Fibres can be further categorized for example based on their use of energy, such as “FOG” fast oxidative glycolytic and their speed of contraction, force production and resistance to fatigue.  Finally they can be categorized by staining with different ATPase at different pH, and at the molecular level by their myosin heavy chain (MHC) isoforms, MHC-I, MHC-IIA and MHC-IIB.  An additional type is called type IIX muscle fibre.  Note that fibres may be in transition from one type to another, so there’s a I-IIA and a IIA-IIB “hybrid” type.

The fibre types differ in activation and maximum power output and velocity.  The conventional theory is that so-called “motor unit” fibres are recruited in order of their capacity.  The fatigue resistant Type I fibres are always recruited while the fast powerful fibres are reserved for when more power or speed are needed.  So the motor units containing slow fibres are recruited first and followed by motor units composed of faster muscle fibres in sequence from type I to IIA to IIB.  The “fast” IIB fibres can generate about twice the power, up to four times as quickly as the slow type I fibres.

Isolated type IIB fibres can generate about 8x the power, while type IIA generate about 5x times the power of slow type I fibres.  In terms of velocity, type IIB fibres shorten almost 4x as quickly and type IIA about 2.5x as quickly as type I.

How does this work in a muscle that is comprised of mixed fibre types?  The makeup of each muscle is different in terms of proportion of fibre types, so that the muscle as a whole has optimal power and speed of contraction for its task.  The “slow” type I fibres which better resist fatigue always contribute power, and do not simply “give up” when faster fibres create more power while shortening at their optimal speed for power generation.  So they keep the tension in the muscle.

Our muscle fibres “automagically” shorten at a speed at which maximum power is developed.  Our training “automagically” makes this happen!

Many people say that one shouldn’t mix  endurance or “aerobic” exercise and resistance or weight training possibly because they think that you won’t get maximal strength or endurance gains.  There are ways to maximise the benefits if they are done in the same day or session.  Watch out for our guide next week.

Aerobics and Weight Training

Although you might think that there is nothing in common between weight training and the aerobic class in the studio, they are just two extremes of the “strength-endurance continuum” and they both involve muscular ad cardiovascular activity.  At one extreme you have high repetitions with body weight and continuous movement; at the other you have low repetitions with heavy weights and long rest periods.  The body’s adaptations are linked to the type, intensity, and number of repetitions performed.

• Endurance training leads to an increase in the proportion of slow/oxidative or Type I muscle fibres, which are high in mitochondrial content.
• Resistance training is associated with an increase in protein synthesis and hypertrophy (enlargement) of fast/glycolytic or Type II fibres.

Both aerobic and anaerobic processes contribute during intense exercise lasting from 30 s to 3 min.  Your trainer can discuss the various energy sources that your body uses, and their limitations.

In resistance training, endurance, or the maximal number of repetitions at 60% of 1RM and aerobic power improves when performing high repetitions (e.g. 20-28) for two sets, with 1 minute’s rest using light weights.  Conversely maximal strength improves more with low repetitions (e.g. 3-5 reps) for four sets with up to 3 minutes rest and higher weights.  In between these extremes would be the typical induction phase of three sets of 9-11 reps and 2 minutes rest.

The regular performance of resistance exercises and ingestion of adequate amounts of protein are needed to gain muscle strength and build muscle. The suggested dietary allowance of 0.8 g protein per kg body weight per day may be met with a normal diet. However, whey protein is often used a supplement, particularly for those aiming to gain muscle mass. With a normal diet, a fairly low amount of whey protein, about 10 g protein, is enough to support a positive net protein balance.

Research has shown that multiple sets are better than single sets for increases in maximal strength. Also there is some evidence that sets should occasionally be performed to the point of failure. This results in greater activation of muscle “motor units” and secretion of growth-promoting hormones. However, training to failure has the potential for overtraining and overuse injuries.

Preparatory Phase

In the first phase the goals will usually be to improve four levels of power (general, strength, endurance, and reactive explosion); to achieve some weight loss and lower blood pressure; to improve endurance; and to increase flexibility.

• We will use some basic assessments of performance, including the bench press; T-bar row; muscular endurance and flexibility at the start and at Weeks 5, 10, 18 and 30. No maximal lifts are needed to do this.
• Exercise will be varied and periodized (i.e. adapted to your progress).
• Types of exercise range from body weight; exercise bands; medicine ball; cable machines; stack-loaded machines; free weights (e.g. dumbbells and barbells, weight plates, and kettle bells); the Swiss Ball, BOSU trainer, balance and agility exercises; to boxing and circuit training.

Over a six month period you may progress through these stages:

• Psychological: develop goals and drive; improve concentration and mental skills; positive imagery.
• Strength: anatomical and neuromuscular adaptation; hypertrophy, strength, conversion to power.
• Endurance: general aerobic endurance; specific endurance (e.g. high intensity interval training); constant endurance.
• Speed: develop fundamental skills; agility and quickness.
• Nutrition: balanced nutrition (protein after workouts) months 1-3; high protein, lower carbs, moderate fat (months 4-6).

