The ultrastructure of the normal human skeletal muscle

SummaryMuscle biopsies were taken from the middle part of the vastus lateralis muscle of 9 men, who were not regularly involved in endurance training (M, average $$\dot V_{{\text{O}}_{\text{2}} } $$ max=61.3 ml/min·kg), 3 sedentary women (W, $$\dot V_{{\text{O}}_{\text{2}} } $$ max=43.7 ml/min·kg) and 5 well trained orienteers (TO, $$\dot V_{{\text{O}}_{\text{2}} } $$ max=76.1 ml/min·kg). Morphometric analysis of 60 electron micrographs per biopsy gave the following significant differneces: 1.The volume density of central mitochondria was 1.47-fold higher in TO than in M, and 1.44-fold higher in M than in W.2.The volume density of peripheric mitochondria was 3.22 times higher in TO compared to M.3.The ratio of the central mitochondrial volume to the volume of myofibrils was 1.54-fold higher in TO compared to M, while the respective ratio was 1.49 for M compared to W.4.The surface of the central mitochondria was 1.28-fold higher in TO than in M and 1.35-fold higher in M than in M.5.The surface of mitochondrial cristae was higher by a factor of 1.62 in TO compared to M and 1.35 in M compared to W.6.The central mitochondria were larger in TO compared to M by a factor of 1.12.7.The volume density of intracellular lipid (triglyceride droplets), was 2.5-fold higher in TO than in M.There were highly significant correlations between $$\dot V_{{\text{O}}_{\text{2}} } $$ max and volume density of central mitochondria (r=0.82), surface of mitochondrial cristae (r=0.80) and the ratio of mitochondrial volume to myofibrillar volume (r=0.78).No quantitative changes could be observed in mitochondrial fine structure. Neither volume density of sarcoplasma nor volume and surface density of the tubular system showed any difference as a function of training and sex.It is postulated thata)an individual's maximum oxygen intake is limited not only by the capacity of the oxygen transport system but also by the oxidative capacity of mitochondria in the skeletal muscles, andb)the skeletal muscle of trained athletes contains a much higher quantity of intracellular lipids (triglyceride droplets) as a substrate directly available for energy production.