This is an excerpt from Athletic Movement Skills by Clive Brewer.
The skeletal and muscular systems work together to produce movement. Collectively, they are referred to as the musculoskeletal system.
As its name suggests, skeletal muscle anchors to bones and is responsible for movement and control of the skeleton. In mechanics, form dictates function. Therefore, studying the form (structure and shape) of the muscle leads to a greater understanding of how it functions and how training can influence it.
Form and Function of Skeletal Muscle
Skeletal muscles attach to bone at either end of the muscle. Connective tissue runs throughout the collection of individual muscle fibres that come together to make a muscle. This connective tissue forms the tendons, which join muscle to bone. The myotendinous junction enables a pulling force to be created between the bones (i.e., if a muscle attaches to different bones at each of its ends, the muscle can exert a pull between bones, creating movement of one bone relative to the other).
This concept is simple when viewed in isolation. To illustrate this point, let's look at the biceps brachii, which attaches to the scapula and the humerus (upper arm bone) at one end and the ulna and radius bones of the forearm at the other end (figure 2.2a). With the shoulder fixed, contraction in the biceps brachii moves the hand towards the shoulder, and the elbow flexes (figure 2.2b); the ulna and radius are moved relative to the humerus. If the hands are fixed (for example, when hanging from a bar in a chin-up) and the biceps are contracted, the humerus is moved closer to the ulna and radius, again through elbow flexion (figure 2.2c).
Muscles exert forces that enable bones to move relative to each other.
Although this explanation is designed to illustrate how bones move relative to each other, the example is oversimplified. Movement doesn't really occur as an isolated action; for example, executing a chin-up requires the work of many muscles. Completely understanding movement means accepting that muscle actions rarely occur in isolation. A complex interaction between muscles exerting differential forces on bones produces the phenomenon that we observe as movement.
For example, to flex (bend) the arm at the elbow from a normal carrying position (assuming a relatively heavy mass) to a position in which the palm of the hand faces the shoulder joint at full flexion, the primary force is created by contracting the biceps brachii and the brachialis, with assistance from the brachioradialis as resistance increases. In this action, the primary stabilizer of the elbow joint is the anconeus. The position of the shoulder - the humeral head is in position against the glenoid fossa of the clavicle - is fixed by contraction of the rotator cuff muscles (infraspinatus, teres minor, subscapularis and supraspinatus). The triceps brachii (the primary extensor muscle of the elbow) also acts as a synergist to fix the position of the humerus relative to the shoulder (figure 2.3).3
Muscle arrangement around the glenohumeral (shoulder) joint.
Another example of how antagonistic muscle groups work in synchronization to bring about coordinated movement can be seen in the vertical jump. The practical considerations for developing and programming this activity are explored in detail in chapter 9.
The vertical jump involves the near simultaneous extension of the hip, knee and ankle joints from the flexed starting position at the beginning of the vertical component of the action (figure 2.4a). The prime extensor group for the knee is the quadriceps muscle group: vastus medialis, vastus intermedius, vastus lateralis and rectus femoris. Within this group, the rectus femoris crosses both the hip and knee joint and is responsible for flexion of the hip in standing (i.e., raising the femur until it is perpendicular to the floor) as well as extension of the knee. The conjoined muscles of the iliopsoas are also powerful hip flexors that bring the upper body forward when the feet are on the floor.
Therefore, if the desired hip extension is to occur, the hamstrings (biceps femoris, semimembranosus and semitendinosus) and the gluteus maximus must concentrically contract to extend the hip joint from the leg to counter the hip-flexing actions of the rectus femoris and iliopsoas. This action brings the trunk into an upright position as the hip extends forcibly at the same time as the knee and ankle (figure 2.4b). The resultant force enables the body to leave the floor (figure 2.4c). The gluteus medius and gluteus minimus stabilize the hip joint in this action.
Coordinated muscle actions in a vertical countermovement jump: (a) starting position; (b) hip, knee and ankle extend; (c) jump.
Note that the respective movements of the joints and the relative positioning of the bones to each other throughout the actions bring about the muscle activation patterns. This observation reinforces a fundamental training philosophy emphasized throughout this book: When athletic development programmes emphasize that technique is based on placing the joints in the right positions by developing the correct movements, muscles are trained functionally. In other words, train movements, not muscles!
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