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Muscle-tendon units (MTUs) in animal limbs can be categorized as monoarticular, biarticular, or multiarticular. A uniarticular muscle is a monoarticular MTU, meaning it crosses only one joint. In contrast, biarticular MTUs cross two joints, and multiarticular MTUs cross three or more joints. Uniarticular MTUs must lengthen or shorten with the flexion or extension of the joint they cross.
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What You'll Learn
Uniarticular muscles only cross one joint
Muscle-tendon units (MTUs) in animal limbs can be categorized as monoarticular (or uniarticular) if they cross only one joint. Uniarticular muscles must lengthen or shorten with the flexion or extension of the joint they cross. In other words, they cause or control motion at only one joint.
Biarticular muscles, on the other hand, cross two joints, and multiarticular muscles cross three or more joints. The hamstrings, for example, are biarticular muscles that cross both the hip and the knee. The function of biarticular muscles is complex and depends on both their anatomy and the activity of other muscles at the joints in question. Their role in movement is not well understood.
Biarticular muscles can transfer mechanical power between distal and proximal joints, although the direction and magnitude of this transfer vary with anatomy, muscle activity level, and joint angles. They can also maintain a relatively constant length due to shortening at one joint and lengthening at another joint. For instance, during the propulsive phase of a jump, the rectus femoris muscle remains unchanged in net length because the proximal and distal attachments are contradicting one another.
While biarticular muscles have certain advantages over uniarticular muscles, such as a greater variety of movement, uniarticular muscles are simpler in structure and function.
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Biarticular muscles cross two joints
A uniarticulate muscle is one that crosses a single joint. On the other hand, biarticular muscles cross two joints in series, usually in a limb. Examples of biarticular muscles include the hamstrings, which cross both the hip and the knee, and the gastrocnemius, which crosses the knee and ankle joints.
The function of biarticular muscles is complex and depends 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 a biarticular muscle play a significant role in determining its function. For instance, the gastrocnemius technically spans both the knee and ankle joints, but due to its origin point being very close to the axis of rotation of the knee joint, it functions primarily as an ankle plantar flexor.
Biarticular muscles can contract isometrically (without changing length), putting the joint into a four-bar linkage. This allows the contraction of muscles at one joint to move the other by a fixed amount. They can also transfer mechanical power between distal and proximal joints, although the direction and magnitude of this transfer vary with anatomy, muscle activity level, and joint angles.
Biarticular muscles are most responsive to upper-body pitch perturbations in human standing. They play a key role in dealing with typical perturbations such as pushing, stumbling, or walking on uneven ground. They contribute significantly to the leg force acting perpendicular to the leg axis, making them especially suitable for postural control.
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Biarticular muscles can maintain constant length
A uniarticulate muscle is a muscle that crosses only one joint. On the other hand, a biarticular muscle crosses two joints, usually in a limb. For example, the hamstrings cross both the hip and the knee.
The function of biarticular muscles is complex and depends on their anatomy and the activity of other muscles at the joints in question. Their role in movement is poorly understood.
Biarticular muscles can maintain a constant length by contracting isometrically (without changing length). This puts the joint into a four-bar linkage, allowing the contraction of muscles at one joint to move the other by a fixed amount. For example, during 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 contradict one another.
Biarticular muscles can also transfer mechanical power between distal and proximal joints, although the direction and magnitude of this transfer vary with anatomy, muscle activity level, and joint angles. This is a crucial consideration when analyzing an organism's movement using inverse dynamics.
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Uniarticular muscles must lengthen or shorten
Muscle-tendon units (MTUs) in animal limbs can be categorized as monoarticular (or uniarticular) if they cross one joint, biarticular if they cross two joints, and multiarticular if they cross three or more joints. Uniarticular muscles must lengthen or shorten with the flexion or extension of the joint they cross. If uniarticular MTUs contain relatively long tendons, lengthening and shortening may occur in series-elastic passive structures, allowing muscle fascicles to contract isometrically. For muscles that lack long tendons, such as many proximal limb muscles, lengthening and shortening must be taken up by muscle fascicles. For example, the vastus lateralis (VL) of many species appears to stretch and shorten with flexion and extension of the knee.
Biarticular muscles, on the other hand, can maintain a relatively constant length due to their ability to shorten at one joint and lengthen at another. They can also transfer mechanical power between distal and proximal joints, although the direction and magnitude of this transfer vary with anatomy, muscle activity level, and joint angles. The role of biarticular muscles in movement is complex and depends on both their anatomy and the activity of other muscles at the joints in question. For example, the human gastrocnemius spans both the knee and ankle joints, but due to its origin point's proximity to the axis of rotation of the knee joint, it functions primarily as an ankle plantar flexor.
The long head of the triceps brachii (TrLONG) in goats is a flexor at the shoulder joint, allowing it to transfer energy from shoulder extension to elbow extension. However, its long fibered architecture and massive relative size suggest a role in work production, which is less commonly ascribed to biarticular muscles.
In summary, uniarticular muscles must lengthen or shorten with the flexion or extension of the joint they cross, while biarticular muscles can maintain a relatively constant length due to their ability to act on two joints. The function of uniarticular muscles is more straightforward than that of biarticular muscles, which have a more complex role in movement and work production.
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Biarticular muscles can transfer mechanical power
Uniarticular muscles, or monoarticular muscles, are those that cross only one joint. In contrast, biarticular muscles cross two joints in series, usually in a limb. Examples of biarticular muscles include the hamstrings, which cross both the hip and the knee, and the gastrocnemius, which spans the knee and ankle joints.
The function and role in movement of biarticular muscles are complex and not yet fully understood. Their function depends on both their anatomy and the activity of other muscles at the joints in question. For example, the gastrocnemius has a much larger lever arm at the ankle than at the knee, allowing it to function primarily as an ankle plantar flexor.
The rectus femoris muscle is another example of a biarticular muscle. During 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. In this case, the muscle remains unchanged in net length due to opposing proximal and distal attachments. By contracting isometrically (without changing length), biarticular muscles can put the joint into a four-bar linkage, allowing the contraction of muscles at one joint to move the other by a fixed amount.
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Frequently asked questions
A uniarticular muscle, also known as a monoarticular muscle, is one that crosses only one joint. Uniarticular muscles must lengthen or shorten with the flexion or extension of the joint they cross.
An example of a uniarticular muscle is the vastus lateralis, which stretches and shortens with the flexion and extension of the knee.
Unlike uniarticular muscles, biarticular muscles cross two joints, usually in a limb. Biarticular muscles can cause and control motion at more than one joint, giving them a greater variety of movement.
Biarticular muscles have two main advantages over uniarticular muscles. Firstly, they can cause and control motion at more than one joint, allowing for a greater variety of movement. Secondly, they are able to maintain a relatively constant length due to shortening at one joint and lengthening at another joint.
