Skeletal muscle capillarization is a central determinant of oxygen and nutrient delivery and removal of metabolites in skeletal muscle and a well-established capillary network has important implications exercise performance. Since the pioneering work from the 1970’s, that revealed that physical activity promotes growth of new capillaries, numerous studies have shown that just a few weeks of regular training result in a 10-30% increase in number of capillaries per muscle fiber in previously untrained subjects and that capillarization can be more than 200% higher in elite athletes compared to untrained counterparts. However, comprehensive information about what is the optimal training intensity, duration and volume are lacking. While high intensity interval training has proven very effective in improving all sorts of health and performance parameters, indications from recent studies are that high intensity training is, at best, not very effective in promoting capillary growth.
 
The growth of new capillaries, angiogenesis, is governed by a large number of pro- and anti-angiogenic factors and the physiological stimuli regulating angiogenesis are the frictional force of flowing blood, known as shear stress, local metabolism and vessel stretch. These factors are stimulated during muscle activity but the magnitude of each may vary according to the intensity and duration of given training session. Ultimately, the stimulated release of pro- and antiangiogenic factors and the growth of new capillaries are dependent on training modality. While we are awaiting the perfect study that will reveal how to train to improve skeletal muscle capillarization, meta-analysis of training studies published over the past ~50 years can give indications as to what training modality is most effective.
 
This talk will cover the physiological stimuli that leads to growth of capillaries in skeletal muscle, the underlying mechanisms and how different training modalities may impose different angiogenic stimuli. In addition, meta-analysis of data on training induced capillarization from more than 50 trails are presented to provide recommendations for training as well as for directions of future research.