Some of the truly fascinating insights into talent and greatness emerge from the realm of human musculature -- how our skeletal muscles are initially formed, the attributes of different muscle fibers, and the different ways muscles can be transformed by activity and training. Reviewing the nature/nurture of muscles is also perhaps the best window into the dynamics of genetic expression. Here's an overview:
The human body contains three basic muscle types:
-- Smooth (involuntary muscles serving the digestive system, blood vessels, airways, etc.)
-- Cardiac (also involuntary; cardiac muscle is self-excitable and designed to function on its own)
-- Skeletal (all voluntary muscles, from eyes to fingers to toes).
This overview concentrates on skeletal muscles -- the muscles we exert direct control over.
The fibers are fed by tiny, blood-filled capillaries, held together with various kinds of connective tissue, and fired ("innervated") by motor neurons -- one neuron firing 600 or so muscle fibers.
Each individual muscle fiber also contains a string of DNA-containing nuclei positioned just underneath and along the entire length of its membrane. The genetic material constantly instructs each fiber how to react *and adapt* to various circumstances.
There are two basic types of muscle fibers:
-- "Slow-twitch" (type I) fibers are designed to contract for long periods of time; packed with mitochondria, they are extremely efficient at converting oxygen to fuel.
----- These fibers enable us to jog, swim, bicycle, and other lengthy activities
-- "Fast-twitch" (type II) fibers contract rapidly and forcefully
for a period of seconds, very quickly using voracious amounts of
(anaerobic) energy, becoming spent and
needing to rest and replenish.
----- These fibers enable us to sprint, jump, lift weights and other short-burst activities.
In musculature, we are not all created equal
Although on average, human beings have about a 50/50 mix of slow and fast-twitch muscle fibers, some are born with differing proportions.
"The 'average' healthy adult has roughly equal numbers of slow and fast fibers in, say, the quadriceps muscle in the thigh. But as a species, humans show great variation in this regard; we have encountered people with a slow fiber percentage as low as 19 percent and as high as 95 percent in the quadriceps muscle." (Anderson et al, 2000)
As anyone might logically expect from the above description of the fiber types, a higher proportion of one or another can offer certain potential advantages to highly-trained athletes. Elite marathon runners and cyclists benefit from a higher proportion of slow-twitch fibers, for example, while sprinters benefit from a higher proportion of fast-twitch fibers. (Anderson et al, 2000).
These genetic differences, however, must be put into careful context.
First, muscle fiber proportion is only one of many performance factors. On its own, it is not a good predictor of individual performance.
Second, muscles are tremendously adaptive to external stimulus, and are designed to be so. The muscles we are born with are merely default muscles -- ready and waiting to recreated in one or another particular direction by active use.
To understand how adaptation is literally built into our muscle DNA, let's look at all the things that happen as a result of training
At any given time, each muscle is adapted to a status quo of activity and exertion -- i.e., each muscle is exactly as big, strong and efficient as it needs to be. When pushed just beyond the ordinary level of exertion, a number of physiological changes begin to unfold:
1. Neural response.
"The first measurable effect is an increase in the neural drive stimulating muscle contraction. Within just a few days, an untrained individual can achieve measurable strength gains resulting from 'learning' to use the muscle." (NSMRC)
2. Genetic response makes muscle fibers more efficient.
In response to extended (aerobic) exercise -- e.g. jogging -- there is a genetic response in the nucleus of each cell fiber that makes it more efficient and enduring: increasing the number of mitochondria and provoking an increase in surrounding capillaries and the accumulation of fats and carbohydrates. (Wiki)
3. Genetic response makes muscle fibers become stronger and grow in size.
In response to overload/resistance exercise -- e.g. weight lifting -- the DNA responds with instructions that will lead to the strengthening and enlarging [hypertrophy] of each fiber.
"As the muscle continues to receive increased demands...upregulation appears to begin with the ubiquitous second messenger system (including phospholipases, protein kinase C, tyrosine kinase, and others). These, in turn, activate the family of immediate-early genes, including c-fos, c-jun and myc. These genes appear to dictate the contractile protein gene response.
"Finally, the message filters down to alter the pattern of protein expression. It can take as long as two months for actual hypertrophy to begin. The additional contractile proteins appear to be incorporated into existing myofibrils (the chains of sarcomeres within a muscle cell). ...These events appear to occur within each muscle fiber. That is, hypertrophy results primarily from the growth of each muscle cell, rather than an increase in the number of cells." (NSMRC)
4. When training is particularly intense and prolonged, slow-twitch muscle fibers can become transformed into fast-twitch fibers, and vice-versa.
"Adult skeletal muscle shows plasticity and can undergo conversion between different fiber types in response to exercise training or modulation of motoneuron activity." (Wang et al, 2004)
A detailed diagram of gene expression at work in muscle fibers:
Exercise, stretches and other muscle activity (LEFT) interacts with DNA in the nucleus (CENTER, circled in red), which in turns interacts with protein translators to effect changes on the cell and surrounding tissue (RIGHT).
(Source of graphic and detailed explanation of genetic transcription: Rennie et al 2004.)
While evolution has given humans some variability in muscle types, the much more powerful product is its adaptivity. Muscles are designed to be rebuilt.
"The ability of striated muscle tissue to adapt to changes in activity or in working conditions is extremely high. In some ways it is comparable to the ability of the brain to learn." (Bottinelli and Reggiani, 2006)