Fig 1.
Modified knee brace used in the experiment.
(a) Side view showing the modified joint-following mechanism; (b) implementation of torsion springs along with the selected moment-angle behavior of the four springs; (c) implementation of rotary dampers along with the moment-speed behavior of the four dampers used (data was imported from damper data sheets–dashed line indicates interpolated data).
Fig 2.
Visualization of the placement of retroreflective markers and EMG sensors on the body along with a table with the corresponding anatomical descriptions.
Retroreflective markers were placed on anatomical landmarks of the lower limb and on the knee brace. Wireless surface EMG sensors were placed on the gluteus medius (GM), vastus medialis (VM), semitendinosus (ST), biceps femoris (BF), and medial gastrocnemius (MG). We added three additional markers to the rigid frame of the brace to measure brace deformation independently of biological knee angle.
Fig 3.
(a) Stiffness and damping randomization procedure. Before each experiment, the stiffness values and damping values are shuffled and randomly assigned to the trial number for both walking and stair trials; (b) the order of experimental conditions completed, where C is the control with no brace, k represents stiffness trials, and b represents damping trials; (c) the walking and stair navigation tasks for each corresponding experiment grouping. The walking tasks and stair tasks were performed sequentially to allow re-calibration of the motion capture cameras for each respective task; (d) the survey questionnaire asked after each trial.
Fig 4.
Group mean knee flexion angle during 1.1 m/s walking (N = 10) for the (a) stiffness and (b) damping conditions, normalized to percent gait.
A separating line between stance phase and swing phase is drawn at 60% gait.
Fig 5.
Group mean results for 1.1 m/s walking experiment (N = 10) for (a) stiffness and (b) damping conditions.
Kinematic variables including maximum hip flexion angle, maximum knee flexion angle, minimum toe clearance, and maximum knee speed are displayed for every experimental condition during swing phase. Change in peak swing phase muscle activity from the brace-only condition (e.g., k0, b0) are shown for the biceps femoris (BF) and semitendinosus (ST) muscles. Finally, peak brace moment during swing phase for each condition is shown. The error bars represent one standard deviation (*p < 0.05, shown for minimum toe clearance variable only).
Fig 6.
Group mean results for (i) stair ascent and (ii) stair descent tasks (N = 10) for each (a) stiffness and (b) damping conditions.
Kinematic variables including maximum knee flexion angle and change in step time from control for each experimental condition during swing phase. Change in muscle activity from the control condition (e.g., k0, b0) during swing phase are shown for the biceps femoris (BF) and semitendinosus (ST) muscles. Finally, the peak brace moment for each stiffness condition during swing phase is shown. The error bars represent one standard deviation.
Fig 7.
Group survey results for different stiffness and damping conditions (N = 10).
(a) Responses after 1.1 m/s walking, (b) responses after stair ascent, (c) responses after stair descent.
Fig 8.
(a) Stiffness thresholds and (b) damping thresholds, separated by activity and basis of threshold.
Individual thresholds (circles) and group thresholds (colors) are shown. Survey based thresholds are defined as the amount of stiffness or damping at which each participant’s survey response crossed from “disagree” to “agree”. Minimum toe clearance (MTC) based thresholds are defined as the stiffness or damping at which the risk of scuffing an unseen 1 cm tall obstacle [30] exceeds 20% per stride. The group mean risk of scuffing an unseen 1 cm object is shown at each stiffness value. Participants were excluded if no threshold could be determined.
Fig 9.
Relationship between change in hamstring muscle activity (averaged BF and ST percent) and mean minimum toe clearance for each participant during the maximum stiffness condition.