Influence of the Shod Condition on Running Power Output: An Analysis in Recreationally Active Endurance Runners

es have already analysed power output in running or the relation between VO2max and power production as factors related to running economy; however, there are no studies assessing the difference in power output between shod and barefoot running. This study aims to identify the effect of footwear on the power output endurance runner. Forty-one endurance runners (16 female) were evaluated at shod and barefoot running over a one-session running protocol at their preferred comfortable velocity (11.71 1.07 km h 1 ). The mean power output (MPO) and normalized MPO (MPOnorm), form power, vertical oscillation, leg stiffness, running effectiveness and spatiotemporal parameters were obtained using the Stryd™foot pod system. Additionally, footstrike patterns were measured using high-speed video at 240 Hz. No differences were noted in MPO (p= 0.582) and MPOnorm (p= 0.568), whereas signi cant differences were found in form power, in both absolute (p= 0.001) and relative values (p<

(MPOnorm), form power, vertical oscillation, leg stiffness, running effectiveness and spatiotemporal parameters were obtained using the Stryd™foot pod system. Additionally, footstrike patterns were measured using high-speed video at 240 Hz. No differences were noted in MPO (p= 0.582) and MPOnorm (p= 0.568), whereas signi cant differences were found in form power, in both absolute (p= 0.001) and relative values (p< 0.001), running effectiveness (p= 0.006), stiffness (p= 0.002) and vertical oscillation (p< 0.001). By running barefoot, lower values for contact time (p< 0.001) and step length (p= 0.003) were obtained with greater step frequency (p< 0.001), compared to shod running. The prevalence of footstrike pattern signi cantly differs between conditions, with 19.5% of runners showing a rearfoot strike, whereas no runners showed a rearfoot strike during barefoot running. Running barefoot showed greater running effectiveness in comparison with shod running, and was consistent with lower values in form power and lower vertical oscillation. From a practical perspective, the long-term effect of barefoot running drills might lead to increased running ef ciency and leg stiffness in endurance runners, affecting running economy. Keywords:barefoot; footstrike; stiffness; sensor; wearable 1. Introduction Endurance running events range from 3000 m to over 160 km in ultra-marathons. Nowadays, both the number of runners in endurance races and the number of organised races have increased. For example, 19,076 runners (19.51% women) nished the half marathon in Valencia in 2020 (Spain). The lower limb muscles execute three distinctive functions during such events: (i) force and power generation; (ii) shock absorption; and (iii) store and release elastic energy [1], thus compromising running economy. Endurance runners experienced improvements in muscle strength and power, among others, after an 8-week training intervention directly affecting running economy and, thus, performance [2]. The novel appearance of wearable devices capable of obtaining kinetic and kinematic data during running offers sports practitioners a new way to quantify workload by acquiring valuable metrics such as spatiotemporal parameters, power, and leg stiffness. Sensors2022,22, 4828.

wearable devices capable of obtaining kinetic and kinematic data during running offers sports practitioners a new way to quantify workload by acquiring valuable metrics such as spatiotemporal parameters, power, and leg stiffness. Sensors2022,22, 4828.

Sensors2022,22, 4828 2 of 9 Although a wide range of endurance runners tend to collide with the ground rst with the heel when shod [3], the switch from shod to barefoot running implies a tendency toward a midfoot (MFS) or forefoot strike pattern (FFS), in uencing factors such as contact (CT) and ight time (FT), step frequency (SF), step length (SL), loading rate and leg compliance [4–7]. While leg stiffness increases with barefoot running in comparison to shod running [8], a signi cant reduction in running dynamic stability was found when changing from shod to barefoot conditions [9]. Moreover, greater activation levels in the intrinsic muscles of the foot have been found with the stance phase in barefoot running, in comparison with shod running, producing adjustment during the compression of the longitudinal arch. This results in a greater ability to recoil elastic energy when running barefoot [10,11]. The recent appearance of wearable power meters on the running scene may change training and competition by providing power output values for endurance runners. These sensors may allow us to monitor and quantify workload from a fair and objective perspec- tive with accurate replication, as they already do in cycling [12]. Velocity and both the body height and weight of a runner, as well as external conditions such as slope and wind, may in uence power output in running [13,14]. Although the level of agreement between power meter systems in running and two theoretical models for power output analysis has been assessed [15], the lack of scienti c evidence for the use and interpretation of such metrics in endurance runners may prevent sport practitioners from adopting them as a means to monitor and assess running performance. A recent wearable system (i.e., Stryd™) calculates power production while running, separating this metric into two parts: power and form power. Apparently, power re ects the power output associated with changes in the athlete's horizontal movement. Form power, however, represents the power output production caused by the combination of the oscillatory up and down movements of the centre of mass and lateral power when the athlete moves

