# Adaptation in Gait to Body-Weight Unloading
## 1 Introduction  
The sensorimotor system is comprised of sensory systems, motor systems, and the central integration processes that help in producing and controlling movements. The vestibular, proprioceptive, and visual systems provide sensory information from the external environment as well as that related to body position and movement. The sensory inputs from these systems are integrated in the central nervous system (CNS) following which appropriate motor commands are generated and sent through the descending pathways to the various body segments. Any changes in the sensory information either due to changes in the environmental condition or body condition will affect movement control. The process that enables us to modify and maintain accurate movements as sensory condition changes is called motor adaptation [1]. Several studies have investigated motor adaptation using a variety of experimental paradigms. One of the ways is by studying the changes in the movement characteristics produced by adding and subsequently removing sensory (visual, proprioceptive, acoustic, and vestibular) distortions or perturbations while performing motor tasks [2; 3; 4; 5]. One of the common locomotor adaptation paradigms involves studying locomotor changes by altering the normal interlimb relationship during walking by changing the speeds of treadmill belts relative to each other (commonly called split-belt treadmillwalking [6; 7; 8]). Adaptive changes in step length and double support time are observed during split-belt walk. Similarly, in another paradigm, the effect of increased trunk rotation during walking was studied by having the subjects walk along the circumference of a rotating disc for 2 h [9]. Subsequently, after the adaptation session, the subjects were found to produce curved walking trajectories. Likewise, adaptive changes in heading direction in response to modifications of the direction of optical flow was also observed after exposing subjects to a visual scene that gave the perception of walking along the perimeter of a room [10]. Body-weight unloading (BWU) using various types of body-weight support systems is used to study locomotor adaptation in response to reduced load input [11; 12; 13; 14]. Load-related sensory information is essential for regulating the timing, phasing, and magnitude of neural activities that generate locomotor patterns during stepping [15; 16; 17]. They also help in maintaining balance control during locomotion and gait termination [18; 19; 20]. Lower-body positive pressure (LBPP) is an emerging technology that is used to provide body-weight support [21]. It consists of an air chamber that covers the lower part of the body. When inflated with positive pressure, the lower part of the body is lifted upwards from the hips and the body weight is reduced. LBPP is regarded as one of the superior methods of unweighting when compared to upper-body harness [21]. Upper-body harness partially supports the body weight and results in the formation of pressure points. LBPP on the other hand results in uniform distribution of air pressure around the entire surface of the body while still maintaining normal muscle activation and is thus considered superior [22; 23]. Body weight as large as 80% can be unloaded in increments as small as 1% using this system. An antigravity treadmill is a special type of treadmill that is equipped with an LBPP air chamber and provides an opportunity to study locomotor adaptation to BWU. Several studies using either the antigravity treadmill or the vertical harness system have investigated the immediate effects of BWU on metabolic energy expenditure and locomotor performance [12; 13; 24; 25; 26]. A linear decrease in stride frequency, vertical contact forces, stance time, peak hip and knee flexion, and extensor and flexor muscles' activity burst during the stance phase with BWU has been reported [24; 26; 27; 28].