**1. Introduction**

Lower extremity peripheral artery disease (PAD) is a chronic atherosclerotic vascular morbidity that leads to the narrowing and/or occlusion of lower-limb arteries [1]. PAD affects more than 200 million people worldwide [2]. Intermittent claudication—a pain occurring in the lower limbs during exercise and resolving with rest—is one of the typical manifestations of PAD [1]. Intermittent claudication has a huge impact on patients' daily life activities, leading to reduced quality of life for these individuals [3,4].

Beyond the well-known manifestations of reduced walking capacities, physical function, altered muscular characteristics, and impaired balance [5–9], gait abnormalities have also been documented in patients with PAD [10–17]. Previous investigations reported a reduction in walking speed and cadence, smaller step length, and greater stance phase duration in patients with PAD compared to age-matched non-PAD individuals [13,17,18]. These changes were also observed during pain-free walking conditions [13,15,18]. The attributes of slower gait speed and stride frequency and shorter stride length were recently associated with higher levels of circulating biomarkers of inflammation and endothelial cell oxidative stress [19].

Cardiovascular risk management, pharmacological treatment, and exercise therapy are the main pillars of the treatment of PAD [1]. Following supervised exercise training (SET), greater treadmill walking performance has been well documented, with an improvement of ~82 m and ~120 m in treadmill pain-free and maximal walking distance, respectively [20]. Although less investigated, the 6 min walk test (6MWT)—an overground submaximal functional walking test—is also an effective tool that had been used to assess walking performance following interventions in patients with PAD [21,22]. A meta-analysis showed a mean improvement of ~35 m in 6 min walking distance (6MWD) following SET in individuals with PAD [23].

The question of whether SET induces gait changes, and whether the latter are related to treadmill performance in symptomatic patients with PAD, remains controversial [24–30]. To the best of our knowledge, the recent study by Lanzi et al. [29] is the only study to have assessed this relationship. Their study [29] showed that SET reduced the duration of the push-off and extended the duration of the foot-flat during a constantspeed treadmill exercise. Interestingly, in treadmill tests, gait changes were found to be significantly related to the delayed onset claudication distance [29]. However, the question of whether gait changes following SET are also related to functional walking improvements assessed by the 6MWT remains to be investigated. The investigation of this issue would be clinically relevant, as the 6MWT is representative of the type of walking one commonly partakes in daily life [21]. Following SET, a greater distance covered during the 6MWT is observed alongside an obvious increase in average walking speed. As spatiotemporal gait parameters (such as stride frequency and length) are influenced by walking speed [31], it is expected that gait pattern changes should be observed following SET during the 6MWT.

Previous investigations showed that the durations of the inner-stance phases (e.g., foot-flat and push-off) were altered during an acute treadmill exercise performed at a constant speed in patients with symptomatic PAD [29,32]. These findings showed that, compared to the pain-free walking condition, the duration of foot-flat phase increased and the duration of the push-off phase decreased with the onset of claudication pain [29,32]. The extended duration of the foot-flat phase during exertion may ameliorate the balance between oxygen supply and demand in the active ischemic calf musculature [13,29]. On the other hand, the reduced duration of the push-off phase during the transition from pain-free to painful walking may be related to exercise-induce ischemia, which may lead to calf muscle strength deficit and affect forward propulsion [8]. Notably, even if the onset of the claudication distance was delayed following SET, similar evolutions were observed during a constant-speed treadmill test regarding temporal gait parameters [29]. This suggests that once claudication is established and it worsens to moderate-to-maximal levels, similar gait adaptations occur during an acute bout of exercise before and following SET [29]. However, the evolution of spatiotemporal gait parameters during acute exercise in the form of the 6 min walk test following SET remain to be investigated in these individuals.

The primary aim of this study was to determine the spatiotemporal gait and foot kinematics parameters during an acute bout of 6MWT (acute adaptations) and in response to 3 month SET (chronic adaptations) in patients with symptomatic PAD. It was hypothesized that (1) SET would improve the 6 min walking distance (chronic adaptations); (2) SET would increase walking speed, and as well stride frequency and length (chronic adaptations); and (3) during the transition from pain-free to painful walking, similar acute gait adaptations would be observed during the 6MWT before and after SET.

#### **2. Methods**

#### *2.1. Participants*

Symptomatic patients with chronic lower extremity PAD were recruited from the Division of Angiology of the University Hospital of Lausanne, Switzerland. As described elsewhere, all the participants were enrolled in the Angiofit study and took part in the SET program [29,33]. For the purpose of this study, we included data regarding all the patients' spatiotemporal gait and foot kinematics parameters during the 6MWT before and following SET. This study was approved by the local ethics committee (study number: 2016-01135) and was conducted in accordance with the Declaration of Helsinki. Before participation, the patients provided written, voluntary, informed consent.

