Background: To date the metabolic response to physical exercise in humans
is a topic of great interest, in view of the importance of sports in modern society.
Oxygen-independent glycogenolysis is satisfactorily reported in textbooks and in
specialized reviews as a paradigm the metabolic response to intense muscle contraction,
as it occurs in the 100 meter sprint.
Focus: Here we propose to carefully analyze the more complex oxygen-dependent glycogen
and free fatty acids utilization in sustained contracting muscle, as it occurs in the
marathon endurance race. The reactions of aerobic glycogenolysis, tricarboxyilic acid
cycle, and free fatty acids catabolism, although well known long time ago, are presened
in a systematic manner, to facilitate understanding of their central role in ATP generation.
The metabolic response of glycogen to intense muscle exercise mainly consites in
the exergonic oxygen-independent conversion of each glycosyl residue into two pyruvate molecules, with
the concomitant production of ATP, the substrate of mysosin ATPase. The two pyruvate molecule enter the
Tricaboxylic Acid Cycle, and generate additional ATP molecules. Particular emphasis is given to the metabolic
response of fatty acids derived from lipid stores to long distance endurance race.
Prospects: Memorizing a long series of reactions might appear a boring task. However, a long didactic
experience convinced one of us (P. L. I.) that discussing through a guided approach the detailed metabolic
networks responsible for the synthesis of ATP, the substrate of myosin ATPase, is as rewarding as
memorizing the various physiological steps of muscle contraction. Finally, a deep knowledge of the metabolomics
of oxygen-dependent and oxigen-independent muscle contration will greatly help students to
get a clear idea of two important muscle physiological concepts: muscle capacity and muscle power.