Before we started the fartlek training, our bodies were using the aerobic system and bi-products of this system are carbon dioxide (CO2) and water (H2O). Our heart rate (HR) was low, 60-70 beats a minute, and our breathing rate (BR) was slow and deep, 1 breathe about every 6 seconds. The demand for fuel was low because we didn't need to supply the muscles in our body quickly. Once we started walking, the demand for fuel increased but only slightly. Our bodies started using the phosphate system, (anaerobic energy system), but since we were only walking it lasted most of the walk. Our breathing rate, heart rate, blood pressure (BP), cardio output (CO) and stroke volume all increased but only a relatively small amount compared to maximal effort.
At the start of the sit + stand, ATP (adenosine triphosphate) is used with creatine phosphate resynthesising ADP (adenosine diphosphate) and Phosphate back together again.On the first sit and stand, the demand for fuel increased to supply the bodies muscles with oxygen and fuel (hamstrings, quadriceps, solius, etc.). Breathing rate became faster and shallower to help supply the muscles with O2. HR, CO, BP and SV all increase to supply the muscles with O2 and fuel. By about the third sit and stand, the ATP has worn off and lactic acid has built up in legs (hamstrings, quadriceps, solius, sartorius, gastrocnemious). But it does not reach toxic levels. Expelling carbon dioxide at a rapid rate.
The demand for fuel goes down slightly for the jog since in a continuous motion instead of sitting down and standing up. Lactic acid is diluting while jogging. The lactic acid system is cable of releasing energy to resynthesise ATP without the involvement of oxygen which is called anaerobic glycolysis. Glycolysis results in the formation of pyruvic acid and hydrogen ions. The pyruvic acid molecules undergo oxidation in the mitochondrion and the Krebs cycle begins. HR, BR...