An Experimental Approach to Induce Trips in Lower-Limb Amputees

Summary

An experimental approach was developed to induce trips in lower-limb amputees. The goal was to create unexpected trips and induce meaningful tripping/recovery responses. The kinematic data from one transtibial amputee confirmed that such an approach effectively elicits reactive trip recovery responses.

Abstract

Reestablishing balance after a trip is challenging for lower-limb amputees and often results in a fall. The effectiveness of reestablishing balance following a trip depends on factors such as amputation level (transtibial or transfemoral) or which limb is tripped (prosthetic or sound/lead or trailing). Understanding the recovery responses can help identify strategies to avoid a trip becoming a fall and what trip-response functionality could be designed into a prosthesis. This study presents an experimental approach for inducing unexpected trips in individuals with amputation. Tripping was manually triggered by activating an electromagnetic device to raisea polypropylene wire to obstruct (bring to a near halt) theswinging limb during its mid-swing phase. A safety harness attached to a ceiling rail ensured participants did not hit the ground if they failed to reestablish balance following the trip (i.e., it prevented a fall from occurring). One transtibial amputee completed repeated walking trials in which a trip was induced around 1 out of 15 times to avoid it being anticipated. 3D kinematics were determined via two smartphones (60Hz) using the OpenCap software, highlighting that the experimental approach induced meaningful tripping/recovery responses dependent on which limb was tripped (prosthetic or sound). The presented methodology avoids using a rigid obstacle, potentially reducing the risk of injuries, and is inexpensive and easy to set up. Importantly it permits a trip to be unexpectedly introduced during the mid-swing phase of the gait and hence provides an approach for identifying real-world trip recovery responses. When tripping the sound limb, participants could 'disentangle' from the trip-wire (post-trip) by plantarflexing the ankle, but such action was not possible when tripping the prosthetic limb.

Introduction

It has been estimated that 57.7 million people worldwide live with limb amputation, of which ~ 65% occur in the lower limbs¹. Lower limb amputation may derive from several factors (e.g., acute traumatic events, disease progression, health complications, life-saving surgery, and congenital deformity). It has been associated with high mortality and morbidity rates for those with poor health conditions². In addition, mobility reestablishment after amputation is crucial to regaining independent living and life quality and is one of the most significant challenges for prosthesis users³.

After an amputation, mobility limitations are accompanied by a reduced range of motion⁴, decreased strength⁵, diminished confidence in balance⁶, and can lead to a marked joint degeneration in the non-amputated limb⁷. These changes are described as relevant fall risk factors⁸. Indeed, lower limb prosthesis users are twice as likely to fall compared to the general population⁹. Around 40% and 80% of persons with transtibial and transfemoral amputations fallat least once a year^9,10. Falls occur most often during walking^11,12, and amputees with a limited walking ability (adjusted for exposure) are six times more likely to fall and eight times more likely to suffer an injury¹¹. In addition, a lower limb prosthesis user that has experienced a fall in the past year has a 13% likelihood of falling again. The probability rises to 28% if they experienced two falls in the past six months¹³. Thus, falling is a concerning problem for lower-limb amputees.

Tripping while walking is a predominant factor for falls in prosthetic users. During a trip, there is a sudden interruption of the swinging limb (e.g., caused by an obstacle or uneven terrain), making the body rotate forward rapidly on the support limb and causing a large forward thrust^14,15. Maintaining/recovering balance after tripping for prosthetic users can be much more difficult due to the absence of ankle or knee joints, associated musculature, and reduced sensory feedback. An ineffective response to a stumble may culminate in it becoming a fall, which may have significant physical, psychological, and social consequences¹⁶.

Several studies have focused on describing tripping recovery strategies for able-bodied and older adults^17,18,19,20 by inducing a trip in a laboratory-controlled scenario. Several methods have been applied to produce a disturbance to generate a trip. There are many ways to impose a trip disturbance, including obstructing the lower-limb segment during its swing phase using a rope attached to the ankle²¹ or using obstacles unexpectedly placed in front of someone walking on a treadmill^20,22. In addition, some studies have applied sudden changes in the treadmill's speed to disturb dynamic balance (i.e., induce a stumble)²³. Finally, others have used rigid objects that are manually^18,24,25 or automatically^22,26 positioned in the way of the swinging limb to cause a trip event during overground walking.

Despite successfully applying such strategies in older adults, only a few studies have induced a trip in lower limb amputees, with fewer still involving those with transfemoral level amputation^21,25,26. For instance, Crenshaw and colleagues tripped TFA while walking over-ground using a hidden rigid obstacle manually activated to appear from the ground. However, such a way of introducing an obstacle is technically demanding and hence can be expensive to reproduce. Shirota and colleagues induced a trip in TFA while participants walked on a treadmillusing a rope attached to the ankle. Even though a trip was caused, using a rope may have limited the experiment as it likely impeded the participants from walking naturally²¹. More recently, Eveld and colleagues tripped TFA by placing steel blocks on a treadmill conveyer belt using an integrated targeting algorithm to allow the objects to cause the disturbance at different stages of the swing phase (early, mid, late swing)²⁶. However, treadmill-based protocols may not fully reproduce the conditions during over-ground walking²⁷. Using a treadmill-based protocol is also not ideal when investigating TTA or TFA who use microprocessor-controlled foot-ankle or knee devices because the automatic sensors used in such devices are set up for walking on a solid/stationary surface. Hence, when walking on a non-stationary surface, these sensors may trigger the device's hydraulic cylinders to 'self-adjust' their resistances to an incorrect level.

