Thus increase discharge of Group IV (metaboreceptors
) afferent fibers initiate a reflex increasing the activity of the sympathetic nervous system.
The recovery is a primal part of exercise prescription, being affected by compensatory mechanisms such as chemoreceptors, metaboreceptors
and, mechanoreceptors (17,33).
These factors are responsible for the activation of peripheral receptors, such as the mechanoreceptors and metaboreceptors
(III and IV fibers) that send information by the afferent pathway to the central nervous system (ventrolateral medullary region), resulting in increased sympathetic discharge to the cardiovascular system (efferent response) (Mitchell, 1990).
The experimental evidence for a respiratory metaboreflex includes the observation in anesthetized rats that fatigue of the diaphragm increases the firing rate of type IV afferent metaboreceptors
from this muscle, while the activity of type III mechanoreceptors is not altered (8).
The increased work of breathing due to severe obstructive and restrictive breathing in patients with chronic lung disease could lead to sympathetic activation through stimulation of local metaboreceptors
. Oxygen radicals and products of ischemic metabolism generated during muscular contraction (e.g., isometric exercise) have been shown to stimulate local receptors and cause increases in heart rate, arterial pressure, and sympathetic activity (26, 27).
In the patient who describes work or effort when breathing, the sensation is stimulated by respiratory motor muscle contraction and muscle fatigue and is mediated through a combination of central motor discharge, chest wall receptors, and metaboreceptors
located within skeletal muscle, he said at the meeting, which was sponsored by National Jewish Health.
Exercise pressor reflex depends on the stimulation of mechanoreceptors and metaboreceptors
.  Greater the muscle mass involved in the exercise higher would be the stimulation of mechanoreceptors; however, acidosis and lactate concentration occurs more in CE than TM [30,31] that might cause higher stimulation of metaboreceptors
in CE as compared to TM.
The later reduction in post-exercise heart rate is due to the increased parasympathetic activity, which is under the control of the baroreceptors and metaboreceptors
(following changes in the subjects' metabolic products, core temperature, catecholamines, and other hormonal factors) (4,32).
The accumulation of hydrogen ions and C[O.sub.2] in muscle and blood activate the ventilatory response via metaboreceptors
and chemoreceptors, resulting in a nonlinear increase in the slopes of the relationships between pulmonary ventilation and work rate, and between V[O.sub.2] and VC[O.sub.2] (3,4).
Inactive recovery from dynamic exercise is associated with cessation of the primary exercise stimulus from the brain and abrupt changes in the stimuli to baro- and metaboreceptors
) is likely to play a crucial role in evoking this reflex in humans.
However, prior work has suggested that the activation of type III fibers and metaboreceptors
may inhibit the parasympathetic branch of the autonomic nervous system and chemoreflex stimulation, thereby contributing to an increase in human HR (4,5,19).