Gluteus Maximus is very important to running

Muscles can be classified in to groups depends of number of joints its moves. If a muscle moves one joint it is called mono articular muscle and if it muscle moves one or more joints is called bi articular. The position of joint determine which muscle works more efficient.  When the hip is flexed and the knee is extended all group of hip flexor works efficiently but when the hip is flexed and the knee too the only muscle which works efficiently is the gluteus maximus. Like first part of stance in running technique.