We compared maximum active wrist flexion in UEMSD subjects versus control subjects at three forearm postures. None of the UEMSD subjects reported any increase in pain symptoms during testing, indicating that their wrist flexion was not limited by pain. Mean wrist flexion in the UEMSD group was restricted compared to the control group in both wrists at all forearm postures with the exception of the non-dominant wrist with the forearm prone. These flexion limitations are consistent with previous studies of individuals with UEMSDs [13
]. However, the most striking result was the decrease in wrist flexion at the supine forearm posture compared to the prone forearm posture in both wrists of both groups.
The control group's mean flexion data for both wrists at the prone and neutral forearm postures was comparable to previous studies of active wrist flexion [24
]. The control group's flexion decreased at the supine posture compared to the prone or neutral postures in both wrists. Wrist flexion in the UEMSD group also decreased between the prone and supine forearm postures, but to a greater extent than in the control group. Overall, wrist flexion between the prone and supine forearm postures in the UEMSD group decreased 51.5% in the dominant and 49.3% in the non-dominant wrist, and in the control group decreased 26.8% in the dominant and 21.6% in the non-dominant wrist.
We are aware of only one previous study conducted by Hewitt in 1928, who found that mean active wrist flexion decreased approximately 12° between the prone and supine forearm postures in normal subjects [27
]. However, in Hewitt's study active wrist flexion was approximately 10° – 15° greater than that shown in previous studies or our study [25
]. We suspect that this discrepancy may be due to the comparatively young age (mean age < 20 yrs.) and exclusively female gender of the subjects in Hewitt's study, both of which have been associated with increased wrist flexion [28
]. Hewitt [27
] did not explicitly address the decrease in flexion between the prone and supine forearm posture observed in her study, but suggested that agonist muscle efficiency may be greater with the forearm prone versus supine
Another study on the effects of forearm posture found that maximum active wrist flexion decreased slightly (< 5 degrees) when the forearm was semi-prone (45°) versus fully prone (90°) [26
]. This decrease was attributed to possible changes in the articular contact of the carpal bones, or increases in trans-carpal ligament tension [26
]. Our results did not show a significant decrease in wrist flexion between the prone and neutral forearm postures in either group. However, the decrease in wrist flexion between the prone and supine forearm postures that is evident in both groups cannot be readily explained by changes in articular contact or trans-carpal ligament tension. The results of a cadaveric study [29
] examining wrist tendon excursion demonstrated that the normal wrist is capable of 70 degrees of flexion at the prone, neutral, and supine forearm postures when the proximal wrist motor tendons are severed from their origins. Presumably both cadaveric and live wrists possess equivalent trans-carpal ligament and articular contact characteristics.
Forearm posture may exert an effect on wrist flexion due to biomechanical changes that occur with forearm supination. Several cadaveric wrist studies have shown that excursion of the distal tendon of the extensor carpi ulnaris (ECU) muscle during wrist flexion increases approximately three-fold between the prone and supine forearm postures [29
]. These results have been attributed to an increase in the length of the distal ECU tendon's moment arm due to dorsal displacement of the distal ECU tendon relative to the carpus during forearm supination [29
]. A three-fold increase in the length of the distal ECU tendon's moment arm corresponds to a three-fold increase in distal ECU tendon excursion to achieve a given wrist flexion angle. It follows that decreased extensibility of the ECU muscle would resist ECU tendon excursion, limiting wrist flexion to a greater degree with the forearm supine versus prone. Decreased extensibility of the ECU muscle would explain the greater loss of wrist flexion with the forearm supine that we observed in the UEMSD group compared to the control group. This apparent decrease in ECU muscle extensibility may be due to increases in active and/or passive tension.
The presence of ECU muscle tonus and an intact connection to the proximal tendon's origin in live subjects might explain why a supine forearm posture limited wrist flexion in our control subjects, but not in studies of cadaveric wrists [29
]. Similarly, an increase in ECU muscle activity in the UEMSD group compared to controls might also explain why the decrease in flexion between the prone and supine forearm postures in the UEMSD group was approximately double that in the control group. We are not aware of any studies demonstrating increased wrist extensor muscle activity during wrist flexion in UEMSD subjects, although such increased activity may occur. However, it appears that even maximum contraction of the ECU muscle would be incapable of resisting maximum wrist flexion torque.
