Understanding what speeds of locomotion animals choose during interactions with conspecifics (i.e. social or reproductive behaviour), other species (i.e. predation or predator avoidance), and when moving through an often changing environment is paramount to better understanding their biology. Studies of terrestrial animal locomotion, however, are overwhelmingly conducted under laboratory conditions using treadmills . Treadmill studies have facilitated great insight into the biomechanics of locomotion and using this approach the correlation between kinematic parameters like stride length (lstride), stride frequency (fstride) stance (tstance) and swing (tswing) time with speed (U) has been widely reported in the literature across a range of species. For example; treadmill kinematic data exists on a wide range of mammals including polar bears ; horses ; otters ; deer  cats , rodents [7, 8] and monkeys . However, perhaps the most comprehensive research into animal locomotion has been conducted in birds [10-17]. The focus on avian biomechanics is likely due to birds evolutionary link to their theropod ancestors, being bipedal, easy to train, experimentally tractable and exibiting a wide range of adaptations. For example, the Svalbard rock ptarmigan (Lagopus muta hyperborea) has been extensively studied for locomotor adaptations related to energy savings upon gait change , sexual selection , efficient load carriage  and ontogeny .
Studying the locomotion of wild animals in their natural enviroment can be challenging, many animals are elusive and fieldwork can be protracted, expensive and prone to a wide range of factors that cannot be controlled. Trackways are one way to circumvent these issues and may provide insight into the biology of animals in the absence of the animal themselves. To this end tracks have been used to help understand aspects of extinct fauna such as their diversity, the description of new ichnotaxa, and to gain inference into morphological, behavioural, and ecological aspects of the trackmakers [e.g. 22, 23-26]. Trackways have also provided evidence of key evolutionary events such as the transition of tetrapods from water to land [27, see 28] and the first bipedal hominids [e.g. 29, 30, 31]. Outside of the evolutionary insights, perhaps the most common usage of the information gleaned from trackways relates to gait selection and speed [e.g. 23, 32-35]. An established concept for extracting speed (and gait) from trackways uses the Fr [10, 36], defined as:
Where U is speed, g is the acceleration due to gravity and h is the functional hip height. Fr is a dimensionless number that by equalising the centripetal to gravitational force ratio allows the locomotion of terrestrial animals to be compared equally across all sizes. Geometrically similar animals of different sizes will move in a dynamically similar way at any given Fr. In practice, not all animals are geometrically similar, but it was argued that despite this, the ratio of stride length (lstride) to h gave a highly predictable relationship across a broad size range of mammals and birds [10, 11]. By using this empirically derived relationship with the Fr concept, it was further suggested  that the forward U of a terrestrial animal can be calculated from:
Fr may also allow the U at which gait transition occurs (e.g. walking to running) to be estimated . Alexander (10) and Thulborn (37) suggest that gaits will shift from walking to a bouncing gait (e.g. trotting) when lstride/h reaches 2.0, and the transition from trotting to running (or galloping) at an lstride/h of 2.9. Equation 2 is therefore probably applicable to walking animals only . For highspeed gaits where lstride/h is greater than 2.9 the following is advocated as being more appropriate for estimating U [23, 38]:
For trotting U is better estimated by the mean of predictions derived from equations 2 & 3 . Irrespective of the equation used, the reliability of estimates of U may be compromised if there is lack of certainty on h –in particular in extinct animals where h is not available [39-42]– and the use of lstride boundaries that may not be compatible with bipedal gaits . Trackways are therefore restricted in the information that they can provide as much of the information needed for accurate locomotion analysis, such as leg morphology and stride frequency, depends on data from the animal itself. It is worth remembering that anecdotally the vast majority of extant animal movement does not leave evidential tracks. Aside from seeing occasional footprints in the sand or on muddy ground, overwhelmingly animals are not moving over substrates where their feet will leave lasting impressions. An exception to this is locomotion over snow which will, in the vast majority of cases, leave tracks. Regions of the world, like the Arctic are seasonally covered in snow which provides an opportunity to examine trackways and the kinematics of locomotion in context of the real-world influence of variations in substrate. Svalbard rock ptarmigan are endemic to the high Arctic Archipelago of Svalbard meaning they spend approximately half a year locomoting over snow and they are also one of the few species in which a comprehensive laboratory treadmill dataset exists which can be used for comparison. Recently one of the first comparisons of the kinematics of locomotion under field and laboratory treadmill conditions was undertaken in the Svalbard rock ptarmigan . The kinematics of locomotion were conserved for ptarmigan moving in the field and during laboratory treadmills studies but only for walking and aerial running gaits. Important differences were found when the birds were grounded running, with the birds taking faster and shorter steps in the field when compared to the movement on the treadmill. These kinematic differences were attributed to differing substrate when moving over snow compared to a treadmill belt . Our ptarmigan studies also highlighted the importance of understanding the influence substrate can have on locomotion . Studies in extant animals have demonstrated that substrate can influence the neuromuscular control of locomotion to maintain stability [43-45] and can affect the energetic cost [46, 47] and the speed  of locomotion. Furthermore, despite the obvious links between trackways and the ground, substrate is rarely considered when inferences into speed and gait are made from tracks. Not taking any potential effect of substrates into account is surprising as information derived from tracks depends more on the substrate properties than the anatomy of the foot itself . When substrate is considered it is most often examined in terms of the formation of the physical tracks themselves [see 49]. Consideration of substrates and tracks has also been used to demonstrate that in extant species foot morphology can vary with stance and gait [50, 51] and highlighted the interaction of the feet with different sedimentary substrates [52, 53].
The principle objective of our study was to develop a species-specific model to examine gait and speed predictions directly from lstride of trackways of the Svalbard rock ptarmigan. The accuracy of these trackway derived speed predictions and gait transitions was determined by ground truthing data extracted from videos of the birds taken as the tracks were being made. Finally, a comparison between the predictions obtained using our ptarmigan species-specific model and existing Fr based models [10, 23, 38] were made to elucidate the accuracy of each approach. Further, we discuss how reliable information extracted from trackways is for examining the predicted speed of locomotion in both extant and extinct animals.