For instance, the human gastrocnemius technically spans both the knee and ankle joints.
[citation needed] As a result, even though it spans two joints, the strong bias in lever arms allows it to function primarily as an ankle plantar flexor.
Other muscles, such as the hamstrings, do not display such biases, so their function is not immediately evident from anatomy alone.
Unlike four-bar linkages in which all elements are bone and angle relationships are fixed by the relative bone lengths, the biarticular muscle can hold the joint at many different lengths, creating a range of four-bar properties as needed.
Biarticular muscles can also transfer mechanical power between distal and proximal joints, though the direction and magnitude of this transfer varies with anatomy, muscle activity level, and joint angles.