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BackgroundThe heart rate (HR) response at the onset of moderate intensity isometric handgrip (IHG) exercise is related to neural control of the heart, primarily vagal withdrawal. If so, modest modifications in neuro‐cardiac function by postural shifts should modify the HR response to exercise.ObjectiveThis study examined the effect of supine versus seated postures on the changes in HR and heart rate variability (HRV) at the onset of IHG.Methods13 healthy adults (24±3 years; 6 females) performed IHG at 35% and 50% of maximal voluntary contraction strength (MVC), each lasting 30 seconds and separated by at least one minute of rest. HR (electrocardiogram) and root mean square of successive differences (RMSSD) were recorded from the last 30 seconds of the 1 minute baseline prior to handgrip, and the last 10 seconds of each 30 second handgrip.ResultsBaseline HR increased from 68bpm to 76bpm (p<0.001; d=1) and RMSSD decreased from 56ms to 44ms (p=0.25; d=0.4) when shifting from supine to seated positions. Compared to the supine position, the ΔHR with 35% IHG decreased in the seated position (7bpm to 3bpm; p=0.01; d=0.71). Similarly, the ΔRMSSD with 35% IHG tended to be less in the seated versus supine positions (−31ms to −16ms; p=0.07; d=0.7). During 50% MVC work, the seated position exerted only a modest effect on the IHG‐induced ΔHR compared to supine (18bpm to 13bpm; p=0.06; d=0.47) whereas the ΔRMSSD from baseline to IHG was diminished in seated versus supine positions (−47ms to −22ms; p=0.001; d=0.78).ConclusionWith RMSSD as an indicator of changes in cardiovagal influence, these preliminary results indicate that small changes in posture modify parasympathetic modulation of HR responses at the onset of moderate‐intensity isometric handgrip.Support or Funding InformationSupported by the Natural Sciences and Engineering Research Council of Canada.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

BackgroundPharmacological evidence indicates that baseline cardiovagal mechanisms determine the rapid heart rate (HR) response to the onset of moderate intensity isometric handgrip (IHG) exercise. The HR response at exercise onset expresses sex differences, being less in females. Further, somatosensory nerve stimulation (STIM) appears to affect baseline cardiovagal control in some supine participants.ObjectiveThis study explored the effect of STIM on HR and heart rate variability (HRV) during short‐duration moderate‐intensity IHG in supine healthy adults and explored whether this effect expresses sex differences.ParticipantsTwenty‐four young healthy individuals aged 23 ± 1.9 years (18 females (F): 165 ± 7.4cm, 63 ± 13kg, BMI 23 ± 2kg/m2; 6 males (M): 179 ± 2.2cm, 79 ± 11kg, BMI 25 ± 3.3kg/m2).MethodsParticipants performed six isometric handgrips with their right hand at 30 to 40% of their maximal grip strength, each lasting 20 to 30 seconds and separated by one minute of rest. Three of these IHG contractions were performed during simultaneous administration of sub‐motor STIM provided by transcutaneous electrical nerve stimulation (TENS; 100 Hz, 50 μs) on the medial contralateral forearm through anesthetized skin for the duration of the handgrip. A 3‐lead ECG provided R‐R interval data. Systolic (SBP) and diastolic blood pressure (DBP) were monitored, as well as, heart rate (HR), root mean square of successive differences in R‐R intervals (RMSSD) as a measure of HRV, and respiration. Data were analysed using two‐factor ANOVA with Bonferroni correction, and independent t‐tests.ResultsThe ΔHR from baseline to IHG without STIM was greater in males compared to females (12 ± 6bpm versus 6 ± 6bpm, respectively; d = 1.00). No differences in ΔHR were detected for either sex when STIM was added to the IHG. Of the 18 F participants, 4 provided a minimal HR response (ΔHR > 4bpm) to the IHG without STIM (F1: 77 to 79bpm; F2: 69 to 70 bpm; F3: 50 to 50bpm; F4: 62 to 62 bpm) which resolved in 3 of the 4 F participants with application of STIM (F1:73 to 83bpm; F3: 45 to 59bpm; F4: 54 to 60bpm). Further, no statistically significant differences were observed in ΔRMSSD from baseline to IHG without STIM (p = 0.0321; M: 77 ± 68ms to 30 ± 145ms; F: 74 ± 38ms to 54 ± 39ms) and with STIM (p = 0.953; M: −28 ± 61ms and F: −30 ± 21ms).ConclusionOur preliminary findings suggest that, in supine individuals, sex differences exist in the HR response at the exercise onset but these effects are not modified by concurrent sub‐motor somatosensory nerve stimulation.Support or Funding InformationSupported by Natural Science and Engineering Research Council of CanadaThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

BackgroundVagal withdrawal forms the dominant mechanism determining the rapid heart rate (HR) response at the onset of moderate intensity isometric handgrip (IHG) exercise. Previous evidence suggested that the addition of submotor somatosensory stimulation (STIM) can elevate vagal cardiac function in some individuals. This study tested the hypothesis that STIM can interfere with the vagal withdrawal and HR responses at the exercise onset.MethodsYoung healthy individuals (n=7, ages 24 ± 1.2 years) were recruited. STIM was provided by transcutaneous electrical nerve stimulation (TENS; 100 Hz, 50 μs) on the medial forearm through anesthetized skin. A 3‐lead ECG provided R‐R interval data. Participants performed 2–3 trials of IHG at 35–40% maximal voluntary contraction, each lasting 20 seconds and separated by one minute of rest. These were performed both with and without STIM and baseline cardiovagal influence was modified by repeating the tests in seated and supine positions using a varied order of STIM delivery.ResultsIHG elicited increased HR in each posture (supine: baseline=64 ± 9 bpm, IHG=73 ± 7 bpm; p<0.05; seated: baseline=66 ± 6 bpm, IHG=78 ± 8 bpm, p<0.05). In the seated position, STIM reduced the maximum HR response to IHG, compared to no STIM, in five out of seven participants resulting in an overall modest effect (no STIM: 19 ± 10 bpm; STIM: 15 ± 8 bpm; p<0.05; d=0.44). In the supine position, however, the effect of STIM to reduce the HR response was observed in only three out of seven participants. Thus, there was no significant difference between treatments (no STIM: 15 ± 7 bpm; STIM: 16 ± 9 bpm; p>0.05; d=0.12).ConclusionThese data suggests that the HR response to exercise in young healthy adults can be modulated with submotor somatosensory nerve stimulation but that this effect is most apparent in the seated position.Support or Funding InformationSupported by Natural Sciences and Engineering Research Council of CanadaThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.