tennis play. Analysis was performed to determine if the INJ group employed muscle activation patterns that may be counterproductive to speedy recovery from tennis elbow. Additional main effects of

Data collection and processing

The analog outputs of the EMG and accelerometer sensors were collected at a sampling frequency of 1000 Hz for all test conditions. The data logger system collected and stored the data with 8 bit resolution. All channels were polled within 20 micro-seconds during each 0.001 second sampling interval, providing complete synchronization of the data logger sensor data. Two software packages provided with the data logger system (DLM.exe and ADGRAF.exe), assisted in initializing the chip cards,

Results

The variable health status proved to be statistically significant for all muscle activity responses for the extensor group (p<0.05). Activity levels of the flexors and triceps were not statistically different between CON and INJ participants (p>0.05). Table 2 lists the abbreviations for the nine impact and ball velocity impact conditions used during this study. Fig. 1 illustrates the impact locations studied.

Fig. 3Fig. 4Fig. 5 illustrate the responses evaluated during this study. Fig. 3 shows

Discussion

The results of this study indicate that surface EMG provides useful data in evaluating biomechanical changes that take place in the case of tennis elbow. This result is significant since it is often very difficult to quantify the magnitude of changes that result from soft tissue injury.

The present data show that muscle activation patterns are statistically different under identical conditions between tennis elbow sufferers and players not suffering from such an injury. Our findings that there

Dr Jeffrey A. Bauer has been an Assistant Professor and Director of the Biomechanics Laboratory at the University of Florida since 1994. He completed his doctoral work in the “water tower” at Penn State and was selected to start an emphasis of study in biomechanics at the gratuate level UF's department of Exercise and Sport Sciences. His primary research interests are in determining effects of both repetitive and acurate dynamic loading to the human body. His teaching responsibilities include

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    Dr Jeffrey A. Bauer has been an Assistant Professor and Director of the Biomechanics Laboratory at the University of Florida since 1994. He completed his doctoral work in the “water tower” at Penn State and was selected to start an emphasis of study in biomechanics at the gratuate level UF's department of Exercise and Sport Sciences. His primary research interests are in determining effects of both repetitive and acurate dynamic loading to the human body. His teaching responsibilities include an undergraduate Kinesiology course and Graduate courses in Biomechanics and Biomechanical Instrumentation.

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    Robert Murray is a graduate of the College of Health and Human Performance with a major in Exercise and Sport Science emphasizing Exercise Physiology. He minored in Chemistry, Anthropology, and French. His plans to go to medical school and someday practice orthopedic surgery.

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