A study of normal wrists [32
] found that maximum wrist extension torque was less than 60% of maximum flexion torque over the range of flexion angles we measured in our study. Other studies have confirmed the superiority of wrist flexion torque, although to lesser degrees, and all were conducted with the forearm prone [33
]. Presumably the extension torque capability of the ECU muscle would be enhanced when the forearm is supine due to the increase in its moment arm length [29
We are aware of only one study that examined wrist extensor torque with the forearm supine. Ketchum [36
] used measures of wrist extensor muscle masses and tendon excursions from cadavers combined with flexion torque from live subjects to estimate the force generated by each muscle. The tendon excursions were measured with the forearm supine, but the ECU muscle was estimated to contribute less than 30% of the total extensor force [36
It is also possible that the reduction in wrist flexion with the forearm supine was due to a decrease in wrist flexion torque. However, wrist flexor tendon excursion has not been shown to vary significantly between the prone and supine forearm postures [29
], suggesting that flexion torque would not decrease. As the ECU muscle is only one of three muscles contributing to wrist extension torque it appears unlikely that even maximum activation of the ECU muscle would be capable of opposing wrist flexion torque. Therefore, we infer that the limited wrist flexion exhibited by the UEMSD group compared to the control group with the forearm supine is due to increased ECU muscle passive tension.
Maximum active wrist flexion in the dominant wrist of the UEMSD group was also below normal with the forearm prone, in agreement with previous studies [13
]. However, we are unable to attribute this flexion restriction to increased ECU muscle passive tension. It follows from the studies of wrist motor tendon excursion previously discussed [29
] that the required ECU tendon excursion for a given wrist flexion angle is reduced three-fold when the forearm is prone versus supine. Consequently, one would expect the wrist flexion angle to increase three-fold with the forearm prone versus supine if ECU muscle passive tension was the limiting factor. Our results showed that dominant wrist flexion in the UEMSD group increased approximately two-fold between the supine and prone forearm posture, and remained below normal. The fact that wrist flexion increased only two-fold and remained below normal despite a three-fold decrease in the required excursion of the distal ECU tendon indicates that ECU muscle tension is not limiting wrist flexion with the forearm prone.
The limited wrist flexion may have been due to a general impairment in dominant wrist flexor muscle strength in the UEMSD group. A study of subjects with medial and lateral epicondylitis found that peak wrist flexion torque was reduced by 13% in the affected versus the unaffected arm [37
]. However, the same study found that peak wrist extension torque was still less than 60% of peak flexion torque in both arms [37
]. These results suggest that substantial wrist flexor impairment would be required to limit wrist flexion in conjunction with maximum extensor muscle activation. Instead, we propose that dominant wrist flexion in the UEMSD group with the forearm prone is limited by decreased extensibility of the radial wrist extensor muscles: extensor carpi radialis brevis (ECRB) and extensor carpi radialis longus (ECRL). We infer that this is due to an increase in passive rather than active tension because, as described previously, wrist flexion torque exceeds wrist extension torque over the range of wrist joint angles measured in our study [32
Our finding of restricted wrist flexion in UEMSD subjects compared to normal controls is consistent with the idea that wrist extensor muscle passive tension is elevated in this group [17
]. This interpretation agrees with a previous report of palpable increases in wrist extensor muscle tone with wrist flexion in UEMSD patients [13
]. The modulation of maximum active wrist flexion by forearm posture is readily explained by known changes in the length of the distal ECU tendon's moment arm [29
]. The existence of flexion restrictions in the dominant wrist of the UEMSD group at both the prone and supine forearm postures implicates both the ulnar (ECU) and radial wrist extensor muscles (ECRL and ECRB).
Increased wrist extensor muscle passive tension would affect wrist joint dynamics during flexion. We had initially presumed that, as wrist flexion during computer use is minimal, increased joint and muscle/tendon stress might have occurred during domestic or recreational activities, the symptoms of which were then exacerbated by, and perhaps attributed to computer use. However, the evidence for increased ECU muscle passive tension suggests that increased stress could also occur during radial deviation of the wrist, as the ECU muscle despite its name, functions primarily as an ulnar deviator [30
The apparent increase in extensor muscle passive tension exhibited by the UEMSD group may be related to computer use. Mackinnon and Novak [7
] suggest that prolonged abnormal postures may affect muscle tension due to muscle length adaptation. It has been shown that animal skeletal muscle immobilized in a shortened state shortens due to the loss of serial sarcomeres, and the rate of such loss increases when the muscle is chronically activated [38
]. However, the increase in muscle passive tension that follows immobilization has been attributed to qualitative and quantitative changes in the connective tissue surrounding the muscle, and these changes have been shown to precede sarcomere loss [40
]. Conceivably, prolonged wrist extension during computer use could result in muscle length and/or connective tissue changes that increase extensor muscle passive tension.
The cross-sectional design of the current study limits our ability to infer causality, particularly as flexion limitations were also evident in the unaffected wrist of the unilaterally affected UEMSD subjects. However, the results of this study suggest a role for elevated extensor muscle passive tension as a factor in the development of UEMSD symptoms in computer users. Computer usage in the control group was minimal, so we cannot determine from this study whether wrist flexion is similarly affected in asymptomatic computer users. The limitations of this study include a small sample size, and a lack of clear case definitions for the UEMSD subjects. The control group was not age and gender matched to the UEMSD group; however, 6 of the 7 female control subjects were within the age range of the female subjects in the UEMSD group.