Yuting Lin: Hindlimb kinematics, kinetics and muscle dynamics during sit-to-stand and sit-to-walk transitions in emus (Dromaius novaehollandiae)

These behaviours must overcome gravitational constraints to substantially elevate the body's centre of mass (COM) from a flexed initial limb posture, likely resulting in large joint moments and potentially unfavourable effective mechanical advantage (EMA) (Biewener, 1989). In humans, both STS and STW require considerable muscle strength and coordination for task execution and balance control (Dehail et al., 2007; Doorenbosch et al., 1994; Ellis et al., 1984; Riley et al., 1997; Roebroeck et al., 1994; Schultz et al., 1992; Yoshioka et al., 2009). In older adults, these activities even approach the upper limits of muscle capacity (Hortobágyi et al., 2003; Hughes, 1996). However, despite extensive studies on movement patterns and muscle recruitment (Hughes et al., 1994; Perera et al., 2023; Smith et al., 2020), understanding of the control strategies used during these movements remains elusive even for humans (e.g. Actis et al., 2018; Bobbert et al., 2016; Pandy et al., 1995; Shia et al., 2018). Remarkably, research on the biomechanics of STS and STW transitions (henceforth, simply STS and STW) in animals is extremely scarce, with only three studies on dogs as examples (Ellis et al., 2018; Feeney et al., 2007; Triviño et al., 2024).

Birds, especially large, cursorial species, including emus, present a unique opportunity to understand limb structure and locomotor function in both extant and extinct species (e.g. Carrano, 1999). Large, cursorial bird species are known for their remarkable speeds and efficient locomotion as a result of elevated storage and release of elastic energy in tendons, with muscle fibres predicted to act either approximately isometrically or slowly shortening (Badri-Spröwitz et al., 2022; Rankin et al., 2016; Rubenson et al., 2011; Smith and Wilson, 2013). These simulated fibre actions during locomotion are also consistent with studies of in vivo muscle function in other species (e.g. Biewener, 1998; Daley and Biewener, 2003; Fukunaga et al., 2001; Lichtwark et al., 2007; Lichtwark and Wilson, 2006; Roberts et al., 1998). However, when compared with other forms of locomotion, STS and STW impose unique musculoskeletal demands because they require potentially large joint moments at postures with a low strength-to-weight ratio. In particular, the challenges faced by cursorial birds during STS and STW, including substantial fibre length change and force production, are probably compounded by their elongated, flexed limbs and specialised muscular configurations (e.g. allometrically shorter muscle fibres in distal limbs) (Biewener, 2005; Bishop et al., 2021d; Dick and Clemente, 2017; Lamas et al., 2014; Maloiy et al., 1979). Understanding the muscle–tendon dynamics during STS and STW in cursorial birds should provide valuable insights into the biomechanical constraints and allow for investigation into how non-locomotor movements shape locomotor form and function.

This study investigates the movement dynamics, biomechanical constraints and musculotendinous coordination strategies during STS and STW in emus (Dromaius novaehollandiae). Emus serve as an ideal avian model because of their cursorial adaptations, manageability and limb structure, and they offer a compelling basis for comparisons with humans, as shown by the similarities in walking biomechanics (Goetz et al., 2008). Our objectives are twofold: firstly, to quantify the patterns of hindlimb kinematics and kinetics in emu STS and STW behaviours, and secondly, to identify the mechanical constraints and possible musculotendinous coordination strategies used by emus in performing the two tasks. We hypothesised that: (1) both transitions would require high muscle activations in the key hip, knee, and ankle extensor muscles and in other muscles whose primary actions are non-parasagittal (Lamas et al., 2014; and Fig. 1); (2) emu hindlimb muscles would operate near their functional limits, especially for distal muscles (i.e. muscles crossing the ankle and TMP joints) (Lamas et al., 2014; Fig. 1); (3) hindlimb tendons would play important roles in preventing large muscle activations and length changes during the movements; and (4) STW in emus would entail greater demands for muscle capacity than STS.

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