#### *2.2. Experimental Design*

Each participant underwent (i) a pre-SET vascular medicine examination; (ii) a pre-SET 6MWT with gait assessment; (iii) a 3-month SET program; (iv) a post-SET 6MWT with gait assessment; and (v) a post-SET vascular medicine examination.

#### 2.2.1. Vascular Medicine Examination

The medical history of each individual was assessed, and physical and vascular evaluations were performed. The ankle–brachial index (ABI) and toe–brachial index (TBI) were measured in accordance with the guidelines [1]. ABI and TBI values related to the most symptomatic leg were considered for the analyses.

#### 2.2.2. Six Min Walk Test

In an indoor 50 m corridor, the patients were asked to walk as far as possible within 6 min to determine their 6MWD [34]. The patients were told that they were allowed to stop during the test and/or lean against the wall. If they did so, they were instructed to resume walking as soon as they could. During the test, standard phrases of encouragement were used in accordance with the guidelines [34]. The pain-free walking time (PFWT6min) and distance (PFWD6min) during the 6MWT was recorded during the test. These values correspond to the time or distance covered by the patients until the onset of pain. At the end of the test, the rate of perceived exertion on Borg's scale (6: "very very light"; 20: "maximal effort") [35] and the claudication pain severity on the visual analogue scale (VAS; 0: "no pain"; 10: "maximal pain") were also recorded. In the post-SET condition, the 6MWT was performed at least 48 h following the last training session.

#### 2.2.3. Multimodal SET

The patients participated in the clinical multimodal SET program, as previously described [29,33,36,37]. Briefly, the patients performed Nordic walking twice weekly and exercises to strengthen the lower limbs once a week. Each exercise session's duration was 60 min. However, this was the total time available for each training session and does not represent the actual exercising time performed by the patients. Indeed, depending on the exercise tolerance and the baseline functional status of the patient, the actual exercising time at the beginning of the program was around 15–25 min, which increased progressively up to 30–45 min at the end of the program. Each training session started with a 5–10 min warm-up and ended with a 5 min cool down. A clinical exercise physiologist supervised all of the training sessions.

During the outdoor Nordic walking sessions, the patients were asked to walk until they experienced moderate-to-severe claudication leg pain. Subsequently, the patients were asked to rest until they experienced complete (or almost complete) resolution of the pain. To enable complete supervision over the training sessions, patients were asked to walk back and forth over a 100–200 m section of level ground. In addition, the training intensity of the exercise sessions was also monitored using Borg's scale [35]. During the first few weeks of training, patients were asked to exercise at a low intensity (9–11 on Borg's scale). Subsequently, if feasible and safe, the exercise intensity was increased

to a moderate or moderate-to-vigorous intensity (12–16 on Borg's scale). The duration of each walking bout depended on the exercise intensity and the induced claudication pain. In general, walking bouts 5–10 min in duration were performed when the exercise intensity (assessed by Borg) was set at a low-to-moderate intensity. On the other hand, walking bouts 1 to 4 min in duration were performed when the exercise intensity was set at a moderate-to-vigorous intensity. The latter, despite inducing a higher cardiovascular stimulation, usually elicits a rapid increase in, and high levels of, claudication pain in these individuals.

The strengthening of the lower limbs was performed indoors with circuit training composed of 5–6 stations. Each station consisted of (1) a different type of walking, such as toe/heel, high knees, side-to-side, or backward walking, or (2) lower-limb resistance exercises (calf/heel raise, lunges, or squats) using body weight, dumbbells, or elastic bands. During the first few weeks of training, the patients were asked to perform 5–15 repetitions of each exercise using their body weight, interspersed with 30 to 60 s of recovery. The exercise training intensity was mainly set at a low level. In the following weeks, the patients were encouraged to exercise at a moderate intensity (12–14 on Borg's scale). To that end, the patients were asked to increase the number of repetitions (20–30 repetitions using their body weight) or to exercise using dumbbells or elastic bands (10–20 repetitions).

During the program, the patients received 6 h of therapeutic workshops on cardiovascular risk factors and a healthy lifestyle (regarding nutrition, physical activity, and tobacco). Compliance with the SET program was defined by the percentage of attended sessions out of the total number of sessions [29].

#### *2.3. Spatiotemporal Gait and Foot Kinematics Parameters*

During the 6MWT, the patients wore two Physilog® (GaitUp, Lausanne, Switzerland) inertial sensor units (dimensions: 50 mm × 40 mm × 16 mm, weight: 36 g) [38,39]. These sensors were used to evaluate spatiotemporal gait and foot kinematics parameters [38,39]. Physilog® units integrate a microcontroller, a memory unit, a three-axial accelerometer (range ±3 g), a 3-axial gyroscope (range ±800◦ s −1 ), and a battery [38,39]. The inertial sensors displayed good accuracy and precision parameters and showed excellent test–retest reliability [39]. Physilog® units have been validated in young [39] and older adults [38,39]. In addition, these sensors were also validated in individuals affected by stroke [40] and in children with cerebral palsy [41]. Finally, these sensors were previously used in other clinical populations, such as patients with Parkinson's disease [42] and in patients with PAD [29].

