Participants

Eleven (4 females/7 males), elite rowers (18–23 years; 1.70–1.85 m; 58–90 kg) volunteered to participate in this experiment after providing written, informed consent. All participants (also mentioned in the following as either rowers or athletes) have been engaged in competitive rowing for at least four years. One athlete received the gold medal in the 2012 World Rowing Championships (participant 1) and all were gold medallists in national, rowing competitions. Six athletes were starboard rowers and two compete exclusively on sculls. According to the routine medical screening they receive by the National Team Medical Staff, participants did not report any musculoskeletal or foot-related disorders at the occasion of experiments. Experimental procedures conformed to the Declaration of Helsinki and were approved by the Institutional Ethics Committee of Politecnico di Torino, Italy.

Insoles

Textured insoles have been prepared to passively stimulate feet glabrous mechanoreceptors, according to the literature [18,24,25], by distributing 3 mm height nodules over a medical, rapid-prototyping material (bio-compatible photopolymer MED610; Stratasys, Eden Prairie, MN, United States). Three different insoles were prepared with the same material, depending on the number of nodules per mask (Fig 1A): insoles without nodules (Control insole), with nodules having a relatively high (HD insole) and a relatively low (LD insole) density. Nodules were arranged square over the polymer mask, with 1.7 cm and 2.8 cm distance between adjacent nodules for the HD and LD insoles, respectively. This arrangement was thought to ensure there would be at least one nodule per receptor field in the plantar surface [28]. The number of nodules, depending on the insole size, ranged between 48 and 66 for the HD insoles and between 18 and 24 for the LD insoles. The total thickness, both for the control and textured insoles, amounted to 5 mm (mask and nodes).

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Fig 1. Experimental setup.

(A) Insoles for the stimulation of cutaneous receptors (from left to right: control insoles, low-density (LD) insoles, linear spacing between nodules: 2.8cm; high-density (HD) insoles, linear spacing between nodules: 1.7cm. (B) Inertial measurement units (IMUs) arrangement.

https://doi.org/10.1371/journal.pone.0187202.g001

Experimental protocol

Participants were initially instructed to warm up on a rowing machine (Concept II model E, Morrisville, USA) for 5 min. They were asked to self-select and adjust the drag factor of the rowing machine during the warm up session though not during experiments. After that, and based on rolling starts, nine series of at least 30 consecutive strokes each were applied, one for each combination of stroke rate (18, 24 and 32 strokes/min; [11]) and insole (control, LD and HD). Test duration was set at the rowing machine display; 100s, 80s and 60s durations were respectively set for 18, 24 and 32 strokes/min, ensuring the recording of at least 30 strokes per trial. Through the rowing machine monitor, participants were provided with visual feedback on the stroke rate and on the time taken to cover 500 m for individual strokes. Our elite athletes were instructed to maintain the same rate from the beginning of each trial and to minimise the time taken to cover a 500 m distance, i.e. the standard performance metric used by rowers during indoor training. Trials were applied at random order with at least 5 min intervals in-between, to allow athletes to recover and to practice with each of the three insoles prior to starting data collection. Upon completion of the measurement session, participants were asked to indicate whether they felt comfortable or not rowing with each of the three insoles.

Instrumentation

Pressure data were sampled with the Pedar-X system (Novel_GmbH, Munich, Germany) at 50 Hz. The accuracy–higher than 5%—of the exploited capacitive technology [29,30], and the reliability of the specific arrangement of the sensor technology in this flexible, in-shoe measurement system (ICC>0.80 for total foot and all variables [31]) render the Pedar system suitable for the accurate pressure measurement this study relies on. For each foot, the instrumented insole—2mm thick and made of 99 calibrated capacitive sensors—was placed in-between the textured insole and the shoe, after removing the standard shoe insole (i.e. the insole that comes with the shoe). Capacitive sensors, once individually calibrated in factory, do not require further calibration before measurements; the only recommended procedure before starting acquisition with the Pedar insoles is the zeroing procedure aimed at removing any possible pre-loading offset. Within this study, zeroing was correctly implemented for each participant at any replacement of textured insoles. Prior to measurements, assessment trials had been conducted to exclude any possible sources of interference between the Pedar-X and the textured insoles. Moreover, at this preliminary stage, none of the volunteers complained about potential differences in cushioning between the insoles used during experiments (textured and instrumented insoles) and the standard insoles they typically wear during training.

