Anatomical and mechanical changes following repetitive eccentric exertions
Introduction
Power hand tool use has been considered a risk factor for upper extremity musculoskeletal disorders (MSD) because of the associated repetitive motions, forceful exertions, vibration, and posture stress (Armstrong et al., 1993; Keyserling et al., 1993; Muggleton et al., 1999; NIOSH, 1997). Rotation of the forearm (supination/pronation) accompanies pistol grip power tool use and several studies have reported relationships between forceful and repetitive movements and development of forearm musculoskeletal disorders (Haahr and Anderson, 2003; MacFarlane et al., 2000; NRC/IOM, 2001).
Eccentric exertions (muscle lengthening contractions) often accompany power screwdriver use when rapidly rising tool-generated forces exceed the tool operator’s capacity to react against them (Armstrong et al., 1999; Oh et al., 1997; Oh and Radwin, 1998). It is well documented that injury occurs during intense eccentric exercise and is often associated with muscle weakness and soreness that develops 24–48 h following that activity (Clarkson et al., 1992; Cleak and Eston, 1992; Friden et al., 1983).
Both mechanical and anatomical changes have been described in skeletal muscle following intense eccentric activity (Armstrong, 1990; Evans et al., 1998; Foley et al., 1999; Friden and Lieber, 1992). One anatomical change observed following eccentric exercise is an increase in muscle water content or edema (Evans et al., 1998; Foley et al., 1999; Shellock and Fleckenstein, 2000).
Mechanical properties of muscle and tendon are functionally important since they counteract the effects of applied loads. Changes in mechanical properties following eccentric exertions may affect a muscle’s ability to react to rapid forceful loading, resulting in increased strain in the muscle.
The human tool operator has been modeled as a single-degree of freedom second order system mechanical system consisting of an element for stiffness, effective mass and viscous damping (Lin et al., 2001, Lin et al., 2003a, Lin et al., 2003b, Lin et al., 2003c). An apparatus delivers an external perturbation to the hand through a handle resulting in oscillations that the person actively attempts to stop. Since the apparatus has known mechanical stiffness, damping and effective mass, any change in the system response when the hand is coupled to the handle is attributed to the person. The mechanical elements are therefore identified from the handle displacement by calculating the frequency changes of the externally loaded system in oscillation.
This apparatus was used for model parameter identification by Lin et al. (2001), and the correlation between measured and predicted frequency was 0.9. Good test–retest reliability was found with controls demonstrating less than a 5% difference in mechanical stiffness 24 h later (Sesto, 2003). The apparatus was employed in the current study to investigate the mechanical changes in muscle following repetitive submaximal eccentric loading.
Magnetic resonance imaging (MRI) is sensitive to acute and chronic variations in muscle water content or edema (Fleckenstein and Shellock, 1991). An increase in the T2 relaxation time correlates with an increase in edema and has been observed following muscle strain and delayed onset muscle soreness (Fleckenstein et al., 1989; Shellock and Fleckenstein, 2000). The T2 relaxation time has also been used to assess edema in muscle following eccentric exertions. Time to peak T2 relaxation time varies, but increases have been observed 48 h after submaximal eccentric activity (Evans et al., 1998).
The level of exertion considered in most previous eccentric exertion studies are rarely observed occupationally or in daily living activities, making it difficult to extend their outcomes to these activities. Recently, several investigations have considered changes following submaximal eccentric muscle activity at exertion levels comparable in occupational settings (Nosaka and Newton, 2002; Sesto, 2003). Maximal eccentric exercise has been compared to submaximal eccentric exercise levels at 50% of isometric maximum voluntary contraction (MVC) and a similar magnitude of initial muscle injury has been reported although secondary damage was less in the submaximal group (Nosaka and Newton, 2002). Reductions in mechanical stiffness (41%) and effective mass (40%) were reported following short duration eccentric activity at 50% of isometric MVC but there were no change in these properties for the isometrically exercised group (Sesto, 2003).
The purpose of the current study was to investigate the effects of short duration repetitive submaximal eccentric exertions at levels comparable to industrial power hand tool use on dynamic mechanical properties (stiffness, effective mass and damping) and on forearm edema. It was hypothesized that changes in anatomical (edema) and mechanical properties of stiffness, damping and effective mass occur in muscles repeatedly exposed to submaximal eccentric exertions at levels typically encountered occupationally. Anatomical measures using magnetic resonance T2 relaxation time and dynamic mechanical properties were measured before, immediately after and daily for three days following submaximal eccentric exertions.
Section snippets
Protocol
Eight healthy right-handed male volunteers were recruited as subjects (mean age = 26.4 years, SD = 5.92 years). The study was limited to males due to strength requirements for the test apparatus in the particular posture used. Informed consent was obtained in accordance with the University of Wisconsin guidelines for the protection of human subjects. A general health status questionnaire was administered to all subjects immediately prior to testing. Subjects were excluded if they reported upper
Data analysis
Repeated measures analysis of variance (ANOVA) was used to investigate the effect of exercise on mechanical and anatomical measures over time. Square root transformation of the mechanical variables was used due to moderate skewness observed in some of the post-exercise mechanical values. Post hoc analysis was done using the Bonferroni multiple pair-wise comparison method.
Strength and symptoms
Forearm supination static strength, measured before, immediately after, and daily for three days following exercise, is shown in Fig. 1. Static forearm supinator MVC was significantly less following exercise (P < 0.01). A 42% loss of static strength was observed in the exercised muscles immediately after exercise (P < 0.01). No other change in static strength occurred following exercise and static strength returned to within 10% by day three (P > 0.05).
Pain levels, (using a 0–10 analog visual scale),
Discussion
The purpose of this study was to investigate the effects of short duration repetitive submaximal eccentric exertions at levels comparable to industrial power hand tool use on dynamic mechanical properties (stiffness, effective mass and damping) and forearm edema. A decrease in mechanical stiffness and effective mass and a subsequent increase in MRI T2 relaxation constant were observed after 30 min of submaximal eccentric activity.
The changes in mechanical stiffness (r = 0.56) and effective mass (r =
Conclusions
Changes in both mechanical and anatomical properties were observed following short duration submaximal eccentric activity. The combination of these findings, a decrease in mechanical properties and a subsequent increase in edema, suggest that short duration submaximal activity may have a negative short-term effect on an extremity that is eccentrically exercised. It is plausible that the physical demands associated with torque producing power tool operation have a detrimental impact on the
Acknowledgments
This research was partially supported by the National Institute for Occupational Safety and Health and the Center for Occupational Health and Safety Engineering, University of Michigan Grant P.O. 30000183598.
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