The Effect of Endurance Exercise on the Morphology of Muscle Attachment Sites: An Experimental Study in Sheep (Ovis Aries)

The morphology of muscle attachment sites, or entheses, has long been assumed to directly reflect in vivo muscle activity. However, upon close consideration it becomes obvious that this idea is likely an over-simplification of what appears to be a complex functional relationship. It is clear from previous studies that muscle load does not always induce hypertrophy, and skeletal resorption has been reported at actively loaded attachment sites. This study is the first to document the relationship between known muscle load and the morphology and mineralization rates of the relevant attachment sites.

This study tests a number of hypotheses that are based on the idea that an increase in the number and magnitude of applied muscular forcfs will hypertrophy the muscles’ attachment sites in a number of ways. The size (2D area, 3D area and volume), rugosity (3D area: 2D area ratio and fractal dimension) and mineral apposition rates (MAR) at the sites of attachment are all predicted to increase in response to the increased load. The attachment sites of six limb muscles and one muscle of mastication (control) in mature female sheep were measured and compared in exercised (weighted treadmill running for one hour/day for 90 days) and sedentary control animals.

Results of this study demonstrate no response to the exercise treatment used in this experiment in any measure of enthesis morphology or in MAR. Further tests examined the idea that attachment sites reflect differences in relative lifetime stress levels, but found no significant differences between such entheses. These results indicate that the attachment site parameters measured in this study do not reflect the forces produced by muscle contractions. Potential explanations of these surprising results include the mature age of the animals, inadequate experiment length, and inappropriate load for inducing morphological change. The results of this study reinforce the idea that the relationship between in vivo loads and attachment site morphology is not as straightforward as has long been assumed. Ideas for future studies that may help further elucidate this relationship are discussed

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Year

Entry

2002

Currey JD. Bones: Structure and Mechanics. Princeton, NJ: Princeton University Press; 2002.

1998

Benjamin M, Ralphs JR. Fibrocartilage in tendons and ligaments: an adaptation to compressive load. J Anat. 1998;193(4):481-494.

1978

Uhthoff HK, Jaworski ZF. Bone loss in response to long-term immobilisation. J Bone Joint Surg. August 1978;60B(3):420-429.

1987

Frost HM. The mechanostat: a proposed pathogenic mechanism of osteoporoses and the bone mass effects of mechanical and nonmechanical agents. Bone Miner. April 1987;2(2):73-85.

1992

Rubin CT, Bain SD, McLeod KJ. Suppression of the osteogenic response in the aging skeleton. Calcif Tiss Int. April 1992;50(4):306-313.