Training machines or trainers are usually found in fitness centers. These devices are designed for workout and provide good control of motion. This control is provided through a selection of load sizes throughout the whole amplitude of movement, direction of force or torque, and amplitude and movement plane. Trainers are generally intended to strain individual muscles or muscle groups.
Trainers are divided into uni-articular and multi-articular categories, or to work in an open or closed kinetic chain. Uni-articular trainers are designed to strain each muscle and muscle group (agonist), which help the trainer in the execution of movement (synergist). Their main feature is the movement of one joint, where circular movement is implemented as stretching/bending, moving away/moving closer, and so on. A typical example is a trainer for knee stretching. Multi-articular trainers involve movement of multiple joints and require linear movement. A typical example is the leg press machine for stretching legs.
A trainer is especially useful when we want to strain specific muscles, work out safely and are not interested in complex movements with complex coordination.
Specific guidelines must be followed for proper installation of a trainer. In uni-articular trainers, we first align the axis of the joint with the trainer axis. In trainer for knee stretching this means to align knee axis with the working axis of the trainer. Next, support surfaces are set to support the appropriate parts of the body. With knee stretching this means that the thigh is supported by the seat throughout its length, with the back leaning against the backrest at least up to the shoulder blades. Then, align the working support for the correct position. Stretching of the knee is just above the ankle. As a general rule, there is no pressure on the muscles and the lever should be as big as possible to minimize pressure under the working surface. Set the burden by moving the bolt in the stack of weights. Some fitness machines allow setting limits for largest amplitudes. This makes sense for safety reasons. If we want to set the initial position of working support, then limit the amplitude of movement. If there is a counterweight, place it in the proper position. The purpose of the counterweight is to neutralize the weight of the working handle and working support (it works only when the speed of movement is slow and steady).
Installation of a multi-articular trainer has basically the same requirements as those of a uni-articular trainer, but it is also necessary to consider direction or line of movement. The line of movement in the leg press is from the pelvis to the front part of the foot, for example.
An important part of the trainer is the so-called kidney. It is an eccentric that allows relatively even strain of the muscle throughout the whole amplitude (this works only when the speed of movement is slow and steady). By varying the length of the lever force of the weights, we adjust torque to match the torque that muscles are able to develop in an individual area. Since it is not possible to ensure torque adjustment with levers and bars, that is the reason trainers allow for higher work amplitude.
During the implementation of the movement, the body remains stable by resting on supports. Some trainers have handles for this purpose, to push against the seat. Others have belts that attach the body to the trainer.
In addition to trainers, you can also find pulleys, bars and handles in fitness centers. Movement with them is less controlled than with trainers and therefore requires greater involvement of synergists. Exercises are therefore more complex, which is clearly an advantage from a safety point of view, as it increases stability of joints. This is especially true for exercises with a bar, due to the increased weight we move at that time and generally explosive movements. It is important to master the techniques of exercises that we perform, otherwise such exercise can be very dangerous.