Alterations to plantar loading and ankle range of motion of the contralateral foot during assisted walking in patients with Type 2 Diabetes Mellitus

Abstract

Aims:

Patients with diabetic foot ulcers are instructed to be non-weight bearing on the affected limb to promote healing. Therefore, the aim of this study was to investigate the effect of different assistive devices on whole foot plantar loading, peak forefoot force, ankle range of motion, and locomotion speed during gait in patients with Type 2 Diabetes Mellitus.

Methods:

Participants walked normally, with crutches, a walker, and a wheeled knee walker (WKW) in randomized order. Force sensitive insoles and 3D motion capture were used to record plantar normal force and ankle kinematics. Force sensitive pads were wrapped around handles of the crutches and walker to measure bodyweight offloaded onto the assistive device. An instrumented WKW was used to measure bodyweight offloaded onto the handlebars and knee cushion.

Results:

Locomotion with the WKW produced the lowest whole foot plantar loading and peak forefoot force in the propulsive limb, while also producing the greatest ankle range of motion and locomotion speed amongst assistive devices.

Conclusions:

This pre-clinical study found that the WKW could be the preferred assistive device for total unilateral offloading of diabetic foot ulcers as it reduced propulsive limb whole foot and forefoot plantar loading while retaining ankle range of motion and locomotion speed.

1. Introduction

Approximately 7 % of patients with Type 2 Diabetes Mellitus (DM) will suffer from a Diabetic Foot Ulcer (DFU) in their lifetime [1]. Established methods of treating DFUs include partial bodyweight (BW) offloading via knee high total contact casts and felted foam inserts [2]. However, iatrogenic ulceration can occur due to poorly fitted casts, even when casts are fitted by skilled professionals [2]. A substitute for casting incorporates total offloading of DFUs via assistive devices such as crutches, walkers, or wheeled knee walkers (WKW). These aids allow patients to transfer BW onto the device and maintain stability but leave the contralateral (i.e., propulsive) foot exposed to ground reaction forces alone during walking. Previous research investigating the effects of assistive devices on propulsive foot plantar loading report that the WKW reduced peak whole foot force by 0.29 and 0.35 BW compared to crutches and walkers, respectively [3]. However, the amount of BW transferred to the assistive device during gait is unknown for persons with Type 2 DM. Excessive force exerted on the hands during assistive device use can cause crutch palsy due to nerve compression, pain, and discomfort in the hands during extended periods of use [4–6]. Symptoms of crutch palsy can be amplified in the DM population due to reduced upper body strength and increased body mass relative to healthy counterparts [7], particularly in the 6–8 weeks when patients are prescribed an assistive device to offload a DFU [8].

Differences in ground reaction forces could influence ankle joint range of motion (ROM) during the stance phase of gait. Previous studies have found that DM patients have 49 % stiffer ankles in extended knee positions and 29 % stiffer ankles in flexed knee positions than healthy counterparts [9]. Locomotion with different assistive devices significantly changes peak plantar loading during stance [3]. Therefore, it is reasonable to believe that stance phase ankle ROM could be altered due to changes in plantar loading. There is a well-established relationship between limited ankle dorsiflexion and increased forefoot plantar pressure during stance [10–14]. Thus, ankle mobility during assistive device prescription should be considered to reduce the risk of forefoot DFU formation in the propulsive limb. However, alterations to ankle ROM when using different assistive devices have not been investigated in the DM population.

Maintaining non-weight bearing prescription compliance is crucial for improving outcomes in patients recovering from DFUs [2]. Quality of life is an important factor in maintaining patient prescription compliance. One way quality of life can be quantified is locomotion speed during assistive device use. This metric can be used to indicate mobility retained from normal gait during assistive device use. A study in healthy adults found that participants had a 67 % faster self-selected walking speed with a WKW compared to bilateral crutches during a 6-minute walk test [15]. The same patients reported 44 % lower rates of perceived exertion during the 6-minute walk test [15]. Similar comparisons have been made between normal walking and assisted gait with crutches and walkers in patients with spinal cord injuries. During a 10 m walk test, locomotion speed with walkers and crutches was 63 % and 50 % slower than normal walking, respectively [16]. However, locomotion speed while using assistive devices has not been investigated in the DM population to our knowledge.

