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Passive joint stiffness can influence the risk of injury and the ability to participate in sports and activities of daily living. However, little is known about how passive joint stiffness changes over time with intensive repetitive exercise, particularly when performing unilateral activities using the dominant upper limb.
This study aimed to investigate the difference in passive wrist quasi-stiffness between the dominant and nondominant upper limb of competitive squash players, compare these results with a previous study on young unskilled subjects, and explore the impact of aging on wrist stiffness.
A total of 7 healthy, right-side dominant male competitive squash players were recruited and examined using the Massachusetts Institute of Technology Wrist-Robot. Subjects were aged between 24 and 72 years (mean 43.7, SD 16.57) and had a mean of 20.6 years of squash playing experience (range 10-53 years, SD 13.85). Torque and displacement data were processed and applied to 2 different estimation methods, the fitting ellipse and the multiple regression method, to obtain wrist stiffness magnitude and orientation.
Young squash players (mean 30.75, SD 8.06 years) demonstrated a stiffer dominant wrist, with an average ratio of 1.51, compared with an average ratio of 1.18 in young unskilled subjects. The older squash players (mean 64.67, SD 6.35 years) revealed an average ratio of 0.86 (ie, the nondominant wrist was stiffer than the dominant wrist). There was a statistically significant difference between the magnitude of passive quasi-stiffness between the dominant and nondominant wrist of the young and older squash player groups (
Findings from this pilot study are novel and contribute to our understanding of the likely long-term effect of highly intensive, unilateral sports on wrist quasi-stiffness and the aging process: adults who participate in repetitive sporting exercise may experience greater joint quasi-stiffness when they are younger than 45 years and more flexibility when they are older than 60 years.
Joint stiffness is a biomechanical feature of human anatomy that is both essential and potentially detrimental to participation in sports, leisure activities such as music, and activities of daily living. Reduced joint stiffness may increase the risk of injury because of poor stabilization of joints and an inability to maintain joint postures [
The term joint stiffness has various definitions depending on the discipline and nature of the research. In physical education, sports medicine, and allied health disciplines, joint stiffness is more commonly referred to as the flexibility, or ROM, of a joint or group of joints [
Extensive research has been conducted on passive joint quasi-stiffness of the lower limb and the impact on gait [
Within the literature on wrist joint stiffness, there is large variability in the magnitude and orientation of passive joint quasi-stiffness, likely because of differences in study methodology (alignment and orientation of the starting position, ROM, number of degrees of freedom assessed, and the method of data analysis) as well as subject characteristics (including sex, hand dominance, participation in sporting and leisure activities, and age). Previous studies have investigated the impact of study methodology [
Squash players were chosen as the target population for this study because of the highly repetitive, intensive, unilateral nature of the sport and for the vital role the wrist and forearm play in generating high racquet head speeds during the forehand and backhand stroke actions [
Volunteer competitive squash players were recruited with a study flyer posted on a local Web-based squash association newsletter. The inclusion criteria were as follows: (1) male, (2) aged 18 years or older, (3) no prior wrist surgery or injury, (4) English-speaking, and (5) a minimum of 5 years of playing experience. Only male subjects were recruited to reduce the risk of data variability, as previous work had demonstrated a difference in the magnitude and direction of wrist quasi-stiffness between the male and female sex [
Subject demographics and age group allocation.
Group allocation | Young players | Older players | ||||||
Subject 3 | Subject 5 | Subject 6 | Subject 7 | Subject 1 | Subject 2 | Subject 4 | ||
Age (years)a | 31 | 42 | 26 | 24 | 61 | 72 | 61 | |
Years of playing experience | 20 | 25 | 12 | 10 | 37 | 53 | 20 | |
Squash sessions/week (n) | 5 | 2 | 4 | 4 | 3 | 3 | 4 | |
US squash rating criteria by skill level | 5.5 | 5 | 4.5 | 5.12 | 4.47 | 3 | 3.5 | |
Hours post upper limb exercise | 24 | >48 | >24 | >48 | 24 | 24 | >48 |
aSubject demographics were collected by interview before commencing the experiment.
A total of 7 healthy male competitive squash players aged between 24 and 72 years (mean 43.7, SD 16.57) volunteered to participate (
Data from 7 right-hand dominant males (mean age 28.57, SD 12.11, range 19-55 years) reported in the study by Durand et al [
Wrist quasi-stiffness was evaluated using a 3 degree of freedom Wrist Robot (
Massachusetts Institute of Technology Wrist-Robot and experimental forearm, wrist, and hand position (excluding finger strap).
Subject demographics were collected in the form of an interview (
Torque and displacement data were processed using a customized program in MATLAB 2016b [
Size: stiffness magnitude (ellipse surface [Nm/ rad]2)
Orientation: stiffness orientation (angle in degrees between radial deviation (RD) direction and ellipse major axis direction toward RD, counterclockwise angles are considered positive)
Shape: the ratio of the major axis of the stiffness ellipse to the minor axis
Equilibrium position: the offset of the ellipse center corresponding to the FE and RUD offset angles.
