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  Last updated: 5/22/2018


There are 2 types of muscle fibers in the human body - smooth muscle and striated (or skeletal) muscles. Smooth muscles are found in the digestive tract, lining of blood vessels, and other internal organs. Skeletal muscles are, as the name suggests, the muscles that are involved in voluntary movement of our core and limbs. Unless otherwise noted, when I say "muscle" I will be talking about the skeletal muscles, those muscles that power cycling activities.

Skeletal muscles makes up over 1/2 of the body weight in a lean individual. All muscles (quadriceps, biceps, etc.) are composed of thousands of muscle cells. And these individual muscle cells contain two proteins - actin and myosin - which chemically interact and shorten the cell (and along with it the muscle itself) when the muscle cells are stimulated by the release of calcium initiated by a nerve impulse. The interaction of the actin-myosin complex, which results in the shortening or contraction of the muscle cell, requires the energy in the form of ATP.

Interestingly, a recent study has demonstrated a direct positive effect of caffeine on the muscle fiber itself resulting in an increase in the mobilization of calcium from the sarcoplasmic reticulum of the muscle cell. The result was a reported 7% increase in power output over a 6 second cycle exercise task.


Skeletal muscle cells (or muscle fibers) come in two varieties.

Type I (slow twitch, SO fibers) - These muscle cells shorten at a relatively slow speed and generate energy from both fats and carbohydrates via aerobic metabolism . They are the major muscle fiber in use at 70-80% VO2max. Type I cell characteristics include:

Type II (fast twitch, FG fibers) - These muscle cells are less efficient than the slow twitch cells and are almost entirely dependent on glycogen as fuel. They are called into action for sprints when the athlete approaches 100% of their maximum performance (and are working in the anaerobic range above 100% VO2max). Type II cell characteristics include: The relative proportion of type I and type II fibers that make up an individual muscle (biceps, quadriceps, etc.) varies from person to person and is initially determined by our genetic inheritance from our parents). However, with limits, this ratio can be modified with exercise and training. Successful endurance athletes have a preponderance of slow twitch muscle fibers (up to 90% of the fibers in the calf in cross country skiers) while sprinters have more fast twitch fibers. Short term studies in bicyclists (5 months) failed to show a change in the ratio of cell types (percentage of slow vs fast twitch fibers) in leg muscles, but a longer multi-year study has suggested that this ratio can change with time, continuing to change for at least 5 years with regular training.

But even without a change in the ratio of muscle fiber types, there is no question that both slow and fast twitch fibers can markedly improve their metabolic capacity with training. (see also Principles of Training)

But all training may not be positive for muscle cell adaptation. A recent article (Derman et al, Journal of Sports Medicine, 15:341-351, 1997) described muscle cell biopsy changes in athletes that:

Muscle biopsies from the vastus lateralis demonstrated cell structure abnormalities. They speculated that repeated bouts of high volume training over years (with repeated micro trauma) might lead to chronic muscle structure changes and symptoms. At this time there is not enough evidence to call this, but it may represent a unique subset of elite athletes that present with training problems.


We know that physical trauma and micro tears are a result of training - and are the likely explanation for post exercise muscle pain (DOMS). It makes sense that muscle injury should lead to a "leaking" of muscle cell enzymes into the blood stream, and an elevation of CK levels in blood samples. This blog , references articles providing some hard numbers supporting that assumption.

Assuming that we all respond to exercise differently, significant elevations of a blood CK might suggest overtraining to the point that limits in recovery may outweigh the benefits of the stress-improvement training strategy, and it is time to back off a bit.


Before you read further, note that the condition I am going to describe is quite rare. But I found it fascinating that most of the cases occurred in cyclists and especially in cyclists in spin classes. It may be that spinning allows the untrained to expend much more energy (abuse their muscles) without significant discomfort (which would make them stop) than other aerobic sports (such as running).

Rhabdomyolysis results when muscle breakdown and damage allow the injured muscles to release their contents into the bloodstream. These in turn can harm the kidneys leading to kidney failure in up to 40% of cases.

Rhabdomyolosis can be seen after trauma, with drug use, with medications such as statins, and with severe dehydration and at extreme temperatures. But it can also be seen after intense exercise that involves repetitive motion of a muscle - and especially a new movement.

Before you get too confident that this is not a worry for you, a regular cyclist, it has also been reported in people who stop cycling for some time and then jump back on the spin bike and try to immediately achieve their old levels of performance. And it can also happen in professional athletes in their sport (snowboarding for example).

The moral here is that that this although this syndrome is rare, you should always be cautious about rigorous exercise when you are just starting out.

Here are few common sense suggestions that might minimize that risk even further:


Energy output (or work) is expressed as power (the amount of work done during a specified unit of time). Power output can be measured as steady state power output (maintaining a steady speed for minutes to hours) or maximal power output - which require maximal activation of the ATP-CP energy system. The latter reflects the maximal muscle power of the athlete and is limited by the amount of ATP and CP available in the cell - about 6 seconds.

Curt Austin has put together a nice calculator to estimate power output (in Watts - you enter your own parameters) on his website. Malcolm Firth also published some comparative numbers in an online coaching forum. (As the amount of ATP-CP available to the muscle cell is limited, Malcolm's maximum power output over several minutes would be lower than that achievable in a brief sprint lasting 5 to 5 seconds):

"In February 1998 I did a small research project in which a group of 24 cyclists were asked to do two tests on a CompuTrainer (an electromagnetically braked turbo trainer made by RacerMate of Seattle, USA). The first of these was a step increased load test to voluntary exhaustion in which the load began at 100 watts and was increased at approx 20 watts per minute. After a break of at least three hours the cyclists then rode a simulated ten miles time trial on the CompuTrainer with the instruction to complete the distance as quickly as possible. Some of the data is summarized below:

The average 10 mile heart rate worked out at 94.5% of the mean max heart rate.(st dev 2.81%, range 88.41%-97.41%). If you go to my web site at you will find an article giving details on how to use the average ten miles heart rate to estimate heart rates for other training and racing intensities."


Q.I was hoping you could shed some light on how I might be able to shed some upper body muscle mass to lighten up a bit. I have 7% body fat and have not been to the gym in a year, although most of my life has been dedicated to weights until now. I don't want to lose any leg strength in the process.

A. Except for inactivity (accelerated by overall weight loss) , I am unaware of any way to decrease muscle mass. Thus I think you are looking at a long term program of increasing or maintaining lower body stress on the muscles (biking, weight lifting) to maintain or increase mass there while discontinuing all upper body weight work.


Questions on content or suggestions to improve this page are appreciated.

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