Biarticular muscles are

muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

s that cross two

joint A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw- ...

s rather than just one, such as the

hamstring A hamstring () is any one of the three posterior thigh muscles in human anatomy between the hip and the knee: from medial to lateral, the semimembranosus, semitendinosus and biceps femoris. Etymology The word " ham" is derived from the Old ...

s which cross both the hip and the knee. The function of these muscles is complex and often depends upon both their anatomy and the activity of other muscles at the joints in question. Their role in movement is poorly understood.

Anatomy

Biarticular muscles cross two joints in series, usually in a limb. The details of the origin (proximal attachment) and insertion (distal attachment) can play a large role in determining muscle function. For instance, the human gastrocnemius technically spans both the knee and ankle joints. However, the origin point of the muscle is so close to the axis of rotation of the knee joint that the muscle's effective lever arm would be very small, especially compared to its large lever arm at the ankle. 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

s, do not display such biases, so their function is not immediately evident from anatomy alone. Another important concept of biarticular muscles (consider the rectus femoris muscle for this example) is the change in muscle length when motion at the proximal and distal ends of the muscle is happening. For example, in the propulsive phase of a jump, the thigh is extended at the coxal joint, and the shank (lower leg) is extended at the tibiofemoral joint. These joint positions cause the rectus femoris muscle to remain unchanged in net length, because the proximal (Eccentric action) and distal (concentric action) attachments are contradicting one another.

Function

Biarticular muscles can fulfill a range of functions during movement. By contracting isometrically (without changing length), they put the joint into a four-bar linkage, allowing the contraction of muscles at one joint to move the other by a fixed amount. 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. This is a crucial consideration when analyzing an organism's movement using inverse dynamics.

References

* * * * * {{cite journal , last1=Savelberg , first1=H. H. C. M. , last2=Meijer , first2=K. , title=Contribution of mono- and biarticular muscles to extending knee joint moments in runners and cyclists , journal=Journal of Applied Physiology , date=1 June 2003 , volume=94 , issue=6 , pages=2241–2248 , doi=10.1152/japplphysiol.01001.2002 , pmid=12533502 , url=http://jap.physiology.org/content/94/6/2241.full , accessdate=20 October 2020 , issn=8750-7587 Muscular system