Smooth changes in the EMG patterns during gait transitions under body weight unloading

Abstract

During gradual speed changes, humans exhibit a sudden discontinuous switch from walking to running at a specific speed, and it has been suggested that different gaits may be associated with different functioning of neuronal networks. In this study we recorded the EMG activity of leg muscles at slow increments and decrements in treadmill belt speed and at different levels of body weight unloading. In contrast to normal walking at 1 g, at lower levels of simulated gravity (<0.4 g) the transition between walking and running was generally gradual, without systematic abrupt changes in either intensity or timing of EMG patterns. This phenomenon depended to a limited extent on the gravity simulation technique, although the exact level of the appearance of smooth transitions (0.4-0.6 g) tended to be lower for the vertical than for the tilted body weight support system. Furthermore, simulations performed with a half-center oscillator neuromechanical model showed that the abruptness of motor patterns at gait transitions at 1 g could be predicted from the distinct parameters anchored already in the normal range of walking and running speeds, whereas at low gravity levels the parameters of the model were similar for the two human gaits. A lack of discontinuous changes in the pattern of speed-dependent locomotor characteristics in a hypogravity environment is consistent with the idea of a continuous shift in the state of a given set of central pattern generators, rather than the activation of a separate set of central pattern generators for each distinct gait.

Changes of Gait Kinematics in Different Simulators of Reduced Gravity

“…According to the principle of dynamic similarity, geometrically similar bodies that rely on pendulum-like mechanics have similar gait dynamics at the same Froude number, which is inversely proportional to gravity. Indeed, the optimal walking speed and the speed of gait transitions decrease monotonically with body unloading consistent with the predictions of dynamic similarity Minetti 2001;Sylos Labini et al 2011). Additional inertia of the tilted chassis (~20% of subjects' mean body mass) or that of the exoskeleton (1.5 kg for each leg) could affect metabolic rate or gait kinetics (Chang et al 2000;Browning et al 2007), although their influences on the speed and abruptness of gait transitions are minor .…”

“…Additional inertia of the tilted chassis (~20% of subjects' mean body mass) or that of the exoskeleton (1.5 kg for each leg) could affect metabolic rate or gait kinetics (Chang et al 2000;Browning et al 2007), although their influences on the speed and abruptness of gait transitions are minor . During locomotion in humans and other animals, changes in stepping rate typically result from a change in the duration of the extensor (stance) phase, whereas the flexor (swing) phase duration remains relatively constant (Juvin et al 2007;Sylos Labini et al 2011). Nevertheless, the swing phase adapts to reduced gravity.…”

“…Investigating human locomotion in a hypogravity environment has received significant attention in view of the biomechanical principles that determine the kinematics of different gaits (Donelan and Kram 1997;Ivanenko et al 2002;Everett et al 2009), adaptability of central pattern generation networks to limb loading and clinical implications for gait rehabilitation (Dietz and Duysens 2000;Dominici et al 2007;White et al 2008;Selinov et al 2009;Sylos Labini et al 2011). Gravity significantly affects the optimal speed and the characteristics of the walk-run transition as shown in theoretical and experimental studies using reduced gravity simulators Minetti 2001;Ivanenko et al 2011), during parabolic flights (Newman 1996;Cavagna et al 2001) and lunar exploration (Carr and McGee 2009).…”

Human Locomotion under Reduced Gravity Conditions: Biomechanical and Neurophysiological Considerations

“…These simulators, that have been used in the past by both Roscosmos (Russian Federal Space Agency) and NASA to train astronauts before space flights [15–17], are based on the idea of neutralizing the component of the gravity force normal to the lying surface [mg · cos(α), where α is the angle of inclination], while the component of the gravity force acting on the body and swinging limbs in the sagittal plane is reduced in relation to the tilt angle [mg · sin(α)]. A similar concept has been used in the reduced gravity simulator (Figure 1(d)) designed by Ivanenko et al (Italian patent number Rm2007A000489): the subject lies on the side on a tilted couch (up to 40° from the horizontal position) with both legs suspended in the exoskeleton and steps on the treadmill, which is tilted to the same angle [7, 18, 19]. This simulator included additional mass of the tilted chassis (~15 kg) and exoskeleton (1.5 kg for each leg).…”

“…As already discussed (Figure 2(a)), gravity has a strong effect on the speed at which gait transitions occur (Fr ~0.5). Surprisingly, however, we found [18, 19] that at lower levels of simulated gravity the transition between walking and running was generally gradual, without any noticeable abrupt change in gait parameters or EMG bursts (Figure 4(a)). This was associated with a significant prolongation of the swing phase, whose duration became virtually equal to that of stance in the vicinity of the walk-run transition speed, and with a gradual shift from inverted-pendulum gait (walking) to bouncing gait (running).…”