power production while running, separating this metric into two parts: power and form power. Apparently, power re ects the power output associated with changes in the athlete's horizontal movement. Form power, however, represents the power output production caused by the combination of the oscillatory up and down movements of the centre of mass and lateral power when the athlete moves forward. This system employs mathematical calculations to estimate these two parameters from kinematic data collected from the described movements executed by the runner's foot [16]. In addition, in a recent review [17], the Stryd foot pod was noted for its reliability and compatibility with metabolic power, compared to other commercially available portable running power devices. While several studies have already analysed power output in running [18,19] and others have investigated the relation between VO2max and power production [16,20], to the best of the authors' knowledge, there are no studies assessing the difference in power output between shod and barefoot running. In order to bridge this gap, this study aims to identify the effect of footwear on power output in endurance runners. It is hypothesised that increased effectiveness, leg stiffness and power production would be identi ed in barefoot running. 2. Materials and Methods 2.1. Subjects Forty-one recreationally active endurance runners (25 males; age = 28.5 6.9 years; height = 1.73 0.08 m; body mass = 68.2 11.6 kg), recruited by convenience, volunteered to take part in this study. All the participants were 18 years of age or older, capable of running 10,000 m in under 50 min (44.02 4.22 min), injury-free for the last 6 months and were completing no fewer than 2 running sessions per week, therefore meeting the inclusion criteria. Every participant signed a formal consent form, aligned with the bioethics of the World Medical Association's Declaration of Helsinki (2013). Once the objectives and procedures of the study were explained, participants were assured that they were free to leave the study at any time. The study was approved by the Ethics Committee of San Jorge University (009-18/19), from which sport sciences students were recruited. 2.2. Procedures Participants completed

with the bioethics of the World Medical Association's Declaration of Helsinki (2013). Once the objectives and procedures of the study were explained, participants were assured that they were free to leave the study at any time. The study was approved by the Ethics Committee of San Jorge University (009-18/19), from which sport sciences students were recruited. 2.2. Procedures Participants completed two testing trials over a one-session running protocol at their preferred comfortable velocity (11.71 1.07 km h 1 ) for data collection at the San Jorge University Biomechanics Laboratory (Zaragoza, Spain) in April 2019. Both trials were

Sensors2022,22, 4828 3 of 9 completed on a motorised treadmill with a slope maintained at 0% (HP cosmos Pulsar 4P; HP cosmos Sports & Medical, Gmbh, Nußdorf, Germany). Participants warmed up for 5 min on the treadmill where the velocity was increased and decreased several times until a comfortable velocity was achieved [21]. For each trial, participants completed two successive 3 min running bouts (i.e., shod for the rst and barefoot for the latter), separated by a 2 min period to change from shod to barefoot condition. Since power output [19] and spatiotemporal parameters [22] reach a steady state in less than 2 min, data were recorded during both running trials and 6–8 strides were analysed [23]. 2.3. Materials and Testing Both body weight and height were measured for each participant, utilising a weigh- ing scale (Tanita BC-601; TANITA Corp., Maeno-Cho, Itabashi-ku, Tokyo, Japan) and a stadiometer (SECA 222; SECA Corp., Hamburg, Germany), respectively. For this study, a commercially available wearable power meter, Stryd™(Stryd Summit Powermeter; Stryd, Inc., Boulder, CO, USA), was clipped on the laces of the runner's shoe when running shod and placed and secured with tape on the runner's instep during barefoot running (Figure). This lightweight, reinforced carbon- bre foot pod (weight: 9.1 g) is based on a 6-axis inertial motion sensor (3-axis gyroscope, 3-axis accelerometer) and provides kinetic and kinematic data. During barefoot running, participants ran with socks to avoid friction injuries to the soles of their feet caused by the treadmill belt. When running shod, participants wore their traditional training shoes. The power meter was linked to the manufacturer's mobile application (StrydApp, version 5.13), downloaded on a smartphone (iPhone 8, Apple Inc., Cupertino, CA, USA), for recording data.Sensors 2022, 22, x FOR PEER REVIEW 3 of 9 University Biomechanics Laboratory (Zaragoza, Spain) in April 2019. Both trials were completed on a motorised treadmill with a slope maintained at 0% (HP cosmos Pulsar 4P; HP cosmos Sports & Medical, Gmbh, Nußdorf, Germany). Participants warmed up for 5 min on the treadmill where the velocity was increased and decreased several times until a comfortable velocity was achieved

