Animal Behavior Analysis

High resolution gait analysis system analysis was the most commonly used functional assessment in the studies included in our review. High resolution gait analysis system describes the kinematic and kinetic changes observed in walking gait. Stride length, step width, GRF/contact intensity, stance, paw print area, and speed were the most commonly reported parameters. Each parameter represented different aspects of gait, but only the stride length and GRF/contact intensity were reliably and specifically observed to reflect the changes in shoulder function after RC tears or repair.

The forward stride of the forelimb in a rat could be analogous to shoulder abduction in humans when the scapular plane is taken as a reference . Stride length has been defined as the distance between paw strikes , which represents the forelimb's ability for active forward flexion. These results indicated that the RC tendon injury reduced the active forward flexion, and the extent of injury correlated to the extent of functional loss. These changes also were similar to the clinical observations that decreases in active ROM are more commonly seen in patients with massive RC tear than in patients with nonmassive tears. This observation indicated that the stride length could resemble the human clinical condition by demonstrating active ROM loss in RC injury models. On the other hand, the step width (distance between the front paws) usually was found not affected in the cases where the stride length was drastically reduced. It was suggested that the stride width was impaired because the normal forelimb shifted medially to support more body weight, instead of being caused by the limited ROM of the injured forelimb . Therefore, it is reasonable to postulate that step width may not be a reliable parameter to estimate the degree of function of an injured shoulder.

Because strength is another important aspect of shoulder function, researchers have developed several methods to indirectly measure the shoulder strength. In rats, the body weight is loaded on the shoulder joints and transmitted to the ground during walking, which helped the GRF to reveal the loading capacity of the shoulder. Similarly, the light intensity that is generated in a fully automated gait analysis system could reflect the loading capacity of the shoulder because the light intensity correlates well with GRF. Investigators have used the light intensity of a rat's footprint to assess its shoulder's loading capacity.

Three studies measured the GRF/light intensity, and they demonstrated a notable decrease in the shoulder loading capacity in the RC tear/repair models. A substantial decline in GRF values was reported with no change in the temporal and spatial gait results in the model with massive RC tears and delayed repair . Based on a comprehensive comparison between GRF and the temporal and spatial parameters, the GRF was acknowledged to be the most sensitive parameter to reveal impairment of shoulder function. Moreover, the decrease in loading capacity correlates with human clinical outcomes that indicated patients lost 60–70% of their shoulder strength after RC tears. Thus, GRF and light intensity are reliable and representative parameters that can be used to reveal the shoulder loading capacity in RC injury models.

Pain is another crucial factor that modifies the functional performance, and clinically, pain is reported by patients. Although pain cannot be assessed directly in animal studies, it can be reflected in changes in walking gait. The influence of pain on the shoulder function was limited to the first four days postoperatively.

Objective gait analysis can provide clinicians with important information for therapeutic decision-making. It can be used not only to quantify and differentiate gait for diagnosis, but also to monitor rehabilitation and treatment efficacy. In addition, objectively collected data can provide important information for breeding decisions.

Animal treadmill gait analysis systems currently used in veterinary medicine to collect kinematic and kinetic data are either camera-based systems, force plate systems, accelerometer-based systems, surface electromyography measurement systems or instrumented treadmills. Camera-based systems that track optical, active, or passive markers attached to the dog's body are commonly used in research facilities but rarely in veterinary clinics because they are very expensive and require a dedicated space to set up the system. Ground reaction force measurement systems, such as force plates, have been shown to be accurate indicators of irregular gait patterns or lameness, especially when combined with camera-based motion tracking devices, but require a long acclimatization period and training of the dog to the walking surface.

Several studies indicate that inertial measurement unit systems provide valuable information for the analysis of the canine gait . In a study the peak vertical forces (PVF) measured with a force platform were compared with measurements from a triaxial accelerometer placed dorsally over the thoracic or lumbar region. There was positive and significant agreement between the PVF of the accelerometer and the force platform for the forelimbs and positive and low agreement for the hindlimbs. described the use and reliability of accelerometers in gait assessment of healthy dogs and dogs with a diagnosis of muscular dystrophy. It reported that kinematics recorded with an inertial measurement unit (IMU) in the sagittal plane in dogs, showed good correlation with optically recorded kinematics, so the use of IMU sensors could provide an alternative to optical kinematic gait analysis while allowing data collection outside the laboratory. It presented an IMU sensor-based gait measurement system for dogs that demonstrated good sensitivity and repeatability with a precision likely sufficient to detect clinically relevant gait abnormalities in dogs. They concluded that, with further development, the system could have a wide range of applications in both research and clinical practice.

During everyday life, natural head movements in mammals cover a large range of frequencies and velocities. To avoid blurry vision, image displacements on the retina are minimized by compensatory eye movements. These eye-in-space movements are referred to as gaze stabilization eye movements, which result from the transformation of sensory signals into extraocular motor commands. Vertebrates possess two gaze stabilizing reflexes -the optokinetic reflex (OKR) and the vestibulo-ocular reflex (VOR)—that act synergistically to compensate for environmental and self-movements. The OKR responses rely on direction-selective retinal ganglion cells that are efficient for relatively slow motions of the visual scene (± 3º/s in mice). Consequently, the OKR gain is inversely proportional to the velocity of the visual stimulus.

On the other hand, the vestibular acceleration-sensitive neurons responsible for VOR are more sensitive to mid-to-high frequency range head motions8. In addition, the OKR can respond to constant-velocity visual motions while the vestibular system encodes only non-constant, transient head velocities. The optokinetic and vestibulo-ocular reflexes are therefore functionally complementary, their combination enables efficient gaze stabilization and allows to discriminate self-generated from externally imposed movements in most naturally encountered situations.

The VOR works as an open-loop system: it is completely functional in the dark, i.e., inner ear vestibular signals generate compensatory eye movements even in the absence of visual feedback. In rodents, the initial development of the VOR relies on the early maturation of the vestibular circuitry even before eye-opening. Nevertheless, visual inputs are critical for the development and proper functioning of VOR: its fine-tuning depends on the visual feedback that informs on the efficacy of the compensatory eye movements. In the absence of vision, such as in congenitally or adventitiously blind people, the VOR is impaired. The gain of the vestibulo-ocular reflex improves after the opening of the eyes in mice, while the phase shifts toward smaller phase leads. In addition, vision critically influences the time constant of the velocity storage16, the development of vestibular nuclei neurons and the acquisition of their plastic properties.