Offloading DFUs using an assistive device that simultaneously reduces propulsive limb plantar loading, increases ankle ROM, and retains locomotion speed is crucial to reducing DFU risk in the propulsive limb and maintaining prescription compliance. Therefore, the primary purpose of this study was to compare whole foot and regional plantar loading during normal and assisted walking in patients with Type 2 DM. We hypothesize that whole foot plantar impulse will be lowest when using the WKW due to the most BW being offloaded onto the assistive device. We also hypothesize that forefoot peak plantar loading will be lowest when using the WKW due to the least amount of propulsive force needed for locomotion. Our secondary objective was to compare ankle ROM and locomotion speed during normal and assisted walking in the Type 2 DM population. We hypothesize that locomotion with the WKW will produce the most reduced stance phase ankle ROM due to the lowest amount of plantar loading. We also hypothesize that locomotion speed with the WKW will be greatest compared to the crutches and walker.

2. Materials and methods

2.1. Participants

Twelve female and eight male participants were recruited for this study (Table 1). Informed consent was obtained before data collection per the University of Nebraska Medical Center Institutional Review Board. Inclusion criteria included people older than 40 years of age with Type 2 DM, A1c ≤ 10 % (≤ 86 mmol/mol), and the self-reported ability to walk for 5 min at a self-selected speed without any support. Exclusion criteria consisted of a history of lower extremity joint replacement or amputation, unexplained dizziness in the last 6 months, use of an assistive device during the previous 12 months, patients currently in treatment for a DFU, and BW > 136 kg (weight capacity of assistive devices). Participants also presented with a number of other comorbidities for which we did not control for.

Table 1.

Mean(standard deviation) and range of participant descriptives and anthropo-metric information.

Mean (SD) Range	Male n = 8	Female n = 12	Total n = 20
Age (yrs)	64.25(10.08)	58.83(10.38)	61.0(10.36)
	47–79	43–75	43–79
Mass (kg)	96.24(15.55)	86.74(10.68)	90.54(13.34)
	70.34–112.05	68.85–103.83	68.85–112.05
Height (m)	1.75(0.06)	1.69(0.09)	1.71(0.08)
	1.66–1.82	1.46–1.78	1.46–1.82
Diagnosis Duration (yrs)	15(7.98)	13.10(5.49)	13.94(6.57)
	1–27	9–26	1–27
HbAlc (%)	7.44(0.73)	6.65(0.84)	7.0(0.87)
	6.5–8.6	5.5–8	5.5–8.6
HbA1c (mmol/mol)	57.88(7.77)	49.20(9.09)	53.06(9.40)
	48–70	37–64	37–70

2.2. Experimental protocol

Participants were screened for eligibility via a medical history questionnaire before data collection. Height and weight were then recorded. A familiarization period took place where crutches, a walker, and a WKW were fitted to each participant to ensure comfort and proper use. Participants also chose their propulsive limb at this time, which was consistent for all assistive device trials. This study consisted of four walking conditions: 1) unassisted (Normal) walking, 2) Crutches, 3) Walker, and 4) WKW (Fig. 1). Participants were instructed to walk under each condition back and forth along a straight 10 m path for 200 m or until they could not continue. A total walking distance of 800 m was chosen to represent community-based walking bouts [17].

Fig. 1.

Fully instrumented participant performing the A) Normal walking, B) Crutches, C) Walker, and D) Wheeled Knee Walker conditions.

Wireless force-sensitive insoles (loadsol, Novel Electronics Inc., St. Paul, MN) were placed in the participant’s left and right shoes to measure plantar normal force at 100 Hz. Insole size ranged from US Men’s 6–14 and US Women’s 5.5–11. Insoles used in this study are segmented into thirds by the manufacturer, based on sensor length, to record rearfoot, midfoot, and forefoot forces. Insoles were calibrated to each participant’s BW prior to data collection, following the manufacturer’s protocol. Previous research has shown that these insoles produce accurate and reliable plantar normal force measurements compared to force plates during walking, running, and jumping [18,19]. Flexible force sensing pads sized 158.75 mm × 107.95 mm (loadpad, Novel Electronics Inc., St. Paul, MN) were calibrated to the participant’s BW using quiet toe standing on the pad. Participants were instructed to stand on a pad while balancing on their metatarsophalangeal joints. The contralateral foot was totally offloaded during this process to help ensure 100 % BW was transmitted through the sensor. Participants were given a walker to aid with balance. This procedure was done with each pad separately. Pads were then wrapped around handlebars of the crutches and walker to measure BW being offloaded to the assistive device at 100 Hz in sync with the insoles. A custom made WKW instrumented with uniaxial strain gauges under the handlebars (TAL206, HT Sensor Technology Co., Ltd, China, capacity: 20–300 kg) and knee cushion (TAL208, HT Sensor Technology Co., Ltd, China, capacity: 60–800 kg) was used during this study. Force data from the strain gauges were recorded at 50 Hz. 54 reflective markers were placed on the participant’s torso and lower legs to record motion capture via 20 infrared cameras (Motion Analysis Corp., Ronhert Park, CA) at 100 Hz.