After this phase, which might last 4-6 months in total, we can consider improving fundamental skills (e.g. performing complex lifts like the “clean”); train in fatigue situations; improve power movements; and increase range-of-motion (ROM) and sport-specific or competitive training.

Many factors affect the response to training, for example:

• Range of motion
• Number of repetitions and sets
• Load magnitude and duration of each repetition – time under tension
• Rest interval both between repetitions and in-between sets
• Number of interventions/week and duration of training period
• Muscular failure and recovery time.

The length of the rest interval is based on the training goal.

• When training for strength, heavier loads are lifted and the rest interval between might be as long as 3-5 minutes.
• When training for muscular power, a minimum of 3 minutes rest is allowed between sets of repeated almost maximal effort movements.
• When training for muscular hypertrophy (size), shorter rest intervals of 30-60 seconds between sets are associated with higher acute increases in growth hormone.

Body fat reduction

In addition to aerobic or endurance training, strength training may be of surprising benefit to overweight individuals.

• An increase in glucose uptake by muscle leads the pancreas to secrete less insulin and more glucogen.
• Muscle growth leads to an improvement in whole-body metabolism associated with an increase in fatty acid beta-oxidation in liver, the production of ketone bodies, and the resolution of hepatic steatosis, and promotes lipolysis in adipocytes.

Sugar.  When we consume highly refined, energy dense food or drink, particularly if it contains sugar or HFCS we become “awash” in excess calories.  Our metabolism no longer has to be efficient and instead has to try to “waste” calories.  If we are also inactive and sitting at a desk all day, it has no way to get rid of the calories but to store them as body fat. Indeed, storage in the adipose tissue is promoted by insulin, the activity of which is stimulated by high blood sugar.

But fat cells are not inert – they are almost like the “Adipose”, a race of vaguely humanoid blobs of fat from the Dr. Who TV program. It is a complex and highly active metabolic and endocrine organ producing hormones and cytokines, with connective tissue, nerves, immune cells, and its own blood supply. 

Normally fat cells have an important role in maintaining triglyceride and free fatty acid levels, which are the preferred source of energy for muscle contractions at low activity level.  The heart, for example, normally gets 60-70% of its energy from fatty acids and triglycerides and 35% from carbohydrates, and the heart can also use ketone bodies.

In skeletal muscle at low exercise intensities, slow oxidative (type I) muscle fibers with a high capacity for fat oxidation are recruited first.  As intensity increases, faster (type II) fibers with higher glycolytic capacity are activated.  This parallels the changes in fuel selection from mainly fatty acids to mainly carbohydrate.

Fat is stored as triglycerides and has to be converted to fatty acid and glycerol, and transported and the release and availability of fatty acids is a relatively slow process. To keep the process going, at rest we continually recycle triglycerides to fatty acids and back.

When we do moderate exercise the recycling drops from about 70% to about 25% – the process to provide FFAs doesn’t have to do a “cold start”.   When we stop exercising, the recycling quickly increases back to 80% to recover the fatty acids into triglyceride stores and to protect against an overshoot of plasma FFA levels.

Even so…

Fatty acids have more energy per gram than carbohydrates, but produce a little less ATP per “cycle” and consume a little more oxygen per ATP produced, but overall this is still a very efficient process.   The energy yield from a gram of fatty acids is approximately 9 Kcal, compared to 4 Kcal for carbohydrates.

Cardiac muscle has a large number of mitochondria supporting continuous aerobic respiration via oxidative phosphorylation and is highly aerobic and resistant to fatigue. The heart can also recycle lactate, which is very energy efficient when lactate is oxidized to pyruvate, which itself can then be used aerobically in the TCA cycle.  So the “lactate” by-product of exercise is an efficient source of energy for the heart, which has to increase its output to support the exercise that’s producing the lactate.  Isn’t that quite amazing?  Both lactate and ketone bodies are also usable by the brain, but that’s another story.

The metabolism of glucose requires less oxygen than metabolism of fatty acids to produce the same amount of energy stored in the form of ATP.  However each gram of palmitate produces significantly more energy (3.6 times greater) than that derived from a gram of glucose.  So it makes sense that we get about 70% of our energy from fatty acids at low levels of activity.

When we go to high intensity activity, we switch to more glucose metabolism, and this occurs very rapidly if, for example, we are in the ”fight or flight”  mode brought about by adrenaline (epinephrine).

Weight loss

With the proper training we can stimulate biogenesis of more mitochondria and more efficient use of fatty acids at higher levels of activity.  We can use more fat as we exercise, and that is very efficient.  Think of it like a hybrid car engine that more efficiently uses high-energy stores (battery) and adds the gasoline motor when needed.  And the gasoline engine tops up the batteries.  In our case we are quite capable of producing glucose from stored fat (gluconeogenesis), but if we always have excess glucose, we never need to.

