Internals

"I am clearly not an expert and would welcome meaningful input" OK here's the crash course: All work of muscle fibers is done utilizing an energy source called ATP (adenosine triphosphate) available in limited quantities (enough for only a couple seconds of work) in each muscle cell. When a molecule of ATP is broken down it looses one of the phosphate molecules. This breakdown yields energy used to power the contraction of the fibre and create heat. The molecule of ATP must be recycled (rebuilt by using energy from another source to reattach that wandering phosphate molecule) before it can be used again to power more muscle contractions. The different types of muscle metabolism are really just different ways derive energy needed to recycle those ATP molecules. Nonaerobic Metabolism - Creatine Phosphate (CP), also present in each muscle cell, is the front-line energy source for recycling ATP. The chemical reaction that does this is simple (one step) and fast, generating lots of energy in a very short time - it can recycle ATP molecules as fast as the athlete can break them down. No oxygen is required for CP/ATP reactions and no nasty byproducts (lactic acid) are created. But a muscle cell can only store enough CP for 10-15 seconds of work. All energy required to recycle ATP for work beyond that first 10-15 seconds must be supplied by metabolizing glucose stored in or transported to the muscle cell (glycogen), fats or protein. Glycogen metabolism - There are hundreds of steps in the complete metabolism of glycogen. The first eleven steps are referred to as glycolysis, more commonly referred to as anaerobic metabolism. The remaining steps are what is commonly referred to as aerobic metabolism. Anaerobic Metabolism - The 11-step chemical reaction, glycolysis, can provide energy at close to the same rate as the CP/ATP reaction. Anaerobic metabolism of one molecule of glycogen provides enough energy to recycle 2 molecules of ATP. The near-end products of glycolysis are pyruvic acid and hydrogen ions (H+). If these are not metabolized immediately (see Aerobic Metabolism below) they meet to form lactic acid (LA). As LA accumulates, muscle pH goes down, which impedes the ATP recycling process (acidosis), reducing the muscle's ability to contract (which you experience as muscle fatigue and slowing of motions). At high rates of exertion pH drops to the acidosis point in 60 seconds or less. LA is removed from muscles via several mechanisms which are beyond the scope of this post. Aerobic Metabolism - During glycolysis, when the first pyruvic acid and H+ are formed, they are immediately funneled into two, more complex (hundreds of steps), slower systems called the Krebs Cycle and the Electron Transport Chain (ETC). These are the two major phases of aerobic metabolism. Aerobic metabolism of one molecule of glycogen releases enough energy to recycle 36 ATP molecules. The Krebs Cycle metabolizes pyruvic acid to carbon dioxide and the ETC combines hydrogen ions (H+) with available oxygen to produce H2O, releasing energy in the process to fuel the recycling of ATP. The ETC can only accept new H+ for processing at the front end as quickly as it can combine H+ with oxygen and dump it as water out the other end. Because oxygen is required at the endpoint of the ETC, its supply is one of many bottlenecks to aerobic metabolism. If there is not a sufficient quantity of oxygen available at the endpoint of the reaction then H+ concentration increases at the front end. This allows the pyruvic acid and H+ generated by glycolysis to form LA. Only to the extent that oxygen is freely available at the end of the ETC can aerobic metabolism prevent or reduce the formation and accumulation of LA. If you followed the flow of chemicals and energy in the above description you will understand that aerobic and anaerobic metabolism are active at all points of the exercise continuum beyond the first 2 or 3 seconds. What changes is the relative energy contribution of each system. At the beginning, it is all nonaerobic, then shortly it becomes predominantly anaerobic and eventually, in prolonged exercise, predominantly aerobic. In endurance activities the ETC provides around 90% of total exercise energy. The more oxygen your cells are able to 1) get and 2) process, the greater will be the energy contribution of the aerobic system at any point in exercise past the first 10 or 15 seconds. Different types of training are intended to stress the different systems in hopes of eliciting a desirable training response. There are MANY different desirable metabolic training responses and each type of training can elicit only a subset of those responses. Hence, various types of training are required to maximize performance. Can you do work that is 100% aerobic? No, because you cannot stop glycolysis (anaerobic metabolism). In fact glycolysis is required in order to provide the pyruvic acid and H+ that feed the aerobic processes. Can you do work that is 100% anaerobic? Not beyond the first few seconds, as soon as ANY glycolysis takes place, aerobic processes are sure to follow, albeit later and at a slower rate. Anaerobic Threshold - If you are swimming along at precisely the fastest rate at which the muscles you are using can process oxygen (at the end of the ETC) you are said to be working at Anaerobic Threshold (AT). If you speed up a bit, H+ will begin to backup, LA will be produced and soon accumulate to a concentration that will slow you down again. AT is the highest intensity you can maintain for a long duration. This is considered to be the most effective type of training for the aerobic systems. Doing intervals with large work/rest ratios (10/1, 6/1 - i.e. lots of work and very short rests as in swimming 100s on 5-10 seconds rest) is predominantly aerobic work. Doing intervals with smaller work/rest ratios (1/1, 1/3, 1/6 etc - work followed by rest in equal or multiple amounts - say, swimming all-out 100s with 5-10 minutes rest between) is more anaerobic in nature. Note that ANY significant anaerobic exercise bout requires the involvement of the aerobic system to clear the H+ of LA accumulation (recovery). This is why low-level aerobic intensity swimming allows you to recover from highly anaerobic exercise more rapidly than by simply sitting still. That's all I've got time for. I left out loads of detail but hit most of the basics needed to understand what kind of work is going on during workouts.

