Factors leading to falls in transfemoral prosthesis users: a case series of sound-side stumble recovery responses

Abstract

Background: Transfemoral prosthesis users' high fall rate is related to increased injury risk, medical costs, and fear of falling. Better understanding how stumble conditions (e.g., participant age, prosthesis type, side tripped, and swing phase of perturbation) affect transfemoral prosthesis users could provide insight into response deficiencies and inform fall prevention interventions.

Methods: Six unilateral transfemoral prosthesis users experienced obstacle perturbations to their sound limb in early, mid, and late swing phase. Fall outcome, recovery strategy, and kinematics of each response were recorded to characterize (1) recoveries versus falls for transfemoral prosthesis users and (2) prosthesis user recoveries versus healthy adult recoveries.

Results: Out of 26 stumbles, 15 resulted in falls with five of six transfemoral prosthesis users falling at least once. By contrast, in a previously published study of seven healthy adults comprising 214 stumbles using the same experimental apparatus, no participants fell. The two oldest prosthesis users fell after every stumble, stumbles in mid swing resulted in the most falls, and prosthesis type was not related to strategy/fall outcomes. Prosthesis users who recovered used the elevating strategy in early swing, lowering strategy in late swing, and elevating or lowering/delayed lowering with hopping in mid swing, but exhibited increased contralateral (prosthetic-side) thigh abduction and trunk flexion relative to healthy controls. Falls occurred if the tripped (sound) limb did not reach ample thigh/knee flexion to sufficiently clear the obstacle in the elevating step, or if the prosthetic limb did not facilitate a successful step response after the initial sound-side elevating or lowering step. Such responses generally led to smaller step lengths, less anterior foot positioning, and more forward trunk flexion/flexion velocity in the resulting foot-strikes.

Conclusions: Introducing training (e.g., muscle strength or task-specific motor skill) and/or modifying assistive devices (e.g., lower-limb prostheses or exoskeletons) may improve responses for transfemoral prosthesis users. Specifically, training or exoskeleton assistance could help facilitate sufficient thigh/knee flexion for elevating; training or prosthesis assistance could provide support-limb counteracting torques to aid in elevating; and training or prosthesis assistance could help initiate and safely complete prosthetic swing.

Keywords: Fall prevention; Stumble recovery; Transfemoral prosthesis; Trip.

Fig. 1

Experimental setup for stumble recovery experiments. Perturbations in early swing (top), mid swing (middle), and late swing (bottom) are pictured here. The stumble perturbation system and experimental protocol are detailed in [14]. Video clips of each stumble trial are included in the Additional files 1, 2, 3

Fig. 2

Summary of outcomes for each stumble for each participant. Fall versus recovery, recovery strategy attempted, swing percentage of perturbation, and number of steps prior to a fall (i.e., loading the harness with >50% bodyweight) are provided for each stumble. Video clips of each stumble trial are included in the Additional files 1, 2, 3. If the participant experienced more than one perturbation in a particular bin of swing phase, it is identified by a lower-case letter which is used in subsequent figures and Additional files 1, 2, 3

Fig. 3

Kinematic characterization for early swing perturbations. Lower-limb kinematics: Sagittal-plane thigh, knee, and ankle angle trajectories for the ipsilateral (tripped/sound, top) and contralateral (support/prosthetic, bottom) limbs after the perturbation. Supplementary kinematics, from left to right: sagittal-plane trunk angle, frontal-plane contralateral thigh angle, contralateral forearm COM trajectory in sagittal and transverse planes. Positive angles indicate sagittal plane joint flexion, frontal plane thigh abduction, and forward trunk angle deviation from vertical. Positive arm positions indicate superior, anterior, and medial to position at perturbation. Arm positions are normalized from the position at perturbation, so trajectories begin at position (0, 0). Lower-limb and trunk trajectories are plotted from the instant of perturbation (Time 0) to either 1.5 s after perturbation (recoveries) or until the participant loaded the harness with >50% bodyweight (falls). Arm trajectories are plotted from the instant of perturbation to either one second after perturbation (recoveries) or until the participant loaded the harness with >50% bodyweight (falls)

Fig. 4

Early swing discrete summary metrics: Lower-limb dynamics. These plots highlight the differences in lower-limb motion between transfemoral prosthesis user falls versus recoveries, with healthy control recovery data included as a reference. Peak thigh and knee flexion in the ipsilateral (tripped/sound-side) step were calculated from perturbation to first ipsilateral foot-strike. Peak thigh flexion in contralateral (prosthetic-side) step was calculated from first ipsilateral foot-strike to first contralateral foot-strike or fall, whichever index occurred first. An “x” in a marker indicates the prosthesis user did not foot-off prior to falling. Capital letters above each plot correspond to letters that are marked in Fig.  3 for reference

Fig. 5

Early swing discrete summary metrics: Foot-strike states. These plots capture the body’s state at each foot-strike after the perturbation, highlighting the differences in falls vs. recoveries for transfemoral prosthesis users. Each metric was computed at the indicated foot-strike. Positive values indicate anterior position, forward trunk flexion, and forward trunk flexion velocity