In previous studies that induced a trip during overground walking, the trip disturbance was caused by the lead limb contacting a solid obstacle that appeared in front of them. However, using such rigid objects may cause foot injuries due to impact forces²⁵. Here we describe an experimental approach for tripping the swinging limb that avoids the issue of the foot hitting something solid. The tripping mechanism is formed by an electromagnetic system that controls the release of a movable spring-operated plate. When the electromagnetic device is deactivated, the spring-operated plate positioned on one side of the walkway is pulled upwards, raising a polypropylene wire (4 mm diameter)positioned perpendicularly to the walking direction. The wire is anchored to the opposite side of the walkway and is raised to a height of 0.1 m. Dummy wires (3 to 4, spaced at least 1 m apart) are positioned across the walkway so that participants cannot guess which wire would cause the disturbance. The experimenter manually deactivates the electromagnetic device with the contralateral limb positioned on the ground, slightly ahead of the wire, just after the instance of toe-off of the swinging limb. Therefore, when the wire is raised, the swinging segment is consistently caught during the mid-swing phase²⁸. The mid-swing phase was selected because the horizontal velocity of the swinging foot at this phase is close to its maximal (~3 times CoM forward speed) and is at its minimum clearance above the ground, and hence is the period when most trips occur in real-world conditions. The height of the wire (i.e., 0.1 m) is sufficient to allow the foot to be consistently caught (on approximately shoe-laces area). The study aimed to establish if the proposed protocol could create a trip disturbance and induce meaningful/real-life recovery responses. Only a TTA was analyzed in the present protocol, as higher-level amputations represent the more complex cases and present higher fall rates.

Protocol

The University's ethics committee approved procedures, and the participant signed an informed consent form before participating.

1. Participant

NOTE: One Transtibial (TTA) amputee attending a local amputee rehabilitation center was invited and agreed to participate in the study. The participant was able to walk independently. Exclusion criteria were clinical conditions other than their amputation that could affect balance and mobility (e.g., neurological, orthopedic, or rheumatic disorders); ongoing pain, phantom pain, or pressure sores on the prosthetic limb, and difficulties understanding simple commands (i.e., less than 24 points in the Mini-Mental State Examination²⁹). In addition, the participant had over six years of experience with the current prosthesis.

1. Prosthetic details
   1. Request the prosthetic details from the TTA. Note the experience of the TTA with the prosthesis. Ensure that the participant has a high ability to walk using the prosthesis.
      ​NOTE: The TTA used a prosthesis, a silicone suction socket (silicon liner with five sealing rings), and a carbon fiber foot (Table of Materials). Experience with the current prosthesis was six years. The amputation was due to trauma, and the participant was classified as level K4 according to the Medicare Functional Classification³⁰. According to the standardized functional classification, the participant had a high ability to walk using the prosthesis and was considered a young active adult³¹.

