What Are Biarticular Muscles and How Do They Work?

Biarticular muscles are a class of muscle tissue distinguished by crossing two joints instead of just one. This structural characteristic allows them to influence movement at both of the joints they span. This configuration enables them to perform complex actions and contribute to movement in ways that single-joint muscles cannot.

How Biarticular Muscles Function

The function of a biarticular muscle is complex, as its action depends on the position of both joints it crosses. When a biarticular muscle contracts, it can cause movement at either or both joints simultaneously. The force the muscle produces at one joint is directly influenced by the muscle’s length, which is determined by the other joint’s position.

A biarticular muscle’s length changes based on the combined movement of the two joints. If both joints move to stretch the muscle, it will be under high tension. Conversely, if both joints move to shorten the muscle, its ability to generate force may be reduced. This dynamic relationship means the muscle’s effectiveness changes with the action being performed.

These muscles can also act isometrically, contracting without changing length to stabilize one joint while allowing movement at another. This stabilization creates a rigid link that helps transfer force generated by other muscles across the skeleton. This capability is important for producing powerful movements like jumping or throwing, where force needs to be channeled through multiple body segments.

Key Examples in the Human Body

The lower limbs contain several well-known examples of biarticular muscles.

• The hamstrings, a group on the back of the thigh including the biceps femoris, semitendinosus, and semimembranosus, cross both the hip and knee joints. Their actions are to extend the hip and flex the knee.
• The rectus femoris, one of the four quadriceps muscles, is on the front of the thigh. It is biarticular because it spans both the hip and the knee, functioning to flex the hip and extend the knee.
• The gastrocnemius is the larger of the two major calf muscles. It originates above the knee and inserts into the heel, crossing both the knee and ankle to point the foot downward and assist in flexing the knee.
• The biceps brachii in the upper arm is another example. It crosses the shoulder and elbow joints to help raise the arm and bend the elbow.

Unique Contributions to Movement

The design of biarticular muscles allows them to contribute to movement efficiency and power. Their primary advantage is the ability to transfer mechanical energy between joints. For instance, during running, the gastrocnemius can transfer energy generated at the knee to the ankle, contributing to a more powerful push-off. This transfer means single-joint muscles do not have to do all the work, distributing the load more effectively.

This energy redistribution enhances the power of explosive movements. During a vertical jump, muscles like the rectus femoris and hamstrings coordinate the extension of the hip and knee. The muscles can maintain a relatively constant length as both joints move, allowing them to efficiently transmit force from large hip muscles to the lower leg, resulting in a more forceful launch.

Biarticular muscles also improve movement economy by reducing the muscular work required for certain tasks. By linking the motion of two joints, these muscles help control the body’s momentum with less energy expenditure. This coordination comes from their ability to generate motion at one joint while controlling or resisting movement at another.

Considerations in Exercise and Injury Prevention

The mechanics of biarticular muscles have direct implications for exercise and injury prevention. When stretching a biarticular muscle, its position across both joints must be considered to achieve a full range of motion. To properly stretch the hamstrings, for example, the knee must be extended while the hip is flexed, as a bent knee will not place the muscle under enough tension.

Strengthening exercises can be tailored to target these muscles more effectively. For the rectus femoris, performing a leg extension with the hip extended (such as when lying prone) can increase its activation compared to a standard seated leg extension. Extending the hip pre-stretches the muscle, allowing it to contract more forcefully at the knee.

Biarticular muscles are more susceptible to strain injuries because they are subjected to forces from two different joints. The hamstrings are particularly vulnerable during activities involving rapid acceleration and deceleration, like sprinting. These injuries often occur when the muscle is forcefully lengthened while it is contracting, a scenario common when the hip is flexing and the knee is extending simultaneously. Proper warm-ups, targeted strengthening, and maintaining flexibility are important strategies for injury prevention.