Wrist torque control and strength are critical for performing everyday activities as well as racket sports and other sports that require wrist strength and control. Depending on the motor task, activities involving the wrist require specific direction and combinations of movements of the joint as in flexion, extension, pronation, supination, radial deviation, and ulnar deviation. In contrast to other joints of the upper extremity such as the shoulder and the elbow, the wrist has a significantly smaller muscle mass controlling the joint (9). Because of the repetitive use as well as ballistic actions of the joint in sports, hand and wrist injuries are common occurrences in all sports resulting from trauma or overuse (13). Musculoskeletal disorders including injuries resulting from trauma and overuse affect a large number of individuals as shown by the fact that 22.3% of Ontario, Canada's population, 2.8 million people, sought medical attention for musculoskeletal disorders in the 2006–2007 fiscal year (10). Musculoskeletal disorders are prevalent in the working population as well, with a previous study finding that 56% of participants who reported upper extremity disorders experienced work-related productivity loss (11). Hand and wrist musculoskeletal disorders are associated with longer absences from work, and greater loss in productivity and wages when compared with other anatomical locations of the body (1). Whether in sports or day-to-day activities, the wrist joint is susceptible to injury because the wrist is required to generate large magnitude of torques with 3 degrees of freedom: flexion-extension, ulnar-radial deviations, and pronation-supination.
Adequate wrist strength can help prevent individuals from experiencing wrist injuries or injury-related discomfort (5) because wrist injuries often arise from weak muscle strength or lack of joint control as the wrist moves through its range of motion (6). An important modality to increase muscle strength and joint control is by direction-specific joint strength training. Previous studies on strength training of hand and finger joints have reported a significant increase in the force production and torque control with strength training (14). In addition to the hand and fingers, the wrist joint also plays a critical role in completing everyday manipulation and dexterity tasks, but the role of wrist joint strength training on the torque production and control is not yet known. The purpose of this study was to examine changes in wrist torque control and strength after 6 weeks of strength training. Based on previous research on strength training of hands and fingers (14), we hypothesized that the experimental group will have a greater improvement in the strength and torque control of the wrist joint after wrist resistance training as compared with the control group.
Experimental Approach to the Problem
This study was designed to measure 2 characteristics of wrist motor performance: isometric torque control and isokinetic maximum strength. A between subjects experimental design was used. Strength training was administered to the training group for 6 consecutive weeks, whereas the second group consisted of age-matched controls that did not undergo any training. The details of the strength training protocol have been explained in the sections that follow. Both training and control groups were evaluated during visits at weeks 0 (before training), 2 (2 weeks of training), 4, and 6. Assessments for wrist strength and control of submaximal accurate torque in flexion, extension, pronation, supination, radial deviation, and ulnar deviation were performed on separate days, within 72 hours of a training session. This method was chosen to enable adequate recovery time in between the testing sessions. Subjects were instructed to refrain from any excessive physical activity during these 3 days of testing and also asked to maintain their regular diet and abstain from nutritional supplements during the training. Subjects were tested at the same time of day for each testing session to maintain consistency.
Nineteen right-handed, nonathlete young men between the ages of 23 and 26 years with no history of serious upper extremity or hand injuries participated in this study after providing written informed consent. The mean (±SE) age, height, body mass, hand length, and hand width of the subjects were 24.1 (±0.2) years, 174.8 (±1.4) cm, 68.7 (±1.3) kg, 17.3 (±0.2) cm, and 9.5 (±0.1) cm, respectively. Handedness was determined by the Edinburgh Handedness Inventory (12). Subjects were randomly assigned to either the wrist training or control groups. Nine subjects in the wrist training group and 10 subjects in the control group completed the study. Physical characteristics of the subjects were not statistically different between the groups (Table 1). Each subject was familiarized with the training protocol, testing procedures, and equipment before training and data collection. Before the experiment, this study was approved by the University of Maryland's Institutional Review Board. Subjects were informed of the benefits and risks of the investigation before written informed consent. Subjects had the choice to drop out of the study at any time. No subjects reported pain or discomfort during or after the training.
Custom-made wrist training devices (Figure 1) were used for all training exercises. The load of the wrist training device was adjusted to determine each subject's initial 1-repetition maximum (1RM) in each direction, by adding or removing small weights in the increments of 100 g. The 1RM was used to determine the subsequent load for actual resistance training.
