Key result
Heat load of 40°C significantly increased sympathetic and decreased parasympathetic nervous system activities during handgrip work compared to 25°C.
Why the study?
How do different handgrip work patterns and heat loads affect heart rate variability in healthy men?
How do different handgrip work patterns and heat loads affect heart rate variability in healthy men?
Heat load significantly alters autonomic nervous system balance during isometric handgrip exercise, increasing sympathetic and decreasing parasympathetic activity.
Heat may promote sympathetic dominance during handgrip work; leaves open effects on occupational cardiovascular risk.
The purpose of the present study was to evaluate cardiac sympathetic (SNS) and parasympathic (PNS) nervous system activities during isometric continuous and intermittent handgrip with power spectrum analysis of heart rate variability (HRV). Eight healthy men performed work at 10% maximum voluntary contraction. The sequence of tests included three different work patterns: 1) the continuous handgrip for 30 min, 2) the intermittent handgrip (10 sec contraction +5 sec relaxation) for 45 min, 3) the intermittent handgrip (10 sec contraction +10 sec relaxation) for 60 min. These experiments were performed at two different heat loads (25 degrees C and 40 degrees C, R.H. 50%). To evaluate HRV, low frequency component (0.03-0.15Hz: LFP) and high frequency component (0.15-0.5Hz: HFP) power were calculated. The SNS and PNS activities were evaluated by LFP/HFP and HFP/(HFP + LFP), respectively. Summary of the results was shown below. 1) The change of the SNS and PNS activities were not significant among the different work patterns, due to the lower work load in the present experiment. 2) The increase of HR at 25 degrees C was mediated primarily by the decrease of the PNS activities during the work periods. 3) The effects of heat loads on the SNS and PNS activities were significant. Briefly, at the same work pattern, the SNS activities at 40 degrees were higher than those at 25 degrees C, but the PNS activities at 40 degrees C were lower than those at 25 degrees C.
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Katsuura et al. (1994) studied Healthy (n=8). Isometric continuous and intermittent handgrip at different heat loads (25°C and 40°C) vs. Different work patterns and temperatures (25°C vs 40°C) was evaluated on Cardiac sympathetic (SNS) and parasympathetic (PNS) nervous system activities. Heat load of 40°C significantly increased sympathetic and decreased parasympathetic nervous system activities during handgrip work compared to 25°C.
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