Acceleration of the rowing machine handle was measured with an inertial measurement unit (IMU) (MTx, XSens, Enschede, the Netherlands; angular resolution 0.05°; repeatability 0.2°; dynamic accuracy 2° RMS). The sensor was secured tightly to the machine handle to minimise any relative movements between the sensor and the handle. Kinematic data of the left side were acquired with three additional IMUs of the same IMU system–whose accuracy had been proved to be adequate for the study purposes [32] –to monitor the range of knee and trunk flexion-extension. IMUs were secured laterally to the leg and thigh and centrally to the lumbar spine [11,33]. Acceleration and kinematic data were sampled at 50 Hz (output frequency) with a 12 bit A/D converter and offline synchronised with foot pressure data, via an external trigger pulse (TTL signal) provided by the Pedar-X system. A schematic view of the measurement arrangement is provided in Fig 1B.

Quantifying rowing cycles and pressure parameters

Handle acceleration was considered for the identification of individual rowing cycles. A single rowing cycle is composed of the recovery and drive phases [3]. During recovery, rowers move the handle of the oar (or rowing machine) forward with a sequential, proximal-distal joint movement; shoulder flexion and elbow extension followed by trunk flexion and then hip and knee flexion. The drive phase commences once rowers reach their maximal forward position (catch instant). During the drive phase, the handle is moved backward with a coordinated, distal-proximal joint movement; hip and knee extension followed by trunk extension and then shoulder extension and elbow flexion. The drive phase ends (recovery phase starts) when the handle is brought to rest, roughly at the chest height (finish instants). Catch and finish instants, which define the transition between rowing phases, were defined as peak instants in handle acceleration data [11]. Peak instants correspond to zero crossings in handle velocity and therefore posit the onset of recovery (finish instants: positive peaks) and drive phases (catch instants: negative peaks; cf. Fig 1 in [11]). Peaks were identified with a custom Matlab script (The MathWorks Inc., Natick, Massachusetts, USA). The duration of recovery and drive phases was then computed for each insole and stroke rate. Finish and catch instants were finally considered to segment foot pressure and kinematic data into individual rowing cycles.

Key variables characterising the rowers’ interaction with the foot stretchers were quantified from the pressure distribution. For each foot we calculated the perpendicular component of the lumped, foot stretcher force vector, the centre of pressure position (CoP) in the rearfoot-forefoot direction, and the contact area (Fig 2A). For each time sample, foot force was defined as the spatial integral of the pressure data whereas CoP was computed as the average of the sensors’ sagittal coordinates weighted by the pressure data. The contact area was defined by summing, for each time sample, the area of sensors providing pressure figures over the 15kPa threshold. Foot force, CoP and contact area time profiles were partitioned into individual rowing cycles and then averaged (Fig 2B). From these average profiles we computed the instant of peak force to test whether rowers pushed maximally at the same relative instant with the textured insoles. Force, CoP and contact area at the peak instant were considered to quantify how strongly and where on the foot stretcher rowers pushed at the instant of maximal force. Finally, during the drive phase, the average force, CoP and contact area profiles were calculated to provide a general indication on how pressure distribution changed with cutaneous stimulation. Variables were quantified for both feet to test for whether the textured insoles could attenuate potential asymmetries in foot pressure during rowing.

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Fig 2. Quantification of foot loading distribution during rowing.

(A) Force, contact area and CoP computed for the instant of peak force as well as over the whole drive phase. Data are referred to the right side of participant 6 (male, 18 years, 84 kg, 1.85 m) while rowing at 18 strokes/min with the control insoles. Profiles have been averaged over the 30 central rowing cycles of the trial (mean and standard deviation are respectively shown with continuous and dashed lines). Vertical dashed lines indicate the instant of peak force (black) and the catch instant (grey). (B) Pressure values detected by each of the 99 sensors of the instrumented insole (darker intensities indicate lower pressure values). Circles denote the sensors which pressure value was greater than the 15 kPa threshold, defining the absence of foot contact, whereas the crossed circle indicates the coordinate of pressure distribution along the foot fore-rear direction.

https://doi.org/10.1371/journal.pone.0187202.g002

Trunk and knee kinematics were computed from the inertial sensors data. Orientation matrix data were converted to Euler angles, which were then considered to quantify changes in trunk and knee angles in the sagittal plane. The range of flexion and extension motion throughout the rowing cycle was considered to assess the effect of insoles on trunk and knee kinematics.

Statistics

After ensuring the homogeneity of variance (Levene’s test; W values >0.24 for all cases) and the data Gaussian distribution (Shapiro-Wilk statistics; P>0.15 for all cases), parametric tests were applied to compare foot pressure data for the different insoles and stroke rates. Multi-factorial ANOVA was applied, with insoles as repeated measures (2 sides x 3 stroke rates x 3 insoles). Bonferroni correction was considered for post-hoc analysis. Differences in the duration of rowing cycles and the effect of textured insoles on kinematics, distance travelled and mean power were tested with two-way ANOVA (3 stroke rates x 3 insoles), with insoles as repeated measures. Based on the error variance and the variance associated with the insoles, we estimated the effect size to range from 31% (contact area at peak force) to 61% (average force during drive; [34] Thirty rowing cycles ensured a high/moderate (range: 46–91%) statistical power [34].

