Effects of body orientation, load and vibration on sensing position and movement at the human elbow joint

Experiments were carried out to study the ability of human subjects to matc h the position of their forearms relative to the horizontal. The normal, an us-in-front position with the hands aligned and Little forward flexion at t he shoulder was called the reference position. When the arms were rotated t o the side, one arm was raised, or both arms were raised, matching ability deteriorated compared with the reference position, when expressed as an inc rease in the standard deviation of matching errors. It was concluded that p articular significance was assigned by the brain to the arms-in-front posit ion, with the hands in their normal working space. Increases in errors were also observed when the reference arm was made weightless or its weight was increased by means of an adjustable load. This suggested that lifting the arm against gravity provided additional positional information. In a second experiment, dependence of the illusion of muscle lengthening evoked by vib ration was tested after two different forms of muscle conditioning, a co-co ntraction of elbow muscles with the arm held flexed or with it held extende d. The speed of the illusory extension of flexor muscles during their vibra tion increased three-fold after flexion conditioning compared with extensio n conditioning. Since after flexion conditioning, muscle spindles in flexor muscles are expected to be-more sensitive to vibration than after extensio n conditioning, this observation provides additional support for the view t hat muscle spindles make an important contribution to kinaesthesia at the e lbow joint.