The Genius in All of Us: New Insights Into Genetics, Talent, and IQ (46 page)

    
A seven- or fourteen- or twenty-eight-year-old outfitted with a certain height, shape, muscle-fiber proportion, and so on is not that way merely because of genetic instruction
.

   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 and 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. Each skeletal muscle is a bundle of thousands of specialized elongated cells called muscle fibers.

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 six hundred or so muscle fibers.

Each individual muscle fiber also contains a string of DNA-filled 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 engage in other lengthy activities.
  

• “Fast-twitch” (type II) fibers contract rapidly and forcefully for a period of seconds, very quickly using huge amounts of (anaerobic) energy, becoming spent and needing to rest and replenish. These fibers enable us to sprint, jump, lift weights, and engage in other short-burst activities.
  

In musculature, we are not all created equal. Although on average, human beings have about a fifty-fifty 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., “Muscle, Genes and Athletic Performance.”)

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., “Muscle, Genes and Athletic Performance.”)

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. (Quinn, “Fast and Slow Twitch Muscle Fibers.”)

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 be re-created 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.

3. 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.

4. 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. (National Skeletal Muscle Research Center, “Hypertrophy.”)

  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., “Regulation of muscle fiber type and running endurance by PPAR.”)

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), 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., “Control on the size of the human muscle mass,” p. 802.)

In sum, while evolution has given humans some variability in muscle types, perhaps its 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, eds.,
Skeletal Muscle Plasticity in Health and Disease
.)

Citations

Among humans, great variation in muscle-fiber ratios

Anderson, Jesper L., Peter Schjerling, and Bengt Saltin. “Muscle, Genes and Athletic Performance.”
Scientific American
, September 2000.

DIFFERENT FIBER RATIOS PROVIDE ADVANTAGES AND DISADVANTAGES FOR CERTAIN SPORTS

Anderson, Jesper L., Peter Schjerling, and Bengt Saltin. “Muscle, Genes and Athletic Performance.”
Scientific American
, September 2000.

MUSCLE-FIBER TYPE IS A POOR PREDICTOR OF PERFORMANCE

Quinn, Elizabeth. “Fast and Slow Twitch Muscle Fibers: Does Muscle Type Determine Sports Ability?” Published on the
About.com
Sports Medicine Web site, October 30, 2007.

Articles cited by Quinn for further reference

Anderson, Jesper L., Peter Schjerling, and Bengt Saltin. “Muscle, Genes and Athletic Performance.”
Scientific American
, September 2000.

McArdle, W. D., F. I. Katch, and V. L. Katch.
Exercise Physiology: Energy, Nutrition and Human Performance
. Williams & Wilkins, 1996.

Lieber, R. L.
Skeletal Muscle Structure and Function: Implications for Rehabilitation and Sports Medicine
. Williams & Wilkins, 1992.

Thayer, R., J. Collins, E. G. Noble, and A. W. Taylor. “A decade of aerobic endurance training: histological evidence for fibre type transformation.”
Journal of Sports Medicine and Physical Fitness
40, no. 4 (2000): 284– 89.

NEURAL RESPONSE AND GENETIC RESPONSE

National Skeletal Muscle Research Center. “Hypertrophy.” Published on the UCSD Muscle Physiology Laboratory Web site.

Genetic response makes muscle fibers more efficient

Russell, B., D. Motlagh, and W. W. Ashley. “Form follows function: how muscle shape is regulated by work.”
Journal of Applied Physiology
88, no. 3 (2000): 1127–32.

Conversion between different fiber types

Wang, Yong-Xu, et al. “Regulation of muscle fiber type and running endurance by PPAR.” Published on the Public Library of Science Web site, August 24, 2004.

Kohn, Tertius A., Birgitta Essén-Gustavsson, and Kathryn H. Myburgh
. “
Do skeletal muscle phenotypic characteristics of Xhosa and Caucasian endurance runners differ when matched for training and racing distances?”
Journal of Applied Physiology
103 (2007): 932–40.

Coetzer, P., T. D. Noakes, B. Sanders, M. I. Lambert, A. N. Bosch, T. Wiggins, and S. C. Dennis. “Superior fatigue resistance of elite black South African distance runners.”
Journal of Applied Physiology
75 (1993): 1822–27.

Andersen, J. L., H. Klitgaard, and B. Saltin. “Myosin heavy chain isoforms in single fibres from m. vastus lateralis of sprinters: influence of training.”
Acta Physiologica Scandinavica
151 (1994): 135–42.

Pette, D., and G. Vrbova. “Adaptation of mammalian skeletal muscle fibers to chronic electrical stimulation.”
Reviews of Physiology, Biochemistry and Pharmacology
120 (1992): 115–202.

Trappe, S., M. Harber, A. Creer, P. Gallagher, D. Slivka, K. Minchev, and D. Whitsett. “Single muscle fiber adaptations with marathon training.”
Journal of Applied Physiology
101 (2006): 721–27.

    
This nongenetic aspect of inheritance is often overlooked by genetic determinists
:
culture, knowledge, attitudes, and environments are also passed on in many different ways: See
chapter 7
.

    
“The large variance in both the global and individual admixture estimates
”:
Benn-Torres et al., “Admixture and population stratification in African Caribbean populations,”.

    
The annual high school Boys’ and Girls’ Athletic Championships
:
Rastogi, “Jamaican Me Speedy.”

    
“dozens of small children showed up for a Saturday morning youth track practice
”:
Layden and Epstein, “Why the Jamaicans Are Running Away with Sprint Golds in Beijing.”

    
Dennis Johnson did come back to Jamaica to create a college athletic program
:
Clark, “How Tiny Jamaica Develops So Many Champion Sprinters”; Rastogi, “Jamaican Me Speedy.”

    
“We genuinely believe that we’ll conquer,” says Jamaican coach Fitz Coleman
:
Clark, “How Tiny Jamaica Develops So Many Champion Sprinters.”

    
a person’s mind-set has the power to dramatically affect both short-term capabilities and the long-term dynamic of achievement
:
Dweck,
Mindset
; Elliot and Dweck, eds.,
Handbook of Competence and Motivation
.

    
Bannister himself later remarked that while biology sets ultimate limits to performance, it is the mind that plainly determines how close individuals come to those absolute limits
.