Effects of simulated hypo-gravity on lower limb kinematic and electromyographic variables during anti-gravitational treadmill walking

Abstract

Introduction: This study investigated kinematic and EMG changes in gait across simulated gravitational unloading levels between 100% and 20% of normal body weight. This study sought to identify if each level of unloading elicited consistent changes-particular to that percentage of normal body weight-or if the changes seen with unloading could be influenced by the previous level(s) of unloading. Methods: 15 healthy adult participants (26.3 $\pm$ 2.5 years; 53% female) walked in an Alter-G anti-gravity treadmill unloading system (mean speed: 1.49 $\pm 0.37$ mph) for 1 min each at 100%, 80%, 60%, 40% and 20% of normal body weight, before loading back to 100% in reverse order. Lower-body kinematic data were captured by inertial measurement units, and EMG data were collected from the rectus femoris, biceps femoris, medial gastrocnemius, and anterior tibialis. Data were compared across like levels of load using repeated measures ANOVA and statistical parametric mapping. Difference waveforms for adjacent levels were created to examine the rate of change between different unloading levels. Results: This study found hip, knee, and ankle kinematics as well as activity in the rectus femoris, and medial gastrocnemius were significantly different at the same level of unloading, having arrived from a higher, or lower level of unloading. There were no significant changes in the kinematic difference waveforms, however the waveform representing the change in EMG between 100% and 80% load was significantly different from all other levels. Discussion: This study found that body weight unloading from 100% to 20% elicited distinct responses in the medial gastrocnemius, as well as partly in the rectus femoris. Hip, knee, and ankle kinematics were also affected differentially by loading and unloading, especially at 40% of normal body weight. These findings suggest the previous level of gravitational load is an important factor to consider in determining kinematic and EMG responses to the current level during loading and unloading below standard g. Similarly, the rate of change in kinematics from 100% to 20% appears to be linear, while the rate of change in EMG was non-linear. This is of particular interest, as it suggests that kinematic and EMG measures decouple with unloading and may react to unloading uniquely.

Keywords: EMG; anti-gravity treadmill; gait; hypogravity; hysteresis; kinematics; unloading.

FIGURE 1

Each plot contains the average kinematic waveforms for its respective unloading (in red) and loading (in blue) condition, along with a 2-standard deviation shaded area around each waveform. All 20% load conditions are in black to avoid any confusion, as only a single waveform is present. Note the low variability of the knee waveforms across loading levels, and irrespective of absolute level of load. Conversely, the ankle shows higher variability between like levels, though it remains similar across absolute levels of load.

FIGURE 2

Each plot contains the average EMG waveforms for its respective unloading (in red) and loading (in blue) condition, along with a 2-standard deviation shaded area around each waveform. All 20% load conditions are in black to avoid any confusion, as only a single waveform is present. Though the phasic properties of these muscles appear to be robust with unloading, note the clear peak differences in the medial gastrocnemius as well as in the rectus femoris at 40% load depending on whether participants were loaded or unloaded previously.

FIGURE 3

The graphs in this figure depict both the average 100% (baseline, red) and 20% (black) unloading conditions for the hip, knee, and ankle. In the hip, smaller joint angle values correspond with increased extension, while higher joint angle values correspond with increased flexion. In the knee, higher values are flexion, and lower are extension. In the ankle, higher values indicate plantar flexion, while lower values indicate dorsiflexion. The table below shows the mean, maximum, and minimum joint angle values (in degrees) for these conditions.

FIGURE 4

The coordination strategy between the joints of the lower extremities appears to be generally robust as load was decreased. However, there is clear stretching and translating in the coordinative strategies between the hip and knee as well as the hip and ankle. The hip and ankle, in particular, demonstrates a marked shift in coordinative strategy as the hip enters hyperextension. Coordination between the knee and ankle appears to be mostly preserved between 100% and 20% loads, though there is some stretching and shifting as load is decreased.

FIGURE 5

Phase portraits for the hip, knee and ankle suggest that the hip and ankle are most sensitive to shifts in load. Both the hip and ankle demonstrate notable expansion of the phase space (and thus possible states), while the available states of the knee are almost entirely unaffected by the decrease in load.