The spatiotemporal gait and foot kinematics parameters were recorded during the whole 6MWT. For the analyses, ten consecutive strides were selected 1 min after the beginning of the test (pain-free walking: pain-free) and before the end of the 6 min walk test during the painful walking condition (pain).

During the 6MWT, walking speed, spatiotemporal gait, and foot kinematics parameters were assessed. Stride length was the only spatial parameter. The temporal parameters were stride duration and frequency (i.e., cadence) and the relative duration of the swing, stance, and double support phases (% of gait cycle duration). In addition, the relative duration of the inner-stance phases (i.e., loading response, foot-flat and push-off) were also reported [38,39]. The foot kinematics parameters were the heel-strike pitch angle (the positive angle formed between the level ground and the foot during heel-strike), the toe-off pitch angle (the negative angle formed between the level ground and the foot during toe-off), and the foot clearance (the foot's height during the swing phase). Details regarding the estimation of the spatiotemporal gait and foot kinematics parameters are presented in the supplementary materials.

The symmetry between the legs was assessed by dividing the values of the most symptomatic leg by those of the less or non-symptomatic leg [29].

## *2.4. Statistical Analysis*

The sample size was estimated using our previous data [36], showing that 23 patients were necessary (power 80%; α = 5%). The Kolmogorov–Smirnov test was used to assess the normality of the distribution. First, a two-way repeated measures analysis of variance (ANOVA) (time (before SET vs. after SET) × duration (pain-free vs. pain)) was used to evaluate the symmetry of the spatiotemporal gait and foot kinematics parameters between legs. Second, a two-way ANOVA was also used to compare the gait pattern in the most symptomatic leg alone. If the ANOVAs showed a significant main effect (time or duration) or interaction effect (time × duration), multiple comparison analyses with Bonferroni adjustments were performed to detect the differences. Paired t-tests were used to compare the 6MWT and vascular parameters before and following the multimodal SET program. To determine the relationship between the spatiotemporal gait and foot kinematics changes (i.e., delta; post- minus pre-training values) and changes in 6MWD following SET, partial correlations, controlled for gait baseline values, were performed. The data are expressed as the mean ± SD. The level of significance was set at *p* ≤ 0.05. SPSS 27 software (IBM Corporation, Armonk, NY, USA) was used for the statistical analyses.

## **3. Results**

#### *3.1. Participants*

Twenty-nine symptomatic patients with chronic PAD were included. All the patients completed the 3 month SET program. Their general characteristics are reported in Table 1. A similar pharmacological therapy was observed before and after SET, except that one patient started antidiabetic therapy during SET. The compliance of the participants with the SET program was 98 ± 4%.


**Table 1.** Characteristics of the participants.

BMI: body mass index; CVD: cardiovascular disease.

#### *3.2. Vascular Parameters*

The values regarding the ABI (before SET: 0.79 ± 0.14 after SET: 0.78 ± 0.14; *p* = 0.829) and TBI (before SET: 0.60 ± 0.15, after SET: 0.60 ± 0.18; *p* = 0.971) were unchanged following SET.

#### *3.3. Six Min Walk Test*

Following SET, a significant increase was observed in the 6MWD values (+10%; Table 2). The values regarding PFWT6min and PFWD6min did not change significantly (Table 2). The RPE at the end of the 6MWT was significantly higher after SET (Table 2). Values relating to claudication pain at the end of the 6MWT were unchanged (Table 2).

**Table 2.** 6 min walk test before and after supervised exercise training.


6MWD: 6 min walking distance; PFWT6min: pain-free walking time during the 6 min walk test; PFWD6min: pain-free walking distance during the 6 min walk test; 6MWTRPE: rate of perceived exertion at the end of the 6 min walk test; 6MWTVAS: claudication pain at the end of the 6 min walk test. Bold *p* value is statistically significant (*p* ≤ 0.05).

#### *3.4. Spatiotemporal Gait and Foot Kinematics Parameters: Acute and Chronic Adaptations*

The symmetry of the spatiotemporal gait and foot kinematics parameters between legs showed no significant time, duration, or time × duration interaction effect (data not shown). This suggests that similar acute and chronic adaptations were present in both legs of the participants. Therefore, for sake of clarity, only the results regarding the most symptomatic leg were presented.

#### 3.4.1. Spatiotemporal Gait Parameters (Acute Adaptations)

During the 6MWT, a significant duration effect was observed for all the spatiotemporal gait parameters (Table 3). Multiple comparison analyses demonstrated that walking speed, stride duration, stride frequency, stride length, duration of swing, loading response duration, and push-off phase duration significantly decreased during the transition from the pain-free to painful walking condition, whereas the duration of the stance, foot-flat, and double support phases significantly increased during the 6MWT (Table 3).
