The sense of force or sense of resistance is a perceptual quality of interoceptive depth sensitivity and forms part of the kinesthetic system. Through the sense of force, humans can estimate their own effort during movements and thus coordinate pull and push. In extrapyramidal lesions, the sense of force is impaired.
What the sense of force?
The sense of force allows humans to estimate their own effort during movements and thus coordinate pull and push. Human perception is divided into interoception and exteroception. Exteroception is the perception of external stimuli. Interoception corresponds to the perception of stimuli from one’s own body. Proprioception is a part of interoception. This type of perception encompasses all sensory impressions for the perception of one’s own body movements and position in space. For proprioception, the so-called depth sensitivity is crucial. This is the muscle sense (sense of force), which has different perceptual qualities. In addition to estimating one’s own muscular strength, the muscle sense allows people to move specifically against resistance. The muscle sense enables the movement of individual muscle groups and the perception of pressure distributions. It thus enables the dosage of pressure and tension during muscle movement. These perceptual qualities of depth sensitivity are called the sense of force or the sense of resistance. Together with the position sense for the perception of the current initial position and the movement sense for the reception of movement dimensions or position changes, the force sense forms the totality of the kinesthetic system.
Function and task
The sense of force allows humans to precisely plan the effort of each muscle contraction and match it to the movement goal. Thanks to this perceptual quality, humans do not involuntarily crush an object when reaching for it, for example. Proprioceptors in the muscles and tendons provide permanent information about the tension state of the muscles. Proprioceptors include the muscle and tendon spindles. The muscle spindles detect the length of skeletal muscles. They are formed by intrafusal muscle fibers that have their afferents in Ia class nerve fibers. Secondary afferent innervation of the muscle spindles is given by class II nerve fibers. The efferent innervation of the structures is provided by gamma motoneurons. They mainly control the sensitivity of the spindle. Tendon spindles, in turn, lie between muscle and tendon fibers. They are formed by bundled collagenous fibers enclosed by a connective tissue capsule. They are connected to the muscle and tendon fibers and are supplied by afferent Ib nerve fibers. As soon as a muscle is contracted and thus shortened, the collagen fibers in the tendon spindles stretch. As a result, the spindles polarize and transmit the stimulus with information about the stimulus intensity to the spinal cord. There, the Ib neurons exert an inhibitory influence on the motoneurons of the irritated muscle through interneurons and stimulate the motoneurons of the respective muscle antagonist. Via ascending nerve tracts, the impulses reach the tractus spinocerebellaris anteriorly, posteriorly, and the cerebellum. Through the sense of force, humans can estimate resistance to movement and estimate the weight of objects. He obtains this information from the muscle tension involved in a particular movement. The acquired information is stored in a sense-specific memory and helps humans in the future to accurately coordinate and plan muscle force in the context of a particular movement. Without the sense of force, movement planning and force estimation would not be possible. Human movements would be clumsy and not purposeful without the perceptual quality. Although the sense of force is an interoceptive perceptual quality, it also contributes to receptive information acquisition about the external world. This is especially true for information about the weight of certain objects. Estimates of the weight or strength of a resistance depend on the muscular strength of the individual and are accordingly subjectively shaped.
Disease and complaint
In the context of various neurological diseases, so-called extrapyramidal disorders are characteristic. Extrapyramidal, in terms of movement disorders, means a cause outside the pyramidal system. The pyramidal system is the main switching point for all motor activity.It is located in the spinal cord and connects the central motor neurons, which play a role in every type of movement. Damage to the pyramidal system can result in paralysis, muscle weakness or spasticity. The extrapyramidal system captures all movement control processes outside the pyramidal system. Damage to this system is characterized primarily by a lack of the inhibitory influence exerted by the upper motor neuron to coordinate movements. As a result, the movement patterns of patients with extrapyramidal lesions appear greatly exaggerated. Any extrapyramidal damage thus shows effects on the sense of force. The affected persons often feel the weight of their own limbs as heavy and thus apply disproportionately more force to perform the smallest movements. Due to the extrapyramidal damage, they have the feeling that they always perform movements against resistance. The perceived resistance is projected outwards and for this reason the patients expend unnecessarily much force to overcome resistance. As a result, those affected are often no longer able to adequately dose pressure and traction. In some cases, movement is also generally slowed because it occurs against perceived resistance. This type of central nerve damage is characteristic of diseases such as multiple sclerosis. This is an autoimmune disease that causes the immune system to cause inflammation in the central nervous system. The inflammation often causes neurons to die irreversibly. Movement disorders such as the phenomenon described often occur. In addition to this disease, trauma or spinal cord infarction can also cause the described phenomenon. An equally conceivable cause is tumor disease of the spine.
