On this subject, there were a couple of new pieces of research published last week on interval training:
Similar metabolic adaptations during exercise after low volume sprint interval and traditional endurance training in humans.
Low-volume "sprint" interval training (SIT) stimulates rapid improvements in muscle oxidative capacity that are comparable to traditional endurance training (ET) but no study has examined metabolic adaptations during exercise after these diverse training strategies. We hypothesized that SIT and ET would induce similar adaptations in markers of skeletal muscle carbohydrate (CHO) and lipid metabolism and metabolic control during exercise despite large differences in training volume and time commitment. Active but untrained subjects (23+/-1 y) performed a constant-load cycling challenge (1 h at 65% of VO2peak) before and after 6 wk of either SIT or ET (n=5 men and 5 women per group). SIT consisted of 4-6 repeats of a 30 s "all out" Wingate Test with 4.5 min recovery per d, 3 dwk-1. ET consisted of 40-60 min of continuous cycling at ~65% VO2peak per d, 5 dwk-1. Weekly time commitment (~1.5 vs ~4.5 h) and total training volume (~600 vs ~3000 kJwk-1) was substantially lower in SIT vs ET. Despite these differences, both protocols induced similar increases (P<0.05) in mitochondrial markers for skeletal muscle CHO (pyruvate dehydrogenase E1alpha protein content) and lipid oxidation (3-hydroxyacyl CoA dehydrogenase maximal activity) and protein content of peroxisome-proliferator-activated receptor-gamma coactivator-1alpha. Glycogen and phosphocreatine utilization during exercise were reduced after training, and calculated rates of whole-body CHO and lipid oxidation were decreased and increased respectively, with no differences between groups (all main effects, P<0.05). Given the markedly lower training volume in the SIT group, these data suggest that high-intensity interval training is a time-efficient strategy to increase skeletal muscle oxidative capacity and induce specific metabolic adaptations during exercise that are comparable to traditional ET.
So, from a perspective of time efficiency do intervals. You get the same benefits for a fraction of the time investment!
IGF-I and IGFBP-3 during Continuous and Interval Exercise.
The purpose of this research was to compare changes in circulating levels of total IGF-I and IGFBP-3 during continuous, moderate-intensity exercise (CE) and high-intensity interval exercise (IE) of equal duration. Ten healthy males completed 2 exercise sessions and a resting control session (R) in random order. The CE was 20 minutes of cycling at 60 - 65 % of V.O (2max). During IE, subjects cycled at 80 - 85 % of V.O (2max) for 1 minute followed by 40 seconds of active recovery, with the cycle repeated for a total of 20 minutes. In each session blood samples were drawn at - 10, 0, 5, 10, 20 and 30 minutes. Both IGF-I and IGFBP-3 increased during exercise (p < 0.05) and repeated measures ANOVA revealed a significant effect for session (IE, CE > R, p < 0.05). Area under the curve (AUC) analyses showed no difference in IGF-I between sessions, however, the IGFBP-3 AUC was significantly greater during IE than R (p < 0.05). These results suggest interval and continuous exercise will result in similar changes in circulating IGF-I and IGFBP-3. This could be beneficial to individuals who can exercise longer and at a higher intensity in intervals than would be possible using a continuous protocol.
Not sure if this adds to the discussion or not...