By Kirstie Saltsman, Ph.D. | December 20, 2016
Calf muscle fibers (outlined in red) of male rats are on average larger than in females likely placing increased  strain on the Achilles tendon and rendering it more vulnerable to injury.
Calf muscle fibers (outlined in red) of male rats are on average larger than in females, likely placing increased strain on the Achilles tendon and rendering it more vulnerable to injury.
Credit Louis J. Soslowsky, Ph.D., University of Pennsylvania.

A new study in rats, funded in part by the NIH’s National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), has revealed differences between males and females in the physical properties of the Achilles tendon. The results, which appeared in the Annals of Biomedical Engineering, may explain why the incidence of ruptures is about five times higher in men than women. A detailed analysis of the characteristics of the Achilles tendon may lead to insights on how to avoid debilitating tears and possibly to new clinical approaches for repairing them.

The Achilles tendon, the largest tendon in the human body, lies at the back of the ankle and links the calf muscles to the heel bone. When the muscles contract, the tendon tightens and pulls on the heel, enabling us to point our toes and stand on tiptoe. The Achilles tendon is also used for all types of motion, such as walking, running and jumping. It can withstand large stresses, but a sudden strain can cause a tear or rupture. Many injuries occur in middle age while participating in recreational sports like tennis, volleyball or basketball. Achilles tendon ruptures are serious injuries that typically require surgery and many weeks of recovery.

The incidence of Achilles tendon rupture has been rising over the past few decades in both men and women, with about 84 percent of cases occurring in men. Some studies have suggested that female hormones like estrogen reduce the risk of rupture in women, but the hormones’ precise role has been unclear. In addition, some scientists have argued that the typically larger, stronger calf muscles in men would exert greater forces on the tendon and increase the risk of rupture.

To gain a better understanding of the factors influencing sex-specific differences in vulnerability to damage, a team of investigators led by Louis J. Soslowsky, Ph.D., of the University of Pennsylvania, compared the material properties of the Achilles tendon/muscle unit in male and female rats. To specifically test for effects of female sex hormones, they also studied female rats that had been made estrogen-deficient by having their ovaries removed.

Their measurements showed that while Achilles tendons from males are larger, those from females are stronger and remain more elastic during movement. They also noted that muscle fibers were larger in male rats compared to females, as expected. These findings suggest that inferior properties of the tendon coupled with greater muscle size could explain men’s increased susceptibility to Achilles tendon ruptures.

Female sex hormone deficiency did not affect tendon strength, however it did lead to an expansion of calf muscle fibers compared to normal females. The researchers concluded that both hormonal and other sex-specific factors influence the structure of the Achilles-tendon/muscle unit. Additional research is needed to uncover the molecular mechanisms that underlie these effects.

Achilles tendon ruptures are one of the few musculoskeletal injuries that are more common in men, with anterior cruciate ligament (ACL) ruptures, for example, being more common in women. This study contributes to the growing body of data suggesting that a tendon’s local environment influences its biology, and highlights the importance of evaluating each tendon individually for factors that affect its risk of tears or ruptures.

"Rupture of the Achilles tendon is a devastating injury that results in significant pain, disability and healthcare costs," said Dr. Soslowsky. "A better understanding of the tissue-level properties of the tendon may enable us to develop new treatment strategies that would speed patients’ return to normal activities and improve long-term outcomes."

This work was supported by the NIH’s NIAMS (grants R01-AR064216S1, P30-AR050950 and T32-AR007132) and National Center for Advancing Translational Sciences (TL1-TR000138). The National Science Foundation also contributed support.

Males have Inferior Achilles Tendon Material Properties Compared to Females in a Rodent Model. Pardes AM, Freedman BR, Fryhofer GW, Salka NS, Bhatt PR, Soslowsky LJ.Ann Biomed Eng. 2016 Oct;44(10):2901-10. doi: 10.1007/s10439-016-1635-1. PMID: 27150673

The mission of the NIAMS, a part of the U.S. Department of Health and Human Services' National Institutes of Health, is to support research into the causes, treatment and prevention of arthritis and musculoskeletal and skin diseases; the training of basic and clinical scientists to carry out this research; and the dissemination of information on research progress in these diseases. For more information about the NIAMS, call the information clearinghouse at (301) 495-4484 or (877) 22-NIAMS (free call) or visit the NIAMS website at https://www.niams.nih.gov.

Last Reviewed: 12/20/2016