An ankle-type ergometer for measuring a force exerted on the ankle joint of a lower limb of a user by the muscles involved in the mobility of said ankle, said ergometer, in the form of a boot, including: - a first portion of the splint type capable of receiving said lower limb when the knee of said user is extended, and comprising immobilisation means configured to immobilise said lower limb with the extended knee, and - a second portion comprising: a) a main body fixed to the first portion, b) a counter-bearing element secured to said main body, c) a rigid plate forming a bearing surface for the lower surface of said foot, when said lower limb is immobilised with the immobilisation means, said rigid plate being substantially static relative to said counter-bearing element, d) a force sensor interposed between said plate and said counter-bearing element. - said plate being not secured to said main body so that said sensor is capable of measuring the force exerted on the bearing surface of the plate by the muscles involved in the mobility of said ankle, said force being transmitted from the plate to the counter-bearing element. and wherein the ergometer includes: - a rolling or sliding track, - a rolling or sliding member directly or indirectly fixed to the main body, said member being capable of cooperating with said track in order to limit friction between said main body and said track and so said ergometer is configured to accurately measure the forces exerted at the ankle joint by the flexor muscles of the foot of an individual and exclusively the forces exerted by these flexor muscles, and not the forces exerted by the hip or knee extensor muscles. - a first portion of the splint type capable of receiving said lower limb when the knee of said user is extended, and comprising immobilisation means configured to immobilise said lower limb with the extended knee, and - a second portion comprising: a) a main body fixed to the first portion, b) a counter-bearing element secured to said main body, c) a rigid plate forming a bearing surface for the lower surface of said foot, when said lower limb is immobilised with the immobilisation means, said rigid plate being substantially static relative to said counter-bearing element, d) a force sensor interposed between said plate and said counter-bearing element. a) a main body fixed to the first portion, b) a counter-bearing element secured to said main body, c) a rigid plate forming a bearing surface for the lower surface of said foot, when said lower limb is immobilised with the immobilisation means, said rigid plate being substantially static relative to said counter-bearing element, d) a force sensor interposed between said plate and said counter-bearing element. - said plate being not secured to said main body so that said sensor is capable of measuring the force exerted on the bearing surface of the plate by the muscles involved in the mobility of said ankle, said force being transmitted from the plate to the counter-bearing element. - a rolling or sliding track, - a rolling or sliding member directly or indirectly fixed to the main body, said member being capable of cooperating with said track in order to limit friction between said main body and said track and so said ergometer is configured to accurately measure the forces exerted at the ankle joint by the flexor muscles of the foot of an individual and exclusively the forces exerted by these flexor muscles, and not the forces exerted by the hip or knee extensor muscles.