Work and strength of muscles

The main property of muscle tissue that forms skeletal muscles - contractility leads to a change in the length of the muscle under the influence of nerve impulses. Muscles act on the bones of the levers, which are connected with the help of joints. In this case, each muscle acts on the joint in only one direction. In the uniaxial joint (cylindrical, block-shaped), the motion of the bone levers occurs only around one axis, so the muscles are located in relation to such a joint on both sides and act on it in two directions (flexion-extension, reduction-retraction, rotation). For example, at the elbow joint, some muscles are flexors, others are extensors. A friend to the friend, these muscles, acting on the joint in opposite directions, are antagonists. Typically, for each joint in one direction, two or more muscles act. Such sympathetic muscles are called synergists. In the biaxial joint (ellipsoid, condylar, saddle), the muscles are grouped according to its two axes, around which movements are made. To the spherical joint, which has three axes of motion (multi-axis joint), the muscles adjoin from several sides and act on it in different directions. So, for example, the shoulder joint has muscles - flexors and extensors, moving around the front axis, withdrawing and leading - around the sagittal axis and rotators - around the longitudinal axis (inside - the pronators and outwards - the arch supports).

In the group of muscles that perform this or that movement, it is possible to distinguish the main muscles that provide this movement, and auxiliary ones, of which the name itself speaks for an auxiliary role. Auxiliary muscles model movement, give it individual characteristics.

For the functional characteristics of muscles used indicators such as their anatomical and physiological width. The anatomical diameter is the size (area) of the cross section perpendicular to the muscle length and passing through the abdomen in its widest part. This indicator characterizes the size of the muscle, its thickness. The physiological diameter of the muscle is the total cross-sectional area of all muscle fibers that make up the muscle being studied. Since the force of the contracting muscle depends on the number of muscle fibers, the cross section, the physiological diameter of the muscle characterizes its strength. In muscles spindle-shaped, ribbon-shaped with a parallel arrangement of fibers, the anatomical and physiological diameters coincide. A different picture in pinnate muscles, which have a large number of short muscle beams. Of two equal muscles with the same anatomical diameter, in the pinnate muscle the physiological diameter is greater than that of the spindle-shaped one. The total cross section of muscle fibers in the pinnate muscle is larger, and the fibers themselves are shorter than the spindle muscle. In this regard, the pinnate muscle in comparison with the latter has more strength, but the scope of contraction of its short muscle fibers is less. Cirrus muscles are present where a significant force of muscle contractions is required with a relatively small range of movements (leg muscles, feet, some forearm muscles). Muscles spindle-shaped, ribbon-shaped, built of long muscle fibers, shortened by a shortening to a larger size. At the same time, they develop less force than pinnate muscles, which have the same anatomical diameter.

Work of muscles. Since the ends of the muscle are attached to the bones, the points of its beginning and attachment become closer to each other during contraction, while the muscles themselves perform a certain work. Thus, the body of a person or a part of it with the reduction of the corresponding muscles changes their position, they move, overcome resistance to gravity, or, on the contrary, yield to this force. In other cases, when the muscles contract, the body is held in a certain position without performing the movement. Proceeding from this, distinguish overcoming, inferior and retaining the work of muscles.

Overcoming the work of muscles is performed if the force of muscle contraction changes the position of a part of the body, limb or its link, with or without a load, overcoming the resistance force.

Inferior is called work, in which the strength of the muscle is inferior to the action of the force of gravity of the part of the body (limb) and the cargo held by it. The muscle works, but it does not shorten, but, on the contrary, it lengthens; for example, when it is impossible to lift or hold an object with a large mass on its weight. With a large muscle force, you have to lower this body to the floor or to another surface.

Retaining work is carried out if the force of muscle contractions the body or load is held in a certain position without moving in space. For example, a person stands or sits, without moving, or holds the load in the same position. The strength of muscle contractions balances the body weight or weight. In this case, the muscles contract without changing their length (isometric contraction).

Overcoming and conceding work, when the force of muscle contractions moves the body or its parts in space, can be considered as a dynamic work. Retaining work, in which the movement of the whole body or part of the body does not occur, is static work.

Bones, jointed joints, with the contraction of muscles act as levers. In biomechanics, a lever of the first kind is distinguished, when the points of resistance and application of muscular strength are on opposite sides of the fulcrum, and a lever of the second kind, in which both forces are applied on one side of the support point, at different distances from it.

The lever of the first kind of two-arms is called the "lever of balance". The point of support is located between the point of application of force (the force of muscle contraction) and the point of resistance (gravity, body weight). An example of such a lever is the connection of the spine with the skull. Equilibrium is achieved provided that the torque of the applied force (the product of the force acting on the occipital bone by the length of the arm, which is equal to the distance from the fulcrum to the point of application of force) is equal to the torque of gravity (the product of the force of gravity by the length of the arm, points of support to the point of application of gravity).

Leverage of the second kind. In biomechanics (unlike mechanics), it is of two kinds. The type of such a lever depends on the location of the point of application of force and the point of gravity, which in both cases are on one side of the support point. The first kind of lever of the second kind (the lever of force) takes place in the event that the shoulder of application of muscular force is longer than the shoulder of resistance (gravity). Considering the foot as an example, we can see that the fulcrum (the axis of rotation) is the head of the bones of the metatarsus, and the point of application of the muscular force (the triceps muscle of the lower leg) is the heel bone. The point of resistance (the weight of the body) is at the junction of the shin bone with the foot (ankle joint). In this lever, the gain in strength is noted (the arm of the application of force is longer) and the loss in the speed of displacement of the resistance point (its shoulder is shorter). In the second kind of single-arm lever (lever of speed), the arm of the application of muscular strength is shorter than the shoulder of resistance, where the opposing force, the force of gravity is applied. To overcome the force of gravity, the point of application of which is at a considerable distance from the point of rotation in the elbow joint (fulcrum), a much larger force of the flexor muscles attached near the elbow joint (at the point of application of force) is needed. In this case, there is a gain in the speed and span of the movement of the longer lever (resistance point) and the loss in force acting at the point of application of this force.