# A Neuro-Prosthetic Device for Substituting Sensory Functions during Stance Phase of the Gait
## 1 Introduction
### Innovation and Motivation for Current Study  
The central nervous system of quadrupeds, which includes the brain and spinal cord, plays a key role in controlling body movements [21; 22]. Damage to these organs can cause severe permanent disabilities and any treatment is limited in efficacy because it is not possible to regenerate lost nerve cells [23]. To help patients, neural prostheses are used to assist, rehabilitate, reduce pain, and restore normal bodily functions [24; 25; 26; 27; 28]. Functional Electrical Stimulation (FES) as a useful rehabilitation method can apply electrical charge to stimulate peripheral nerves and restore motor functions lost by injury. Dogs are excellent animal subjects in which to study spinal cord disease and rehabilitation because spinal cord injuries in dogs cause the same debilitating effects as in humans. The outcome of this injury for a dog can be weakness in hindlimb muscles and loss of sensation causing disability in walking and maintaining balance. While many dogs suffering such disease will completely lose their ambulatory functions in the hindlimbs, some of them are still able to make walking movements through muscle reflexes elicited when the toes touch the ground. However, due to lack of sensory feedback information ascending to the coordinating centers in the brain, such dogs will lose their balance after a few steps when the hindquarters tilt, and the animal will fall because of an inability to correct this process. To restore walking, our idea is to attach four IMUs on the pelvic, femur, tibia, and metatararsus region of the dog to gain information about angle of rotation at the pelvis and hip, knee, and hock flexion angles (Figure 1). Using this information, we can track dog gait in real time. Figure 1: Position of IMUs on dog hindleg and pelvis. We categorize the walking gait for each leg to two phases: Swing and stance phases (Figure 2). The swing phase commences when the dog lifts its leg from the ground and moves it forward. During the swing phase, the leg is not in contact with the ground, as shown in Figure 2. The stance phase is defined as the time when the leg is in contact with the ground, again shown in Figure 2. During stance phase, the toes of the dog are in contact with the ground. Because of lack of sensation, spinal cord injured-dogs are unable to move their hindlegs to follow normal trajectory lines during the swing phase. This causes them to place their toes in an unusual or unstable position at the start of the stance phase. Because the limb does not have the appropriate sensory feedback and control loop to permit weight-bearing, in this condition, the pelvis rotates and tilts laterally, causing falling. To develop our new balancing device, we propose that both gait phases can be controlled by electrical stimulation to restore walking. During swing phase, IMUs should measure the joint angles and stimulate muscle nerves based on data from normal joint angles of the gait to force the leg to follow normal trajectory lines. This method is not the subject of the current study and will be explained in a separate paper. During the stance phase, our aim is to balance the pelvis to prevent falling. The IMU on the pelvis measures the pelvis rotational angle and, when it reaches a falling threshold condition, the appropriate muscles are actuated to correct pelvis orientation and maintain balance. Since the stimulation strategies during stance and swing phases are different, our balancing device should be programmed to swiftly recognize the stance and swing phase of the gait for each leg. In our previous work [19], we extensively discussed the design of our robotic test-bed and showed the accuracy of the robot in mimicking dog walking gait and the joints' control systems to stimulation commands that resemble the muscle response to stimulation. We also showed two strategies to maintain pelvic equilibrium during the stance phase while the knee joint was fixed.