Given a passage, construct a question on the basis of the information present in the passage. Construct the question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) it answer uses text phrases from the passage. Avoid creating questions that (i) can't be answered correctly without actually understanding the passage and (ii) uses the same words or phrases given in the passage.

Resting heart rate (RHR) declines with exercise training. Possible mechanisms include: 1) increased parasympathetic tone, 2) decreased responsiveness to beta-adrenergic stimulation, 3) decreased intrinsic heart rate or 4) combination of these factors.', 'To determine whether an increase in resting parasympathetic tone or decrease in response to beta-adrenergic stimulation contributes to the decrease in RHR with training.', '51 screened healthy subjects aged 18-32 (n=20, mean age 26, 11 female) or 65-80 (n=31, mean age 69, 16 female) were tested before and after 6months of supervised exercise training. Heart rate response to parasympathetic withdrawal was assessed using atropine and beta-adrenergic responsiveness during parasympathetic withdrawal using isoproterenol.', 'Training increased VO2 max by 17% (28.77.7 to 33.69.20ml/kg/min, P<0.001). RHR decreased from 62.86.6 to 57.67.2 beats per minute (P<0.0001). The increase in heart rate in response to parasympathetic withdrawal was unchanged after training (+37.312.8 pre vs. +36.412.2 beats per min post, P=0.41). There was no change in the heart rate response to isoproterenol after parasympathetic blockade with training (+31.910.9 pre vs. +31.012.0 post beats per min, P=0.56). The findings were similar in all four subgroups.
Is exercise training bradycardia largely explained by reduced intrinsic heart rate?