Prolonged mean response time in older adults with cardiovascular risk compared to healthy older adults

Abstract

Background: During incremental exercise (Inc-Ex), the mean response time (MRT) of oxygen uptake (V̇O₂) represents the time delay before changes in muscle V̇O₂ reflect at the mouth level. MRT calculation by linear regression or monoexponential (τ') fitting of V̇O₂ data are known to be highly variable, and a combination of incremental and constant load exercise (CL-Ex) is more reproducible.

Methods: We evaluated MRT in older adults using linear regression and combination methods. We recruited 20 healthy adults (male: 9, 69.4 ± 6.8 years) and 10 cardiovascular risk subjects (male: 8, 73.0 ± 8.8 years). On day 1, they performed Inc-Ex using a 10W/min ramp protocol, for determination of the ventilatory anaerobic threshold (VAT) using the V-slope method. On day 2, they performed Inc-Ex to VAT exercise intensity and CL-Ex for 25min total. The MRT was calculated from the CL-Ex V̇O₂ average and the time at equivalent V̇O₂ in the Inc-Ex. We also assessed the amount of physical activity using the International Physical Activity Questionnaire short form (IPAQ-SF).

Results: The MRT of healthy participants and those at cardiovascular risk were 49.2 ± 36.3 vs. 83.6 ± 45.4s (p = 0.033). Total physical activity in the IPAQ-SF was inversely correlated with MRT.

Conclusion: The MRT was significantly prolonged in cardiovascular risk participants compared to healthy participants, possibly related to the amount of daily physical activity. Individual MRT may be useful for adjustment of exercise intensity, but this should also be based on daily physical activity and individual condition during exercise.

Keywords: Exercise intensity; Exercise prescription; Mean response time; cardiopulmonary exercise testing; Older population.

Fig. 1

Diagram of Mean Response Time Calculation. (A) (upper panel) shows V̇O₂ during incremental and constant exercise load in a sample case. Rest, warm-up, and a ramp protocol of 10W/min (Ramp10) on day 1 was followed on day 2 by constant load exercise for a total of 25min. (B) (lower panel) shows a diagram of the MRT calculation. The difference in oxygen uptake from VAT on day 1 to V̇O₂ steady state (20–25min) on day 2 was defined as ΔV̇O₂ ① (ΔV̇O₂ after VAT). The difference in oxygen uptake from the day 1 VAT to the earlier MRT was defined as ΔV̇O₂ ② (ΔV̇O₂ before VAT). VAT – ventilatory anaerobic threshold; V̇O₂ – oxygen uptake

Fig. 2

Mean Response Time. The mean response time (MRT) for each group is shown. Green markers indicate individual mean response times for all cases. Blue markers indicate individual mean response times for the healthy group and red markers indicate individual mean response times for the cardiovascular risk group. Patients with cardiovascular risk had significantly prolonged MRT compared to healthy adults

Fig. 3

ΔV̇O₂ before and after VAT. The left figure (A) shows all cases (green) and the right figure (B) shows pre- and post-VAT ΔV̇O₂ for each group (healthy group: blue; risk group: red). The x-axis shows ΔV̇O₂ after VAT and the y-axis shows ΔV̇O₂ before VAT. Dotted lines indicate 95% confidence intervals for each group in their associated color. ΔV̇O₂ showed a significant correlation in both groups. Patients with cardiovascular risk had significantly higher ΔV̇O₂ compared to healthy adults. VAT – ventilatory anaerobic threshold. V̇O₂ – oxygen uptake

Fig. 4

Relationship between mean response time and physical activity. Physical activity (in metabolic equivalents, METs) was calculated using the responses from the International Physical Activity Questionnaire short form (IPAQ-SF). Mean response time (MRT) tended to shorten in both groups (healthy group: blue; risk group: red) as activity level increased