Inter-stride variability triggers gait transitions in mammals and birds

Abstract

Speed-related gait transitions occur in many animals, but it remains unclear what factors trigger gait changes. While the most widely accepted function of gait transitions is that they reduce locomotor costs, there is no obvious metabolic trigger signalling animals when to switch gaits. An alternative approach suggests that gait transitions serve to reduce locomotor instability. While there is evidence supporting this in humans, similar research has not been conducted in other species. This study explores energetics and stride variability during the walk -run transition in mammals and birds. Across nine species, energy savings do not predict the occurrence of a gait transition. Instead, our findings suggest that animals trigger gait transitions to maintain high locomotor rhythmicity and reduce unstable states. Metabolic efficiency is an important benefit of gait transitions, but the reduction in dynamic instability may be the proximate trigger determining when those transitions occur.

Variation in limb loading magnitude and timing in tetrapods

“…During locomotion on level substrates, muscle forces produced by limb muscles must support body weight and propel the animal forward. To optimize energy expenditure, animals should only apply the amount of force necessary to achieve support, balance and propulsion (O'Connor et al, 2012;Taylor et al, 1980Taylor et al, , 1982 as increased variability in muscle force magnitude wastes considerable amounts of energy (Agiovlasitis et al, 2015;Granatosky et al, 2018a;O'Connor et al, 2012;Verdaasdonk et al, 2006). Hence, minimizing variability in muscle force generation contributes to energetic efficiency during steady-state locomotion.…”

“…Differences in rhythmicity between tachymetabolic and bradymetabolic tetrapods have been identified in limb step cycle duration (Granatosky et al, 2018a;Ross et al, 2013), but these data do not directly refer to variability in the locomotor forces. One important question is whether substrate reaction forces are also less variable in taxa with low variation in step cycle duration.…”

Joint angular excursions during cyclical behaviors differ between tetrapod feeding and locomotor systems

“…By comparison, the functions of the locomotor system are to move the organism from place to place to acquire food and mates while avoiding predation and minimizing competition. Locomoting animals confront substrates that vary in regularity, mechanical properties and complexity, requiring limbed tetrapods to modulate overall speed and limb-joint excursion in order to maintain dynamic stability (Daley et al, 2006;Granatosky et al, 2018). Moreover, unlike chewing sequences, the goal of which is to reduce food item size, progression through a locomotor bout is not inevitably associated with progressive reduction in substrate size and complexity: the goal of locomotion is not to make the substrate flatter and/or more homogeneous.…”

Path tortuosity changes the transport cost paradigm in terrestrial animals

“…Not least, we only consider power and COT metrics as bi-dimensional when they will be affected by multiple properties of the environment that modulate power use, such as slope and surface penetrability [24]. This will be further complicated by gait changes [30][31][32][33][34]. This work nevertheless demonstrates the profound effect that turns have on cost of transport for terrestrial animals using humans as a model, although we expect the principle to be the same for flying and aquatic animals.…”