T Nation

Energy Systems

Ok, we know that the Phosphagen system and the fast glycosis systems use carbs/protein as their primary enegery source(intense acitivity) and that the Oxidative/slow glycosis system uses fats in the place of carbs.

So my questions, if you’re doing an endurance actitivty(going for a medium paced few mile jog) which enegery system kicks in first? Would it be the oxidative/slow system or would it but the phosphagen/fast system even though it’s a low intensity acitivity?

You are not entirely correct…

Phosphagen system: uses ATP and creatine to produce energy

Anaerobic alactic system: (technically includes the phosphagen and anaerobic glycolysis) produces energy without the need for oxygen and doesn’t lead to a significant increase in lactic acid. Uses (besides the ATP-CP system) glucose to produce energy.

Anaerobic lactic system: produces energy without the need for oxygen and lead to a significant increase in lactic acid. Uses glucose to produce energy.

Oxydative system: produces energy with the use of oxygen. Uses lipids (free fatty acids), glucose and protein (converted to glucose in the liver) to produce energy.

During endurance work you use primarily free fatty acids and glucose for fuel. Protein can constitute up to 10-15% of the energy production during ultra endurance activities or when the other energy substrates are low.

Here is an approximation of the source of energy production depending on exercise intensity:

65-70% of max heart rate = 40% glucose, 60% fatty acids

70-75% = 50% glucose, 50% fatty acids

75-80% = 65% glucose, 35% fatty acids

80-85% = 80% glucose, 20% fatty acids

85-90% = 90% glucose, 10% fatty acids

90-95% = 95% glucose, 5% fatty acids

100% = 100% glucose, 0% fatty acids

[quote]bigpump23 wrote:
So my questions, if you’re doing an endurance actitivty(going for a medium paced few mile jog) which enegery system kicks in first? Would it be the oxidative/slow system or would it but the phosphagen/fast system even though it’s a low intensity acitivity?[/quote]

It’ll always be the oxidative system to kick in first; anaerobic mechanisms kick in as reinforcements when intensity gets too high or duration is too long.

[quote]Eric Cressey wrote:
bigpump23 wrote:
So my questions, if you’re doing an endurance actitivty(going for a medium paced few mile jog) which enegery system kicks in first? Would it be the oxidative/slow system or would it but the phosphagen/fast system even though it’s a low intensity acitivity?

It’ll always be the oxidative system to kick in first; anaerobic mechanisms kick in as reinforcements when intensity gets too high or duration is too long.[/quote]

Ok but then would it be utterly useless for weight loss, considering that your body still has stored carbs that it is not using?

[quote]Eric Cressey wrote:
bigpump23 wrote:
So my questions, if you’re doing an endurance actitivty(going for a medium paced few mile jog) which enegery system kicks in first? Would it be the oxidative/slow system or would it but the phosphagen/fast system even though it’s a low intensity acitivity?

It’ll always be the oxidative system to kick in first; anaerobic mechanisms kick in as reinforcements when intensity gets too high or duration is too long.[/quote]

All the systems are working but the intensity of the exercise dictates which system dominates.