Abstract
People with rheumatoid arthritis (RA) are typically affected by joint deformities of the thumb (Ratliff, 1971). Forty-two percent of people with RA experience difficulties related to lateral instability of the first metacarpophalangeal (MCP) joint (Brewerton, 1957) and deformity of the thumb carpometacarpal (CMC). Thirty percent of people with RA wear thumb orthoses (Henderson & McMillan, 2002). CMC and MCP immobilization for people with RA is clinically effective (Silva, Lombardi, Breitschwerdt, Poli Araújo, & Natour, 2008).
People with RA should consider features of good design when selecting tools (Hammond, 2013). Previous research has indicated that the optimal diameter of a cylindrical handle is midsized (30–40 mm) for healthy adults (Kong & Lowe, 2005). However, whether a handle of this size is appropriate for people with RA who use CMC and MCP immobilization orthoses is not yet known.
Immobilizing one joint changes the biomechanical mechanisms of adjacent joints (Shu & Mirka, 2006). When the wrist is immobilized, the shoulder abducts and flexes more in activities such as feeding and pouring water (King, Thomas, & Rice, 2003; May-Lisowski & King, 2008). Flexor activity is higher in grasping movements when the wrist is supported by an orthosis than with no orthosis support (Johansson, Björing, & Hägg, 2004). Because the biomechanical mechanisms and grasping patterns of the hand joints are altered with CMC and MCP immobilization, the optimal handle diameter for people who use an orthosis may differ from that for healthy adults. Unfortunately, no current guidelines specify a grasp distance suitable for people with RA, especially in the CMC- and MCP-immobilized conditions.
The ultimate purpose of this study was to investigate the optimal grasp distance for people with RA who wear a CMC- and MCP-immobilization orthosis. Seo and Armstrong (2008) defined optimal grip diameter as that which results in the greatest grip force for healthy adults. Therefore, we anticipated that the optimal grasp distance would allow participants to produce the greatest grip strength with the least loading on the forearm muscles.
Electromyography (EMG) data provide objective and quantitative information about muscle loading during active upper-extremity tasks (Johansson et al., 2004; Shu & Mirka, 2006). Morrison, Short, Ludwig, and Schwab (1947) found lower EMG amplitudes in people with RA than in healthy adults who performed a squeezing task. Because EMG amplitude is proportional to the force, people with RA might have less grip force in a squeezing task. The main purpose of this study was to find the optimal grasp distance for people with RA in the CMC- and MCP-immobilized conditions by investigating the EMG of forearm muscles during grasping activities. We sought to answer three research questions:
Do people with RA require more muscle loading to produce the same grip strength as healthy adults?
Does the optimal grasp distance for people with RA differ when wearing a CMC- and MCP-immobilization orthosis versus not wearing an orthosis?
What is the optimal grasp distance for people with RA?
Method
Design
We first designed a 2 × 2 (Group × Orthosis) mixed factorial experiment to examine the effects of the CMC- and MCP-immobilization orthosis on grip strength and EMG performance in both healthy adults and people with RA. Our first hypothesis was that people with RA would have lower maximal grip strength than healthy adults. We assumed that people with RA would require more muscle activity than healthy adults to reach the same level of grip strength. In this 2 × 2 experiment, the group factor was the between-subjects factor, and the orthosis factor was the within-subject factor. The two groups were (1) people with RA and (2) healthy adults. The two orthosis factor conditions were (1) no orthosis and (2) CMC and MCP immobilization. The two dependent variables were (1) 30% of maximal grip strength measured using a dynamometer and (2) muscle activity measured using normalized surface EMG data.
In the second experiment, we compared the effects of orthosis use and grasp distance with EMG performance only of people with RA. Our second hypothesis was that, with the combination of CMC and MCP immobilization and optimal grasp distance, people with RA would perform the same task with the least muscle load. The second experiment had a 2 × 3 (Orthosis × Grasp Distance) factorial design, with both orthosis and grasp distance as within-subject factors. Again, the two orthosis conditions were (1) no orthosis and (2) CMC and MCP immobilization.
