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The biceps brachii is a large, thick muscle of the arm consisting of two heads: the short head and the long head. Its main function is to flex the forearm at the elbow joint. The biceps are a type of skeletal muscle, which are under conscious control and are therefore considered voluntary muscles. On the other hand, involuntary muscles are controlled by the autonomous nervous system and are involved in the movement of internal organs and the passage of fluids and food in the digestive system. Examples of involuntary muscles include the cardiac muscle and smooth muscle lining the intestinal tracts, blood vessels, and urogenital tracts.
| Characteristics | Values |
|---|---|
| Type of muscle | Skeletal muscle |
| Muscle fibres | Smooth muscle fibres |
| Voluntary control | Yes |
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What You'll Learn
Biceps are skeletal muscles
Skeletal muscles make up about 30% to 40% of an individual's total body mass, with men having about 36% more skeletal muscle mass than women. The biceps muscle in a young adult male, for example, contains around 253,000 muscle fibres. These muscles are powered predominantly by the oxidation of fats and carbohydrates, with anaerobic chemical reactions also contributing to their energy supply.
The structure of skeletal muscles, such as the biceps, consists of long, fine fibres or myofibrils. These myofibrils are composed of actin and myosin filaments, which slide past each other during muscle contraction and expansion. The overlapping regions of actin and myosin filaments are known as Z lines, with the region between two Z lines called a sarcomere. Sarcomeres are considered the primary structural and functional units of muscle tissue.
Skeletal muscles, including the biceps, play a crucial role in producing movement, maintaining body posture, controlling body temperature, and stabilising joints. They are an essential part of the musculoskeletal system, allowing us to perform a wide range of movements and functions.
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They are not under conscious control
Involuntary muscles are those that are not under conscious control. They are innervated by the autonomic nervous system (ANS). The biceps brachii is a skeletal muscle of the anterior compartment of the arm. It has two heads, a short head and a long head, and its primary function is to flex the forearm at the elbow joint.
The biceps are not considered involuntary muscles. They are, in fact, voluntary muscles that are under conscious control. The biceps are controlled by the somatic nervous system, which is responsible for voluntary movements. When a person wants to move their arm, the brain sends a signal through the somatic nervous system to the biceps muscle, instructing it to contract and relax, resulting in the desired movement.
Voluntary muscles, such as the biceps, are typically attached to bones and play a role in movement and posture. They are often found in the limbs and are generally striated, or striped, in appearance due to the arrangement of their muscle fibres. These muscles can be contracted at will, allowing for a wide range of movements, from subtle gestures to more complex and powerful actions.
In contrast, involuntary muscles, such as those found in the walls of internal organs like the heart, stomach, and intestines, are controlled by the autonomic nervous system. These muscles are typically smooth in appearance and work without conscious effort. They are responsible for essential bodily functions such as digestion, breathing, and maintaining a steady heart rate.
While the biceps are not considered involuntary muscles, there are certain situations where they can exhibit involuntary movements or spasms. For example, in the case of cerebral palsy, involuntary activation of the biceps muscle, along with reduced strength, can contribute to limitations in upper limb activity. However, this is not the typical function of the bicep muscle, which is primarily under conscious control.
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Biceps are smooth muscles
Biceps are not smooth muscles. Smooth muscles are derived from both mesoderm and neural crest cells and contribute to many different tissues throughout the body. They are found in the stomach, intestines, urinary system, bladder, uterus, eyes, and heart. Smooth muscles are not under conscious control and contract involuntarily. They are not striated and do not appear striped under a microscope because their cells are arranged in sheets instead of bundles.
Biceps, on the other hand, are skeletal muscles that are under conscious control and contract voluntarily. They are the muscles that you can flex and see push up under your skin when you make a muscle in your arm. Skeletal muscles, in general, are striated or striped because their cells are arranged in bundles.
While biceps are not smooth muscles, it is important to note that the human body has over 600 muscles, including skeletal, smooth, and cardiac muscles. Smooth muscles, such as those found in the stomach and intestines, aid in digestion and nutrient collection. They also line the walls of internal organs, helping them carry out their functions. For example, smooth muscles in the stomach contract to break down food into smaller pieces.
Smooth muscles are essential for survival, even though their movements occur without conscious effort. They play a crucial role in various bodily functions, from digestion and nutrient absorption to maintaining the body's electrolyte balance and facilitating childbirth.