2.3. Data processing

Force data from the insoles, pads, and WKW strain gauges were processed using Matlab (R2021b, The Mathworks Inc., Natick, MA). For Normal, Crutches, and Walker conditions, the insoles and motion capture system were synchronized by stomping on a force plate at the start of a walking trial for the first 13 participants. The force plate was solely used for synchronization in this setup. This stomp produced a ‘spike’ in the insole and force plate signals that were used to synchronize the signals. For the WKW condition, a pad was placed over the handlebars of the WKW in line with the center of a force plate. Participants were instructed to ‘slap’ the pad to register a ‘spike’ in force data through the pad, strain gauge, and force plate to be used to synchronize the signals in Matlab. Strain gauge data were upsampled to 100 Hz in Matlab. For participant 14 onwards, a syncing module (Loadsync, Novel Electronics Inc., St. Paul, MN) was used to time-synchronize insole, pad, and motion capture data in Normal, Crutches, and Walker conditions. Both methods of synchronization produced a delay of ≤ 10 ms, or 1 frame.

A 50 N threshold was used to identify gait events from whole foot force data during walking [3]. This threshold was chosen to avoid false gait event identification from sensor deflection within the shoe. Un-normalized stance phase force profiles were integrated numerically to calculate whole foot plantar impulse. Un-normalized force profiles from both pads were numerically integrated and summed to quantify BW offloaded onto the crutches and walker. The same analysis was done with the handlebar and knee cushion strain gauges in the WKW condition. Following these calculations, all stance phase force profiles were normalized to 101 data points and participant BW. Normalized force profiles were ensemble averaged and a peak magnitude was identified. Gait events identified from insole data were also used to parse force pad and WKW strain gauge data into corresponding stance phases. Propulsive limb joint kinematics and locomotion speed were calculated in Visual3D (Version 2021, C-Motion, Germantown, MD). Locomotion speed was quantified by averaging the instantaneous velocity of the pelvis segment in the direction of travel over time. The pelvis segment was chosen as it is closest to the body’s center of mass.

2.4. Statistical analyses

Statistical analyses were completed using R (RStudio 2022, R Core Team, Vienna, Austria). Separate one-way repeated measures ANOVA were used to compare whole foot plantar impulse, peak forefoot loading, BW offloaded, stance phase ankle ROM, and locomotion speed for all four conditions. Significant main effects of walking condition were further investigated using Bonferroni adjusted pair-wise comparisons. A Pearson product-moment correlation was used to assess the relationship between whole foot plantar impulse and BW offloaded onto the assistive device. Correlation coefficient strengths < 0.09 are considered non-existent, 0.10–0.29 are weak, 0.3–0.59 are moderate, and 0.6–1.00 are strong [20,21]. All statistical tests will be performed using an a priori α = 0.05.

3. Results

One-way repeated measures ANOVA results found a significant main effect of walking condition on all variables of interest except BW offloaded (Table 2). Locomotion with the WKW produced 43.28 %, 71.21 %, and 77.78 % less whole foot plantar impulse than Normal, Crutches, and Walker conditions, respectively (Table 2). Post hoc pairwise comparisons revealed significant differences in whole foot plantar impulse between all walking conditions (p_adj < 0.001).

Table 2.

Mean(standard deviation) variables of interest across all walking conditions. Separate one-way repeated measures ANOVA were used to compared whole foot plantar impulse, peak forefoot force, ankle joint ROM, and locomotion speed. Values that share letters are not significantly different.

	Normal	Crutches	Walker	WKW	p-value	${\eta }_{\mathit{\text{partial}}}^2$
Whole Foot Plantar Impulse (BW*s)	0.67(0.11)	1.32(0.28)	1.71(0.51)	0.38(0.08)	< 0.001	0.878
Peak Forefoot Force (BW)	0.96(0.19)a	0.76(0.15)	0.88(0.22)a	0.46(0.18)	< 0.001	0.796
Bodyweight Offloaded (BW*s)	–	0.65(0.20)	0.69(0.26)	0.66(0.12)	0.845	0.010
Stance Phase Ankle ROM (◦)	23.70(4.86)ade	24.02(3.77)abc	20.73(4.45)bd	26.52(5.93)ce	0.001	0.240
Locomotion Speed (m/s)	1.05(0.20)	0.46(0.15)	0.28(0.06)	0.79(0.25)	< 0.001	0.913

The WKW produced 52.08 %, 39.47 %, and 47.73 % lower peak forefoot force than Normal, Crutches, and Walker conditions, respectively (Table 2). Post hoc pairwise comparisons revealed significant differences between all walking conditions (p_adj < 0.05) except for the comparison between the Normal and Walker conditions (p_adj = 0.073).