The fitting ellipse method (
The MR method (
A 1-sample Kolmogorov-Smirnov test was used to confirm the normal distribution of the data. A 1-way analysis of variance was calculated to determine the statistical difference between the wrist quasi-stiffness of the left and the right arm in the young and older squash player groups and to compare the magnitude of the wrist quasi-stiffness and orientation of the 4 young squash players with the results of the 7 young right-handed dominant, unskilled male subjects [
(a) The fitting ellipse method, where variable τ represents torque, ‘j’ corresponds to 1 of the 12 directions considered through the FE-RUD plane, || means parallel to the wrist movement direction, and ⊥ means perpendicular to this direction. Rj is the rotation matrix for the jth direction, θ represents angular displacement, and ‘K’ represents quasi-stiffness [
For the older adult group, the mean passive wrist stiffness magnitude of the left upper limb was 11.91 Nm/rad2 (SD 1.26) and 9.99 Nm/rad2 (SD 2.12) for the right upper limb. The ratio between right (dominant playing upper limb) and the left arm for the older adult group was 0.86. The older adult demonstrated a higher mean stiffness in the left (nondominant, nonplaying) upper limb (
Mean passive quasi-stiffness ellipses (solid lines) and standard deviations (dotted lines) for the left and right wrist of the older adult squash player group (n=3).
Mean passive quasi-stiffness ellipses (solid lines) and standard deviations (dotted lines) for the left and right wrist of the young squash player group (n=4).
Results from the comparison of the 4 young squash payers (ratio right over left stiffness magnitude of 1.51) with the 7 young unskilled males (ratio right over left stiffness magnitude of 1.18) showed a stiffer dominant wrist for the squash players. There was no statistically significant difference between the young unskilled and the young squash player groups (
The correlation between the passive wrist stiffness of all 7 players and subject characteristics revealed an interesting difference between the dominant and nondominant upper limb. There was a strong positive correlation between passive stiffness of the nondominant left wrist and age (
For the young unskilled group, the orientation of highest quasi-stiffness followed a “dart throwing” pattern, with a mean angle of 14.42° (SD 4.45) for the left wrist and a mean angle of –13.96° (SD 7.06) for the right [
The magnitude of passive wrist quasi-stiffness of the young players confirmed the study hypotheses that competitive squash players demonstrate greater quasi-stiffness in the dominant playing upper limb compared with the nondominant upper limb. Although the ratio of dominant over nondominant wrist quasi-stiffness for young squash players compared with unskilled young subjects did not reach statistical significance, there was a trend favoring the squash player group. Studies investigating other sporting activities have shown that muscle strength, orientation of joint stiffness, and pattern of ROM between the playing and nonplaying upper limb do not consistently increase or decrease on the playing side. Klinge et al [
There have been few studies, to the authors’ knowledge, investigating the effect of exercise or sporting activities on passive wrist stiffness. Leger and Milner [
Although age-related increases in passive joint stiffness may seem clinically obvious, how exercise impacts passive joint stiffness in older adults has not been widely studied and results are inconsistent. Inconclusive results are likely because of variances in the joints evaluated, the study methodology, and the definition of flexibility and stiffness measurements. In research on flexibility, active older tennis players were shown to maintain shoulder flexibility on their playing side [
The effect that squash play appears to have on the passive stiffness of the wrist with aging may partly be because of changes in the physiological properties of muscle fibers. The wrist flexor and extensor muscles are composed of approximately 50% type I and 50% type II muscle fibers in young adults aged 17 to 30 years [
Type I muscle fibers have been reported to have greater passive stiffness, likely because of increased collagen concentration and cross-linking of collagen compared with type II muscle fibers [
One must take this study with the appropriate caveats; several features of our study design limit the impact and generalizability of the results. Notably, the small sample size of this pilot trial significantly limits the power of the results. Other papers within this field of study have also reported small sample sizes ranging from 6 to 15 subjects [
This study provides a valuable initial insight into the possible effect that highly intensive, repetitive, unilateral sports may have over time on wrist quasi-stiffness and reducing the impact of the aging process. Further studies are required to investigate this relationship with a larger sample size and age group analysis. This field would also benefit from the study of passive wrist stiffness in young and older subjects who participate in intensive bilateral sporting activities such as upper limb weight or grip training to determine the magnitude and effect of changes in muscle fiber type across the lifespan. Our findings confirm that the evaluation of passive joint stiffness has relevance and far-reaching value in many fields, from sporting activities to the rehabilitation of the older adults, following surgical interventions, or those with neurological impairments.
flexion-extension
multiple regression
radial deviation
range of motion
radial-ulnar deviation
ulnar deviation
United States Squash Rating Algorithm
The authors would like to thank the study volunteers for their time and participation. The authors would also like to acknowledge Joseph Davidson, Rogerio Goncalves, and the members of The 77 Lab at the Mechanical Engineering Department, Massachusetts Institute of Technology (MIT) for their technical support during the planning and data collection phases of this study.
HIK is a co-inventor of several MIT-held patents for robotic therapy. He was the founder of Interactive Motion Technologies and Chairman of the Board (1998-2016). He successfully sold Interactive Motion Technologies to Bionik Inc. He founded 4Motion Robotics, Inc in 2017. TH is now employed by Bionik Inc, the company that purchased Interactive Motion Technologies.