Dominic Stone
Biarticular muscles, also known as two-jointed muscles, cross two joints in series, usually in a limb. They can fulfill a range of functions during movement, such as transferring mechanical power between distal and proximal joints. All major joints of the body—the shoulders, elbows, hips, knees, and ankles—are governed by at least one two-jointed muscle. For example, the hamstrings cross both the hip and the knee joints. The rectus femoris is another example of a two-jointed muscle, spanning the hip and knee joints.
| Characteristics | Values |
|---|---|
| Name | Biarticular muscles, Two-jointed muscles (TJM) |
| Function | Movement, coordination, power transfer between joints |
| Examples | Hamstrings, Semitendinosus, Semimembranosus, Biceps femoris (long head), Rectus femoris, Gastrocnemius |
| Joints Crossed | Hip and knee, Knee and ankle |
| Training Considerations | Isolating individual functions of TJMs can maximise muscle potential |
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What You'll Learn
Hamstrings cross the hip and knee joints
The hamstrings are a group of three muscles—the biceps femoris, semitendinosus, and semimembranosus—that cross two joints, the hip and the knee. They are located in the back of the thigh, starting at the pelvis and extending to the knee. The hamstrings are skeletal muscles, which means that they are under voluntary control. They are used for many daily activities such as walking, running, jumping, and climbing stairs. They also play a crucial role in sports that require quick stops and starts, such as soccer, basketball, and football.
The hamstrings contract when the knee is bent and lengthen when the knee is extended. They assist with knee flexion (heel to buttocks) and hip extension (extending the thigh backward). The biceps femoris is also responsible for rotating the lower leg from side to side when the knee is bent. The semitendinosus and semimembranosus flex the knee and medially (inwardly) rotate the lower leg when the knee is bent.
The hamstrings are considered biarticular muscles, which means they cross two joints in series, usually in a limb. The functions of biarticular muscles are complex and depend on their anatomy and the activity of other muscles at the joints in question. Their role in movement is not yet fully understood. The origin (proximal attachment) and insertion (distal attachment) of biarticular muscles play a large role in determining their function.
To target the hamstrings in their knee flexion role, they need to be removed from their role as hip extensors. This can be done through exercises such as heel raises or toe presses, which work the gastrocnemius muscle, another biarticular muscle that crosses the knee and ankle joints.
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Gastrocnemius crosses the knee and ankle joints
The gastrocnemius muscle is a two-jointed muscle that crosses the knee and ankle joints. It is a superficial two-headed muscle that is located in the back part of the lower leg of humans. The muscle gets its name from the Latin, derived from the Greek γαστήρ (gaster), meaning 'belly' or 'stomach', and κνήμη (knḗmē), meaning 'leg', thus earning the name 'stomach of the leg' due to the calf's bulging shape.
The gastrocnemius is one of the muscles that make up the calf, along with the soleus and plantaris. The gastrocnemius and soleus come together to attach to the Achilles tendon, which then connects to the heel bone. The gastrocnemius is the larger of the two muscles and is easily visible and touchable at the back of the lower leg.
The gastrocnemius has two heads, with the lateral head originating from the lateral condyle of the femur, and the medial head from the medial condyle. The two heads of the muscle form the lower boundaries of the popliteal fossa. The muscle is fundamental for walking and posture, and it provides a significant amount of propulsive force when running, walking, or jumping.
Gastrocnemius strains are common due to its connection to two joints. A severe ankle dorsiflexion force may result in a medial gastrocnemius strain, which is commonly referred to as a "torn" or "strained" calf muscle. This injury is acutely painful and disabling. To effectively stretch the gastrocnemius, the knee must be in extension while the ankle is dorsiflexed.
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Rectus femoris crosses the hip and knee joints
The rectus femoris is a muscle in the quadriceps group that crosses the hip and knee joints. It is the only muscle in the quadriceps group that crosses the hip. It is situated in the middle of the front of the thigh and is fusiform in shape. Its functions are to flex the thigh at the hip joint and to extend the leg at the knee joint.
The rectus femoris is also known as the "kicking muscle" for its involvement in activities involving forceful knee extension. It is one of the postural muscles that tend to tighten. It is prone to injury, as it crosses both the hip and knee joints. Rectus femoris strain, also referred to as hip flexor strain, is a common injury at the tendon that attaches to the patella or in the muscle itself. The injury is usually a partial tear but could be a full tear. It is caused by a forceful movement related to sprinting, jumping, or kicking and is common in sports like football or soccer.
