Article By : Dr Neeraj Mahorekar, (PT)FITBIT Sports PhysioMPT (Sports & Rehab)CKTP, CMT, CDNP, CCTP

Did you know, Up to 45% of your total mass of the body is made up of skeletal muscles. Skeletal muscle is the tissue responsible for generation of the force needed to move your joints needed for exercise.Athletic performance is a combination of Genetics, Proper training, Rest, Recovery, Hydration & Optimized Nutrition.There are 3 types of exercises which is as follows:-

  • Endurance Exercises which are prolonged activity at lower intensity helps in metabolic adaptation like increase oxygen supply
  • Resistance Exercises which is short, intense burst of power output helps in increase strength, power and muscle mass.
  • Sporting Activities which include combination of strength and endurance helps mostly in sports specific.

Ask yourself which activity do you involve

Requirements for all exercises are Energy, Oxygen, Nutrients with exhaust of Metabolic waste & Heat. Also we have to know what impacts exercise capacity are

  • Muscles total cross sectional area
  • Types of muscle fiber 
  • Numbers of active motor units
  • Motor neuron firing frequency
  • Muscle length
  • Velocity of contractions of muscles

Now lets we discuss some about muscles fibers,  How these helps you to understand which energy systems is best for your muscles. There are 2 types of muscles fibers Slow twitch and Fast twitch.

  1. SLOW TWITCH is Type 1 Muscles Fibers which are fatigue Resistant,Work on Aerobic/Oxidative energy systems, well supplied by capillaries and used for prolonged low intensity activities for e.g. Marathon.
  2. FAST TWITCH is Type 2 Muscles Fibers further divide in to two Fast Twitch A(FTa) & Fast Twitch B(FTb) known to be more fatigable, utilize energy more from anaerobic/ glycolytic systems, less rely on aerobic/oxidative systems and better suited to high intensity exercises for shorter duration for e.g. weight lifting

The types of fibers is determined by GENETICS & TRAINING INDUCED VARIATION. As by research Elite Endurance Athletes consist of 70-90% of slow twitch muscle fibers and Sprint & Explosive sport athletes have more fast twitch fibers approx 70%.You may be asking yourself why we need to more about ENERGY SYSTEMS, knowing which energy system used for various activities and helps us to understand. SO WE CAN TRAIN BETTER


  • ATP-CrP (adenosine triphosphate-creatine phosphate) which function in the absence of oxygen
  • Anaerobic or Glycolytic system also independent of oxygen
  • Aerobic or oxidative system which function in presence of oxygen

The type of system utilized by the body depends on the duration, intensity and time of physical activity that u are take part in.To produce movements our muscles have to contract and relax and these are served by ATP which is found in mitochondria of cells hence mitochondria is known as powerhouse of the cells. Our body relies on continuous supply of energy through ATP. ATP is made up of molecule ADENOSINE and three phosphates, the bond between the last two phosphate is vital because when it breaks, energy is released and subsequently used for muscles contraction. now ADP is left behind which is Adenosine and 2 phosphate. In order to provide energy ATP must be replenished.

Lets explore how ATP replenish through different energy systems.

The first one is ATP-CrP System – CrP is a high energy compound

Which is available in limited amount in body and it is the fastest way to replenish ATP. As the system replenish very quickly it does not rely on oxygen. since the system is quick it burn out very fast. so the body needs to go on a different energy system to continue the activity. That,s why the body is not dependent of one energy system & hence changes the energy system according to intensity and duration of activity.Creatine is made up of amino-acids which are the building blocks of  protein so it can replenish through protein rich food sources. In order for Creatine Phosphate to donate a Phosphate for replenish of ATP it must first be broken down by an enzyme called CREATINE KINASE, it takes Creatine Phosphate and breaks it down in individual Creatine and Phosphate molecules and now Phosphate joins ADP to make ATP. Again revising because of limited amount of CrP and quick system no oxygen is required.

The second one is Anaerobic or Glycolytic system, it means without oxygen.


that means Glycolysis is the breakdown of Glucose to donate a Phosphate to ADP to form ATP in the absence of oxygen. The Glucose is obtained from 2 main sources i.e. Glycogen from muscles stores is first broken in to Glucose and secondly Glucose from dietary intake of Carbohydrates. The body can also obtained Glucose from other sources like the liver also. This system also supply energy for shorter period of time approx 60-120 seconds & depletion started. the Byproduct of this system is Lactic acid/Lactate.

Lactate is not only the waste product of this cycle but also an intermediate nutrients of CORI CYCLE (The Lactate moved from the muscles to the liver which converted in to glucose after getting  recycled  & than again used in glycolysis). Unfortunately this pathway is energy insufficient and can not be sustained for longer period of time. when there is insufficient energy to convert the lactate to glucose, the lactate builds up and causes the muscles to fatigue which will slow or hold the activity.

The last one is Aerobic Energy System which is also known as Oxidative System.

It is activated when activity continue for prolonged time at low intensity with requirement of Oxygen.

The substrate used in this system is Carbohydrates, Lipids and very much lesser extract of Amino-acids. These substrate comes either from exogenous sources i.e. DIET or by endogenous sources i.e. BODY STORAGE. The amount of Carbs vs Lipids used depend on exercise Intensity and Duration, Preceding Diet, Substrate Availability and Training Status & Environmental Factor.

Lets recap- ATP loses the phosphate and generate energy & leaves ADP behind. when a prolonged and sustained energy is needed Aerobic system is used, its a longer and more complicated system that needs Oxygen. Glucose can be obtained from your Glycogen stores or from diet & Lipids obtained from Adipose/fat tissue or diet which broken down by the process Aerobic Glycolysis and Aerobic Lipolysis simultaneously to form the Phosphate which is used to replenish ATP from ADP.

 Glucose is our body preferred fuel which will used first and only when it depletes the body access fat/lipids. The Byproduct of this cycle is CO2  which is breath out, this is low and slow process which is used for long period of time .

Lets understand the energy systems with the help  of diagram

In the last lets summarize  ENERGY SYSTEM in various intensity and duration of exercise.

  • For High Energy and Short Duration we use ATP-CrP and Anaerobic System. Example- Weightlifting
  • For Stop Start Sports we use a combination of Anaerobic & Aerobic Energy System. Example- Basketball, Football, Swimming etc.
  • For Moderate Intensity we use mainly Aerobic Energy System and with some extent of blood glucose & circulating fatty acids. Example- jogging, dance, cycling etc
  • For Moderate to Low Intensity we predominantly use Aerobic Pathways (Glycolysis then Lipolysis). Example- Walking

NOTE- The longer you work or exercise at low intensity the more fat is used.