Is Plantar Loading Altered During Repeated Sprints on Artificial Turf in International Football Players?

Author:Girard, Olivier
Position::Research article - Report
 
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Introduction

Characterisation of the location and magnitude of plantar loading parameters can help with injury prevention (Rice et al., 2016), rehabilitation (Thomson et al., 2017) and soccer footwear design (Hennig and Sterzing, 2010). In-shoe measurement of plantar loading on the actual field of play (football pitch) allows for the analysis of football-specific movements (Eils et al., 2004; Orendurff et al., 2008), different surfaces (Ford et al., 2006) and different outsole cleat configurations at the foot-shoe-surface interface (Wong et al., 2007).

Greater vertical forces, impact loading rates and peak plantar pressure are known to occur when running in football boots compared to running or training shoes (Carl et al., 2014; Smith et al., 2004). These high peak pressures are focused at the forefoot (i.e., hallux, medial, central and lateral forefoot) during acceleration (Orendurff et al., 2008; 2009). Accordingly, it has been suggested that use of football footwear may augment the risk of developing certain lower limb injuries such as metatarsal stress fractures (Eils et al., 2004; lee and Chung, 2016; Oztekin et al., 2009).

While rare in elite male football, injuries to the 5th metatarsal in particular are problematic and often result in long absences from football (Ekstrand and van Dijk, 2013). Intuitively, muscle fatigue is thought to play a role in the development of metatarsal stress fractures as forefoot loading increases in runners who are in a fatigued condition (Weist et al., 2004). However, football-specific studies investigating fatigue-induced alterations in plantar foot-loading are very rare. In one previous study on elite U19 male football players a performance decrement (~3%) was not associated with any significant alterations in plantar pressure patterns during repeated 20-m sprints with 20-s of recovery (6 total) (Girard et al. 2011c). It is also reported that most of sprint mechanical variables differ between early acceleration (5-10 m) and near top speed (30-35 m) over a repeated-sprint series (12 x 40-m sprints) due to the increase in running velocity; these changes, however, were of similar magnitude with sprint repetitions (fatigue) between these 2 sprint phases (Girard et al., 2011b). To date, it is unknown how plantar loading parameters may change with longer (> 20 m) sprint efforts and less rest (

The running anaerobic sprint test or RAST is a protocol (6 x 35 m with 10-s rest) commonly used by football teams to assess repeated-sprint ability (RSA) (Zagatto et al., 2009). The RAST leads to substantial alterations in stride mechanics and leg-spring behaviour (Brocherie et al., 2015) and thereby may be an appropriate test for identifying fatigue-induced changes in plantar loading parameters. In a previous study investigating plantar pressure, however, only three steps close to top speed at the end of each of the six, 20-m sprints were analysed (Girard et al., 2011c).

Therefore, the aim of this study was to test the hypothesis that fatigue-induced changes in plantar loading parameters during the RAST differ depending on the distance interval in International male football players. Assessing pressures distribution during the most demanding periods of a game (i.e., succession of intense efforts), as performed indirectly here from RAST completion, have important clinical implications for protecting regions of the foot that may be susceptible to injury.

Methods

Participants

After being informed of the potential risks and benefits involved, twelve International male football outfield players (mean [+ or -] SD; age 28.3 [+ or -] 5.3 years, stature 1.78 [+ or -] 0.04 m, body weight 72.4 [+ or -] 3.1 kg) belonging to the national 'A' squad of the Qatar Football Association provided their written consent to participate in this study. All players had a minimum of 5 years of experience in the domestic 1st division league (i.e., Qatar Star League), with an average participation of 10-12 h of training and competitive play per week. The experiment was approved by the hospital scientific and Ethics committee and conformed to the current Declaration of Helsinki guidelines.

Experimental set-up

After a standardized warm-up of ~20 min, the participants performed the RAST involving 6 x 35 m straight-line maximal sprints in alternating directions interspersed by 10 s of recovery (Brocherie et al., 2015). After deceleration until a cone placed at 10 m, players jogged back to the starting line and assumed a standing ready position for 2 s before the next sprint. The RAST was conducted between 5 and 7 pm on a third-generation artificial turf indoor football field (FIFA approved for international football; Classic series, 3G, Mondo, Italy), while players were wearing their preferred moulded stud football boots. High reliability (r = 0.90) has been reported in young basketball players (Balciunas et al., 2006) or in armed force members (intraclass correlations > 0.65) (Zagatto et al., 2009) using the RAST.

Repeated-sprint ability

Sprinting times were measured to the nearest 0.01 s using dual-beam electronic timing gates (TAC System, TT Sport, Galazzano, Republic of San Marino), which height was adjusted according to the height of the participant's hip and placed at 0, 17.5 and 35 m. Each sprint was initiated from an individually chosen standing position with their leg foot in front, 50 cm behind the first timing gate. RSA was assessed using three scores: the fastest (i.e., initial in all cases) sprint time, the cumulated sprint time (i.e., sum of the six sprints) and the percentage decrement score calculated as {[100 - [(fastest sprint time x 6) / (cumulated sprint times)] x 100} (Girard et al., 2011a).

Instrumentation

Insole plantar pressure distribution was recorded for each sprint using the PedarX Mobile System (Novel GmbH, Munich, Germany). Each pressure insole consists of a 2-mm-thick array of 99 capacitive pressure sensors. After calibration, the insoles were placed bilaterally between the sock and shoe with no other manufacturer's insoles or foot orthotics in place so that the Pedar-X insoles were flat (Spooner et al., 2010). No participants used orthotic supports. The data logger (weight ~400 g) for data storage was in a harness on the players' waist. Plantar pressures were sampled at 100 Hz via Bluetooth technology. Data from the left and right foot were averaged for subsequent analysis. The validity (McPoil et al., 1995) and reproducibility (Kernozek and Zimmer, 2000) of the capacitive sensors in the Pedar-X have...

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