Logan Steele
The human body contains three types of muscle: skeletal, cardiac, and smooth. Skeletal muscles are the muscles that connect to bones, creating and stopping movement in response to signals from the nervous system. The nervous system causes the thick and thin filaments to slide across each other, shortening the muscle and causing it to contract. Tendons help connect skeletal muscles to bones. A muscle's origin is its attachment to an immobile bone, and its insertion is its attachment to a mobile bone. There are four categories of muscles: agonists, antagonists, synergists, and stabilisers. When creating animations, computer animators usually start with a virtual wireframe model of a character’s outer appearance. By starting from the inside with a skeleton and working outwards by adding muscles, it is possible to make very realistic animations.
| Characteristics | Values |
|---|---|
| Types of muscle | Skeletal, cardiac, and smooth |
| Types of muscle action | Agonists, antagonists, synergists, and stabilizers |
| Role of agonists | Primary force that drives the action |
| Role of antagonists | Provide resistance and/or reverse the movement |
| Role of synergists | Help the agonist |
| Role of stabilizers | Maintain the position of the body or limb during muscle actions, keep muscle movements controlled |
| How muscle movement is animated | Start with a virtual wireframe model of a character’s outer appearance, then use preprogrammed limb movements to work out how the character’s outer layer should bend before overlaying skin and hair |
| How muscle movement works in the body | Signals from the nervous system cause the thick and thin filaments to slide across each other, shortening the muscle and causing it to contract |
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What You'll Learn
The four categories of muscles: agonists, antagonists, synergists, and stabilisers
Muscles are categorised into four groups: agonists, antagonists, synergists, and stabilisers. Each of these groups plays a different role in the movement of bones.
The agonist, also known as the prime mover, is the muscle that provides the primary force driving the action. It is the main muscle that tries to create a movement at a joint. The prime mover is usually the largest muscle that provides the most force to produce the movement.
The antagonist is the main muscle that does the opposite action at the joint. It provides some resistance and/or reverses a given movement. Prime movers and antagonists are often paired up on opposite sides of a joint, with their roles reversing as the movement changes direction.
Synergists are muscles that assist the prime mover in its role. They are often smaller and produce less force, but they contribute to the movement by refining the angle of movement so that the desired movement occurs. A synergist that makes the insertion site more stable is called a fixator.
Stabilisers, or stabilizers, are muscles that keep everything else in place while the agonist and synergists are trying to do their work. They act to keep bones immobile when needed, such as when your back muscles are keeping your posture sturdy.
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Skeletal muscle microanatomy
Skeletal muscles are voluntary muscles, meaning you control how and when they move and work. They are located throughout the body, between bones. They consist of flexible muscle fibres that range from less than half an inch to just over three inches in diameter. Each muscle can contain thousands of fibres.
The muscles at play in any given action can be divided into four categories: agonists, antagonists, synergists, and stabilisers. An agonist is the primary force that drives the action. The agonist is also called the prime mover. An antagonist provides resistance and/or reverses the movement. These muscles help maintain the position of the body or limb during muscle actions, and they also keep muscle movements controlled. Synergists help the agonist.
Signals from the nervous system cause the thick and thin filaments to slide across each other, shortening the muscle and causing it to contract. Tendons help connect skeletal muscles to bones. A muscle's origin is its attachment to an immobile bone, and its insertion is its attachment to a mobile bone.
Different types of sheaths, or coverings, surround the fibres: Epimysium is the outermost layer of tissue surrounding the entire muscle. Perimysium is the middle layer surrounding bundles of muscle fibres. Endomysium is the innermost layer surrounding individual muscle fibres. The terminal arteriole, along with the capillaries that it supplies, is known as a microvascular unit. It is the smallest unit in the skeletal muscle where the blood flow can be regulated.
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How signals from the nervous system cause muscles to contract
The process of muscle contraction can be summarised in three steps. Firstly, a message travels from the nervous system to the muscular system, triggering chemical reactions. The brain sends electrochemical signals through the somatic nervous system to motor neurons that innervate muscle fibres. A single motor neuron with multiple axon terminals can innervate multiple muscle fibres, causing them to contract at the same time.
Secondly, the chemical reactions lead to the muscle fibres reorganising themselves in a way that shortens the muscle, causing the contraction. The connection between a motor neuron axon terminal and a muscle fibre occurs at a neuromuscular junction site. This is a chemical synapse where a motor neuron transmits a signal to a muscle fibre to initiate a muscle contraction.
Thirdly, when the nervous system signal is no longer present, the chemical process reverses, and the muscle fibres rearrange again, causing the muscle to relax. ATP is needed for normal muscle contraction, and as ATP reserves are reduced, muscle function may decline, leading to muscle fatigue.
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Muscle attachments
Skeletal muscles are attached to bones and create and stop movement in response to signals from the nervous system. A skeletal muscle attaches to bone (or sometimes other muscles or tissues) at two or more places. If the place is a bone that remains immobile for an action, the attachment is called an origin. If the place is on the bone that moves during the action, the attachment is called an insertion. The muscles surrounding synovial joints are responsible for moving the body in space. Tendons are a common tissue that connect muscle to bone. At either end of the tendon, its fibres intertwine with the fascia of a muscle or the periosteum (a dense fibrous covering of a bone), allowing force to be dissipated across the bone or muscle. Aponeuroses are large, sheet-like layers of connective tissue with a similar composition to tendons. Aponeuroses can also attach to bone, as in the scalp aponeuroses, and to the fascia of other muscles or tissues, such as the anterior abdominal aponeuroses. Their large form and shape provide structure and distribute tension across a wider area or large number of muscle groups. Muscles can also attach directly to other tissues, which is most evident in the face.
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How animators create realistic movement
Animators use a variety of techniques to create realistic movement. One of the most important is the use of reference and observation. This involves studying how real objects and creatures move in different situations and environments, using videos, photos, live models or even acting out the movements themselves. This helps animators to capture the details, nuances and subtleties of realistic movements, such as weight, balance, momentum and personality.
Another technique is to use layers and breakdowns. Layers allow animators to separate different aspects of their animation, such as the body, head, arms, legs, hands and facial expressions, and work on them individually or together. Breakdowns are the poses that connect the key poses and define the direction, speed and shape of the movement. By using layers and breakdowns, animators can refine their work and add more complexity and variation to their movements.
Animators also use the 12 principles of animation, which include squash and stretch, anticipation, staging, straight ahead and pose to pose, follow through and overlapping action, ease in and ease out, arcs, secondary action, timing, exaggeration, solid drawing, and appeal. By following these principles, animators can create movement that is fluid, dynamic and expressive, regardless of the medium or style of their animation.
Finally, animators should use the animation tools available to them effectively. This includes features such as keyframes, curves, graphs, layers, rigs, bones, controllers, or modifiers, which can be used to manipulate the position, rotation, scale, and deformation of animation elements, as well as to adjust the timing, spacing, and interpolation of animation frames.
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Frequently asked questions
Skeletal muscles are the muscles that connect to bones, creating and stopping movement in response to signals from the nervous system.
Signals from the nervous system cause the thick and thin filaments to slide across each other, shortening the muscle and causing it to contract.
Tendons help connect skeletal muscles to bones.
The four categories of muscles are agonists, antagonists, synergists, and stabilizers.
Agonists are the primary force that drives the action.