The rectus femoris originates at the iliac crest (pelvis) and inserts on the tibia (lower leg). It has two origins: the "direct head" originates from the anterior inferior iliac spine, while the superior acetabular ridge is the origin of the "indirect head". The direct head is more often involved than the indirect head. Acute single-head proximal rectus femoris tears may be managed conservatively, but they are prone to recurrence. Surgery is appropriate for total avulsion and recurrent single-head tears.
To assess muscle strength for the rectus femoris, the patient should be positioned with the hip and knee flexed to 90 degrees for grades 5, 4, and 3, while grade 2 is assessed in a side-lying position with the test limb uppermost and the knee flexed to 90 degrees.
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Semitendinosus, semimembranosus, and biceps femoris are lower-body two-jointed muscles
The three hamstring muscles are the biceps femoris, semitendinosus, and semimembranosus. These muscles are located at the back of the thigh and are used for walking, climbing stairs, running, doing squats, and performing other leg movements. The biceps femoris is on the outside of the back of the thigh, while the semimembranosus is on the innermost side. The semitendinosus sits between the semimembranosus and the biceps femoris.
The hamstring muscle group plays a crucial role in hip extension, which is the posterior movement of the femur, and knee flexion, which is the posterior movement of the tibia and fibula. These muscles also help to stabilise the knee joint, working together with the anterior cruciate ligament (ACL) to resist the anterior translation of the tibia during the heel strike phase of the gait cycle.
The semitendinosus, semimembranosus, and the long head of the biceps femoris cross the hip joint proximally and the knee joint distally. This configuration makes them primarily responsible for flexion of the knee joint and extension of the hip joint. The short head of the biceps femoris is an exception, as it only crosses the knee joint and is involved in knee flexion.
The hamstring muscles are biarticular muscles, which means they cross two joints rather than just one. Biarticular muscles can have a range of functions during movement, such as transferring mechanical power between distal and proximal joints. The role of biarticular muscles in movement is complex and not yet fully understood.
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Biarticular muscles can contract isometrically, creating a four-bar linkage
Biarticular muscles, also known as two-jointed muscles, cross two joints in series, usually in a limb. They can contract isometrically (without changing length) and play a role in transferring energy and resisting moments across adjacent joints. By contracting isometrically, biarticular muscles can put the joint into a four-bar linkage, allowing controlled movement at one joint to drive movement at the other joint by a fixed amount.
The four-bar linkage created by biarticular muscles differs from traditional four-bar linkages, which are typically composed entirely of bone with fixed angle relationships determined by relative bone lengths. In contrast, the biarticular muscle can hold the joint at various lengths, creating a range of four-bar properties as needed. This adaptability enables the biarticular muscle to transfer mechanical power between distal and proximal joints, with the direction and magnitude of this transfer influenced by anatomy, muscle activity level, and joint angles.
The gastrocnemius, for example, spans both the knee and ankle joints. However, its origin point near the axis of rotation of the knee joint results in a small effective lever arm at the knee compared to its larger lever arm at the ankle. Consequently, despite spanning two joints, the gastrocnemius functions primarily as an ankle plantar flexor.
The rectus femoris is another example of a biarticular muscle, crossing both the hip and knee joints. It acts as an extensor of the leg. The role of biarticular muscles in movement is complex and depends on their anatomy and the activity of other muscles at the joints they cross. Their function may also be influenced by joint moment arms and muscle length, which are dynamic factors that change as joint angles are altered.
Understanding the behaviour of biarticular muscles is crucial when analysing an organism's movement using inverse dynamics. By studying the contractile behaviour and energy transfer characteristics of these muscles, we can gain insights into their role in coordinating human leg movements, as described by Felix E. Zajac in his research on the topic.
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Frequently asked questions
They are called biarticular or two-jointed muscles.
Examples of muscles that cross two joints include the hamstrings, which cross the hip and knee, and the rectus femoris, which crosses the hip and knee as well.
The function of these muscles can be complex and depend on their anatomy and the activity of other muscles at the joints in question. They can also influence movement at both joints.
To train muscles that cross two joints, it is important to understand their specific functions and the joints they govern. For example, to target the rectus femoris as a lower leg extensor, the sissy squat is an ideal exercise.