When oxygen is abundant (e.g. we’re only walking!) and food is scarce, there is an advantage in utilizing fatty acids for fuel as opposed to using glucose.

So our bodies make the most out of such energy sources as are available, and mitochondria are the place where this energy is made available.  Fatty acids must be activated in the cytoplasm by conversion to the acyl-CoA derivative (which uses some energy) and then enter mitochondria for beta-oxidation, which takes place in the mitochondrion.

The total ATP per mol of tripalmitoyl glycerol is 3 x 129 = 387 (for the fatty acid) plus 20 for glycerol = 407.  Subtract the 2 mol ATP for each FA to form acetyl – CoA derivative = -6, and the total yield is 401 ATP per mol of triglyceride.  Compare this with the 36 mol of ATP per mol of glucose.

So how much energy do we have stored?  If I weigh 68 kg and have 14% body fat that’s about 9.5 kg of fat.  I could go down to about 11% body fat and the difference is about 2.2 kg of fat.  If my basal (resting) metabolism is 1500 kcal a day and I have 2,200 g of triglycerides available, I have sufficient stored energy for about 11 days.   If I was at 22% body fat, I’d have more stored energy.  So being at the top end of the “normal” body fat range might be an advantage if something such as an illness, accident, earthquake, or surgery prevents me from being able to eat much for 10 days.

Normally we use more of whatever nutritional energy is available (in our body), so if we have low blood glucose and insulin levels and high free fatty acids, we’ll use mainly fatty acids. If we consume carbohydrates, then blood glucose levels will rise stimulating insulin secretion and depressing fatty acid levels.  So we’ll use mostly glucose.  And if there’s too much glucose to use, we’ll store it in adipose tissue.

In fact storing energy “wastes” about 5-10% of the energy, but that’s no problem when energy is available in excess.  There’s also a “loss” in recovering the stored energy from fat stores, but again that’s an unavoidable “cost” of “banking” and transporting the energy.

Whole body lipid oxidation increases progressively during prolonged aerobic exercise, even when work rate is maintained constant.  It has to, because we can’t store enough energy in the form of glycogen to sustain long duration exercise.  Our capability to do long duration exercise is thus dependent on our ability to use FA as a fuel.  An increased capacity of mitochondria is therefore an advantage for endurance exercise and we can train so as to upregulate the mitochondrial capacity to oxidize FA.

During exercise at moderate intensity (60% VO2 max) we get about a 40% increase in free fatty acid (FFA) availability which remains significantly elevated for at least 3 to 6 hours after exercise, before decreasing to about 10% after 24 hours.   The increase is proportional to the net energy used in the exercise and is higher in people who have a low resting plasma FFA.   So why is this of interest when we exercise?

The best intensity for weight loss?

You will no doubt read about the differences in opinion regarding weight loss – whether exercise at low, moderate, or high intensity, or interval training has the best effect.

It is all to do with calories!

But it’s not a simple equation.  Carbohydrate is the major fuel source during moderate- to high-intensity exercise, although the contribution of fatty acids increases with training. What is interesting is that substantial fatty acid use continues after exercise. It’s called post-exercise lipid oxidation (PELO) and this uses about the same energy during recovery regardless of whether exercise was at 55% or 75% of HRR when the amount of energy used during exercise is the same.  So it is efficient to use high-intensity interval training because you will use the energy more quickly, and get a better cardiovascular benefit, and still get the same PELO effect and contribution to weight loss.

For a simple calculation, during moderately intense exercise at 65% HRR we may use about 10 kcal a minute for 30 minutes. That’s 300 kcal, with perhaps 100 kcal of those coming from lipids. After exercise, at rest over the next three hours, we will use about the same total of 300 kcal, of which about 80-90% or about 250 kcal come from lipids providing that the energy used is not replenished with carbs or fat straight after exercising! [Note below.] This increased use of lipids continues for many hours.

So if someone is trying to lose weight it may be appropriate to have a small (typically half-portion) protein shake after exercise but to wait a few hours more before eating. If you are also on a calorie restricted diet, your body will increase its lipid oxidation over a period of time (it takes at least a few days or weeks) as its limited carbohydrate energy stores are reduced and glucose homeostasis becomes increasingly dependent on gluconeogenesis.

[Note:] Most of us are eating almost all the time and so this effect does not happen. If we want the beneficial effect of exercise on total energy expenditure and from post-exercise lipid oxidation, we need to be in a temporary fasting state.  We can quickly get used to the feeling – it will soon become a friend!  It’s NOT hunger.  You can take advantage of this process in the 2MD weight loss program.

Agree? Disagree? Do share your opinion.