The training load of the device was adjusted such that 70% of the 1RM for each wrist exercise was prescribed. Subjects then performed the following 6 wrist exercises: flexion, extension, pronation, supination, radial deviation, and ulnar deviation in randomized orders. Three sets of exercise were performed in each direction, and 2 minutes of rest were allotted between sets. Each repetition lasted approximately 4 seconds: 2 seconds for the concentric phase and 2 seconds for the eccentric phase. One repetition involved rotating the device through the full range of motion.
The training group had 3 training sessions per week over 6 weeks. For the first 2 weeks of training, subjects performed 3 sets of 3–5 repetitions of each exercise (Table 2). During the third and fourth weeks, subjects performed 3 sets of 6–8 repetitions of each exercise. During the fifth and sixth weeks, subjects performed 3 sets of 8–10 repetitions of each exercise. The control group did not receive training in between evaluation time points. For the duration of the 6-week training program, subjects were instructed to maintain normal dietary habits, and were not allowed to participate in any other strength training exercises. Both training and testing were performed in the spring.
Isometric Torque Control Testing
This assessment was performed before and after 6 weeks of resistance training and at 2-week intervals during the training. The subjects sat in a chair and placed their right arm into a wrist-forearm brace fixed to a table while looking at a 21″ computer monitor (Dell, Round Rock, TX, USA) (Figure 2A). A 6-dimensional torque sensor (Model PY6; Bertec Corporation, Columbus, OH, USA), equipped with a handle was used to measure the torques. Signals from the sensors were amplified and conditioned. A customized program in LabView 7.0 (National Instruments, Austin, TX, USA) was used to provide the live feedback of the produced torque on the computer monitor. First, the subject's maximum voluntary torque (MVT) was measured in flexion, extension, pronation, supination, radial deviation, and ulnar deviation. For the wrist torque control task, the computer monitor showed a sinusoidal time profile of torque in a yellow line (Figure 2B). Subjects were given live feedback of the torque produced with a trailing cursor in red. Thus, to produce the 20% of MVT in a given direction, subjects had to trace the profile of the yellow line with the red line by controlling their wrist torque. For each torque production task, 2 directions of torque that share the same axis of rotation were combined (i.e., flexion-extension, pronation-supination, and radial-ulnar deviations). The 3 positive-negative torques included flexion-extension, pronation-supination, and radial-ulnar deviations. Each subject performed 12 trials for each sinusoidal profile.
The absolute deviation of the torque produced by the subject from the torque template (root-mean-square error or RMSE) was postprocessed in MATLAB 7.13 (Mathworks, Natic, MA, USA) and averaged over the whole time series to estimate the accuracy of the task.
T(t) is the time trajectory of torque produced by a subject, Ttarget(t) is the target torque (i.e., sinusoidal torque profile), and Δt is 12 seconds in which the analysis was performed (Δt = 12). The absolute difference between the time trajectory of the produced torque and target torque was calculated and averaged across time and all trials. The calculated error was normalized by MVT.
Isokinetic Torque Testing
The maximum isokinetic torque production task about the wrist joint was measured on a Kin-Com dynamometer (Model 125E plus; Chattecx, Chattanooga, TN, USA) (Figure 3). The subject sat in a chair that was stabilized by a seatbelt strap and a cross shoulder harness. The right arm was placed into a wrist-forearm brace that was fixed to a table, and the forearm was fastened with Velcro straps. The subjects secured their right hand on a grip handle attached to the Kin-Com load cell and rested in a neutral position. Isokinetic strength was tested on the dynamometer at the angular velocity of 60° per second. For each trial, the subjects performed maximum concentric torque contractions and were given at least 5 minutes of rest between trials to minimize fatigue. Isokinetic peak torques during flexion, extension, pronation, supination, radial deviation, and ulnar deviation were quantified and considered as the maximum isokinetic strength.
Both RMSE and peak torque values were collected at bi-weekly intervals (week 0, week 2, week 4, and week 6) and each was normalized by the cross-subject averages of individual RMSE and peak torque values collected at the baseline (week 0) to minimize the subject-to-subject variability and examine relative changes of RMSE and peak torque with respect to the initial testing (week 0) (7,14). For statistical analysis, 2-way repeated-measures (RM) analysis of variance (ANOVA) was performed with the within-subject factor, SESSION (4 levels: week 0, week 2, week 4, and week 6) and the between-subject factor, GROUP (2 levels: wrist training group and control group). These variables allow for the examination of the differential changes of RMSE and peak torque over training between the wrist training group and the control group. Thus, the independent variables were SESSION and GROUP and dependent variables were RMSE and peak torque. The level of significance was set at p ≤ 0.05. All values are expressed as means ± SE.
