Introduction: The present study investigated the role of training intensity in the
dose–response relationship between endurance training and cardiorespiratory
fitness (CRF). The hypothesis was that beginners would benefit from an
increase in training intensity after an initial training phase, even if the energy
expenditure was not altered. For this purpose, 26 weeks of continuous
moderate training (control group, CON) was compared to training with
gradually increasing intensity (intervention group, INC) but constant energy
expenditure.
Methods: Thirty-one healthy, untrained subjects (13 men, 18 women; 46 ± 8
years; body mass index 25.4 ± 3.3 kg m−2
; maximum oxygen uptake, VO2max
34 ± 4 ml min−1 kg−1
) trained for 10 weeks with moderate intensity [3 days/
week for 50 min/session at 55% heart rate reserve (HRreserve)] before allocation
to one of two groups. A minimization technique was used to ensure
homogeneous groups. While group CON continued with moderate intensity
for 16 weeks, the INC group trained at 70% HRreserve for 8 weeks and
thereafter participated in a 4 × 4 training program (high-intensity interval
training, HIIT) for 8 weeks. Constant energy expenditure was ensured by
indirect calorimetry and corresponding adjustment of the training volume.
Treadmill tests were performed at baseline and after 10, 18, and 26 weeks.
Results: The INC group showed improved VO2max (3.4 ± 2.7 ml kg−1 min−1
) to a
significantly greater degree than the CON group (0.4 ± 2.9 ml kg−1 min−1
) (P =
0.020). In addition, the INC group exhibited improved Vmax (1.7 ± 0.7 km h−1
)
to a significantly greater degree than the CON group (1.0 ± 0.5 km h−1
) (P =
0.001). The reduction of resting HR was significantly larger in the INC group
(7 ± 4 bpm) than in the CON group (2 ± 6 bpm) (P = 0.001). The mean heart
rate in the submaximal exercise test was reduced significantly in the CON
group (5 ± 6 bpm; P = 0.007) and in the INC group (8 ± 7 bpm; P = 0.001),
without a significant interaction between group and time point