Learning Outcomes: Students will be able to:
Uniarticular muscles are those that cross and move only one joint. Biarticular muscles are those that cross and move two different joints. As such, biarticular muscles may contract to cause, control, or prevent movement at either one or both of the joints it crosses. Biarticular muscles allow for fewer muscles being required for the same movements (which can improved range of motion), while providing some unique characteristics designed to enhance certain movements.
Active and Passive Tension
Tension (a pulling force) can be created in a muscle via two types of forces: active tension when the muscle contracts, and passive tension when the muscle (and its fascia) is being stretched beyond is resting length.
Therefore, total tension (or force) that a muscle can generate is due to 1) the active tension of a muscle generated by its contractile actin and myosin proteins, and 2) the passive tension of a muscle due to the natural elasticity (an elastic recoil force) of the muscle’s fascia, tendons, and structural proteins.
Muscle Length – Tension Relationship & Insufficiencies
Generally speaking, and somewhat depending on the particular muscle involved, the greatest amount of active tension through muscle contraction can be developed when the muscle is slightly stretched beyond its resting length. By placing a muscle on slight stretch, “slack” is taken up so that when the muscle contracts all of the contractile force is applied to the lever to cause movement, and the elastic recoil properties of its fascia are also applied. It should be noted that the active strength of a muscle’s contraction is based on the number of cross-bridges that exist between the myosin and actin filaments in the sarcomere of a muscle. In the illustration below (A), when a muscle is placed on slight stretch all of the binding sites are accessible.
In the preparatory or cocking phase of most athletic activities we see this principle being applied to enhance performance. Whether slightly bending down just before jumping for a rebound in basketball or cocking the arm backward prior to throwing a ball, the involved muscle groups are stretched creating a more effective muscle length to produce both active and passive tension within the muscles.
How is this principle being applied in these exercises?
If a muscle is “stretched beyond the point of slightly stretched”, the amount of active tension it can generate decreases due to insufficient sarcomere overlap resulting in inaccessible binding sites. When a muscle is significantly stretched beyond its resting length its ability to generate force is reduced (weak/insufficient) due to a decrease in the actin-myosin cross-bridging and is referred to as lengthened active insufficiency (Illustration B). The actin filaments are pulled so far from the center of the sarcomere that some of the myosin heads cannot reach the actin filaments to form cross-bridges.
Likewise, a proportional decrease in the ability to develop tension occurs as a muscle is “shortened”. When a muscle is significantly shortened from its resting length, its ability to develop contractile tension is reduced due to a decrease in the actin-myosin cross-bridges. In the shortened condition, some of the actin binding sites are not accessible by the myosin heads. In essence, the muscle has shortened to the point that it cannot generate or maintain max active tension and becomes weakened or “insufficient”. This is called shortened active insufficiency (Illustration C). In addition, in the shortened position the muscle fascia is now no longer being stretched and the elastic recoil properties are diminished (no passive tension).
(Note: The active strength of a muscle’s contraction is based on the number of cross-bridges that exist between the myosin and actin filaments).
Passive insufficiency, on the other hand, is when the opposing muscle to a specific movement (the antagonist) becomes stretched to the point at which it can no longer lengthen and allow further movement. Part of this is due to the tension created from the elastic recoil properties of the fascia reaching its maximum length thereby restricting further movement.
An example of passive insufficiency can be seen in the movements of wrist flexion and extension – involving the finger flexor and extensor muscles. These muscle groups cannot be stretched over the wrist, MP, PIP, and DIP joints at the same time.
Passive insufficiency is usually seen in biarticular muscles and often when performing a countercurrent movement pattern. An example of this type of movement pattern is punting a football. The antagonistic muscles to the movement of punting the football (hip flexion) are the hamstrings and they eventually reach passive insufficiency. On the other hand, the hip flexor muscle group (including the rectus femoris) performing the movement eventually reaches a state of shortened active insufficiency.
Figure 4 Figure 5
A countercurrent movement pattern
Note: As a joint moves through a range of motion due to muscle contraction the tension that is created will fluctuate even though the resistance may remain the same. This fluctuation in muscular force is due to, 1) the change in the length of the muscle, and 2) the change in the angle of pull as the bone (lever) is moved. Because each joint is different (bones and musculature, forming and acting on the joint), a unique strength curve exists for each muscle group.
Example: The maximal amount of force that can be generated by the hamstring muscle group is when the knee is flexed approximately 10-20⁰ from full extension.
The real advantage of biarticular muscles is that in a certain type of movement pattern, both sets of biarticular muscles (which are antagonistic to each other) can maintain a relatively constant length, and therefore a relatively constant tension, as the movement is performed through a full range of motion. This type of movement is referred to as a concurrent movement pattern. An example of this type of movement pattern is performing a squat.
One might think of it as the muscle is “shortening” at one joint while “lengthening” at the other joint. However, because muscles shorten simply by pulling toward their center, the muscle does not actually shorten at one end and lengthen at the other; instead, the concentric shortening of the muscle to move one joint is offset by motion of the other joint. In essence the attachment sites of the involved muscles maintain a relatively constant distance apart throughout the range of motion and therefore a relatively constant tension.
Kinetic Chain Concept
The segments of the body (or links) are joined by a series of joints. These are likened unto the linkage system of a chain. Because these links are connected by joints, movement of one link causes motion of other links and joints in a predictable way. There are two types of kinetic chain movements.
Open Kinetic Chain Movement: Occurs when the distal end of an extremity is “not fixed” to any surface, allowing any one joint in the extremity to move or function separately without necessitating movement of other joints in the extremity. Characteristics of an open kinetic chain movement include:
Closed Kinetic Chain Movement: Occurs when the distal end of an extremity is “fixed” to a surface, preventing movement of any one joint unless predictable movements of the other joints in the extremity occur.
Characteristics of a closed kinetic chain movement include:
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