Effects of Different Types of Mechanical Loading on Trabecular Bone Microarchitecture in Rats

Abstract

Mechanical loading is generally considered to have a positive impact on the skeleton; however, not all types of mechanical loading have the same beneficial effect. Many researchers have investigated which types of mechanical loading are more effective for improving bone mass and strength. Among the various mechanical loads, high-impact loading, such as jumping, appears to be more beneficial for bones than low-impact loadings such as walking, running, or swimming. Therefore, the different forms of mechanical loading exerted by running, swimming, and jumping exercises may have different effects on bone adaptations. However, little is known about the relationships between the types of mechanical loading and their effects on trabecular bone structure. The purpose of this article is to review the recent reports on the effects of treadmill running, jumping, and swimming on the trabecular bone microarchitecture in small animals. The effects of loading on trabecular bone architecture appear to differ among these different exercises, as several reports have shown that jumping increases the trabecular bone mass by thickening the trabeculae, whereas treadmill running and swimming add to the trabecular bone mass by increasing the trabecular number, rather than the thickness. This suggests that different types of exercise promote gains in trabecular bone mass through different architectural patterns in small animals.

Keywords: Cancellous bone; Jumping; Rats; Running; Swimming.

Fig. 1

Effects of hindlimb unloading on trabecular bone microarchitecture in rats. Skeletal unloading was simulated using the tail-suspended rat model. The 5-week-old male Wistar rats completed tail suspension for 2 weeks and were compared with sedentary control rats. After tail suspension for 2 weeks, to assess the 3-dimensional (3D) architecture of trabecular bone, the distal femurs were scanned using micro-computed tomography (micro-CT) and analyzed using 3D image analysis software. The tail-suspended rats exhibited a loss of trabeculae, particularly from the central zone of the femur with a loss of trabecular number, although the remaining trabeculae did not show significant differences in thickness compared to sedentary control rats.

Fig. 2

Effects of treadmill running exercise on trabecular bone microarchitecture in rats. The 11-week-old male Wistar rats performed the treadmill running exercise of 30 m/min for 60 min, 5 times a week for 10 weeks. The 3-dimensional (3D) trabecular bone microarchitecture was evaluated at the distal femoral metaphysis using micro-computed tomography. Treadmill running exercise increased cancellous bone mass in the distal femurs, primarily by increasing trabecular number in growing rats.

Fig. 3

Effects of jumping exercise on trabecular bone microarchitecture after tail suspension in rats. Tail suspension was conducted when rats were 5 weeks old, for 2 weeks. After removal from the tail suspension apparatus, rats jumped upwards 10 times/day, 5 days/week for 5 weeks with a jumping height of 40 cm and were compared with spontaneous recovery rats. The 5 weeks of spontaneous recovery after 2 weeks of skeletal unloading by tail suspension did not sufficiently recover femoral trabecular architecture in young growing rats. On the other hand, subsequent jumping exercise after tail suspension completely restored the deteriorated bone microarchitecture with a subsequent increase in trabecular thickness. [Modified from “Differential effects of jump versus running exercise on trabecular architecture during remobilization after suspension-induced osteopenia in growing rats.”, by Ju YI, et al., 2012, J Appl Physiol (1985), 112, pp. 766–772. Copyright 2012 by the American Physiological Society. Reprinted with permission].

Fig. 4

Effects of swimming exercise on trabecular bone microarchitecture in ovariectomized rats. Ovariectomies and sham operations were conducted on 18-week-old female rats. After postoperative recovery for 2 weeks after the operation, rats performed swimming in a water bath for 60 min/day, 5 days/week, for 12 weeks. Trabecular bone thickness and number in the distal femoral metaphysis were decreased by ovariectomy. On the other hand, these changes were suppressed by swimming exercise. In sham-treated rats, swimming exercise also induced a significant increase in trabecular bone mass, primarily via significant alterations in trabecular number. [Modified from “Effect of swimming exercise on 3-dimensional trabecular bone microarchitecture in ovariectomized rats.”, by Ju YI, et al., 2015, J Appl Physiol (1985), 119, pp. 990–997. Copyright 2015 by the American Physiological Society. Reprinted with permission].