The grasp distance was the distance between the handle and the main shaft of the dynamometer. We designated the first, second, and third handle positions on the dynamometer as the grasp distances: Distance 1 (D1) was 42.25 mm, Distance 2 (D2) was 54.64 mm, and Distance 3 (D3) was 68.90 mm. D1, D2, and D3 were greater than the optimal grasp distance of 30–40 mm in Kong and Lowe’s (2005) results, for two reasons. First, the grasp distances were preset by the handle setting on the dynamometer. Second, according to joint protection principles, people with RA should use enlarged handles (Hammond, 2013; Penrose, 2013). Given that 30–40 mm is considered the optimal distance for healthy adults, it was reasonable to begin measuring the grasp distances at 42.25 mm. The dependent variable of muscle activity in the second experiment was measured using normalized surface EMG.
Participants
People with RA were recruited from a hospital in Taiwan through convenience sampling. The five inclusion criteria were (1) diagnosis of RA according to the 1987 American College of Rheumatology revised criteria (Arnett et al., 1988), (2) stable disease-modifying antirheumatic drug therapy within the preceding 6 mo, (3) age ≥20 yr, (4) right hand dominant, and (5) prescription of an immobilization orthosis by a doctor. Potential participants received a recruitment letter from a researcher who was not their therapist or doctor in charge. Healthy adults were recruited by means of advertisements on a university campus. The four inclusion criteria were (1) age ≥20 yr, (2) right hand dominant, (3) no history of right upper-extremity injury, and (4) normal functional performance of the right upper extremity.
The experimental procedure was approved by the ethics committee of the hospital, and we obtained informed consent from all participants. Of the 36 participants, 18 had RA and 18 were healthy adults. The average age of the participants with RA (5 men and 13 women) was 55.22 (±16.60) yr and of the healthy adults (8 men and 10 women), 52.44 (±15.81) yr.
Procedure
Grip Strength Measurement.
Participants sat on a chair, and the seat height was adjusted so that their feet were flat on the floor with their knees and hips flexed at about 90°. They then placed their right forearm on the table with the shoulder in adduction, the elbow at 60°–90° flexion, the forearm in neutral position between pronation and supination, the wrist in neutral position between the ulnar and radial deviation, and the wrist at 10°–20° extension, and they concurrently grasped a Baseline digital hand dynamometer (Model 12-0112; Fabrication Enterprises, White Plains, NY). The values displayed by the dynamometer were recorded. This participant positioning was adopted from Mathiowetz et al. (1985); however, the dynamometer was reversed to allow participants to see the readings. The forearm was placed on the table to help support the weight of the dynamometer.
First, the participants produced maximal grip (maximum voluntary contraction [MVC]) for 5 s. In MVC, participants performed isometric maximal contraction. Then the participants practiced limiting their grasp force to 25%–35% of their maximum by watching the digital dynamometer readings. When they achieved proficiency, the participants were asked to exert 30% of their maximal grip (±2 g) for 5 s, the amount of maximal grip required in the largest group of general activities of daily living (Faes et al., 2006; Steultjens et al., 2004). Brookham, Wong, and Dickerson (2010) used this amount to simulate an exertion task with a light hand tool. EMG data were recorded for the three grasp distances and two orthosis conditions.
The CMC- and MCP-immobilization orthosis was a short thumb apparatus that excluded the thumb interphalangeal and wrist joints, and the orthosis was custom molded by an occupational therapist with more than 6 yr of experience. Each participant performed three trials per condition for a total of 21 trials (maximal grip × 3 trials and 3 distances × 2 orthoses × 3 trials). The order of the Distance × Orthoses conditions was randomly assigned. Rest periods of 1 min were given between trials, and rest periods of 5 min were given between each condition. The participants could prolong the rest period if they became tired. All procedures were performed by the same research assistant and supervised by a principal investigator to ensure interrater reliability. The participants and research assistants were blind to the purpose of the study.
Electromyography Data Collection.
The surface EMG data were recorded by a commercial recording system (Nexus-10, Mind Media BV; Roermond, the Netherlands) and the original 5-mm electrodes. The electrodes were spaced 2 cm apart over the extensor digitorum communis (EDC), abductor pollicis longus (APL), flexor digitorum superficialis (FDS), and flexor pollicis longus (FPL) muscles of the right hand. In the orthosis condition, the action of the APL was restricted by the orthosis, the EDC and FDS were free from restriction, and the FPL was partially restricted. The skin was cleaned with alcohol, and the electrodes were placed on the thickest area of the muscle belly in accordance with Delagi and Perotto’s (1996) suggestion. The earth electrode, placed on the clavicle, was used as a reference to subtract the ambient noise signal from the ground. Electrode placement was confirmed by isolated contraction of each muscle.