No significant effect of walking condition was observed when comparing BW offloaded between assistive devices (p = 0.85). A Pearson product-moment correlation revealed a non-existent correlation between whole foot plantar impulse and BW offloaded during the Crutches condition (R = 0.05, p = 0.84), a weak negative correlation in the Walker condition (R = −0.18, p = 0.46), and a non-existent correlation in the WKW condition (R = 0.033, p = 0.9) (Fig. 2).

Fig. 2.

Pearson product-moment correlation between whole foot plantar impulse and bodyweight offloaded during stance phase. No statistical significance or correlation was observed. Shaded region represents a 95 % confidence interval.

Locomotion with the WKW produced 11.90 %, 10.41 %, and 27.93 % greater ankle ROM compared to Normal, Crutches, and Walker conditions, respectively. Post hoc pairwise comparisons only found a significant difference between the Walker and WKW conditions (p_adj = 0.002).

Locomotion with the WKW was 0.26 m/s slower than Normal. However, participants walked 0.33 m/s and 0.51 m/s faster with the WKW compared to Crutches and Walker conditions, respectively. These differences are above the minimal clinically important difference of 0.16 m/s established in other adult clinical populations [22]. Post hoc pairwise comparisons found significant difference between all walking conditions (p_adj < 0.001).

4. Discussion

The purpose of this study was to investigate the effect of crutches, walkers, and WKW on whole foot plantar loading, peak forefoot force, stance phase ankle ROM, and locomotion speed during gait. We hypothesized that 1) whole foot plantar impulse and 2) peak forefoot force would be lowest when using the WKW. We also hypothesized that 3) stance phase ankle ROM would be most reduced when using the WKW, as well as 4) the WKW having the greatest locomotion speed of the three assistive devices used in this study. All but hypothesis 3 were supported.

To our knowledge, this study is the first to investigate whole foot plantar impulse amongst assistive devices in an adult clinical population’s gait. Previous research investigating plantar force during assistive device use in healthy older adults found that peak whole foot plantar force during Normal and Crutches conditions were not significantly different, although all other comparisons between devices had significant differences [3]. In the present study, we observed significant differences in whole foot plantar impulse between all combinations of conditions, with the greatest plantar loading being experienced in the walker condition (Table 2). This finding could be due to greater stance times during locomotion, although we did not explicitly investigate that gait parameter. We attribute this finding to longer stance times because of our method of calculating impulse, where we integrated plantar normal force during stance with respect to time. Thus, a longer stance time would incur a greater impulse during a step. Moreover, this finding could suggest that prescribing walkers to offload a DFU could pose the greatest risk of ulceration in the healthy propulsive limb, as excessive plantar force is associated with an increased risk of ulceration [23–26]. Alternatively, prescribing a WKW to patients needing to offload a DFU could reduce the risk of ulceration in the propulsive foot compared to crutches and walkers.

On a similar note, peak forefoot force during stance was 37–47 % lower in the WKW condition relative to Normal, Crutches, and Walker conditions (Table 2), suggesting that the WKW requires the least amount of propulsive force for locomotion. We can speculate that the peak forefoot force is related to overall propulsive force during gait because since peak force in this region occurs during the latter half of stance for all conditions (Fig. 4). Interestingly, forefoot force sharply increased after heel strike in the crutches and walker conditions, followed by a gradual increase until about 75 % stance (Fig. 4). This finding suggests that participants were landing more flat-footed during gait, which could also explain why the ankle is dorsiflexed for much of stance during the crutches and walker conditions (Fig. 3). Locomotion speed with the WKW was the greatest among the assistive devices despite the lowest propulsive force. We speculate that this finding is due to the low rolling resistance of the WKW on flat ground.

Fig. 4.

Mean forefoot plantar force profiles for throughout stance across all four conditions. Force measurements are normalized to 101 data points and bodyweight. Peak forefoot force occurred earlier in stance compared to all other conditions, followed by normal walking, crutches, and walkers.

Fig. 3.