Fig. 6

Kinematic characterization for late swing perturbations. Lower-limb kinematics: Sagittal-plane thigh, knee, and ankle angle trajectories for the ipsilateral (tripped/sound, top) and contralateral (recovery/prosthetic, bottom) limbs after the perturbation. Supplementary kinematics, from left to right: sagittal-plane trunk angle, frontal-plane contralateral thigh angle, ipsilateral forearm COM trajectory in sagittal and transverse planes. Positive angles indicate sagittal plane joint flexion, frontal plane thigh abduction, and forward trunk angle deviation from vertical. Positive arm positions indicate superior, anterior, and medial to position at perturbation. Arm positions are normalized from the position at perturbation, so trajectories begin at position (0, 0). Lower-limb and trunk trajectories are plotted from the instant of perturbation (Time 0) to either 1.5 s after perturbation (recoveries) or until the participant loaded the harness with >50% bodyweight (falls). Arm trajectories are plotted from the instant of perturbation to either one second after perturbation (recoveries) or until the participant loaded the harness with >50% bodyweight (falls)

Fig. 7

Late swing discrete summary metrics: Lower-limb dynamics. These plots highlight the differences in lower-limb motion between transfemoral prosthesis user falls versus recoveries, with healthy control recovery data included as a reference. Peak thigh flexion in contralateral (prosthetic-side) step was calculated from first ipsilateral foot-strike to first contralateral foot-strike or fall, whichever index occurred first. An “x” in a marker indicates the prosthesis user did not foot-off prior to falling. Peak thigh and knee angle in subsequent ipsilateral (sound-side) step were calculated from ipsilateral foot-off after lowering to foot-strike. Metrics are only plotted if they occurred before the participant loaded the harness with >50% bodyweight (fall). Capital letters above each plot correspond to letters that are marked in Fig.  6 for reference

Fig. 8

Late swing discrete summary metrics: Foot-strike states. These plots capture the body’s state at each foot-strike after the perturbation, highlighting the differences in falls vs. recoveries for transfemoral prosthesis users. Each metric was computed at the indicated foot-strike. Positive values indicate anterior position, forward trunk flexion, and forward trunk flexion velocity

Fig. 9

Kinematic characterization for elevating strategies after mid swing perturbations. Lower-limb kinematics: Sagittal-plane thigh, knee, and ankle angle trajectories for the ipsilateral (tripped/sound, top) and contralateral (support/prosthetic, bottom) limbs after the perturbation. Supplementary kinematics, from left to right: sagittal-plane trunk angle, frontal-plane contralateral thigh angle, contralateral forearm COM trajectory in sagittal and transverse planes. Positive angles indicate sagittal plane joint flexion, frontal plane thigh abduction, and forward trunk angle deviation from vertical. Positive arm positions indicate superior, anterior, and medial to position at perturbation. Arm positions are normalized from the position at perturbation, so trajectories begin at position (0, 0). Lower-limb and trunk trajectories are plotted from the instant of perturbation (Time 0) to either 1.5 s after perturbation (recoveries) or until the participant loaded the harness with >50% bodyweight (falls). Arm trajectories are plotted from the instant of perturbation to either one second after perturbation (recoveries) or until the participant loaded the harness with >50% bodyweight (falls)

Fig. 10

Kinematic characterization for lowering/delayed lowering strategies after mid swing perturbations. Lower-limb kinematics: Sagittal-plane thigh, knee, and ankle angle trajectories for the ipsilateral (tripped/sound, top) and contralateral (recovery/prosthetic, bottom) limbs after the perturbation. Supplementary kinematics, from left to right: sagittal-plane trunk angle, frontal-plane contralateral thigh angle, ipsilateral forearm COM trajectory in sagittal and transverse planes. Positive angles indicate sagittal plane joint flexion, frontal plane thigh abduction, and forward trunk angle deviation from vertical. Positive arm positions indicate superior, anterior, and medial to position at perturbation. Arm positions are normalized from the position at perturbation, so trajectories begin at position (0, 0). Lower-limb and trunk trajectories are plotted from the instant of perturbation (Time 0) to either 1.5 s after perturbation (recoveries) or until the participant loaded the harness with >50% bodyweight (falls). Arm trajectories are plotted from the instant of perturbation to either one second after perturbation (recoveries) or until the participant loaded the harness with >50% bodyweight (falls)

Fig. 11

Mid swing discrete metrics: Lower-limb dynamics. These plots highlight the differences in lower-limb motion between transfemoral prosthesis user falls versus recoveries, with healthy control recovery data included as a reference. Peak thigh and knee flexion in the ipsilateral (tripped/sound-side) step were calculated from perturbation to first ipsilateral foot-strike. Peak thigh flexion in contralateral (prosthetic-side) step was calculated from first ipsilateral foot-strike to first contralateral foot-strike or fall, whichever index occurred first. An “x” in a marker indicates the prosthesis user did not foot-off prior to falling. Peak thigh and knee angle in subsequent ipsilateral (sound-side) step were calculated from ipsilateral foot-off after lowering to foot-strike. Metrics are only plotted if they occurred before the participant loaded the harness with >50% bodyweight (fall). Capital letters above each plot correspond to letters that are marked in Fig.  9 and  10 for reference

Fig. 12

Mid swing discrete metrics: Foot-strike states. These plots capture the body’s state at each foot-strike after the perturbation, highlighting the differences in falls vs. recoveries for transfemoral prosthesis users. Each metric was computed at the indicated foot-strike. Positive values indicate anterior position, forward trunk flexion, and forward trunk flexion velocity

Fig. 13

Discrete summary metrics comparing transfemoral prosthesis user recoveries versus healthy control recoveries. Peak trunk flexion and flexion velocity were calculated as the peak value from perturbation to 1.5 seconds after the perturbation. Peak prosthetic-side thigh abduction was calculated as peak frontal-plane thigh angle from contralateral foot-off to contralateral foot-strike. Capital letters above each plot correspond to letters that are marked in Figs.  3,  6,  9, and  10 for reference