Sports Medicine: Just the Facts

• According to the sliding filament theory, actin and
  myosin slide past each other to produce sarcomere
  shortening. Ca2+is released in the sarcomere in response
  to an action potential that exposes myosin cross-bridge
  binding sites on actin. Myosin cross bridges bind to
  actin and pull actin filaments closer to the center of each
  sarcomere producing force and stiffness within the
  skeletal muscle (Lorenz and Campello, 2001).
  •Force production by a skeletal muscle can be volun-
  tarily graded; however, the muscle fibers innervated
  by one motor neuron (i.e., a motor unit) act in an all
  or none fashion. As more motor units are recruited in
  a particular skeletal muscle, the muscle produces
  more force (Lorenz and Campello, 2001).

SKELETAL MUSCLE FIBER TYPES

• There are several different types of muscle fiber based
  on structure and function. These include Type I, Type
  IIA, and Type IIB.
  •Type I muscle fibers (slow twitch oxidative fibers)
  have high mitochondria content and a rich blood
  supply. They are difficult to fatigue but do not provide
  high amounts of muscle force or tension. Type I fibers
  act in an aerobic capacity during activity.
  •Type IIA muscle fibers (fast twitch oxidative-
  glycolytic fibers) also have high mitochondria content
  and are moderately capable of performing aerobic and
  anaerobic activities.
  •Type IIB muscle fibers (fast twitch glycolytic fibers)
  have sparse mitochondria content and blood supply.
  They fatigue easily but are capable of producing
  higher force and tension. Type IIB fibers act in an
  anaerobic capacity during activity.


• All muscle fibers within a motor unit have the same
  metabolic characteristics (fiber type); however, skele-
  tal muscles can be composed of different types of
  muscle fibers based on the metabolic demand of the
  muscle (Lorenz and Campello, 2001).
  •Muscle fiber type composition in human skeletal
  muscle is genetically determined (Lorenz and
  Campello, 2001). There is little information regarding
  muscle fiber type transformations in response to train-
  ing/exercise.


• Heavy resistance training caused increased type IIA
  and decreased type IIB fibers, while type I fiber com-
  position in human skeletal muscle was unchanged
  (Adams et al, 1993).


• Structural and genetic characteristics of muscle fiber
  types have been modulated with fiber-specific stimu-
  lation in vitro (Liu et al, 2001); however, it is
  unknown whether such changes occur in all muscle
  fiber types or if the transformation will be sustained
  over time in vivo.
  
  
  • Since muscle fiber composition is genetically deter-
    mined, athletes may participate in sports or activities
    that involve muscle contractions that are more natural.
    Whether a distance runner can train to be a successful
    power lifter or vice versa is an issue that has not yet
    been clearly elucidated.
  
  
  
  

TYPES OF SKELETAL MUSCLE CONTRACTION

•Skeletal muscle can produce joint movements through

concentric and eccentric movements. Skeletal muscle

contractions produce muscle tension and control body

and joint movement.

• Concentric contractions describe a movement that
  involves shortening of a muscle against a load,
  whereas eccentric contractions involve controlled
  lengthening of a muscle against a load.


• Eccentric muscle contractions produce greater muscle
  force and more myofibrillar disruption than concen-
  tric exercise (Gibala et al, 1995).


• Eccentric muscle contractions (often referred to as
  negativesin weightlifting) are more effective in pro-
  ducing strength gains and hypertrophy than concen-
  tric contractions (Higbie et al, 1996); however, both
  eccentric (negative) and concentric (positive) contrac-
  tions elicit gains in skeletal muscle strength and size
  (Higbie et al, 1996).


• Isometric muscle contractions produce muscle tension
  without joint movement. For example, pushing
  against a wall or contracting the quadriceps muscle
  while holding the knee motionless at a particular point
  in the knee range of motion.


• Isotonic muscle contractions produce muscle tension
  and joint movement against a constant load where rate
  of movement is variable. For example, a dumbbell
  curl is a contraction against a constant load that can be
  voluntarily moved at a self-selected rate. This is the
  most typical contraction in weightlifting.


• Isokinetic muscle contractions involve a constant rate
  of joint displacement that is maintained by varying
  amounts of resistance based on muscle effort. This is
  uncommon in weightlifting or athletic settings.
  Isokinetic exercise requires expensive machinery and
  is usually most applicable in the rehabilitation setting.


• Isotonic and isokinetic muscle movements can be per-
  formed through concentric or eccentric muscle con-
  tractions.

MUSCLE RESPONSE TO RESISTANCE TRAINING

•Improvements in muscle strength, power, or

endurance are best achieved by overloading the

muscle(s) being trained.

544 SECTION 6 • SPORTS-SPECIFIC CONSIDERATIONS