Proceedings of The Physiological Society
Trinity College Dublin (2003) J Physiol 551P, C48
Heart rate and haemodynamic changes differ following voluntary and electrically evoked exercise with maintained chemoreflex activation in humans
James P. Fisher, Charlotte A. Carrington and Michael J. White
School of Sport and Exercise Sciences, University of Birmingham, Birmingham B15 2TT, UK
Following a period of isometric exercise, heart rate (HR) falls more rapidly when circulation to the previously exercising muscle is occluded (PECO) as compared to recovery with open circulation (Coote & Bothams, 2001). This may reflect increased baroreflex stimulation of cardiac vagal tone motoneurones when blood pressure is raised above baseline by PECO, at a time when vagal inhibition by the muscle mechanoreceptor (MM) and/or central command (CC) is removed by cessation of voluntary exercise. To separate the roles of MM and CC in this off transient, well established methods were employed (Bull et al. 1989) to examine HR changes immediately following 2 min of exercise with PECO. Exercise was either electrically evoked (STIM) and voluntary (VOL) isometric contraction of the human triceps surae performed at 30% of maximal voluntary force.
All protocols were performed according to local ethics committee guidelines. Thirteen healthy subjects of mean age 24.1 ± 1.4 years (11 males) gave informed written consent for participation. HR (ECG) and blood pressure (BP) (Finapres) were continuously monitored throughout the protocol. Haemodynamic indices were measured using impedance cardiography (Minnesota Impedance Cardiograph, Model 304B). All values are presented as 15 s ensemble average changes from rest ± S.E.M. Statistical analysis was performed using two-way ANOVA (P < 0.05) and post hoc Student's paired t test with Bonferroni correction for multiple comparisons.
During exercise the heart rate changes in STIM and VOL were not significantly different. However over the first 15 s of PECO, the change in HR from rest was significantly different between the VOL (2.9 ± 1.25 beats min-1) and STIM (-3.7 ± 1.29 beats min-1) conditions (P < 0.01). Also the change from rest in pre-ejection period (PEP), an index of the inotropic state of the heart, was significantly greater in VOL (-12 ± 1.4 ms) than STIM (-5 ± 1.1 ms). BP was not different between conditions during PECO.
Lower HR and longer PEP over the 15 s following stimulated exercise are compatible with the idea that cessation of MM activation allows a greater recovery of cardiac vagal tone (Lewis et al. 2001) than occurs on cessation of both MM and the CC associated with VOL (Coote & Bothams, 2001).
Where applicable, experiments conform with Society ethical requirements