In top-level sport and, logically, in the Olympic Games, nothing is left to chance. Everything is studied, everything is prepared with greater rigour every day. Even the relationship between the menstrual cycle and the hormones of athletes is measured in order to achieve greater efficiency in sporting performance.

One example is French swimmer Caroline Jouisse, who has been tracking her periods on her mobile phone for the past year to provide her coaches with information ahead of the 2024 Paris Olympics. The data helps her plan the best time to work on building her muscles, ideally between the middle and end of her menstrual cycle when her testosterone levels are at their highest.

"It's important to know when my testosterone peaks because that's when you feel the best and are the strongest in training," said the 29-year-old swimmer, who will compete in the 10km open water event at the Paris Games on Thursday morning, 8 August 2024.

Research in sport is overwhelmingly focused on male athletes, and the effect of menstrual cycles on athletic performance remains under-studied. Hoping to fill this gap, the French National Institute of Sport (INSEP) has launched a programme called Empow'her in 2020 to track and learn from the menstrual cycles of female athletes.

"There is no need to be embarrassed about your cycle - it is part of your performance, just like your diet or training," said INSEP gynaecologist Carole Maitre.

For six months last year, Jouisse's training sessions were analysed daily. Her hormonal, cardiac and psychological data were then compared with the different stages of her menstrual cycle. "Before I started the programme, I was not aware that all these phases existed," says Jouisse, who is currently doing 10 swimming sessions and three weight training sessions a week in preparation for the Paris Games, which officially begin on 26 July.

Another French athlete, cross-country skier Juliette Ducordeau, said the Empow'her programme had helped her understand her body better and identify "quite impressive trends" in her performance. "The optimal times for my training sessions are during the ovulation phase, from the first to the fifteenth day of my cycle," said the 25-year-old, who took part in last year's World Championships in Planica, Slovenia.

Since its launch in 2020, 130 French sportswomen have taken part in Empow'her, which aims to help fill significant gaps in scientific research into the female body.

The programme's director, Juliana Antero, said that only nine per cent of sports science studies published in the last five years were about women - compared to 71 per cent for men. "There are very few high-quality studies, so there is currently no consensus on the impact of menstrual cycles on athletic performance," Antero said.

While symptoms such as headaches and pain are relatively similar, she said, their intensity and duration can vary significantly between athletes. A 2021 study of elite female footballers in the UK found that around 90 per cent suffered from fatigue and loss of performance during menstruation.

Two-thirds said their confidence and concentration were affected, while 13 per cent missed training or matches, according to the study led by researchers at Queen Mary University of London. Period tracking hit the headlines in 2019, when the US women's soccer team credited it with helping players train for their victorious World Cup campaign.

Alpine skier Clara Direz, a former Empow'her participant, said her mostly male coaches were still "embarrassed to talk about menstrual cycles - and don't show much interest".  "It's important to raise awareness among athletes - but first and foremost we need to raise awareness among coaches," said swimmer Jouisse.

Talking about periods is still "taboo" in her sport, she added. However, with the Olympic Games due to start in Paris in July, more attention is being paid to the issue. The French Cycling Federation recently took part in a study which showed that, on average, cyclists perform better in the middle of their menstrual cycle.

"In the past, there was discomfort and athletes had to ask for help, but now we're working to give them systematic support," concludes gynaecologist Carole Maitre.