Introduction: 

The number of capillaries surrounding a muscle fiber is an important determinant of maximal O₂ flux capacity in skeletal muscle. In response to moderate endurance training an increase of capillarity in skeletal muscle is well documented, however chronic response to high intensity exercise remains tentative. The aim of this study was to investigate the load-dependent adaptive response on capillary supply in human vastus lateralis muscle of 10 young male competitive cyclists.

Methods: 

We studied the training period (Nov-Feb) and the competitive season (CS, Feb-Oct). Before (PRE) and after (POST) CS the subjects underwent an incremental cycle ergometer test with spirometry and muscle biopsies were obtained from the vastus lateralis. Semi-thin sections (0.6 mm) were analyzed and capillary contacts (CC), capillary density (CD), sharing factor (SF) and the individual capillary-to-fiber ratio (C/F_i) have been quantified.

Results: 

After changing chronic training regimens at the end of the CS the CC and CD increased significantly (P < 0.01; CC; PRE: 5.69±1.03 vs. POST: 6.86±0.99 capillaries/fiber; CD; PRE: 296.10±17.84 vs. POST: 338.14±33.16 capillaries/mm²), accompanied by a significant increase in C/F_i (P < 0.05; PRE: 3.05±0.30 vs. POST: 4.016±1.03 individual capillaries/fiber). There was no significant change in SF (P > 0.05; PRE: 2.35±0.17 vs. POST: 2.38±0.04 fibers/capillary).

Conclusion: 

According to well investigated adaptation to moderate endurance training this study shows an increase of muscle capillarization in response to chronic high intensity exercise. The unchanged sharing factor induces a homogeneous relative increase of number of capillaries/fiber. This specific kind of muscle plasticity assumes the potential for an increased capacity for oxygen flux.