Kangaroos and othre macropods are unique in both their morphology and their locomotor style. At slow speeds, they use a pentapedal gait, were the forelimbs, the hindlimbs, and the tail all contact the ground, while at faster movement speeds, they use their distinctive hopping gait (Dawson and Taylor, 1973; O’Connor et al., 2014). Their uniqueness extends into their energetics of locomotion. as far back as the 19^th century, researchers noticed that the metabolic cost of running in quadrupeds and bipeds, like dogs, horses and humans, increased linearly with speed (Zuntz, 1897; Taylor et al., 1970; Heglund et al., 1982; Taylor et al., 1982). To explain why metabolic rate increased at faster running speeds among quadrupeds and bipeds,Kram and Taylor,1990 refined the ‘cost of generating force’ hypothesis (Taylor et al., 1980). They reasoned that the decrease in contact time with increased speed, reflects an increase in the rate of generating muscle force, and the rate of cross-bridge cycling (Kram and Taylor, 1990). This was supported for a diverse range of running and hopping animals, suggesting that metabolic rate was inversely proportional to contact time (Kram and Taylor, 1990).Yet hopping macropods appear to defy this trend. On treadmills,both red kangaroos (~20 kg) and tammar wallabies (~5 kg) showed little to no increase in the rate of oxygen consumption with increased hopping speed (Dawson and Taylor,1973; Baudinette et al.,1992; Kram and Dawson,1998). The underlying mechanisms explaining how macropods are able to uncouple hopping speed and energy cost is not fully understood (Thornton et al., 2022).

The ability to uncouple speed and energy expenditure in macropods is likely related to the behaviour of their ankle extensor muscle-tendon units, wich store and return elastic strain energy (morgan et al., 1978; Biewener et al., 2004b; McGowan et al., 2005). In tammar wallabies, ankle tendon stress increases with hopping speed, leading to a greater rise in elastic strain energy return than muscle work, which increases the proportion of work done by tendon recoil while muscle work remains near constant (Baudinette and Biewener, 1998; Biewener et al., 1998). Size-related differences in ankle extensor tendon morphology (Bennett and Taylor, 1995; McGowan et al., 2008), and the resultant low strain energy return, may explain why small (<3 kg) hopping macropods and rodents appear not to be afforded the energetic benefits observed in larger macropods (Thompson et al.,1980; Biewener et al., 1981; Biewener et al., 1998) (but see christensen et al., 2022). However,tendon morphology alone is insufficient to explain why large macropods can increase speed without cost,while large quadrupeds with similar tendon morphology cannot (Dawson and Webster,2010). The most obvious difference between macropods and other mammals is their hopping gait, but previously proposed mechanisms to explain how hopping coudl reduce the metabolic cost of generating muscle force, such as near-constant stride frequency (

Kangaroo Hopping: How Posture Impacts Energy and Tendon Stress

Kangaroos are renowned for their efficient hopping locomotion, a feat largely attributed to the elastic energy storage and return within their tendons. However, this efficiency comes with a surprising characteristic: kangaroos operate with relatively low tendon safety factors. This means their tendons experience high stresses during hopping. Recent research suggests kangaroos may actively adjust their posture to manage these stresses and maximize energy return.

Previous studies have shown that both young and adult western grey kangaroos,ranging in mass from 5.8 to 70.5 kg, all hop with gastrocnemius and plantaris tendon safety factors less than two (Snelling et al., 2017). Large tendon stresses may not only be a natural result of their crouched posture and tendon morphology, but also be adaptively selected.Considering this, kangaroos may adjust their posture to increase tendon stress and its associated elastic energy return. If so, there would likely be systematic variation in kangaroo posture with speed and mass which is yet to be fully explored.

In this study, we investigated the hindlimb kinematics and kinetics in kangaroos hopping at various speeds. Specifically, we explored the relationship between changes in posture, elastic muscle activation (EMA), joint work, and tendon stress across a range of hopping speeds and body masses. to do this,we built a musculoskeletal model of a kangaroo based on empirical imaging and dissection data (Figure 1a). We used the musculoskeletal model to calculate ankle EMA throughout the stride.

Key Takeaways

• Kangaroos hop with surprisingly low tendon safety factors, meaning their tendons experience high stress.
• This high stress might potentially be a natural consequence of their anatomy or an adaptation to maximize energy return.
• kangaroos likely adjust their posture to influence tendon stress and energy efficiency.
• A musculoskeletal model was used to analyze the relationship between posture, muscle activation, and tendon stress during hopping.