Moderate- and High-Speed Treadmill Running Exercise Have Minimal Impact on Rat Achilles Tendon

Abstract

Exercise influences clinical Achilles tendon health in humans, but animal models of exercise-related Achilles tendon changes are lacking. Moreover, previous investigations of the effects of treadmill running exercise on rat Achilles tendon demonstrate variable outcomes. Our objective was to assess the functional, structural, cellular, and biomechanical impacts of treadmill running exercise on rat Achilles tendon with sensitive in and ex vivo approaches. Three running levels were assessed over the course of 8 weeks: control (cage activity), moderate-speed (treadmill running at 10 m/min, no incline), and high-speed (treadmill running at 20 m/min, 10° incline). We hypothesized that moderate-speed treadmill running would beneficially impact tendon biomechanics through increased tenocyte cellularity, metabolism, and anabolism whereas high-speed treadmill running would cause a tendinopathic phenotype with compromised tendon biomechanics due to pathologic tenocyte differentiation, metabolism, and catabolism. Contrary to our hypothesis, treadmill running exercise at these speeds had a nominal effect on the rat Achilles tendon. Treadmill running modestly influenced tenocyte metabolism and nuclear aspect ratio as well as viscoelastic tendon properties but did not cause a tendinopathic phenotype. These findings highlight the need for improved models of exercise- and loading-related tendon changes that can be leveraged to develop strategies for tendinopathy prevention and treatment.

Keywords: Achilles tendon; biomechanics; exercise; kinematics and kinetics; mechanobiology.

Figure 1

Running impacted body weight but not ankle joint function. Body weight for animals completing 8‐week running protocols was generally reduced in the high‐speed compared to control group (statistically significant at 2, 4, 5, 7, and 8 weeks) and was transiently lower in the high‐ compared to moderate‐speed group at 2 weeks (A). Treadmill running did not affect passive ankle joint function including range of motion (B), dorsiflexion stiffness (C), and plantar flexion stiffness (D). Likewise, running had a minimal impact on hindlimb gait. No running‐based changes were observed in stride length (E) or stride speed (F). Ground reaction (G) and propulsion (H) force were also unaffected by running level. Data are presented as mean ± standard deviation.

Figure 2

High frequency ultrasound did not demonstrate running‐based structural tendon changes. Maximum frequency as a percent of the total FFT spectrum, indicative of the strength of tendon alignment, was unaffected by running group at all timepoints (A). Axis ratio, indicative of the uniformity of tendon alignment, was also unaffected by running group at all timepoints (B). Data are presented as mean ± standard deviation. Representative images with the Achilles tendon outlined are shown for the 8‐week timepoint (C).

Figure 3

Eight weeks of running impacted metabolomic signature of Achilles tendon dialysate. Principal component analysis of untargeted metabolomic profiles demonstrated separation between the control compared to moderate‐ and high‐speed groups along the first principal component (PC 1) as well as separation between the moderate‐ and high‐speed groups along the second principal component (PC 2, A). Targeted metabolomic analysis of organic acids demonstrated decreased citrate concentration in the high‐speed compared to control and moderate‐speed groups as well as decreased succinate concentration in the control compared to moderate‐ and high‐speed groups (B). Concentrations of 3‐hydroxybutyric acid (3‐HBA), α‐ketoglutaric acid (α‐KG), fumarate, lactate, malate, and pyruvate were unaffected by running. Data are presented as mean ± standard deviation.

Figure 4

Running impacted nuclear shape but not cellularity. Cellularity did not vary based on running group (A). Nuclear aspect ratio was increased in the high‐speed group at both 4 and 8 weeks and decreased in the moderate‐speed group at 8 weeks (B). Data are presented as mean ± standard deviation. Representative images for each group are shown (C, blue indicates nuclear staining and white indicates background).

Figure 5

Running had a minimal impact on gene expression. Principal component analysis of gene expression for both 4‐ (A) and 8‐week (B) groups demonstrated poor separation between running groups. Fold change relative to the control group is presented for genes differentially expressed in moderate‐ or high‐speed groups at 8 weeks (C).

Figure 6

Running impacted viscoelastic mechanical properties. Achilles tendon cross‐sectional area (A), stiffness (B), and elastic modulus (C) were not different between running groups at 4 or 8 weeks. Percent relaxation at 9% strain was lower in the moderate‐ and high‐speed groups compared to control at 8 but not 4 weeks (D). Dynamic modulus at 9% strain was lower in the high‐ compared to moderate‐speed group at all frequencies at 4 weeks (E), but this difference was not observed at 8 weeks (F). After 4 weeks, phase shift at 9% strain was increased in the high‐ compared to moderate‐speed group at 0.1, 1, and 5 Hz (G). In 8‐week groups, phase shift at 9% strain was decreased in the moderate‐ (0.1, 1, and 5 Hz) and high‐speed (0.1 and 5 Hz) groups compared to control (H). Data are presented as mean ± standard deviation.

Figure 7

Running had a minimal impact on fatigue mechanical properties. Cycles to failure was unaffected by running group at 4 and 8 weeks (A). Peak cyclic strain was increased in the high‐ compared to moderate‐speed group at 5% and 50% of fatigue life (FL) at 4 weeks whereas no running‐based differences were present at 8 weeks (B). Laxity (C), hysteresis (D), and tangent modulus (E) at 5% and 50% of fatigue life were not significantly different between running groups at both 4 and 8 weeks.