Partial reductions in mechanical loading yield proportional changes in bone density, bone architecture, and muscle mass

Abstract

Although the musculoskeletal system is known to be sensitive to changes in its mechanical environment, the relationship between functional adaptation and below-normal mechanical stimuli is not well defined. We investigated bone and muscle adaptation to a range of reduced loading using the partial weight suspension (PWS) system, in which a two-point harness is used to offload a tunable amount of body weight while maintaining quadrupedal locomotion. Skeletally mature female C57Bl/6 mice were exposed to partial weight bearing at 20%, 40%, 70%, or 100% of body weight for 21 days. A hindlimb unloaded (HLU) group was included for comparison in addition to age-matched controls in normal housing. Gait kinematics was measured across the full range of weight bearing, and some minor alterations in gait from PWS were identified. With PWS, bone and muscle changes were generally proportional to the degree of unloading. Specifically, total body and hindlimb bone mineral density, calf muscle mass, trabecular bone volume of the distal femur, and cortical area of the femur midshaft were all linearly related to the degree of unloading. Even a load reduction to 70% of normal weight bearing was associated with significant bone deterioration and muscle atrophy. Weight bearing at 20% did not lead to better bone outcomes than HLU despite less muscle atrophy and presumably greater mechanical stimulus, requiring further investigation. These data confirm that the PWS model is highly effective in applying controllable, reduced, long-term loading that produces predictable, discrete adaptive changes in muscle and bone of the hindlimb.

Fig. 1

(A) Partial weight-bearing harness maintains the mouse in a quadrupedal orientation while partially offloading body weight using a spring connected to a support rod via a wheel. Reproduced from J Appl Physiol. 2010;109:350–7. (B) Hindlimb unloading harness suspends the mouse by the tail to prevent the hind paws from touching the ground.

Fig. 2

Effect of unloading intervention on body mass. Each data point represents the mean of ±2 days ±SEM bars. Asterisk denotes significant difference (p <0.005) from baseline at day 21.

Fig. 3

(A) Soleus and (B) gastrocnemius wet mass normalized by body mass (mean ±SEM). ^* =Different from CON (p <0.05), # =HLU different from PWB20, PWB40, PWB70, and PWB100 (p <0.05), brackets =pairwise differences between PWB groups (ANOVA with Scheffé post hoc, p <0.05).

Fig. 4

Relationship between weight bearing and the changes in bone and muscle outcomes after 21 days of partial weight bearing at 20%, 40%, 70%, or 100% of body weight. (A) Soleus and gastrocnemius muscle mass normalized by body mass. (B) Bone mineral density (BMD) of the total body and hindlimb. (C) Trabecular bone volume fraction (BV/TV) and thickness of the distal femur. (D) Cortical area and thickness of the femur midshaft. Equations of the best fit line are displayed for linear regressions that were significant and residuals were normally distributed with equal variance over the range of expected values.

Fig. 5

Percent change in (A) total body and (B) hindlimb bone mineral density from baseline to day 21 (mean ± SEM). ^* =Different from CON (p <0.05), § =HLU different from PWB20, PWB100 (p <0.05), # =HLU different from PWB70, PWB100 (p <0.05), brackets =pairwise differences between PWB groups (ANOVA with Scheffé post hoc, p <0.05).

Fig. 6

Femur microarchitectural measurements of (A) trabecular bone volume fraction at the distal metaphysis and (B) cortical thickness of the mid-diaphysis (mean ± SEM). ^* =Different from CON (p <0.05), # =HLU different from PWB100 (p <0.05), brackets =pairwise differences between PWB groups (ANOVA with Scheffé post hoc, p <0.05).

Fig. 7

Mean ± SEM concentrations of fecal corticosterone metabolites from (A) days 1 to 3 and (B) days 5 to 10. ^* =Different from CON1 (p <0.05); brackets =pairwise differences between PWB groups (p <0.05); § =HLU different from PWB20, PWB40, PWB70, PWB100; # =HLU different from PWB70.