Root-mean-square error values decreased with the resistance training in the training group, whereas the RMSE values did not show significant change in the control group (Figure 4A–C, Table 3). Root-mean-square error of the training and control groups were statistically different after 2 weeks of training, and the difference became greater as the training period continued. This result was supported by the 2-way RM ANOVAs with the between-subject factor, GROUP (2 levels), and the within-subject factor, SESSION (4 levels), which showed significant GROUP effects for flexion-extension (F[1,17] = 14.1, p < 0.01), pronation-supination (F[1,17] = 13.6, p < 0.01), and radial-ulnar deviation (F[1,17] = 6.6, p ≤ 0.05), SESSION effects for flexion-extension (F[3,51] = 25.9, p < 0.001), pronation-supination (F[3,51] = 9.8, p < 0.001), and radial-ulnar deviation (F[3,51] = 26.0, p < 0.001), and significant GROUP × SESSION interactions for flexion-extension (F[3,51] = 23.3, p < 0.001), pronation-supination (F[3,51] = 10.6, p < 0.001), and radial-ulnar deviation task (F[3,51] = 23.1, p < 0.001).
Peak Isokinetic Torque
The isokinetic maximum strength, assessed by the normalized peak torque of all six-directional torques (i.e., flexion, extension, pronation, supination, radial deviation, and ulnar deviation) during 60° per second isokinetic wrist movements, increased with the resistance training in the training group, whereas the control group did not show any statistically significant changes (Figure 5A–F, Table 4). Strength differences between the training and control group were statistically significant after 4 weeks of training, and the difference increased as the training period continued. This result was supported by the 2-way RM ANOVA's with the between-subject factor, GROUP (2 levels), and the within-subject factor, SESSION (4 levels), which showed significant GROUP effects for flexion (F[1,17] = 4.8, p ≤ 0.05), extension (F[1,17] = 4.7, p ≤ 0.05), pronation (F[1,17] = 10.6, p < 0.01), supination (F[1,17] = 6.1, p ≤ 0.05), radial deviation (F[1,17] = 6.9, p ≤ 0.05), and ulnar deviation (F[1,17] = 6.9, p ≤ 0.05), SESSION effects for flexion (F[3,51] = 5.9, p < 0.01), extension (F[3,51] = 3.7, p ≤ 0.05), pronation (F[3,51] = 7.7, p < 0.001), supination (F[3,51] = 5.4, p < 0.01), radial deviation (F[3,51] = 6.4, p < 0.01), and ulnar deviation (F[3,51] = 6.4, p < 0.01), and significant GROUP × SESSION interactions for flexion (F[3,51] = 4.1, p ≤ 0.05), extension (F[3,51] = 4.3, p < 0.01), pronation (F[3,51] = 6.3, p < 0.01), supination (F[3,51] = 4.8, p < 0.01), radial deviation (F[3,51] = 6.2, p < 0.01), and ulnar deviation (F[3,51] = 6.2, p < 0.01).
This study was conducted to determine whether a wrist resistance training program, consisting of all 6 directional motion of the wrist (flexion, extension, pronation, supination, radial deviation, and ulnar deviation) over 6 weeks, would significantly improve the isometric torque control and isokinetic maximum torque of the wrist joint.
We hypothesized that with wrist resistance training, both wrist control and strength would improve significantly. Consistent with our hypothesis, the training group showed a significant decrease in isometric torque control error at each 2-week interval (weeks 2, 4, and 6) in all coupled measurements, with the most significant improvement at the 6-week time point when compared with the control group. These data also indicate that the wrist motor control capabilities as quantified in this study significantly increased starting in the first 2 weeks of training and continued to improve throughout the entire 6-week resistance training program. Specifically, for the training group, the flexion-extension and radial-ulnar deviation measurements revealed the most improvement at week 2 when compared with the pronation-supination measurements. However, the pronation-supination measurement for the week 6 evaluation had the highest significance of improvement in the training group when compared with that of the other directions. Conversely, the training group did not show significant improvement in the isokinetic maximum strength after the first 2 weeks of resistance training in any of the 6 directions. However, significant improvement in maximum strength was noted at the week 4 and week 6 evaluation time points for the training group.