The EMG data for maximum voluntary contraction during the maximal grip (maximal grip × 3 trials) were used to normalize the muscle contraction data during the condition-specific 30% maximal grip (Shu & Mirka, 2006). The EMG data during each condition-specific 30% maximal grip were also recorded during the 18 trials (3 distances × 2 orthoses × 3 trials).
Data Processing.
Grip strength data were averaged across the three trials under the same condition. EMG raw signals were recorded and processed with a 10- to 10,000-Hz band-pass filter, amplified by a gain of 500. Filtered signals were full-wave rectified and smoothed with a 50-ms time constant to produce a linear envelope. Raw EMG is an alternating current signal, varying in both positive and negative voltages. Rectification converts negative voltages to positive values. A linear envelope is a procedure for estimating the volume of muscle activity (Kamen, 2013). The EMG signals were quantified in terms of root mean square, a measurement used in EMG studies to quantify the energy that a muscle generates in a specific time window (Barbero, Merletti, & Rainoldi, 2012).
Raw EMG data were collected at 1,024 Hz using BioTrace+ software (MindMedia, Herten, the Netherlands) through a data acquisition card on a notebook computer. For the voluntary contraction trial, EMG activity for each muscle was quantified in terms of root mean square measured over a 3-s period, and the data for the first and last seconds were deleted. The curve of the rectified EMG signal was integrated, that is, the muscle activity value was the area under the 3-s period. The rectified EMG data were averaged across the three trials under the same condition. The dependent variable, normalized EMG (nEMG%), was an indicator of exertion level as a percentage of the MVC, that is, nEMG% = (EMG for 30% grip/EMG for maximal grip) × 100.
Statistical Analysis.
We conducted a 2 × 2 mixed analysis of variance to analyze the maximal grip strength data and nEMG% data of each muscle for the Group (2) × Orthosis (2) experimental design. A 2 × 3 repeated-measure analysis of variance was used to analyze the nEMG% data for each muscle for the orthosis (2) and grasp distance (3) experimental design. We used a least significant difference test as a follow-up for pairwise comparison of means. Significance was set at 5% (p < .05). Because the distributions were skewed to the right, log transformation was applied. Data were analyzed using IBM SPSS Statistics (Version 20; IBM Corp., Armonk, NY). Means and standard deviations were data transformed back to the original scale.
Results
Effect of Group × Orthosis
A significant main effect of group was found on participants’ maximal grip strength, F(1, 35) = 20.307, p < .001. Means and standard deviations (SDs) for maximal grip strength were 20.04 g (SD = 2.16) for the RA–orthosis condition, 23.78 g (SD = 1.14) for the RA–no orthosis condition, 50.54 g (SD = 3.78) for the healthy adult–orthosis condition, and 58.65 g (SD = 2.66) for the healthy adult–no orthosis condition (Figure 1). Maximal grip strength was greater in the healthy adults than in the participants with RA. The main effect of orthosis condition on maximal grip strength was not significant, F (1, 35) = 2.844, p = .093. No interaction effect between group and orthosis was found, F (1, 35) = 0.231, p = .631.
Mean maximal grip strength for participants with RA and healthy adults in the splint and free conditions.
A significant main effect of group was found on the nEMG% data for 30% of maximal grip for all four muscles (Figure 2). The main effect of orthosis condition on the nEMG% data was not significant for any muscles. No interaction effect between group and orthosis was found in any of the muscles. All of the nEMG% data were higher for people with RA than for healthy adults in both orthosis conditions.
Normalized electromyography (nEMG %) results for the four forearm muscles of participants with rheumatoid arthritis and healthy adult participants in the orthosis and no-orthosis conditions.
Effect of Orthosis × Grasp Distance for Participants With Rheumatoid Arthritis
A main significant effect of grasp distance was found only on nEMG% of the EDC. The post hoc test showed that nEMG% of the EDC was higher in the D2 and D3 conditions than in the D1 condition for people with RA (p < .05), but the difference between the D2 and D3 conditions was not significant (p > .05). The post hoc test showed that nEMG% of the APL was higher in the D2 and D3 conditions than in the D1 condition for people with RA (p < .05), but this difference between the D2 and D3 conditions also was not significant (p > .05; Figure 3). No interaction effect was found between orthosis condition and grasp distance in any muscles (see Table 1). The nEMG% data for the EDC and APL were higher for D2 and D3 than for D1 in the immobilized condition for people with RA.
