The study of animal locomotion is a branch of biology that investigates and quantifies how animals move.
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.
Legged locomotion is the dominant form of terrestrial locomotion, the movement on land. The motion of limbs is quantified by the kinematics of the limb itself (intralimb kinematics) and the coordination between limbs (interlimb kinematics).[1][2]
To quantify the intralimb kinematics and interlimb coordination during walking, the stance and swing phases of the step cycle must be isolated.[2][3][4][5] Stance is associated with the portion of the step where the leg contacts the ground, whereas, swing is where the leg lifts off the ground and moves forward along the body. High-speed videography is used to record the motion of the legs. Pose-estimation methods are then used to track key point(s) on each leg, typically at the joints of the leg.[6][7][8][9] After extracting the positions of each leg throughout a recording, there are several ways of determining the stance and swing phases of the step cycle. One approach involves using peak and trough detection of the leg tip positions in ego-centric coordinates and after the animal has been aligned to a common heading (Fig. 1). Alternatively, swing and stance can be classified as leg tip velocities above and below a chosen threshold, respectively. In this case, leg tip velocities are calculated in allocentric, or world-oriented, coordinates. Once swing and stance phases are determined, the following kinematic and coordination parameters can be calculated.
Static stability: minimum distance from the center of mass (COM) to any edge of the support polygon created by the legs in stance for each moment in time.[13] A walking animal is statically stable if there are enough legs to form the support polygon (i.e. 3 or more) and the COM is within the support polygon. Moreover, static stability is at its maximum when it lies at the center of the support polygon. Steps to calculate static stability are as follows:
Dynamic stability: dictates the degree to which deviations from periodic movement during walking will result in instability.[14]
Quantifying walking often involves assessing the kinematics of individual steps. For more information on methods for acquiring this data, see Methods of Study. The first task is to parse walking data into individual steps. Methods for parsing individual steps from walking data rely heavily on the data collection process. At a high-level, walking data should be periodic with each cycle reflecting the movements of one step, and steps can therefore be parsed at the peaks of the signal. It is often useful to compare or pool step data. One difficulty in this pursuit is the variable length of steps both within and between legs. There are many ways to align steps, the following are a few useful methods.
Fruit flies have six legs and four joints per leg with many joints moving in multiple planes. Thus, there are many kinematic degrees of freedom. Therefore, the continuous variability in coordination patterns across walking speeds and across individual flies can be visualized in a low dimensional embedding,[8] using techniques such as principal components analysis and UMAP.
In addition to stability, the robustness of a walking gait is also thought to be important in determining the gait of a fly at a particular walking speed. Robustness refers to how much offset in the timing of a legs stance can be tolerated before the fly becomes statically unstable. For instance, a robust gait may be particularly important when traversing uneven terrain, as it may cause unexpected disruptions in leg coordination. Using a robust gait would help the fly maintain stability in this case. Analyses suggest that flies may exhibit a compromise between the most stable and most robust gait at a given walking speed.[15]
Many animals alter walking kinematics as they modulate walking speed.[16][17][18] An interlimb kinematic parameter that is commonly speed dependent is gait, the stepping pattern across legs. While some animals alternate between distinct gaits as a function of speed,[19] others move along a continuum of gaits.[20] Similarly, animals commonly modulate intralimb parameters across speed. For example, fruit flies decrease stance duration and increase step length as forward speed increases.[21] Importantly, kinematics are not only modulated across forward velocity, but also rotational and sideslip velocities.[22] In these cases, asymmetry in the modulation between left and right legs is common.
Aerial locomotion is a form of movement used by many organisms and is typically powered by at least one pair of wings. Some organisms, however, have other morphological features that allow them to glide. There are many different flight modes, such as takeoff, hovering, soaring, and landing.[23] Quantifying wing movements during these flight modes will provide insight about the body and wing maneuvers that are required to execute these behaviors.[23] Wing orientation is quantified throughout the flight cycle by three angles that are defined in a coordinate system relative to the base of the wing.[24][25] The magnitude of these three angles are often compared for upstrokes and downstrokes.[24][25][26][27] In addition, kinematic parameters are used to characterize the flight cycle, which consists of an upstroke and a downstroke.[24][26][27][25] Aerodynamics are often considered when quantifying aerial locomotion, as aerodynamic forces (e.g. lift or drag) are able to influence flight performance.[28] Key parameters from these three categories are defined as follows:
Wing orientation is described in the coordinate system centered at the wing hinge.[24] The x-y plane coincides with the stroke plane, the plane parallel to the plane that contains both wing tips and is centered at the wing base.[24] Assuming the wing can modeled by the vector passing through the wing base and wing tip, the following angles describe the orientation of the wing:[24]
Aquatic locomotion is incredibly diverse, ranging from flipper and fin based movement[29] to jet propulsion.[30] Below are some common methods for characterizing swimming:
Body, tail, or fin angle: the curvature of the body or displacement of a fin or flipper.[31]
Tail or fin frequency: the frequency of a fin or tail completing one movement cycle.
Jet propulsion consists of two phases - a refill phase during which an animal fills a cavity with water, and a contraction phase when they squeeze water out of the cavity to push them in the opposite direction. The size of the cavity can be measured in these two phases to compare the amount of water cycled through each propulsion.[30]
A variety of methods and equipment are used to study animal locomotion:
Many of the above methods can be combined to enhance the study of locomotion. For example, studies frequently combine EMG and kinematics to determine motor pattern, the series of electrical and kinematic events that produce a given movement. Optogenetic perturbations are also frequently combined with kinematics to study how locomotor behaviors and tasks are affected by the activity of a certain group of neurons. Observations resulting from optogenetic experiments may provide insight into the neural circuitry that underlies different locomotor behaviors. It is also common for studies to collect high-speed videos of animals on a treadmill. Such a setup may allow for increased accuracy and robustness when determining an animal's poses across time.
Models of animal locomotion are important for gaining new insights and predications on how kinematics arise from the interactions of the nervous, skeletal, and/or muscular systems that would otherwise be difficult to glean from experiments. The following are types of animal locomotion models:
Neuromechanics is a field that combines biomechanics and neuroscience to understand the complex interactions between the physical environment, nervous system, and the muscular and skeletal systems that consequently result in anticipated body movement.[51] Therefore, neuromechanical models aim to simulate movement given the neural commands to specific muscles, and how those muscles are connected to the animal's skeleton.[52][53][54] The key components of neuromechanical models are: