Muscle work and strength

The main property of muscle tissue that forms skeletal muscles is contractility, which leads to a change in muscle length under the influence of nerve impulses. Muscles act on the bones of levers that are connected by joints. In this case, each muscle acts on the joint in only one direction. In a uniaxial joint (cylindrical, block-shaped), the movement 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; adduction - abduction, rotation). For example, in the elbow joint, some muscles are flexors, others are extensors. In relation to each other, these muscles, acting on the joint in opposite directions, are antagonists. As a rule, two or more muscles act on each joint in one direction. Such muscles, friendly in the direction of action, are called synergists. In a biaxial joint (ellipsoid, condylar, saddle-shaped), the muscles are grouped according to its two axes, around which movements are performed. In a ball-and-socket joint, which has three axes of movement (a multiaxial joint), the muscles are adjacent from several sides and act on it in different directions. For example, the shoulder joint has muscles - flexors and extensors, which perform movement around the frontal axis, abductors and adductors - around the sagittal axis, and rotators - around the longitudinal axis (inward - pronators and outward - supinators).

In a group of muscles that perform a particular movement, we can distinguish the main muscles that provide the given movement, and the auxiliary muscles, the auxiliary role of which is indicated by the name itself. The auxiliary muscles model the movement, giving it individual characteristics.

For the functional characteristics of muscles, such indicators as their anatomical and physiological cross-section are used. The anatomical cross-section is the size (area) of the cross-section perpendicular to the long axis of the muscle and passing through the belly in its widest part. This indicator characterizes the size of the muscle, its thickness. The physiological cross-section of the muscle is the total cross-sectional area of all muscle fibers that make up the muscle under study. Since the strength of a contracting muscle depends on the number of muscle fibers and the size of the cross-section, the physiological cross-section of the muscle characterizes its strength. In fusiform, ribbon-shaped muscles with parallel fiber arrangement, the anatomical and physiological cross-sections coincide. A different picture is in pennate muscles, which have a large number of short muscle bundles. Of two equal muscles with the same anatomical cross-section, the pennate muscle has a larger physiological cross-section than the fusiform muscle. The total cross-section of muscle fibers in a pennate muscle is larger, and the fibers themselves are shorter than in a fusiform muscle. In this regard, a pennate muscle has greater strength than the latter, but the range of contraction of its short muscle fibers is smaller. Pennate muscles are found where significant force of muscle contraction is required with a relatively small range of motion (muscles of the lower leg, foot, some muscles of the forearm). Fusiform, ribbon-shaped muscles, built from long muscle fibers, shorten by a greater amount during contraction. At the same time, they develop less force than pennate muscles, which have the same anatomical cross-section.

Muscle work. Since the ends of the muscle are attached to the bones, the points of its origin and attachment come closer to each other during contraction, and the muscles themselves perform a certain amount of work. Thus, the human body or its parts change their position when the corresponding muscles contract, move, overcome the resistance of gravity or, conversely, yield to this force. In other cases, when the muscles contract, the body is held in a certain position without performing a movement. Based on this, a distinction is made between overcoming, yielding, and holding muscle work.

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

Inferior work is work in which the muscle strength yields to the force of gravity of the body part (limb) and the load it holds. The muscle works, but it does not shorten, but rather lengthens; for example, when it is impossible to lift or hold an object with a large mass. With great muscle effort, the body must be lowered to the floor or another surface.

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

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

Bones connected by joints act as levers when muscles contract. In biomechanics, a first-class lever is distinguished, when the points of resistance and application of muscle force are on different sides of the fulcrum, and a second-class lever, in which both forces are applied on one side of the fulcrum, at different distances from it.

The first type of two-armed lever is called the "balance lever". The fulcrum is located between the point of application of force (the force of muscle contraction) and the point of resistance (gravity, organ mass). An example of such a lever is the connection of the spine with the skull. Equilibrium is achieved under the condition 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 the force) is equal to the torque of gravity (the product of gravity by the length of the arm, equal to the distance from the fulcrum to the point of application of gravity).

The second kind lever is single-armed. In biomechanics (as opposed to mechanics), it comes in two types. The type of such a lever depends on the location of the point of application of force and the point of action of gravity, which in both cases are on the same side of the fulcrum. The first type of the second kind lever (lever of force) occurs when the arm of application of muscle force is longer than the arm of resistance (gravity). Considering the foot as an example, we can see that the fulcrum (axis of rotation) is the head of the metatarsal bones, and the point of application of muscle force (the triceps surae muscle) is the calcaneus. The point of resistance (body gravity) is at the junction of the shin bones with the foot (ankle joint). In this lever, there is a gain in force (the arm of application of force is longer) and a loss in the speed of movement of the point of resistance (its arm is shorter). In the second type of single-arm lever (speed lever), the arm of application of muscle force is shorter than the arm of resistance, where the opposing force, gravity, is applied. To overcome gravity, the point of application of which is at a considerable distance from the point of rotation in the elbow joint (the fulcrum), a significantly greater force of the flexor muscles attached near the elbow joint (at the point of application of force) is required. In this case, there is a gain in the speed and range of motion of the longer lever (the point of resistance) and a loss in the force acting at the point of application of this force.