Somatosensory feedback from the limbs exerts inhibitory influences on central neural drive during whole body endurance exercise

Abstract

We investigated whether somatosensory feedback from contracting limb muscles exerts an inhibitory influence on the determination of central command during closed-loop cycling exercise in which the subject voluntarily determines his second-by-second central motor drive. Eight trained cyclists performed two 5-km time trials either without (5K(Ctrl)) or with lumbar epidural anesthesia (5K(Epi); 24 ml of 0.5% lidocaine, vertebral interspace L(3)-L(4)). Percent voluntary quadriceps muscle activation was determined at rest using a superimposed twitch technique. Epidural lidocaine reduced pretime trial maximal voluntary quadriceps strength (553 +/- 45 N) by 22 +/- 3%. Percent voluntary quadriceps activation was also reduced from 97 +/- 1% to 81 +/- 3% via epidural lidocaine, and this was unchanged following the 5K(Epi), indicating the presence of a sustained level of neural impairment throughout the trial. Power output was reduced by 9 +/- 2% throughout the race (P < 0.05). We found three types of significant effects of epidural lidocaine that supported a substantial role for somatosensory feedback from the exercising limbs as a determinant of central command throughout high-intensity closed-loop cycling exercise: 1) significantly increased relative integrated EMG of the vastus lateralis; 2) similar pedal forces despite the reduced number of fast-twitch muscle fibers available for activation; 3) and increased ventilation out of proportion to a reduced carbon dioxide production and heart rate and increased blood pressure out of proportion to power output and oxygen consumption. These findings demonstrate the inhibitory influence of somatosensory feedback from contracting locomotor muscles on the conscious and/or subconscious determination of the magnitude of central motor drive during high intensity closed-loop endurance exercise.

The role of active muscle mass in determining the magnitude of peripheral fatigue during dynamic exercise

“…Further indirect indices of a higher central motor drive are provided by the greater central cardiovascular and respiratory responses during double-leg KE ( Figs. 1 and 2), both of which are determined by feedback and feedforward mechanisms (8). In the presence of greater central motor drive, less overall afferent feedback from the exercising limbs would be required to reach the sensory tolerance limit during doubleleg KE.…”

Cerebral perturbations during exercise in hypoxia

“…Whereas blockade of neural feedback from working muscles with epidural anesthesia does not impair hypoxia-induced increases in systemic cardiovascular and neuroendocrine responses (57), evaluation of the central motor drive during hypoxic exercise when manipulating muscle afferent signals is needed to confirm their impact on the CNS in hypoxic conditions. This should include selective blockade of ascending sensory pathways and avoid the confounding effect of motor nerve activity or maximal force output impairments (10,11,57).…”

Physiological Responses Underlying the Perception of Effort during Moderate and Heavy Intensity Cycle Ergometry

“…In addition, comparison of the O2 associated with the initial stage of each constant RPE ride indicated that subjects maintained a higher O2 cost (p < 0.01; d = 0.67) during the first 20 min of rides at RPEGET+15% than those at RPEGET. There was no significant difference between RPEGET and RPEGET+15% for O2 at min [21][22][23][24][25][26][27][28][29][30][31][32][33] …”

“…These findings were consistent with those of Cochrane et al [6] who reported a close agreement between RPE and , but dissociations for both RPE and vs. metabolic ( O2 and RER), cardiovascular (HR), and ventilatory ( E) responses during constant RPE cycle ergometry within the moderate intensity domain. It was hypothesized [6] that reflex actions associated with the exercise pressor reflex model of fatigue [12,[29][30][31] best explained the similarity for RPE and during the constant RPE rides. According to the exercise pressor reflex model [12,[29][30][31], afferent signals from small group III (mechanical changes) and IV afferents (metabolic changes) from working thigh and respiratory muscles mediate the ventilatory, cardiovascular, and RPE responses during moderate and heavy intensity exercise [2,12,32,33].…”

“…It was hypothesized [6] that reflex actions associated with the exercise pressor reflex model of fatigue [12,[29][30][31] best explained the similarity for RPE and during the constant RPE rides. According to the exercise pressor reflex model [12,[29][30][31], afferent signals from small group III (mechanical changes) and IV afferents (metabolic changes) from working thigh and respiratory muscles mediate the ventilatory, cardiovascular, and RPE responses during moderate and heavy intensity exercise [2,12,32,33]. It has also been suggested [11] that in addition to mediating cardioventilatory responses, group III and IV afferents may have a "critical role" in the regulation of the perception of effort during dynamic exercise.…”