For each test and evaluation time point, the control group showed no significant changes in isometric torque control and isokinetic maximum strength. Therefore, with resistance training, wrist motor control ability significantly improves within 2 weeks and wrist strength significantly improves within 4 weeks. These findings are consistent with Szymanski et al. (15) where wrist strength in the dominant hand of high-school baseball players improved in all 6 directions with wrist and forearm training. Although interim time point data were not provided for comparison to our study, their study concluded that a 12-week training program is effective for a healthy, noninjured athlete. Furthermore, that study reported that the ten-repetition maximum (10RM) for ulnar deviation in the wrist training group had the highest percentage increase in strength compared with all other directions for the dominant hand. Other studies have shown significant improvements in isometric force of an intrinsic hand muscle after a 12-week training program in the elderly (7) and increases in 1RM of the wrist after a 4-week strength training program in healthy subjects (3). In addition, our results are consistent with previous studies of finger joints that showed strength increases within 2 weeks (14) and improvements in finger-pinch force control in 6 weeks (8).
Resistance training also induces adaptations that can influence the manner in which trained muscles are recruited by the central nervous system (CNS) during related functional tasks (4). Carroll et al. (4) also suggest that adaptations at a number of sites in the neuromuscular system are likely to contribute to changes in movement execution and control. Barry et al. (2) further describes how neural adaptations are primary means by which power and strength are enhanced. Although this study did not directly capture neuromuscular adaptations of wrist resistance training, our findings demonstrate that neural adaptations likely occurred with wrist resistance training given the rapid improvement in motor control within the first 2 weeks.
Biomechanically, the wrist is one of the weakest torque generators in the serial chain of the arm, and the whole arm actions may be limited if there is a deficiency in the wrist for strength and control. For example, when a tennis player applies spin to a tennis serve, each joint in the arm is required to have sufficient strength and control to apply large magnitudes of torque and force on the ball. By applying spin to the tennis serve, muscles about the wrist need to be able to generate large torques while simultaneously rotating in all 3 degrees of freedom. If the wrist with the smallest muscle mass acts as the weakest link among all joints in the arm, the wrist joint will not be able to generate sufficient torque or control and would fail before the elbow or the shoulder, which can easily result in an unsuccessful tennis serve. Our study suggests that racquetball sports and other sports that require extensive use of the whole arm including the wrist such as weight lifting, baseball, field/ice hockey, golf, gymnastics, tennis, water polo, etc. may benefit from even a few weeks of wrist training.
For low intensity, repetitive motion tasks such as typing or playing the piano, typically flexion-extension and radial-ulnar deviation of the wrist joint are needed. Results from this study suggest that strength training of the wrist joint in these directions could be beneficial to improve the strength and control of the joint in these tasks. The progressive improvement in the wrist control and strength throughout the 6 weeks of training is encouraging for both athletes and nonathletes.
There were some limitations to this study. This study only included right-handed younger adult male subjects and did not account for any age, sex, or hand dominance differences in results. Therefore, the results in this study may not apply to these varying populations, which Barry et al. (2) describe as needing specialized resistance training strategies to maximize their benefits. This study also did not examine the effect of wrist resistance training on the subjects' nondominant hand, which according to Szymanski et al. (15) may yield different results compared with dominant hand training. This study did not control for the training backgrounds or physical activity levels of participants, but the random assignment of the participants to the training group and the control group might have reduced the effects of their training backgrounds and physical activity levels between the groups. In addition, data for the study were collected on a given day and the test-retest reliability and intraclass correlations were not evaluated.
Future studies should examine longer training periods using sport and daily activity specific movements to determine the optimal training time frame for peak performance for athletes' periodization and to determine the effectiveness and retention of improved strength and torque control of wrist strength training. Future studies should also examine the effect of strength and control training that combines various joints, such as elbow and shoulder, in conjunction with wrist training to examine the total effect of rehabilitation and athletic training programs.
The wrist joint involves a small muscle mass controlling the joint, and most likely the wrist would act as a weakest link in racket sports. This study demonstrates that the weakest link can be improved or overcome through wrist strength training. This study suggests that a relatively short training (2 weeks) can improve wrist torque control performance significantly while a month-long training is required to provide significant benefits of training for strength improvements. Based on the findings of this study, coaches and trainers should consider wrist resistance training to improve athletes' muscular strength and control of the wrist muscles.
This study was supported in part by Maryland Industrial Partnerships (MIPS).
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