Effect of age and exercise on the viscoelastic properties of rat tail tendon

Abstract

Tendon mechanical properties are thought to degrade during aging but improve with exercise. A remaining question is whether exercise in aged animals provides sufficient regenerative, systemic stimulus to restore younger mechanical behaviors. Herein we address that question with tail tendons from aged and exercised rats, which would be subject to systemic effects but not direct loading from the exercise regimen. Twenty-four month old rats underwent one of three treadmill exercise training protocols for 12 months: sedentary (walking at 0° incline for 5 min/day), moderate (running at 0° incline for 30 min/day), or high (running at 4° incline for 30 min/day). A group of 9 month old rats were used to provide an adult control, while a group of 3 month old rats provided a young control. Tendons were harvested at sacrifice and mechanically tested. Results show significant age-dependent differences in modulus, ultimate stress, relaxation rate, and percent relaxation. Relaxation rate was strain-dependent, consistent with nonlinear superposition or Schapery models but not with quasilinear viscoelasticity (QLV). Trends in exercise data suggest that with exercise, tendons assume the elastic character of younger rats (lower elastic modulus and ultimate stress).

FIGURE 1

Relaxation as a function of strain. Relaxation data were plotted on a log–log scale and fitted with power law curves (results from a single 3 month old rat tendon without smoothing shown) (a); the slope of which is the relaxation rate (b). In post hoc analysis, relaxation rates decreased significantly from 3 to 9 months and from 9 to 36 months (p<0.001 in both cases) at strains of 1, 2, 3, 4, 5, and 6%. Error bars represent 1 SE.

FIGURE 2

Low strain smoothed relaxation of 3 month old tendons. Results shown were normalized to peak stress for visual comparison. Relaxation rate at lowest, 0.5%, strain was noticeably faster.

FIGURE 3

Representative isochronal results obtained from relaxation tests (a) showing strain-stiffening behavior typical of tendon. A comparison of age groups (b) shows that young (3 month) tendons exhibit different mechanical behavior than adult (9 month) and elderly (36 month) tendons. Maturation causes the most dramatic changes in tendon mechanical properties. Error bars represent 1 SE.