Normalized electromyography (nEMG%) results for participants with rheumatoid arthritis in the orthosis and no-orthosis conditions at three grasp distances.
Results of a Two-Way Analysis of Variance for Orthosis and Grasp Distance Conditions for Each Muscle
Note. APL = abductor pollicis longus; EDC = extensor digitorum communis; FDS = flexor digitorum superficialis.
p < .05.
Discussion
We examined three main questions. Our evidence suggests that people with RA have lower grip strength and that they exert greater muscle loads to equal the strength of healthy adults. However, our results did not support our second hypothesis. The optimal grasp distance for people with RA was the same with and without the immobilization orthosis but smaller than that for healthy adults.
We demonstrated that the grip strength of participants with RA was significantly lower than that of healthy adult participants. This finding is consistent with those of previous studies (Silva, Jones, Silva, & Natour, 2008). In addition, we observed that grip strength did not differ significantly with versus without use of a CMC and MCP immobilization orthosis. Studies of the effect of this orthosis on grip strength have yielded mixed results. A review by Steultjens et al. (2004) of studies of the effect of an orthosis on grip strength reported that “splints can decrease pain and improve the strength of one’s grip” on the basis of two high-quality studies: Nordenskiöld (1990) found that an elastic orthosis significantly improved grip strength for people with RA, and Kjeken, Møller, and Kvien (1995) also reported positive effects of an orthosis on grip strength after 6 mo of application for people with RA. However, other studies have found no effect of orthosis use on grip strength. Rennie (1996) stated that an MCP ulnar deviation orthosis had no impact on grip strength. Callinan and Mathiowetz (1996) compared two types of resting orthosis (soft and hard) and found no change in grip strength relative to the no-orthosis condition. Veehof, Taal, Heijnsdijk-Rouwenhorst, and van de Laar (2008) reported that wearing an orthosis for 4 wk did not affect grip strength in people with RA.
The present study’s results are consistent with those of Rennie (1996), Callinan and Mathiowetz (1995), and Veehof et al. (2008). However, those studies compared different types of orthoses, evaluated different time effects (immediate vs. short-term vs. long-term), and used different types of measuring instruments (Jamar dynamometer vs. Martin Vigorimeter vs. Gripit); any of these factors can cause different results (Massy-Westropp, Rankin, Ahern, Krishnan, & Hearn, 2004). For example, whereas Stern, Ytterberg, Krug, Mullin, and Mahowald (1996) reported no immediate or short-term effect of three commercial orthoses on grip strength after 1 wk of wearing, Haskett, Backman, Porter, Goyert, and Palejko (2004) showed positive and significant long-term effects of commercial orthoses on grip strength after 4 wk and 6 mo. Therefore, further evidence is needed to determine the effect of orthosis use on grip strength.
A significant group effect was found in our study: Muscle activity in all tested muscles was higher in people with RA than in healthy adults. This higher activity indicates that the participants with RA had to exert more effort than the healthy adult participants to achieve 30% exertion in a task. Brorsson, Nilsdotter, Thorstensson, and Bremander (2014) studied muscle activity in four daily tasks and found that people with RA required higher muscle activity to cut with scissors and pull up a zipper compared with healthy adults. Swezey and Fiegenberg (1971) noticed overactivity, or “spasm,” of intrinsic muscles in people with RA, and the intrinsic overactivity contributed to swan neck deformity. Further investigation is needed to determine the impact on joint alignment of this higher muscle activity in the extrinsic muscles.
We found that wearing the thumb orthosis did not affect muscle activity. This finding is inconsistent with Johansson et al.’s (2004) finding that the forearm muscle activity of healthy adults while gripping was higher with a wrist orthosis than with no orthosis or a commercial orthosis. We suggest two possible explanations for this inconsistency: (1) The orthosis in our study restricted the movements only of the CMC and MCP joints and did not constrain the wrist or other parts, and therefore muscle activity in the forearm was not affected, or (2) the overload muscle activity affected the intrinsic muscles, which we did not measure.
