Motor Activity
From a military point of view, the effects of a cold environment have their greatest overt effect on the motor system. Troops are not able to move as fast and fine coordination is impaired. Cold hands make it difficult to pull a trigger or operate a keyboard. Cold stress and hypothermia influence motor function by way of the neural and cardiovascular systems and on the muscle cell itself. As a person is initially cold stressed and then becomes hypothermic tension leads shivering, which continues until core temperature reaches 29oC to 31 oC. Preshivering tone, of which cold stressed subjects are usu ally unaware, normally precedes shivering. In part, this tonic muscle activity is the basis for the feeling of stiffness that most people experience when they get cold si Increased motor tone has been reported to a pear first in extensor and proximal muscles, which are the same muscles in which the amplitude of shiver is largest. However, humans vary greatly in their shivering patterns, with some human sub jects shivering first in their chest muscles Shiver has been defined as involuntary rhythm waxing and waning muscular contractions that are used to maintain a normal body temperature These oscillations are modulated by reflex loops, because deafferentation will cause the frequency characteristics of shiver to become irregular However, shivering can be influenced b cerebral cortex. A subject can temporarily turn off shivering by relaxing, doing exercises or modify ing the breathing pattern. These techniques are in valuable for field operations, because they allow troops to conduct certain aspects of their mission even when they are cold-stressed. in- From a thermogenic point of view, shivering in creases heat production 2- to 5-fold more than is necessary for normal body heat production. Dur- ing different phases of shivering, both agonist and antagonist muscles contract periodically but not necessarily reciprocally. Thus there will be an in crease in muscle tension, but the limbs do not move effectively. The frequency of shiver varies from muscle to muscle but is considered fairly low, be tween 5 and 10 Hz. In laboratory experiments, cold stressed subjects will demonstrate synchronized muscle contraction of all muscles monitored. If the antagonistic muscles were to be coactivated at higher rates or to elicit contracture (ie, sustained force production without associated electrical ac tivity), then the heat that could be generated would be proportionally greater. However, a major draw back to this type of activation would be the high degree of resultant limb stiffness that would limit one's ability to make superimposed voluntary movements.