Ankle angle and plantar normal force throughout stance phase. Ankle kinematics and plantar normal force are normalized to 101 data points and participant bodyweight (BW) where applicable. No distinct heel-strike or toe-off peaks can be seen throughout stance with the wheeled knee walker (WKW). Positive and negative ankle angle represents Dorsiflexion (DF) and Plantarflexion (PF), respectively.

In this study, we sought to attribute reduced whole foot plantar impulse to increased BW offloaded onto the assistive device. However, our results suggest show that there is no difference in BW being offloaded onto each assistive device during stance. Further analyses investigating the correlation between BW offloaded and whole foot plantar impulse found no significant correlations in any assistive device condition. Therefore, we cannot conclude that reduced plantar loading is caused by increasing BW offloaded onto the assistive device.

Ankle joint ROM while using the walker was about 12–28 % lower during stance relative to Normal, Crutches, and WKW conditions. One explanation for this finding could have motor control implications, as walkers are not commonly prescribed to people who need temporary assistance with ambulation. Because of this novel task of using a walker, participants may co-contract the ankle dorsi/plantarflexion muscles during gait to reduce degrees of freedom used during walking, a common neuromechanical approach when motor learning. This phenomenon of co-contraction has been observed as a mechanism of modulating impedance and stability of upper limb joints while learning novel tasks [27–30]. We can only speculate this to be one cause of reduced ankle ROM as we did not measure muscle activity during gait. Another explanation for differences in ankle ROM between assistive devices could be walking patterns used for locomotion. When using the crutches, participants used a swing-through gait where they place both crutches some distance in front of their propulsive foot and swing their body through the crutches. This gait pattern could elicit greater ankle ROM because of greater degrees of freedom allowed by the crutches. The structure of the WKW could have a similar effect on lower limb degrees of freedom. The BW support afforded by the knee cushion and handlebars when using the WKW could allow for greater freedom of the propulsive hip, knee, and ankle. Contrarily, because of the front brace joining the legs of the walker, users cannot use a swing-through gait pattern. Instead, a ‘hopping’ motion could be used with the walker because of the reduced degrees of freedom available, resulting in reduced ankle range of motion.

Previous literature has found that DM patients have significantly stiffer ankles compared to healthy counterparts [9], leading us to believe that locomotion with the WKW would produce reduced ankle joint ROM given that previous findings found overall reduced plantar normal force during locomotion with this device [3]. Surprisingly, our results show that locomotion with the WKW produced the greatest ankle ROM despite producing the lowest whole foot plantar impulse. Previous research has reported that a therapy regimen aimed to increase ankle dorsiflexion range of motion reduced plantar pressure in DM patients had a positive effect [10], while other research has found that reduced foot segment excursion was correlated with increased plantar pressure under the corresponding segment [14]. Upon visual inspection, it appears that locomotion with the WKW results in the lowest peak dorsiflexion despite producing the lowest plantar normal force during stance. The mechanism behind this finding could be relaxation of the ankle dorsi/plantarflexion muscles during gait with the WKW, as users may feel more confident maintaining stability using the WKW, although we did not collect patient-reported system usability scores for this study.

Limitations of this study include the size of the force sensing pads used, lack of ground reaction force measurements in the anterior-posterior and medial-lateral directions, low resolution of the insoles used, and diabetic sample population. The size of the force sensing pads allowed patients to completely wrap them around the handles of the assistive devices, thus force measurements could be influenced by any gripping force participants applied to the handles. Additionally, the insoles used in this study were only able to record force normal to the surface of the sensor, therefore we are not certain how anterior-posterior and medial-lateral ground reaction forces change when using these assistive devices. The insoles used were also relatively low spatial resolution compared to other systems, which did not allow us to analyze force underneath specific regions of the foot. Finally, although this sample population had Type 2 DM, they had good disease management based on our A1c inclusion criterion and did not have active DFUs. The target group, patients with a DFU that receives an unloading prescription, may have comorbidities that impairs their walking ability and result in different plantar loading profiles when using assistive devices.

5. Conclusion

Locomotion with the WKW resulted in the reduced whole foot plantar loading and peak forefoot force, as well as the greatest ankle ROM and locomotion speed among the assistive devices investigated in this study. These results suggest that the WKW could be the preferred assistive device to prescribe to patients suffering from DFUs to reduce the risk of ulceration in the propulsive foot while maintaining ambulation and ankle mobility.

Abbreviations:

DFU

Diabetic Foot Ulcer

WKW

Wheeled Knee Walker

BW

Bodyweight

ROM

Range of Motion

DM

Diabetes Mellitus