EFFECTS OF SIMULATED GRAVITATIONAL LOADING AND UNLOADING ON KINEMATIC AND ELECTROMYOGRAPHIC VARIABLES DURING WALKING

Gravity is a major factor in human gait and movement. Changes in body weight have been shown to drive adaptations in leg swing speed, foot placement and pressure distribution, as well as affect joint angles and muscle activity during walking. However, it is unclear how quickly the body adapts to new gravitational environments and, in particular, if the adaptations seen are consistent and robust at certain gravitational levels, or if they are influenced by previous gravitational loads. This project was composed of two complementary studies that examined kinematic and electromyographic adaptations to simulated gravitational environments both greater than (hyper-gravitational) and less than (hypo gravitational) standard earth conditions. In this project, 15 healthy, young adults were asked to walk in two treadmill-based loading and unloading systems. These systems allowed for investigators to manipulate the weight of each participant without changing their mass. Each individual was asked to walk under normal loading conditions for 5 minutes to allow time for their gait to stabilize. In the loading protocol, each individual walked for 1 minute each at 110%, 120% and 130% of their body weight before unloading to 120%, 110% and 100%, in that order. In the unloading protocol, individuals were unloaded from 100% down to 20% of their normal body weight, in 20% increments, before returning to 100% of body weight in the same way. Each individual spent 1 minute walking at each level of unloading. Lower body kinematics were collected by an array of inertial measurement units; muscle activity was evaluated with four surface electromyography sensors. This dissertation project found that kinematic and electromyographic responses differed with both loading and unloading during walking. Furthermore, not all levels of load elicited changes in both kinematics and electromyography, but often one or the other, regardless of whether these responses were driven by hyper- or hypo-gravitational environmental stimuli. Though coordinative movement patterns remained largely stable across levels of load, phase analysis revealed marked expansion and contraction of available states in the hip and knee with gravitational changes. These results imply that previous studies utilizing the addition of external mass to simulate increased load should not be generalized to encompass environments in which weight is increased or decreased without additional mass. Similarly, evidence of hysteretic adaptations suggest that rehabilitation protocols should not assume that the increasing or decreasing of effective weight are equivalent phenomena, even if they result in the same externally measured level of load.