A bi-articular muscle is one that crosses two joints. There are, in the legs, three primary muscles that fit the definition bi-articular. They are the rectus femoris at the front of the thighs, the biceps femoris or hamstrings at the back of the thighs, and the gastrocnemius, or calf muscles. (There are four others, partners to those in the upper leg, but it will suffice to work with these three to describe general principles of running mechanics.) Bi-articular muscles function to link the action of muscles along a limb so that the work of powerful mono-articular muscles is transmitted to an extremity. In the case of a leg, this means that it is possible to deliver the power of the gluteus muscles to the feet without changing the lengths of the major leg muscles. For example, in a pure vertical jump, the length of the bi-articular muscles will/can remain constant throughout all phases of the movement, allowing mono-articular muscles, particularly the gluteus maximus (butt) muscles, to extend the entire leg. Stated otherwise, the gluteus maximus muscles, the largest and strongest muscles in the body, when contracted, extend the whole leg through the linking action of the bi-articular muscles. (I am oversimplifying things a bit, as there are other mono-articular muscles that are important in running, such as the vastus lateralis,intermedius and medialis, which are increasingly active in sprinting. David Hopper
This link above is me experiencing these extra blood channels. The modern chair made it easy for mankind not to use this Bi-articular and Mono-articular understanding of anatomy.