Both HIIT (high intensity interval training) and LSD endurance training are important components of endurance training. HIIT training has recently been shown to set in motion the process a sequence of metabolic signals necessary for mitochondrial biogenesis to occur, and you can read about that here. Although HIT research is in its infancy, these preliminary findings will be of interest to athletes/general population whose training time is limited.
LSD endurance demonstrates changes at the level of the heart which have not been demonstrated in HIT training: increased ventricular volume leading to better filling in between heartbeats, a large ventricular chamber and as a result an increased ejection fraction.
Few studies have compared the effects of HIT v LSD training on pulmonary function or time trial performance, until this neat little study by Dunham et al. (2011). Two groups of untrained subjects were divided into a HIIT group (n = 8) or LSD endurance (n=7) and were tested for VO2max , maximum inspiratory pressure & maximum expiratory pressure (both measures of lung strength) and flow volume loops before and after the experiment. The HIIT group performed 5 * 60s cycling at 90% VO2max for four weeks, while the endurance group performed 45min cycling at 60-70% VO2max for four weeks. Incidentally, both groups performed a time-trial before, two weeks into, and after the experiment.
The results showed that both HIT and endurance improved VO2max, but no significant difference was observed between groups. While inspiratory pressure was significantly increased in the HIIT group, this was not the case for the endurance group. Maximal expiratory pressure showed no difference between groups.
The conclusion that can be drawn is that both HIT and LSD endurance are equally capable of increasing VO2max in untrained subjects. HIIT might therefore be viewed as a time-efficient way of developing cardio-respiratory endurance.