Grasp distance did alter muscle activity in some of the muscles tested. The muscle activity of the EDC and APL was lower at the shortest grasp distance (42.25 mm) than at the medium (54.64 mm) and longest (68.90 mm) grasp distances. In Kong and Lowe’s (2005) study, flexor and extensor activity in healthy adults remained the same when grasping handles 25–50 mm in diameter. In contrast, Seo, Armstrong, Ashton-Miller, and Chaffin (2007) reported that grip force tested with a split cylinder decreased when grasp diameter was increased from 45.1 mm to 83.2 mm.
Grip force has also been measured using Seo and Armstrong’s (2008) torque model, which measures friction and normal forces on the fingertips to calculate the torque. In Seo and Armstrong’s study, grip force decreased as diameter increased, a result inconsistent with our findings. However, they also used a Jamar dynamometer to measure grip force and found that it was lower at the smallest grasp distance (35 mm) than at larger grasp distances (49, 63, and 77 mm); these measurements are consistent with our data. This variation in findings may result from differences in the measuring tools. As Seo et al. (2007) noted, only 61% of the variance in dynamometer grip strength can be explained by the split-cylinder grip strength. The results of this study might be explained by the length–tension relationship of the skeletal muscles.
At the shortest grasp distance (42.25 mm), the EDC and APL of our participants with RA were at the ideal length to produce force, resulting in lower contraction loads and thus higher grip strength. Ikeda, Kurita, and Ogasawara (2009) found that when pinching involves a wider grasp diameter (>50 mm), a large antagonist force is necessary to open the thumb wide, and thus the muscle activity of the thumb (the adductor pollicis muscle) is more dominant than that of the index finger FDS. Unlike Ikeda et al., we used CMC and MCP immobilization, and as a result, we found compensatory muscle contraction in the EDC and APL rather than in the adductor pollicis muscle. We also observed no additional muscle contraction during gripping of small objects (42.25 mm in diameter), but for large objects (54.64 mm and 68.90 mm in diameter), extra muscle contraction (of the EDC and APL) was observed. Because more additional muscle contraction was found at the larger grasp diameters, tools that allow effort-saving grasp patterns should be used during the performance of activities of daily living.
Limitations
This study has several limitations. Use of surface EMG limited us to investigating the forearm muscle groups; future research can investigate the intrinsic muscle groups or contact areas and hand force distribution with grasp or finger joint angles while grasping. Second, grasp distance was limited to three conditions (42.25, 54.64, and 68.90 mm) by the design of the hand dynamometer. Future studies can investigate distances of less than 42.25 mm. Third, we measured the immediate effect of an orthosis. Because a custom-made orthosis might require adjustment to fit to the hand movements used in daily activities, investigation of the long-term effect of orthoses on muscle loads, grip strength, and grasp distance might be needed. Fourth, we measured the effect of thumb orthosis use on grip strength using a Baseline digital dynamometer. In gripping the dynamometer, the four fingers (index, middle, ring, and little finger) may play a major role. Therefore, pinch strength might be a better indication of the effect of an orthosis on the thumb. Although pinch strength was measured in the beginning of this study, several participants with RA did not complete 21 trials of pinch grasp because of muscle fatigue.
The research design of this study, including the inclusion criteria, could be adjusted to facilitate the gathering of further evidence. Finally, to allow participants to see the readings on the dynamometer as real-time feedback on their exertion, we instructed them to hold the dynamometer with the direction reversed. The curve of the handle was not fitted to the hand arches, so this ill-fitting arrangement may have reduced participants’ grip strength and affected EMG results.
Implications for Occupational Therapy Practice
The results of this study have the following implications for occupational therapy practice:
People with RA require higher muscle loads to equal the strength of healthy adults.
A grasping handle 42.25 mm in diameter is recommended for people with RA.
The grip strength of people with RA is not improved by a CMC and MCP immobilization orthosis.
Conclusion
This study demonstrates a relationship between grip strength and muscle activity in people with RA. The results support those of other studies, suggesting that such people need well-designed grasping handles to reduce workload during daily tasks. We suggest that a handle with a diameter of 42.25 mm (compared to 54.64 mm and 68.90 mm) is best for people with RA because this size allows them to grasp the handle without extra exertion by the forearm. This suggestion can be used in selecting assistive devices that require grasping, such as jar openers and bars for opening doors.
Footnotes
Acknowledgments
We acknowledge the contribution of the study participants. Funding was provided by Grant 700359CTH-102-1-2B04, jointly supported by the Fu Jen Catholic University and Cardinal Tien Hospital.