of 9 University Biomechanics Laboratory (Zaragoza, Spain) in April 2019. Both trials were completed on a motorised treadmill with a slope maintained at 0% (HP cosmos Pulsar 4P; HP cosmos Sports & Medical, Gmbh, Nußdorf, Germany). Participants warmed up for 5 min on the treadmill where the velocity was increased and decreased several times until a comfortable velocity was achieved [21]. For each trial, participants completed two suc- cessive 3 min running bouts (i.e., shod for the first and barefoot for the latter), separated by a 2 min period to change from shod to barefoot condition. Since power output [19] and spatiotemporal parameters [22] reach a steady state in less than 2 min, data were recorded during both running trials and 6–8 strides were analysed [23]. 2.3. Materials and Testing Both body weight and height were measured for each participant, utilising a weigh- ing scale (Tanita BC-601; TANITA Corp., Maeno-Cho, Itabashi-ku, Tokyo, Japan) and a stadiometer (SECA 222; SECA Corp., Hamburg, Germany), respectively. For this study, a commercially available wearable power meter, Stryd™ (Stryd Sum- mit Powermeter; Stryd, Inc., Boulder, CO, USA), was clipped on the laces of the runner’s shoe when running shod and placed and secured with tape on the runner’s instep during barefoot running (Figure 1). This lightweight, reinforced carbon-fibre foot pod (weight: 9.1 g) is based on a 6-axis inertial motion sensor (3-axis gyroscope, 3-axis accelerometer) and provides kinetic and kinematic data. During barefoot running, participants ran with socks to avoid friction injuries to the soles of their feet caused by the treadmill belt. When running shod, participants wore their traditional training shoes. The power meter was linked to the manufacturer’s mobile application (StrydApp, version 5.13), downloaded on a smartphone (iPhone 8, Apple Inc., Cupertino, CA, USA), for recording data. Figure 1. Representation for the placement of the Stryd™ power meter clipped on the laces of the runner’s shoe (left picture) and placed and secured with tape on the runner’s instep during barefoot running (central and right picture). Average power output (w; ratio of total of watts generated to total run time), form power (w; previously

USA), for recording data. Figure 1. Representation for the placement of the Stryd™ power meter clipped on the laces of the runner’s shoe (left picture) and placed and secured with tape on the runner’s instep during barefoot running (central and right picture). Average power output (w; ratio of total of watts generated to total run time), form power (w; previously described), mean power output (w (MPO)), normalised MPO (w/kg (MPOnorm)), vertical oscillation (cm; quantity of up and down movement generated dur- ing running), leg stiffness (kN/m; ratio of the maximal force at the initial touchdown to the maximum leg compression at the middle of the stance phase) and running effective- ness (kg/N; ratio of speed to power) were obtained using the Stryd™ power meter. Additionally, the running spatiotemporal parameters of contact time (time the foot spends in contact with the ground (CT)), flight time (time from toes-off to initial contact of the same foot (FT)), step length (distance covered between initial contact of one foot and the initial contact of the other foot (SL)) and step frequency (number of ground con- tacts that occurred in a minute (SF)) were also measured utilising the Stryd™ system, which has been previously validated for such purposes [18]. The foot strike pattern (FSP) exhibited by the participants was recorded using high- speed video at 240 Hz (Imaging Source DFK 33UX174, The Imaging Source Europe Figure 1. Representation for the placement of the Stryd™power meter clipped on the laces of the runner's shoe (left picture) and placed and secured with tape on the runner's instep during barefoot running (central and right picture). Average power output (w; ratio of total of watts generated to total run time), form power (w; previously described), mean power output (w (MPO)), normalised MPO (w/kg (MPOnorm)), vertical oscillation (cm; quantity of up and down movement generated during running), leg stiffness (kN/m; ratio of the maximal force at the initial touchdown to the maximum leg compression at the middle of the stance phase) and running effectiveness (kg/N; ratio of speed to power) were obtained using the Stryd™ power meter. Additionally, the running

(MPO)), normalised MPO (w/kg (MPOnorm)), vertical oscillation (cm; quantity of up and down movement generated during running), leg stiffness (kN/m; ratio of the maximal force at the initial touchdown to the maximum leg compression at the middle of the stance phase) and running effectiveness (kg/N; ratio of speed to power) were obtained using the Stryd™ power meter. Additionally, the running spatiotemporal parameters of contact time (time the foot spends in contact with the ground (CT)), ight time (time from toes-off to initial contact of the same foot (FT)), step length (distance covered between initial contact of one foot and the initial contact of the other foot (SL)) and step frequency (number of ground contacts that occurred in a minute (SF)) were also measured utilising the Stryd™system, which has been previously validated for such purposes [18]. The foot strike pattern (FSP) exhibited by the participants was recorded using high- speed video at 240 Hz (Imaging Source DFK 33UX174, The Imaging Source Europe GmbH; Bremen, Germany). The camera was placed perpendicular to the treadmill from a sagittal

