For more than a decade, worldwide participation in ultra-marathons and ultra-trails (running races longer than the 42.195 km marathon) has been increasing. Although considered benign, blisters are a common problem, with serious consequences and limiting performance.

Although blisters are a major factor limiting endurance performance, they are a reason for only 5.8% of ultra-trail runners to quit.

While equipment is improving and participants are increasing and intensifying their training, there is no consensus on the prevention of blisters in ultra-trail running, even though it is the number one factor limiting sports performance. There are few prospective interventional studies on the prevention of blisters during outdoor activities.

On many running forums, there is the hypothesis that lemon applied to the feet prevents the appearance of blisters. This hypothesis has never been scientifically studied.

The investigators hypothesize that the local application of Lemon allows a reduction in the number of blisters on ultra-trail.

The objective of this Blisters-stop 2 study is to evaluate the efficacy of local application of Lemon in preventing the appearance of blisters.

The patients included will be runners registered in a trail of a distance greater than 40km for whom Lemon will be applied on the friction zones of one foot as a preventive measure (28 days).

The study is coordinated by IFREMMONT and the Pays du Mont Blanc Hospitals and 82 patients must be included in order to obtain statistical significance.

Inclusion Criteria:

• Ultra-trail participants with validated registration
• Major (≥ 18 years old)

Exclusion Criteria:

• Presence of blisters or other skin lesions on the feet 28 days before the race
• Amputation of one or two feet
• Allergy to lemon
• Psychiatric disorders making comprehension impossible
• Lack of social security coverage