Effect of textured insoles on plantar pressure distribution during rowing

The textured insoles affected the distribution of plantar pressure during rowing. When rowing with textured material inserted into their athletic shoes, participants could develop greater forces than when rowing pushing on a smooth surface (Figs 3 and 4). In agreement with previous reports [17,20], such effect was however statistically significant only for the insoles with more nodules (HD insole; Fig 1). Watanabe and Okubo [17], for example, observed the number of neuronal discharges in the tibial nerve was greater for insoles with a greater number of cutaneous, stimulation points. It is possible the HD insole sensitised a greater number of mechanoreceptors [28], thereby increasing sensory feedback on the spacing and orientation of the texture pattern and consequently on the efferent responses produced by the lower limbs over the foot stretcher. Differently from the hand [36], the position of receptive fields is randomly distributed throughout the plantar surface [28]. Such a wide dispersion of receptive fields may serve well for sensing fine changes in plantar pressure distribution, both within and between strokes. The effect of HD insoles was evident both at the instant of peak force and during the whole drive phase (Figs 3 and 4), when rowers push strongly against the foot stretcher through vigorous extension of their legs, trunk and arms. Even though the values of peak force reported here are within the documented range of foot stretcher peak forces (300-650N; [1,3,8]), figures on the mean force across drive as well as on foot contact area and CoP (cf. Fig 2) during rowing were not identified in the literature. Here we show rowers seem to use a small area (~40%), in relation to the average size (~165cm²) of their feet, to push against the foot stretcher. And they push predominantly with their forefoot (Figs 2B, 4C and 4F). Our results further show the variations in total force (Fig 4A and 4D) with insole type were accompanied by proportional variations in foot contact area (Fig 4B and 4E). This indicates the textured insoles resulted in greater forces being applied over greater foot stretcher area.

An alternative competing explanation for the altered plantar pressure distribution could have been discomfort. Textured insoles have been used indeed to induce discomfort and therefore to improved kinematics. Aruin and Kanekar [21] used textured insoles alternatively on a single foot to induce discomfort; similarly, Ritchie and colleagues [19] argued the lower ankle pronation they observed during gait could have been partly due to an individual attempt to minimise discomfort resulting from the insole nodules in the midfoot. We however believe discomfort unlikely explains results presented in Figs 2 and 4. If for example participants had increased the foot contact area in an attempt to reduce pressure under individual nodules, greater contact area would have been expected for LD insoles; condition of maximal pressure under individual nodules. Moreover, major, kinematic changes presumably resulting from discomfort were not observed and none of the study volunteers reported discomfort with the insoles upon the end of experiments.

The potential benefits of textured insoles in rowing

Textured insoles have been used in different fields with the twofold aim of improving performance and preventing injuries [20,23]. Performance is typically conceived in terms of better discrimination of foot orientation in space [26], reduced postural sways [24,25] and reduced gait variability [20]. Improvements in such performance variables may in turn reduce the risk of injuries, as more accurate information about foot position in space and better balance and gait skills may assist people in safely interacting with surroundings (e.g. successful obstacle clearance). In the present study, better performance of the indoor rowing athletes was associated with improved foot loading on an increased plantar pressure distribution surface (Fig 2). The increased foot forces observed in response to the use of HD insoles (Fig 4) is likely associated with greater forces applied to the machine handle, possibly explaining the non-significant though consistently greater average power measured for the denser insoles (Table 1). Given the duration of rowing cycles did not change across insoles, the slight increase in the measured power could be alternatively explained by greater stroke lengths. This possibility seems however unlikely as cutaneous stimulation did not affect the range of knee and trunk motion in the sagittal plane. Notwithstanding the potential sources accounting for increased power, our results suggest athletes could optimise the application of force to the foot stretcher when provided with enhanced sensory feedback of plantar pressure.

A note should be made here on the relevance of the statistically small changes with foot insole reported in Table 1, both for power and distance travelled. Overall power depends on the force applied to the machine handle, which in turn depends on inter-segmental forces developed in response to the foot stretcher reaction forces. It is therefore possible that the increased foot force observed for denser insoles was not sufficiently high for the overall power and distance travelled to increase significantly. Nevertheless, the consistently longer, and likely relevant, distance travelled with the insoles with more nodules suggests the increased foot force may contribute to improving performance. In fact, considering roughly 200 strokes take place in a 2000 m race, small changes in power may represent major changes in general performance. Indeed, when considering the average distance travelled for the different trials, participants reached distances from 3 to 19 m longer when rowing with the HD rather than the control insole (Table 1). In relation to the latter, and considering an average 4 m/s boat speed [3,13], we estimate our participants would have travelled equal distances by a shorter duration, from 0.75s to 4.75s, with the HD insole. In agreement with this view, Smith and Hopkins [37] observed minor increases in boat speed (~0.5%) may impact crucially in race times. While the association between foot stretcher forces and performance during indoor rowing remains the subject of future investigations, our results show passive stimulation of foot sole does lead to application of greater forces to the foot stretcher.

