Adaptive Changes in Gait and Balance Control to Unloading

Adaptive motor learning is a process that enables us to modify and maintain accurate movements to changes of both the body and the environment. Spaceflight provides a unique opportunity to study sensorimotor adaptation since astronauts must adapt to microgravity and readapt back to Earth’s gravity upon return. Within the sensorimotor system, both peripheral (functional or structural) and central adaptive changes are reported to occur, which produces gait and balance impairments. Thus, there is a need to understand the mechanisms underlying sensorimotor adaptation to prolonged unloading. Ground based models such as bed rest and dry immersion are helpful in isolating the sensorimotor impairments unique to modifications in the somatosensory system. However, since the participants are inactive and their limb movements are constrained while being unloaded in these models, the changes observed could be due to “passive unloading”. Given that astronauts are constantly moving and actively interacting with the environment while in space, studying the changes produced by “active unloading” may provide additional insights into the adaptation process. To date, there is no systematic research conducted to study motor adaptation to active unloading or unloaded walking. This dissertation was conducted with the main goal of evaluating and understanding the mechanism of motor adaptation to unloaded walking. The primary focus was to evaluate the adaptive changes in dynamic balance control and locomotion produced by prolonged vertical unloaded walking. Secondly, to determine the role of plantar cutaneous receptors’ sensitivity and body weight perception in balance and locomotor adaptation to unloaded walking. Changes in dynamic balance control were evaluated by measuring body sway response and lower limb muscle activity to repeated forward support surface translations in the absence of vision. Changes in locomotion were evaluated by measuring temporal gait parameters, lower limb joint kinematics and muscle activity during treadmill-walking at normal body weight before and after 30 minutes of both loaded (control condition) and unloaded walking (experimental condition) at 38% body weight. Changes in foot sensitivity was assessed by measuring vibration perception threshold and touch detection threshold at the great toe, heel and 5th metatarsal head and correlated with changes in balance and gait performance. Perception of body weight was assessed by using a custom-made weight perception scale.
The results indicate that 1) the postural control system learns to efficiently restore balance with repeated forward support surface translations by attenuating the postural and neuromuscular responses when body weight is unaltered, and fails to do so after 30 minutes of unloaded walking; 2) there are alterations in lower limb kinematics and neuromuscular activation patterns during walking at normal body weight after 30 minutes of unloaded walking; 3) unloaded walking does not affect foot sensitivity measures; and 4) there is a reduction in perceived body weight during unloaded walking, which returns to baseline or increases after unloaded walking depending on the movement context. These findings indicate that behavioral changes in balance and gait performance produced by unloading are not produced by changes in foot sensitivity nor by changes in conscious percept of body weight; instead due to central reinterpretation and recalibration of load related inputs. This study provides insights about the effects of active unloading on gait and balance control, which will aid in developing effective sensorimotor countermeasures against the deleterious effect of prolonged unloading.