"I am clearly not an expert and would welcome meaningful input" OK here's the crash course: All work of muscle fibers is done utilizing an energy source called ATP (adenosine triphosphate) available in limited quantities (enough for only a couple seconds of work) in each muscle cell. When a molecule of ATP is broken down it looses one of the phosphate molecules. This breakdown yields energy used to power the contraction of the fibre and create heat. The molecule of ATP must be recycled (rebuilt by using energy from another source to reattach that wandering phosphate molecule) before it can be used again to power more muscle contractions. The different types of muscle metabolism are really just different ways derive energy needed to recycle those ATP molecules. Nonaerobic Metabolism - Creatine Phosphate (CP), also present in each muscle cell, is the front-line energy source for recycling ATP. The chemical reaction that does this is simple (one step) and fast, generating lots of energy in a very short time - it can recycle ATP molecules as fast as the athlete can break them down. No oxygen is required for CP/ATP reactions and no nasty byproducts (lactic acid) are created. But a muscle cell can only store enough CP for 10-15 seconds of work. All energy required to recycle ATP for work beyond that first 10-15 seconds must be supplied by metabolizing glucose stored in or transported to the muscle cell (glycogen), fats or protein. Glycogen metabolism - There are hundreds of steps in the complete metabolism of glycogen. The first eleven steps are referred to as glycolysis, more commonly referred to as anaerobic metabolism. The remaining steps are what is commonly referred to as aerobic metabolism. Anaerobic Metabolism - The 11-step chemical reaction, glycolysis, can provide energy at close to the same rate as the CP/ATP reaction. Anaerobic metabolism of one molecule of glycogen provides enough energy to recycle 2 molecules of ATP. The near-end products of glycolysis are pyruvic acid and hydrogen ions (H+). If these are not metabolized immediately (see Aerobic Metabolism below) they meet to form lactic acid (LA). As LA accumulates, muscle pH goes down, which impedes the ATP recycling process (acidosis), reducing the muscle's ability to contract (which you experience as muscle fatigue and slowing of motions). At high rates of exertion pH drops to the acidosis point in 60 seconds or less. LA is removed from muscles via several mechanisms which are beyond the scope of this post. Aerobic Metabolism - During glycolysis, when the first pyruvic acid and H+ are formed, they are immediately funneled into two, more complex (hundreds of steps), slower systems called the Krebs Cycle and the Electron Transport Chain (ETC). These are the two major phases of aerobic metabolism. Aerobic metabolism of one molecule of glycogen releases enough energy to recycle 36 ATP molecules. The Krebs Cycle metabolizes pyruvic acid to carbon dioxide and the ETC combines hydrogen ions (H+) with available oxygen to produce H2O, releasing energy in the process to fuel the recycling of ATP. The ETC can only accept new H+ for processing at the front end as quickly as it can combine H+ with oxygen and dump it as water out the other end. Because oxygen is required at the endpoint of the ETC, its supply is one of many bottlenecks to aerobic metabolism. If there is not a sufficient quantity of oxygen available at the endpoint of the reaction then H+ concentration increases at the front end. This allows the pyruvic acid and H+ generated by glycolysis to form LA. Only to the extent that oxygen is freely available at the end of the ETC can aerobic metabolism prevent or reduce the formation and accumulation of LA. If you followed the flow of chemicals and energy in the above description you will understand that aerobic and anaerobic metabolism are active at all points of the exercise continuum beyond the first 2 or 3 seconds. What changes is the relative energy contribution of each system. At the beginning, it is all nonaerobic, then shortly it becomes predominantly anaerobic and eventually, in prolonged exercise, predominantly aerobic. In endurance activities the ETC provides around 90% of total exercise energy. The more oxygen your cells are able to 1) get and 2) process, the greater will be the energy contribution of the aerobic system at any point in exercise past the first 10 or 15 seconds. Different types of training are intended to stress the different systems in hopes of eliciting a desirable training response. There are MANY different desirable metabolic training responses and each type of training can elicit only a subset of those responses. Hence, various types of training are required to maximize performance. Can you do work that is 100% aerobic? No, because you cannot stop glycolysis (anaerobic metabolism). In fact glycolysis is required in order to provide the pyruvic acid and H+ that feed the aerobic processes. Can you do work that is 100% anaerobic? Not beyond the first few seconds, as soon as ANY glycolysis takes place, aerobic processes are sure to follow, albeit later and at a slower rate. Anaerobic Threshold - If you are swimming along at precisely the fastest rate at which the muscles you are using can process oxygen (at the end of the ETC) you are said to be working at Anaerobic Threshold (AT). If you speed up a bit, H+ will begin to backup, LA will be produced and soon accumulate to a concentration that will slow you down again. AT is the highest intensity you can maintain for a long duration. This is considered to be the most effective type of training for the aerobic systems. Doing intervals with large work/rest ratios (10/1, 6/1 - i.e. lots of work and very short rests as in swimming 100s on 5-10 seconds rest) is predominantly aerobic work. Doing intervals with smaller work/rest ratios (1/1, 1/3, 1/6 etc - work followed by rest in equal or multiple amounts - say, swimming all-out 100s with 5-10 minutes rest between) is more anaerobic in nature. Note that ANY significant anaerobic exercise bout requires the involvement of the aerobic system to clear the H+ of LA accumulation (recovery). This is why low-level aerobic intensity swimming allows you to recover from highly anaerobic exercise more rapidly than by simply sitting still. That's all I've got time for. I left out loads of detail but hit most of the basics needed to understand what kind of work is going on during workouts.