Adaptations to free-fall impact are different in the shafts and bone ends of rat forelimbs

Abstract

Welch, J. M., C. M. Weaver, and C. H. Turner. Adaptations to free-fall impact are different in the shafts and bone ends of rat forelimbs. Impact exercise can have beneficial effects on the growing skeleton. To understand what changes it promotes in the shafts and ends of weight-bearing bones, we measured the effects of impact from repetitive free falls in growing rats. Fischer 344 female rats, 6.5 wk old, were assigned to one of three groups (n ϭ 10 each). Controls were not dropped, whereas those subjected to impact were dropped from 30 or 60 cm. Rats in both free-fall groups were dropped 10 times per day for 8 wk. Leg bones were mechanically tested, and their cross-sectional area (CSA), cross-sectional moments of inertia, and volumetric bone mineral density (BMD) were measured by peripheral quantitative computed tomography. In the shafts of the forelimbs, but not the hindlimbs, free-fall impact resulted in greater ultimate breaking force, minimum and maximum second moments of area, and CSA but not BMD. In the bone ends of the forelimb and tibial bones, trabecular BMD increased but CSA did not. Landing from 30 and 60 cm produced peak impact forces of 12.0 and 16.7 times the standing forefoot weight for each front leg and of 4.5 and 7.7 times the standing hind foot weight for each hind foot. Overall, free-fall impact affected the forelimbs by increasing trabecular bone density in the bone ends and improving the strength at the shaft as a result of geometric improvements. These results indicate that adaptation to impact may occur by different mechanisms in bone end and shaft regions. exercise; bone strength; ground reaction force; peripheral quantitative computed tomography; rats Values are means Ϯ SE. ND, sufficient cortical bone not available for analysis of image by software. Significant difference from controls: a P Ͻ 0.05; b P Ͻ 0.01; c P Ͻ 0.001. Significant difference between F30 and F60: d P Ͻ 0.05. ADAPTATIONS TO IMPACT IN BONE

The relative importance of genetics and phenotypic plasticity in dictating bone morphology and mechanics in aged mice: Evidence from an artificial selection experiment

“…While this technique does allow for controlled experimental conditions, loading patterns, and very high strains, the loads typically applied (often 10 N or more) are not biologically realistic. Jumpinduced impact loading does provide a viable alternative to artificial loading (e.g., Honda et al, 2001;Umemura et al, 2002;Welch et al, 2004), but wheel running is perhaps the most realistic activity behaviorally. An additional drawback of three-or four-point bending is that, under natural conditions, bones are rarely loaded in pure bending but rather in a combination of axial compression and bending (Lovejoy et al, 1976;Biewener et al, 1983;Alexander, 2003;Lieberman et al, 2003Lieberman et al, , 2004.…”

“…Expanded understanding of the role of mechanotransduction in bone has resulted from comparisons of different genetic strains of mice under artificial loading regimens (e.g., Robling and Turner, 2002;Robling et al, 2003;Kesavan et al, 2005;Li et al, 2005;Kesavan et al, 2006;Lau et al, 2006;Sawakami et al, 2006). Although jump training (i.e., impact loading) has been used to study the effects of naturalistic loading on bone (Honda et al, 2001;Umemura et al, 2002;Welch et al, 2004), relatively rarely have the combined effects of genetic background and exercise been addressed under natural, musculoskeletal loading. One exception is a study by Kodama et al (2000), who compared the response of bone to four weeks of jump training in C57BL/6J and C3H/HeJ mice.…”

The Effect of Level and Downhill Running on Cortical and Trabecular Bone in Growing Rats

“…Due to the difficulties in determining in vivo stresses and strains at the bones under physiological conditions, there is an apparent lack of agreement with regard to the most effective exercise mode. In order to identify such a mode of exercise that reveals optimal bone formation, many in vivo animal models with various natural exercise types, like resistance training [7,8], running [9], jumping [10], free-fall landing [11], and swimming [12,13], have been investigated. Weight-bearing, impact-like exercises, such as running and jumping, are particularly known for their high osteogenic potential [9][10][11].…”

“…In order to identify such a mode of exercise that reveals optimal bone formation, many in vivo animal models with various natural exercise types, like resistance training [7,8], running [9], jumping [10], free-fall landing [11], and swimming [12,13], have been investigated. Weight-bearing, impact-like exercises, such as running and jumping, are particularly known for their high osteogenic potential [9][10][11]. On the other hand, it is generally accepted that muscle forces during physical activity, which actively subject bones to stresses and strains, are a primary determinant in providing the morphological and mechanical properties of bone [14].…”

Force platform for rats measures fore and hind forces concurrently

“…High impact activities such as those delivered by the track and field sport of triple jump are highly osteogenic (Heinonen et al, 2001), while low impact activities, such as walking, produce little change in bone (Palombaro, 2005). Axial impact forces can improve long bone strength (Welch et al, 2004;Heinonen et al, 1996) and are typically measured using a force platform. Resultant force values are typically normalized using standing forces and reported as either a multiple of body weight (Quatman et al, 2006) or as the ratio of forces, N landing/N standing (Bassey and Ramsdale, 1995).…”