One: sudden leg weakness during running belongs to muscle failure. When muscles are exercising, it is impossible to mobilize all muscle fibers to contract at once, but part of them. When this part of muscle fiber can't work, they are mobilizing another part. When all muscle fibers can't contract normally, they are exhausted. At first, the thigh can also mobilize the recently recovered muscle fibers. When these muscle fibers are exhausted, the subsequent muscle fibers are not ready, resulting in weakness. Two: When exercising, people should carry loads in multiple groups to achieve the cause of exhaustion. Pain in thigh muscles indicates that the practitioner's recent thigh muscles have reached a certain limit. Because thigh muscles recover slowly in the body, it takes more time to recover and replenish muscle glycogen. Otherwise, there will be a decline in endurance.