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. 2015;53(3):206-21.
doi: 10.2486/indhealth.2014-0159. Epub 2015 Feb 9.

Ergonomic Task Reduction Prevents Bone Osteopenia in a Rat Model of Upper Extremity Overuse

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Free PMC article

Ergonomic Task Reduction Prevents Bone Osteopenia in a Rat Model of Upper Extremity Overuse

Mary F Barbe et al. Ind Health. .
Free PMC article

Abstract

We evaluated the effectiveness of ergonomic workload reduction of switching rats from a high repetition high force (HRHF) lever pulling task to a reduced force and reach rate task for preventing task-induced osteopenic changes in distal forelimb bones. Distal radius and ulna trabecular structure was examined in young adult rats performing one of three handle-pulling tasks for 12 wk: (1) HRHF, (2) low repetition low force (LRLF); or (3) HRHF for 4 wk and than LRLF thereafter (HRHF-to-LRLF). Results were compared to age-matched controls rats. Distal forelimb bones of 12-wk HRHF rats showed increased trabecular resorption and decreased volume, as control rats. HRHF-to-LRLF rats had similar trabecular bone quality as control rats; and decreased bone resorption (decreased trabecular bone volume and serum CTX1), increased bone formation (increased mineral apposition, bone formation rate, and serum osteocalcin), and decreased osteoclasts and inflammatory cytokines, than HRHF rats. Thus, an ergonomic intervention of HRHF-to-LRLF prevented loss of trabecular bone volume occurring with prolonged performance of a repetitive upper extremity task. These findings support the idea of reduced workload as an effective approach to management of work-related musculoskeletal disorders, and begin to define reach rate and load level boundaries for such interventions.

Figures

Fig. 1.
Fig. 1.
Experimental design showing onset of food restriction, training period, duration of repetitive task performance, and number of animals utilized. FR: food restricted; training period: rats were trained to perform a high force or a low force task (Trained to high force (HF) or Low force (LF)). After training, rats performed a high repetition high force task (HRHF) or a low repetition low force (LRLF) task for 6 or 12 wk. HRHF-to-LRLF rats performed the HRHF task for 4 wk before switching to a LRLF task for 2 or 8 wk. Ends of arrows indicate points of euthanasia. (B) Rats were weighed weekly from the naïve time point to euthanasia. n.s.: not significant. The switch from HRHF to LRLF occurred at beginning of week 5, as indicated. (C) The biomechanical exposure per week was estimated by multiplying mean reach rate per minute, duration of task performance per week, mean voluntary grasp force in grams, and mean voluntary grasp time in seconds that rats held the force lever. &&: p<0.01, compared to age-matched HRHF rats. Arrow indicates onset of intervention in HRHF-to-LRLF rats. ANOVA and posthoc findings are shown in individual panels; and mean + SEM shown here and hereafter.
Fig. 2.
Fig. 2.
MicroCT analysis of trabeculae of the distal radial metaphysis. Results for trabecular bone volume (BV/TV), trabecular number (Tb.N), trabecular separation (Tb.Sp), trabecular thickness (Tb.Th), degree of anisotropy (DA) in which 0=isotropic 1=anisotropic, and structural model index (SMI) in which 1=plates, 2=rods, 3=cylinders, are shown. * and **: p<0.05 and p<0.01, compared to age-matched FRC rats; & and &&: p<0.05 and p<0.01, compared to HRHF rats.
Fig. 3.
Fig. 3.
MicroCT analysis of trabeculae of the distal ulnar metaphysial regions metaphysis. Results for trabecular bone volume (BV/TV), trabecular number (Tb.N), trabecular separation (Tb.Sp), trabecular thickness (Tb.Th), degree of anisotropy (DA), and structural model index (SMI) are shown. * and **: p<0.05 and p<0.01, compared to age-matched FRC rats; & and &&: p<0.05 and p<0.01, compared to HRHF rats.
Fig. 4.
Fig. 4.
Representative images of distal radius and ulna, and dynamic bone histomorphometry. (A) Representative 3D models of transaxial microCT slices through the metaphyseal region of the left ulnar and radial bones of FRC, and 12-week HRHF, HRHF-to-LRLF or LRLF rats. These transaxial reconstructions are located from 1.0 to 2.0 mm proximal to the respective growth plates, and are viewed from the bottom looking towards the growth plate. (B) Trabecular mineral apposition rate in the radius (MAR). (C) Trabecular bone formation rate (BRF), normalized to bone surface (BS), in the distal radial metaphyseal trabeculae. (D) Representative microscope images showing calcein double labeling in radial trabeculae of each group. *: p<0.05, compared to age-matched FRC rats.
Fig. 5.
Fig. 5.
Osteoblast and osteoclast histomorphometric counts, serum levels of biomarkers of bone turnover, and radial bone levels of IL-1alpha. (A&B) Cellular density of osteoblasts (N.Ob.) and osteoclasts (N.Oc.), normalized to bone surface (BS), in distal radial metaphyseal trabeculae. (C&D) Serum levels of osteocalcin and CTX1, assayed using ELISA. (E&F) IL-1alpha in distal (metaphyseal) and diaphyseal radius and ulna, respectively, assayed using ELISA. * and **: p<0.05 and p<0.01, compared to age-matched FRC rats; & and &&: p<0.05 and p<0.01, compared to age-matched HRHF rats.

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