In a study “Amateur endurance triathletes’ performance is improved independently of volume or intensity-based training” Clemente-Suarez, V.J., et al., the authors tried to find the physiological responses and performance parameters that two different training programs (one based on intensity and one based on volume) would produce.
They looked at swimming and running performance after four weeks of training with either emphasis on volume or intensity. This was a small study of 32 moderately trained triathletes. It should be noted, cycling was not tested.
The study concluded that there were no significant differences in running or swimming improvement between the group concentrating on high-intensity training and the group focused on high volume training. Even though the authors had predicted that the intensity group would perform better.
The study suggested that;
high-intensity training may be favorable for those with time constraints on training.
The authors also acknowledged other researchers had found that;
over-reaching is a risk with high-intensity training that can result in decreased performance.
REFERENCE: Vicente Javier Clemente-Suarez, Rosa Delgado-Moreno, Beatriz Gonzalea, Javier Ortega,Domingo Jesus Ramos-Campo., Amateur endurance triathletes’ performance is improved independently of volume or intensity based training. Physiology & Behavior, 2018