Comparison of two isometric handgrip protocols on sympathetic arousal in women

Abstract

Isometric handgrip is commonly used in stress research because the task reliably increases sympathetic arousal. Various handgrip protocols have been used; they vary in handgrip strength, duration of grip, and the number of cycles of handgrip and rest. However, most protocols require the calibration of a maximum voluntary contraction (MVC) prior to the handgrip task, which is not always convenient (i.e., in a functional magnetic resonance imaging study). Here, we wanted to test whether two handgrip protocols with different strength, duration and cycle protocols would reliably elicit sympathetic arousal in the absence of calibrating an MVC. Sixty-two healthy naturally cycling women and women on hormonal contraception participated in one of the two isometric handgrip protocols using a hand therapy ball of medium resistance. Women completed one of the following handgrip protocols: 1) 30% of a perceived maximum voluntary contraction for 3 min or 2) 3 cycles of maximum voluntary contraction for 18s with a one minute rest in between. All handgrip blocks were counterbalanced with a control condition. Sympathetic arousal was measured throughout the session via pupil diameter changes and salivary alpha-amylase. Results indicate that in the absence of calibrating an MVC, the handgrip tasks elicited different changes in sympathetic arousal. Pupil dilation responses increased significantly in the handgrip versus control blocks only in participants in the 18-s protocol. Additionally, more participants exhibited a salivary alpha-amylase response to the handgrip block in the 18-s condition compared to the 3-min condition. Thus, these results suggest that neuroimaging and behavioral studies with isometric handgrip should be able to successfully induce sympathetic nervous activity with the 18-s paradigm, regardless of the handgrip device and the ability to calibrate an MVC.

Keywords: Handgrip; Norepinephrine; Pupil dilation; Sympathetic arousal; Women.

Fig 1

sAA levels throughout the experimental sessions. The mean sAA levels ± SEM at each of the seven times points for the 3-min IHG in Block1 (n = 12), 3-min IHG in Block2 (n = 11), 18-sec IHG in Block1 (n = 13), 18-sec IHG in Block2 (n = 13), and Ctrl-Ctrl (n = 13) conditions are presented above.

Fig 2

Pupil responses differ to handgrip versus control in 18-s and 3-min tasks. a) In the whole task pupil response to handgrip analysis, there is a significant interaction between experiment block and handgrip paradigm. b) In the top-20 pupil value response to handgrip there is a significant interaction between experiment block and handgrip paradigm, *p < .05.

Figure 3

Top-20 pupil response by squeeze interval in the 18-s and by minute in the 3-min paradigms. We observed a main effect of experiment trial (p < .01), but there was no significant interaction between experiment trial and handgrip paradigm.

Fig 4

Pupil responses in sAA responders and non-responders of the 18-s task. In the top-20 pupil value response to handgrip, sAA responders had near significantly greater pupil dilation compared to non-responders, p = .056.