Foot fixation – or the feeling that studs are trapped on the grass during a rotational movement – is a mechanism commonly associated with injury to the anterior cruciate ligament in football. Despite this, little information exists regarding the magnitude of traction forces at the shoe-surface interface. This FastFact investigates how traction (grip) between the shoe and playing surface changes over one year of elite football and speculates on what impact this may have on football boot selection1.
A lower knee flexion angle, higher external knee valgus moments, increased knee joint loading and increased distance from the plant foot to the centre of mass during cutting manoeuvres are some of the changes that happen under higher traction conditions at the shoe-surface interface. These movements, along with increased loading, have been implicated in anterior cruciate ligament injury and other lower extremity injuries.
This research from the Aspetar FMCE aimed to assess variation in shoe-surface traction of different football boots throughout a full playing season encompassing climatic and grass species variation. Six different football boots were loaded onto a portable shoe-surface traction testing machine to assess how the studs/cleats released from the surface during a rotational traction test. Testing was repeated at five individual time points to collect data across one season on the Qatar national team training pitch. Surface mechanical properties (hardness and soil moisture) and climate data (temperature and humidity) were assessed during each test to study their possible relationship with shoe surface traction.
The rotational traction varied substantially across different shoe type, outsole groups, and grass species. In general highest rotational traction values were seen with the soft ground outsole (screw-in metal studs) shoes tested on warm season grass. Lowest rotational traction values were seen with the artificial grass outsole (small round moulded studs) shoes tested on cool season grass.
At present, there is little or no objective data to help players (or their support staff) choose the best footwear for a given day. The current study shows that the variability within a single season is large enough to warrant tailoring of footwear across different months. Shoes with lower rotational traction help minimise the risk of the foot becoming trapped on the surface during turning or cutting movements. Obviously too little traction can cause slips, so finding the balance for good straight line (translational) traction with reduced rotational traction may be key. Providing objective data on ground hardness or traction may help inform players and their medical staff about the optimal shoe selection. It is suggested that players be encouraged to choose the shoe with the lowest rotational traction that does not impair their performance. This might be particularly important when players return to field-specific rehab or training after a significant lower extremity injury in which torque is involved as the primary mechanism of injury (such as anterior cruciate ligament tear or syndesmosis ankle injury) is pragmatic. It is important to highlight that evidence linking high traction to injury is limited to American football and European handball. No prospective studies have been conducted in soccer football.
1.Thomson, A., Whiteley, R., Wilson, M. and Bleakley, C., 2019. Six different football shoes, one playing surface and the weather; Assessing variation in shoe-surface traction over one season of elite football. PloS one, 14(4), p.e0216364.