Tendons from kangaroo rats are exceptionally strong and tough

Abstract

Tendons must be able to withstand the forces generated by muscles and not fail. Accordingly, a previous comparative analysis across species has shown that tendon strength ( i . e ., failure stress) increases for larger species. In addition, the elastic modulus increases proportionally to the strength, demonstrating that the two properties co-vary. However, some species may need specially adapted tendons to support high performance motor activities, such as sprinting and jumping. Our objective was to determine if the tendons of kangaroo rats (k-rat), small bipedal animals that can jump as high as ten times their hip height, are an exception to the linear relationship between elastic modulus and strength. We measured and compared the material properties of tendons from k-rat ankle extensor muscles to those of similarly sized white rats. The elastic moduli of k-rat and rat tendons were not different, but k-rat tendon failure stresses were much larger than the rat values (nearly 2 times larger), as were toughness (over 2.5 times larger) and ultimate strain (over 1.5 times longer). These results support the hypothesis that the tendons from k-rats are specially adapted for high motor performance, and k-rat tendon could be a novel model for improving tissue engineered tendon replacements.

Skeletal Muscle Shape Change in Relation to Varying Force Requirements Across Locomotor Conditions

“…This finding indicates an increase in the role of elastic energy return from downhill walk to uphill gallop and adds to the debate about the role of tendon compliance in small mammal locomotion (Ker et al, 1988;Bullimore and Burn, 2005). The strains we recorded for the rat Achilles tendon in vivo match well with the data from recent materials testing of the same tendon (Javidi et al, 2019). Because MG fascicle strains showed little evidence of net lengthening across all gait-slope conditions, energy stored in the MG tendon during force development is presumably recovered to power limb and body movement, rather than being dissipated by doing work on the muscle (via fascicle stretch; i.e., "elastic backfire"; sensu (Roberts and Azizi, 2011); in contrast to the buffering of rapid stretch and energy absorption observed for turkey landings from drops of different heights and for landings from human jumps (Werkhausen et al, 2017;Werkhausen et al, 2018;Helm et al, 2019;Hollville et al, 2019).…”

Elastic energy storage across speeds during steady-state hopping of desert kangaroo rats (Dipodomys deserti)

“…A secondary motivation of the study was to examine the impact of using commonly assumed generic elastic modulus data for mammalian tendon (Ker, 1981;Bennett et al, 1986;Shadwick, 1990;Pollock and Shadwick, 1994b) versus recent species-specific measurements of material properties for kangaroo rat ankle extensor tendons (Javidi et al, 2019a). Based on these new tendon modulus values, our data support the advantageous role of increased tendon compliance for elastic energy recovery, at least in our desert kangaroo rat model.…”

“…However, in 2019, Javidi et al revealed the ankle extensor tendons of desert kangaroo rats (Dipodomys deserti) to be considerably less stiff than originally thought, with elastic moduli values falling close to half the original 1 GPa assumption. In fact, several studies have shown elastic moduli values across other rodent species to be consistently and considerably lower than 1 GPa (Huang et al, 2003;Lavagnino et al, 2005;Legerlotz et al, 2007;LaCroix et al, 2013;Javidi et al, 2019a). Thus, while kangaroo rats do have relatively thicker tendons than larger bipedal hoppers (Biewener et al, 1981;Biewener and Blickhan, 1988), their tendons are considerably more compliant than had been assumed.…”

“…1986; Shadwick, 1990;Biewener et al, 1998). Based on the results of our recent study, the GAS and PL tendons of D. deserti had elastic modulus values of 330 MPa and 528 MPa, respectively (Javidi et al 2019a). These unique elastic moduli were used to calculate strain energy return independently for the two ankle extensor tendons.…”

“…A challenge of conducting a study on an animal of this size is the constraint of a single tendon buckle on both ankle extensor tendons. Because we were unable to determine exact fractional forces exerted by the GAS and PL tendon, we relied on muscle PCSA weighting to partition the force and subsequently calculate strain per individual tendon based on its specific elastic modulus value reported by Javidi et al (2019a). Though commonly used, this method allows for potential inaccuracy as equal activation of the muscles must be assumed (Maas et al, 2004;Herzog, 2017).…”