The kinematic chains in the work of levers of different kinds in human movements

The article examines the movement of limb segments as levers of the 1st, 2nd, and 3rd types and explores the relationship between the structure of these segments and their possible motions in relation to the concept of a lever. The classification of human body segments into different types of levers is analyzed based on the concepts of "equilibrium," "force," and "velocity." It is demonstrated that the laws of classical physics are applicable to the human musculoskeletal system. The human skeleton is one of the main components of the musculoskeletal system, and together with muscles, it provides structural support and mobility. The framework of the body is the skeleton, consisting of approximately 200 bones, connected in such a way that their relative position can change during movement. Bones are moved by skeletal muscles, each of which is attached to two different bones. Limb segments function as a combination of levers and pendulums. Bones, which are movably connected at joints, form the basis of kinematic chains and joint articulations. Forces (muscle-generated forces and external loads) applied to them act on the segments of kinematic chains as levers. This allows for the transmission of force through segments over a distance and modifies the effect of applied forces. Bony levers, which are movably connected at the joints, can maintain or change their position under the influence of applied forces. All forces acting on a bony segment as a lever can be divided into two groups: 1) Forces lying in the plane of the joint axis (these do not affect movement around this axis); 2) Forces with components in a plane perpendicular to the lever's axis (these forces influence movement around this axis in two opposing directions), i.e., propulsive forces (acting in the direction of movement) and resistive forces (acting against movement). Levers are classified as follows: 1st-class levers (when forces are applied on opposite sides of the fulcrum). In other words, if two forces act on opposite sides of the fulcrum of a limb segment where rotation is possible, and they act in the same direction, it forms a first-class lever. 2nd-class levers: When forces are applied only on one side of the fulcrum, acting in opposite directions with equal force arms, the system forms a second-class lever. 3rd-class levers: When forces are applied on the same side, but the force arms are unequal. First-class levers are double-armed, while second- and third-class levers are single-armed. Muscle forces may be applied not perpendicularly to the lever but at an angle. When a force is applied at an acute or obtuse angle, its effect on the lever can be determined by decomposing it into components: one directed along the length of the lever and the other perpendicular to it. The first force component is not considered in movement analysis, as it primarily causes joint compression, increases or decreases joint surface contact pressure, or, if the lever is not securely fixed, causes displacement along its length. The second component is the effective force responsible for movement. For different muscles attached at different points on a bone, the lever type may vary. The article examines how, relative to its flexor muscles, the forearm functions as a third-class lever (single-armed). However, relative to the extensor muscles (when holding the body above the head), the forearm acts as a first-class lever (double-armed). Thus, depending on the simultaneous movement of the shoulder-forearm-hand kinematic chain in flexion-extension, abduction-adduction, and pronation-supination, it is demonstrated that these segments act as both first- and third-class levers.