5.3 Muscular Attachments and Contraction
Skeletal muscles can attach themselves to bone by direct or indirect attachment.
- Direct muscular attachment includes the outer membranes of a muscle attaching themselves to the outer membranes of a bone (i.e., temporalis, trapezius).
- Indirect muscular attachment is more common and includes muscles attaching themselves to bones via tendons (Note: this will be discussed in more detail in Chapter 11).
Regardless of the attachment type, each skeletal muscle has at least two points of attachment with the skeletal system, known as the origin and the insertion.
| Origin | Insertion |
|---|---|
|
|
Video: Origins and Insertions
Watch the video “Origins and Insertions” by The Noted Anatomist [2:38], which is licensed under the Standard YouTube License. The transcript and closed captions are available on YouTube.
Muscle Contraction
When conceptualizing the process of muscular contraction, it is important to note that muscles can only “pull” on bone through their direct or indirect attachment site at the insertion; they can never “push”.
The extent and force of this “pulling” (muscular contraction) relative to a load (whether bodyweight or some external load) can produce these three distinct types of skeletal muscle contraction:
Concentric
Concentric Contraction: Shortening of muscle fibres. (For example, the biceps brachii move a dumbbell so that the joint angle at the elbow decreases.)
Eccentric
Eccentric Contraction: Lengthening of muscle fibres. (For example, the biceps brachii lowers a dumbbell so that the joint angle at the elbow increases.)
Isometric
Isometric Contraction: Static hold of muscle fibres. (For example, the biceps brachii is statically holding a dumbbell such that the joint angle at the elbow is unchanging).
Regardless of contraction type, skeletal muscles are typically arranged as pairs that share opposing actions, which are called antagonist pairs. Although numerous muscles may be recruited for a specific action, the principal or primary mover producing this action is referred to as the agonist muscle. Conversely, a muscle with the opposite action of the primary mover is referred to as the antagonist and is responsible for maintaining limb position and controlling movement.
Video: Your Muscle Screams in Agony
This video explains the agonist-antagonist relationship between the biceps and triceps brachii muscles well. It explains that as the agonist muscle contracts and shortens, the antagonist muscle relaxes and lengthens to facilitate movement.
“Your Muscle Screams in Agony” by Free Animated Education [1:53], which is licensed under the Standard YouTube License. The transcript and closed captions are available on YouTube.
The muscles that are activated during movement but are not included as part of the antagonist pair can be referred to as stabilizer or synergist muscles, which both aid in the facilitation of movement but have the following distinct roles:
| Stabilizer | Synergist |
|---|---|
| A muscle that provides support and holds joints in place during movement. (ex, core musculature during a sprint or rotator cuff muscles during an overhead press). |
Assist the agonist muscle by aiding in force production or by stabilizing the origin or insertion of the agonist muscle. (ex, biceps brachii during a pull-up or triceps brachii during a bench press). |
“10.4 Nervous System Control of Muscle Tension” from Anatomy & Physiology by Lindsay M. Biga, Staci Bronson, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern, Katie Morrison-Graham, Kristen Oja, Devon Quick, Jon Runyeon, OSU OERU & OpenStax is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.
“11.1 Describe the roles of agonists, antagonists and synergists” from Anatomy & Physiology by Lindsay M. Biga, Staci Bronson, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern, Katie Morrison-Graham, Kristen Oja, Devon Quick, Jon Runyeon, OSU OERU & OpenStax is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.
