A key adaptation that occurs with SIT is an increased size and number of mitochondria in the skeletal muscle.6 Citrate synthase (CS) is an important biomarker for mitochondrial density and has been shown to be related to improved VO2max. HIIT has been shown to increase CS activity in the muscles of rats.7 In humans, CS activity also increases following SIT. However, there is an increase in CS during SIT, there is a greater increase with TE.8 LARSEN 2015! In regards to CS activity, SIT can be an alternative to TE, but perhaps the best method of increasing mitochondrial density and thereby improving aerobic performance would be to have a combination of the two forms of training. Enzyme activity and protein content also play a role in skeletal muscle oxidative capacity and metabolism. The enzymes, CS and HAD have both been researched in relation to SIT and HIIT training protocols. As previously discussed, CS has been seen to increase after SIT. However, there is not a concurrent increase in HAD activity.9,10 Although there have been contrasting results that show that HAD is increased after SIT.4 A study that had endurance trained runners perform four weeks of SIT showed that both CS and HAD enzyme activity did not diminish after the training protocol. This indicates that in individuals who are already trained, SIT can help maintain oxidative capacity that has been accomplished through TE.11 Skeletal muscle metabolism has been shown to be improved with SIT. This is seen by a decrease in glycogenolysis as well as decreased lactate accumulation.10 Resting ATP content has been shown to decrease after SIT. Cytochrome c oxidase (COX) activity is increased with SIT, which is important in order for oxidative capacity to improve.4 These increases usually happen early in training. There is a plateau in increased COX activity after about a week of training. The protein, PGC-1a has been shown to increase with SIT.3,4 Increased PGC-1a is an important for regulating muscle metabolism.12 Increased capillary content has been shown to occur in TE which increases overall aerobic capacity.13 However, increased capillarisation can occur in SIT as well despite less time performing the training protocol.14 Capillary density has been shown to increase by 27% in a sedentary population who undergoes a SIT protocol.15 In another study of young sedentary males, muscle capillary content of the vastus lateralis increased after SIT. The increase was almost as much as capillary increase after TE. Capillary supply after completing SIT training is maintained in individuals who are already trained.11 …show more content…
However, this statement comes with caution as well. While both training protocols produce similar benefits, TE does have a slight edge in aerobic adaptations, but requires a larger time commitment than SIT. There is a consensus among authors that while there are similarities between the two, that does not mean you should choose one over the other for every exercise routine. Similarly to strength training and endurance, you don’t perform one type of training every day and disregard the other. While that is different than comparing SIT to TE, the same general rule should apply. I think SIT would be a good alternative to TE when seeking to improve aerobic fitness. However, TE should not just be abandoned altogether. A combination of training regimes would be the optimal choice when improving aerobic