In summary, biceps are skeletal muscles that are under voluntary control, while smooth muscles are a separate type of muscle tissue found in various organs and systems throughout the body, contracting involuntarily to support essential functions.
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Involuntary activation of biceps is seen in cerebral palsy
Biceps are generally considered voluntary muscles, which are under conscious control. However, in some cases, the biceps can exhibit involuntary activation, which is observed in individuals with cerebral palsy.
Cerebral palsy (CP) is a neurological disorder that affects body movement and muscle coordination. It is often associated with spasticity and reduced muscle strength, particularly in the upper limbs. Spasticity refers to the involuntary contraction or tightness of muscles, leading to stiff or awkward movements.
Involuntary activation of the biceps, specifically the biceps brachii muscle, is a notable feature of cerebral palsy. This involuntary activation contributes to limitations in upper limb activity and can negatively impact overall movement and function. A study by Brændvik et al. (2013) examined the relationship between involuntary and voluntary muscle activation in children with unilateral cerebral palsy. The results suggested that both involuntary and voluntary muscle activation influenced activity levels, with voluntary activation showing a stronger correlation. However, the negative relationship between involuntary muscle activation and muscle strength indicated that involuntary activation could further restrict movement.
Additionally, individuals with cerebral palsy may experience sustained involuntary muscle activity, which cannot be exclusively explained by spasticity. This sustained involuntary muscle activity has been observed in both cerebral palsy and stroke patients, indicating a common symptom with diverse underlying mechanisms. The study by Brændvik et al. (2013) found that individuals with cerebral palsy exhibited involuntary muscle activity during attempted rest, suggesting that muscle activation can occur even when individuals are not actively attempting to move.
In summary, involuntary activation of the biceps, particularly the biceps brachii, is a characteristic feature of cerebral palsy. This involuntary activation can impact upper limb activity and overall movement, contributing to the motor impairments associated with the condition. Understanding the role of involuntary muscle activation in cerebral palsy is crucial for developing effective treatment approaches to manage spasticity and improve functional outcomes for individuals with this disorder.
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Biceps help in flexing the forearm
The biceps brachii is a muscle with two heads or points of origin (from the Latin "bis", meaning "two", and "caput", meaning "head"). The biceps brachii is a prominent muscle on the front side of the upper arm, which is located between the shoulder and elbow joints. The biceps tendon reflex tests the spinal cord segment C6.
The biceps muscle has two heads: the short head and the long head. The short head originates from the coracoid process of the scapula (shoulder blade), while the long head originates from the supraglenoid tubercle of the scapula. Both heads insert distally into the radial tuberosity and the fascia of the forearm via the bicipital aponeurosis. The biceps brachii is one of the most variable muscles in the human body and, in 10% of cases, has a third head arising from the humerus.
The biceps work across three joints, with its most important function being to supinate the forearm and flex the elbow. The long head of the bicep prevents the upward displacement of the head of the humerus. The biceps brachii functions as a powerful supinator of the forearm, turning the palm upwards. This action is aided by the supinator muscle and requires the humeroulnar joint of the elbow to be at least partially flexed. When the humeroulnar joint is fully extended, supination is primarily carried out by the supinator muscle. The biceps are particularly powerful supinators due to their distal attachment at the radial tuberosity, on the opposite side of the bone from the supinator muscle.
The biceps are also important for flexing the forearm, especially when the forearm is supinated. This action is performed when lifting objects or when performing a bicep curl. When the forearm is in pronation (the palm faces the ground), the brachialis, brachioradialis, and supinator function to flex the forearm, with minimal contribution from the biceps brachii. Regardless of forearm position, the force exerted by the biceps brachii remains the same. However, the biceps can only exert so much force, and as the forearm position changes, other muscles must compensate.
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Frequently asked questions
No, the biceps are voluntary muscles, also known as skeletal muscles, and are under our conscious control.
Involuntary muscles are those that are not under our conscious control and are innervated by the autonomic nervous system. Examples include the cardiac muscle and smooth muscle lining the intestinal tracts, blood vessels, urogenital tracts, and respiratory tracts.
Individual muscle cells in involuntary muscles are long, thin, and spindle-shaped with a centrally located nucleus. The cell membrane of the muscle fibers, called the myolemma or sarcolemma, connects the muscle fibers to each other. Involuntary muscles have fewer mitochondria as they have lower energy requirements compared to voluntary muscles.
