Three Irish athletes take part in the 50-kilometre walk today
London 2012 bronze medallist Rob Heffernan, Brendan Boyce and Alex Wright are the only Irish athletes in action on Friday at the Olympic Games.
All three men are competing in the 50-kilometre walk. Wright has already competed at these Olympic Games where he finished 46th in the 20-kilometre race.
The longer distance is among the most gruelling of the Games, with the winner expected to complete the race in around three hours and 35 minutes. Race walking is one of the strictest disciplines in athletics, with posture and technique being as crucial as race speed.
Two basic rules cover the race that are crucial to all the competing athletes, and will be widely scrutinised by judges around the course in Brazil.
The first is that the athlete's back toe cannot move off the ground until the heel of the front foot has touched. This rule is called "loss of contact".
The second rule states that the supporting leg must be straight from the point of contact with the ground and remain straightened until the body passes over it.
These rules are maintained around the course by judges, but no technology is used to make the decisions. All calls are made with the naked eye.
Athletes in each race are allowed two red-cards (warnings). A third red-card results in disqualification for the walker involved.
The sport can be notoriously harsh with judges regularly disqualifying athletes for breaking the rules. The most famous incident came at the 2000 Olympic Games in Sydney when Australian Jane Saville was disqualified in the women's 20-kilometre walk while entering the stadium on the way to winning the race.
Savile's reaction was one of the most iconic moments of the Games.