The study of animal locomotion is a branch of biology that investigates and quantifies how animals move.
Kinematics
Kinematics is the study of how objects move, whether they are mechanical or living. In animal locomotion, kinematics is used to describe the motion of the body and limbs of an animal. The goal is ultimately to understand how the movement of individual limbs relates to the overall movement of an animal within its environment. Below highlights the key kinematic parameters used to quantify body and limb movement for different modes of animal locomotion.
Quantifying locomotion
Walking
Legged locomotion is a dominant form of terrestrial locomotion, the movement on land. The motion of limbs is quantified by intralimb and interlimb kinematic parameters. Intralimb kinematic parameters capture movement aspects of an individual limb, whereas, interlimb kinematic parameters characterize the coordination across limbs. Interlimb kinematic parameters are also referred to as gait parameters. The following are key intralimb and interlimb kinematic parameters of walking:
Characterizing swing and stance transitions
The calculation of the above intra- and interlimb kinematics relies on the classification of when the legs of an animal touches and leaves the ground. Stance onset is defined as when a leg first contacts the ground, whereas, swing onset occurs at the time when the leg leaves the ground. Typically, the transition between swing and stance, and vice versa, of a leg is determined by first recording the leg's motion with high-speed videography (see the description of high-speed videography below for more details). From the video recordings of the leg, a marker on the leg (usually placed at the distal tip of the leg) is then tracked manually or in an automated fashion to obtain the position signal of the leg's movement. The position signal associated with each leg is then normalized to that associated with a marker on the body; transforming the leg position signal into body-centered coordinates. This normalization is necessary to isolate the movement of the leg relative to that of the body. For tracking the leg movement of unconstrained/untethered animals, it is important to perform a coordinate transform so that the movement of the animal is aligned along one axis (e.g. a common heading angle). This is also a necessary step because it isolates the oscillatory anterior-posterior movement of the leg along a single axis, rather than being obscured across multiple axes. Once the tracked and normalized leg position is obtained, one way to determine the onsets of stance and swing are to find the peaks and troughs of the leg position signal. The peaks of the leg position signal are the stance onsets, which are also the anterior extreme positions of the leg for each step. On the other hand, the troughs of the leg position signal are the swing onsets as well as the posterior extreme positions of the leg for each step. Therefore, the transitions between stance and swing, and vice versa, are determined by finding the peaks and the troughs of the normalized leg position signal. Alternatively, these transitions can be found by using leg velocity, the derivative of the leg position signal. When using this approach, a threshold is chosen to categorize leg movement into stance and swing given the instantaneous velocity of the leg. This stance and swing classification approach is useful for instances when the interaction between the leg and substrate is unclear (i.e. difficult to tell when the leg truly contacts the substrate). Regardless of the approach, accurately classifying swing and stance is crucial for the above calculations of intra- and interlimb kinematic parameters.