2. Experimental procedures

1. Design a system to induce trips.
   1. Construct a custom-made device in which a spring is electronically released to raise a polypropylene wire (diameter of 4 mm and negligible mass) that catches the trailing limb (sound or prosthetic limb) during the mid-swing phase.
   2. Connect the system to a wooden box that allows a lever (approximately 10 cm) to be rotated upwards around a fixed axis. Connect the polypropylene wire to the end of the lever (away from the axis). Install a spring that pulls on the lever to raise the polypropylene wire about 10 cm from the ground.
      NOTE: Video 1 shows the trigger system and how the wire was positioned to cause the trip (Supplementary Figure 1 and Supplementary Figure 2).
2. Safety harness system
   NOTE: Inducing a trip while a participant is walking requires safety measures to be adopted.
   1. Ensure the participant wears a full-body harness attached via a polyester rope to an overhead rail.
   2. Adjust the length of the safety rope according to the participant's stature.
      NOTE: The safety rope (diameter of 11 mm) is attached to a specially designed four-wheeled device that sits inside the overhead rail (about 2 m above the participant's head). Adjusting the safety rope to the participant's stature prevents any part of their body (apart from their feet) from touching the floor should they fail to restore balance after the trip disturbance.In addition, the length of the overhead rail (8 m) is sufficient to ensure the participants' walking is unencumbered (see Supplementary Figure 3).
3. Experimental procedures
   1. According to the following standardized instruction, ask the participant to walk across the laboratory at their usual speed and looking forward as the participant normally would: "You should walk to the end of the walkway using your own pace as if you were walking on a familiar, flat street and look forward as you normally would".
   2. Adjust the participant's starting point to ensure that the contralateral (non-tripped) limb is positioned on the ground slightly ahead of the polypropylene wire, placed approximately 4 m from the starting position. Therefore, the participant could take 4-5 steps at the usual speed before applying the trip disturbance.
      1. The participant is required to complete two blocks of walking. Let the participant perform up to 15 walks in each block with the stumble/trip disturbance applied between the 5^th and 15^th repetition (randomly determined).
      2. After the trip disturbance, do not let the participant make any further repetitions.
      3. Repeat the same procedures in the second block, which is used to trip the opposite limb to the one tripped in the first block.
         NOTE: The order in which the limbs are obstructed is randomly assigned.
      4. Prior to starting the walking tests, inform the participant that some disturbance could occur, but do not provide any specific information regarding the possibility of tripping. Instead, inform the participant about the possibility of losing balance at some point.
      5. Instruct the participant to recover as best as possible if any disturbance of balance is applied and, if possible, to continue walking to the end of the walkway.
   3. Trigger the system only when the foot of the contralateral (non-tripped) limb is correctly positioned on the ground (i.e., slightly ahead of the wire). Do not activate the system if the participant steps before, on the wire, or if the foot is too far ahead of the wire. These procedures allow the trip disturbance to be applied consistently during the mid-swing phase, reducing the chances of mistrials.
4. Evaluating whether the system can inducemeaningful recovery responses.
   ​NOTE: The study aimed to develop an experimental approach to cause unexpected trip disturbances in lower-limb amputees. Although the approach causes unexpected trips, the use of dummy wires and the laboratory environment does not allow one to assume that all trips will be totally unexpected. 3D kinematic data from one TTA was collected and analyzed to establish if the protocol could create unexpected trips and hence induce meaningful tripping/recovery responses.
   1. Data acquisition
      1. Position two smartphones 5 m ahead of where the trip occurs are used to record each walking trial. Set the smartphones facing the walking progression line at an angle of approximately 30^o.
      2. Synchronize both smartphones, sampling at 60 Hz, using the OneCap software. The OneCap software synchronizes the phones by providing a code that is read by the smartphones. Then, the images are automatically stored on the computer and transferred to be remotely processed. The transfer and successful reconstruction are indicated by the software.
         NOTE: This software automatically recognizes and tracks the limb segments without physical markers and pose detection algorithms transform images to estimate joint centers and provide a relatively accurate kinematic analysis. After being processed, the files can be analyzed using the OpenSim software.
      3. Then, process and transfer the images to the OpenSim software (version 4.4) to perform all kinematic analyses.
         NOTE: A markless system is advantageous, as pilot testing showed that the trip-wire dislodges some physical markers (especially those placed on the foot). A discussion regarding the relative merits of the capture and data processing is beyond the scope of the present protocol. The reader should refer to the work by Uhlrich and colleagues³² for further information.
   2. Data processing and analysis
      NOTE: The OpenSim is a freely available software package that enables one to build, exchange, and analyze computer models of the musculoskeletal system and dynamic simulations of movement. Further details can be obtained on the following site: https://simtk.org/projects/opensim/.

Results

The safety harness system was assumed to cause no interference in walking and proved effective in preventing falling when trip recovery strategies were unsuccessful. In addition, no injuries (e.g., skin abrasions, bruising) were reported. The noise generated by the release of the spring was not considered an intervening factor since the participants did not prevent tripping from occurring. Furthermore, the time between the instant the system was activated and the impact with the wire was around 60 ms. Thus, it was assume...

Discussion

Although the present protocol brings preliminary results of an experiment designed to describe a trip protocol applied on a transtibial amputee, such an approach can also be safely applied to other amputees, e.g., transfemoral amputees, who are likely to have greater difficulties in recovering balance after a trip. The approach allowed the identification of the most pronounced actions executed to regain balance in response to an unexpectedly induced trip. The protocol can generally be deemed replicating real-world trippi...

Materials

Name	Company	Catalog Number	Comments
Electromagnetic plates	Intelbras	https://www.intelbras.com/en/set-of-supports-with-electro-magnetic-lock-fe-150-kt-741-prata	Two electromagnetic plates (a fixed and a movable)
Full body safety harness	Generic	N/A	Safety rope 11 mm attached on a rail running 2 m above the head of the participants
Impact Goggle	Generic	N/A	One goggles with lower and side end closures
Insulator tape	3M	https://www.3m.com/3M/en_US/p/c/tapes/electrical/ptfe/	Used to obstruct vision at the lower and side edges of goggles
Open Pose	OpenPose	https://github.com/CMU-Perceptual-Computing-Lab/openpose	Open Pose is a open Software to movement analysis https://github.com/CMU-Perceptual-Computing-Lab/openpose
Open Sim	OpenSim	https://simtk.org/projects/opensim/	OpenSim is a softwware to analyse several movement parameters https://simtk.org/projects/opensim/
Polypropilene Wire	Generic	N/A	4 mm diameter
Triger system	Generic	N/A	The trigger system was home-made device, formed by a spring that pulls a lever that raises the wire approximately 10cm above the ground level
Video camera	Apple	https://apple.com	The video cameras of two smartphones (apple model 8 and 11) were used.