Left-right asymmetry in plantar pressure distribution may be an additional determinant of performance, as well as of musculoskeletal integrity, during rowing. Among other circumstances, both for novice and elite rowers, asymmetries have been reported in muscle activation [11], oar forces [38], joint kinematics [39] and, most importantly, in foot stretcher forces [6,39]. These side differences may be detrimental to rowing performance, as suggested by the negative correlation between asymmetry and indoor rowing performance reported by Longman et al. [4]. Specifically concerning the foot sole, side differences in foot stretcher forces may lead to greater force application on one blade during sculling, resulting in boat yawing and therefore greater drag forces [3,16]. Moreover, asymmetrical foot forces may demand a differential loading of left-right muscles to optimise force transmission to the oar handle, possibly contributing to the stress injuries and to the frequent episodes of back pain reported for rowers [11,12,40,41]. Even though the left-right differences observed in foot force for the control insole did not reach statistical significance (cf. black bars in Fig 4), they were well in agreement with mean asymmetry values (~10%) reported by others for foot force [6,39]. Strikingly, our results suggest there is a tendency though for denser, textured insoles to attenuate such asymmetry (cf. differences between feet for force and area values shown in Fig 4); side differences in foot force and foot contact area, both as peak and average values, reduced by ~4% (effect size greater than 51% [34]) when comparing HD and control insoles. Additionally, it is worth noting that two out of the 11 subjects tested in our study competed exclusively on sculls; it is thus possible that our results have underestimated the effect of insoles on the reduction of side-differences in foot pressure. Regardless of the causes chiefly accounting for asymmetric rowing [1], and considering hundreds of strokes are performed during competitions and regular training sessions, it seems worth to devise further investigations aimed at clarifying the potential for textured insoles to minimise side differences during rowing.

Although we understand on-water assessment would provide a more comprehensive view of the effect of textured insoles on rowing performance, we limited our investigation to indoor rowing. Two reasons motivated our decision. First, during indoor rowing we could control for the potential effect of confounding factors on the plantar pressure distribution, such as boat roll movements and asymmetric hand movements resulting from oar handles overlapping in sculls. Second, plantar pressure and joint kinematics could not be measured on-water at the time of the assessment without peculiar hardware adaptation. Given rowing simulators seem to reproduce well the lower limb kinematics observed on water [1], one may speculate there is a potential for the effect of textured insoles observed here to apply for on-water rowing as well. However, the above should not be conceived to straight predict on-water performance; rather, they may pose the grounds for devising future protocols aimed at assessing directly the effect of insoles on on-water, rowing performance.

Comparison with existing literature

It is worth mentioning no previous accounts on the use of textured insoles and on the plantar pressure distribution during rowing were found. Direct comparisons between present and previous results are therefore not possible. Differently from previous studies, here we assessed the effect of textured insoles directly on the plantar pressure distribution, at the interface mediating the responses to stimuli provided by the textured material. Insole effect was assessed during rowing because of the pivotal role foot forces may have on general performance [16,39]. Further, forces developed at the foot stretcher crucially affect boat propulsion and may help discriminating sources of left-right asymmetries among elite rowers [9,12,40].

Limitations and future perspectives

Inter-individual differences in foot positioning on the foot stretcher and the general posture of participants during rowing could be argued a potential limitation of the study. Even though we did not measure specific, postural variables, we believe any potential variability between subjects would have not affected our current results. Given we considered insoles as repeated measures, the variability due to inter-individual differences is cancelled [42]. We acknowledge though an exhaustive analysis on the direction and magnitude of the lumped, force vector, not possible with the used experimental setup, could reveal additional, relevant features. A potential limitation might also come from the lack of a sham condition. However, while on one side participants were aware of the experimental intervention, attention was paid when informing them on the study to remain as neutral as possible with respect to the positive, negligible or negative effect of each of the three insoles. Finally, different sources other than textured insoles might have been considered for augmented, foot feedback. With vibrotactile devices, for example, the frequency, amplitude and site of plantar stimulation may be well manipulated [43]. In the present study we considered however testing for the effect of textured rather than of vibrotactile insoles, as the former is less expensive and has been used with beneficial remarks in different circumstances [20,23,26]. Most importantly, the textured insoles considered here may be readily used to investigate the effect of cutaneous stimulation on foot stretcher forces and general performance during on-water rowing, once the instrumentation has been adapted for. Relevant research questions about long-term effects of HD insoles during on-water rowing deserve further investigation.