Sensors2022,22, 4828 4 of 9 view at 2 m from the centre of the treadmill and at a height of 0.30 m, which has been previously validated for such a purpose [24]. Three different FSP were identi ed in the present study [4]: rearfoot strike pattern (RFS), where the heel contacts the ground rst; MFS, in which the outside edge of the foot contacts the ground rst; and FFS, where the forefoot touches down rst. 2.4. Statistical Analysis Descriptive data are shown as mean ( SD), frequency and percentage. To determine the differences between nominal variables, McNemar's test was used. The mean differences between values were analysed via pairwise mean comparisons (t-test) and the magnitude of the differences was expressed by means of the Cohen's d effect size (ES) and interpreted as trivial (<0.19), small (0.2–0.49), medium (0.5–0.79) and large ( 0.8) [25]. All statistical analyses were performed using SPSS (version 25, SPSS Inc., Chicago, IL, USA) and statistical signi cance was accepted at = 0.05. 3. Results Signi cant differences (p< 0.05) were found in spatiotemporal gait characteristics during running when comparing shod and barefoot conditions (Table). When running barefoot, lower values for CT (p< 0.001, ES = 0.46) and SL (p= 0.003, ES = 0.13) were obtained with greater SF (p< 0.001, ES = 0.59), compared to those reported during shod running at the same comfortable velocity. The prevalence of FSP signi cantly differs (p< 0.034) between conditions, with 19.5% of runners showing RF, 56.1% MF and 24.4% FF during the shod condition, whereas no runners showed RF during barefoot running, with 31.8% and 68.2% showing MF and FF, respectively. Table 1. Spatiotemporal gait characteristics during running shod and barefoot at comfortable velocity. Shod Condition Barefoot Condition p-Value (d) FSP (n, %) ˆ RF 8 (19.5) 0 (0) 0.019 MF 23 (56.1) 13 (31.8) 0.033 FF 10 (24.4) 28 (68.2) 0.012 CT (s) 0.261 (0.020) 0.252 (0.019) <0.001 (0.46) FT (s) 0.111 (0.018) 0.108 (0.017) 0.053 (0.17) SL (m) 1.11 (0.15) 1.09 (0.15) 0.003 (0.13) SF (spm) 162.06 (8.06) 166.99 (8.22) <0.001 (0.59) ˆ indicates that a McNemar

(d) FSP (n, %) ˆ RF 8 (19.5) 0 (0) 0.019 MF 23 (56.1) 13 (31.8) 0.033 FF 10 (24.4) 28 (68.2) 0.012 CT (s) 0.261 (0.020) 0.252 (0.019) <0.001 (0.46) FT (s) 0.111 (0.018) 0.108 (0.017) 0.053 (0.17) SL (m) 1.11 (0.15) 1.09 (0.15) 0.003 (0.13) SF (spm) 162.06 (8.06) 166.99 (8.22) <0.001 (0.59) ˆ indicates that a McNemar test was conducted to compare frequencies; d: Cohen's d effect size; FSP: foot strike pattern; RF: rearfoot; MF: midfoot; FF: forefoot; CT: ground contact time; FT: ight time; SL: step length; SF: step frequency. The comparisons between conditions (i.e., shod vs. barefoot) revealed no differences in MPO (p= 0.582, ES = 0.02) and MPOnorm (p= 0.568, ES = 0.03), whereas signi cant differences were found in form power, in both absolute (p= 0.001, ES = 0.14) and relative values (p< 0.001, ES = 0.33), running effectiveness (p= 0.006, ES = 0.36), stiffness (p= 0.002, ES = 0.20) and vertical oscillation (p< 0.001, ES = 0.48) (Table). Table 2. Power output and related parameters during running shod and barefoot at comfortable velocity. Shod Condition Barefoot Condition p-Value (d) MPO (W) 210.05 (44.16) 210.73 (44.24) 0.582 (0.02) MPOnorm(W/kg) 3.07 (0.32) 3.08 (0.32) 0.568 (0.03) Form power (W) 69.95 (12.51) 68.28 (12.19) 0.001 (0.14) Form power (%) 33.6 (2.8) 32.7 (2.7) <0.001 (0.33) Running effectiveness 0.95 (0.05) 0.97 (0.06) 0.006 (0.36) Leg Stiffness (kN/m) 10.26 (1.86) 10.65 (1.93) 0.002 (0.20) Vertical oscillation (cm) 7.93 (0.98) 7.48 (0.90) <0.001 (0.48) d: Cohen's d effect size; MPO: mean power output; MPOnorm: normalised mean power output.

The study compares power output in shod versus barefoot running among endurance runners.