Electrically induced activation of Group III/IV muscle afferent fibres by high-intensity/frequency stimulation produces graded augmentation of systolic blood pressure (SBP) and cardiac output (Q) relative to the intensity of isometric muscle contraction (Turner, 1991; Kaufman & Forster, 1996). In order to test whether this pattern of response occurs in more ‘natural’ conditions of potential muscle afferent activation, we measured the central cardiovascular responses to robust electrically induced H-reflex stimulation (i.e. that which induced small muscle twitches) over the course of 30 min. Eight healthy male subjects (19-38 years old) who gave informed consent participated in the study, which had local ethical committee approval. Participants were instructed to sit comfortably with the left leg extended whilst breathing through a respiratory mouthpiece connected to a low resistance value to monitor airflow. Repetitive intermittent electrical stimulation (~0.3 Hz) of the posterior tibial nerve at the back of the knee was triggered by inspiration onset (4 Ω 1 ms pulses; 13 Hz) at a current sufficient to elicit a maximal soleus H-reflex assessed prior to the experiment. Left soleus EMG (mV) activity was recorded using bipolar active electrodes (sampling rate, 2 kHz). SBP (mmHg), diastolic blood pressure (DBP, mmHg), cardiac output (Q, l min^-1), total peripheral resistance (TPR, a.u.) and fH (min^-1) were recorded before (PRE), and at 2 and 30 min, using a Portapres model 2 (TNO, NL). Differences between means ± S.E.M. were tested using Bonferroni corrected paired t tests with * indicating a significant difference vs. PRE (overall P < 0.05).

H-reflex activation elicited a slowly developing significant augmentation in SBP (Table 1) accompanied by a significant increase in TPR, but no change in Q, fH or DBP.

Intermittent repetitive H-reflex stimulation elicits a slowly developing pressor reflex as a result of an increase in systemic vascular resistance with little or no central cardiovascular component. The relevant afferent fibres eliciting this response cannot be established with this technique.

Kaufman, M.P. & Forster, H.V. (1996). Handbook of Physiology, section 12, Exercise: Regulation and Integration of Multiple Systems, pp. 381-447.

Turner, D.L. (1991). J. Exp. Biol. 160, 309-340.