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Skeletal muscles: How do they work?
 Muscles contract and
relax to allow the
body to perform crucial activity.
Electrical signals tell the muscle when to contract, but when the muscle needs
to relax, the signal is deliberately ignored. Until now scientists have been
unable to understand how the body ignores this signal.
Muscle is an organ specializing in the transformation of chemical energy into
movement. Movement is essential to life, and takes many forms, from cytoplasmic
streaming and the growth of neurones at the cellular level, to the long distance
flight of the albatross or the explosive performance of a sprinter. Although
only a few families of protein are responsible for movement in the
biological world, muscle has developed to optimize this function, and is packed with
movement-related proteins. There are many types of muscles, but they fall into
three categories: skeletal muscle (or striated muscle), responsible for
locomotion, flight etc; cardiac muscle, which has a vital role and is able to
function for a century or more, without ever taking a break, and smooth muscle
(or involuntary muscle) which lines the walls of the arteries to control
blood
pressure, or controls the
digestion of food by causing movement of the
intestine.
Having established that each muscle is comprised of numerous fibres, each of
these in turn consists of many myofibrils, which form the functional units of
muscle and effect the contraction and relaxation process. The functional part of
the myofibrils consists of numerous contractile units, called sarcomere
connected in series. Each sarcomere is composed of different muscle proteins, in
particular the two main contractile proteins (myofilaments) called actin and
myosin. Myosin filaments are thick contractile proteins and remain relatively
stationery during contraction. Actin filaments are thin contractile proteins
that are drawn towards each other from both ends of the sarcomere during muscle
contraction. The actin and myosin filaments lie parallel to each other and
become interlocked during contraction. The two contractile proteins are
connected during contraction by myosin cross-bridges. (These are globular
proteins that originate from the larger myosin filaments and are chemically
bound to the actin filaments during contraction.)
When contracting, the actin myofilaments from both ends of the sarcomere slide
over the myosin filaments towards the middle of the sarcomere, thereby
shortening the contractile unit by pulling the Z-disks towards each other. This
is similar to the forward movement of a millipede. The cross-bridges attach and
detach in different sequences (the way a millipede's legs move along the ground)
to pull the actin filaments over the myosin filaments. Not all myosin
cross-bridges are attached at the same time.
The difference between the millipede and cross-bridges is that the cross-bridge
cycling is significantly faster. As each sarcomere shortens, the entire
muscle fibre contracts. Depending on how many myofibrils are activated, their
respective forces are also summated and transferred by the tendons and tendinous
structures to the bones.
Different forms of muscle contraction
Most people associate muscle contraction with muscle shortening and a
decreasing angle between two bones. However, it also means that the muscle
attempts to shorten against, or actively resists while lengthening against, a
load.
There are four different types of muscle contraction, namely - concentric -
eccentric - isometric - plyometric.
 CONCENTRIC CONTRACTION
The most common muscle contractions in resistance training are concentric and
eccentric. Concentric contraction means that the muscle is activating sufficient
myosin-actin cross-bridges to develop enough muscle tension to overcome the load
or weights applied upon it. The muscle therefore shortens and closes the joint
angle between two bones by pulling the insertion of the muscle towards its
origin. In other words, the muscle generates sufficient force to overcome and
lift the weight. This is also known as a positive contraction. The lifting phase
of resistance training is a concentric contraction. For most concentric
contractions, the duration of the contraction should be roughly one to two
seconds from beginning to end. This is not, however, true for all techniques.
ECCENTRIC CONTRACTION
Once the weight has been lifted, it has to be lowered to its starting
position. This must be done in a controlled manner - for safety reasons as well
as for training benefits. While the muscle lengthens and prevents the weights
from falling down, the muscle is contracting eccentrically and resists
lengthening. The lowering phase of resistance training is an eccentric
contraction. Eccentric contractions should last roughly two to four seconds from
beginning to end, though not for all techniques.
ISOMETRIC CONTRACTION
Isometric contraction is often defined as 'muscle contraction without muscle
shortening and without visible movement' although the muscle does shorten to
some extent as it contracts against the opposing tendons of the origin and
insertion. In this way it applies tension to them and the tendinous structures,
and statically supports a submaximal load (less than the maximum load you can
lift), or tries to overcome a supramaximal load (more than you can lift). The
resistance used, and compliance of the tendons, determine the extent to which
the tendons can stretch and the muscles shorten. A better definition would be 'a
static muscle contraction where no movement of the, limb or body part involved
is visible'. In other words, an isometric contraction is a static contraction,
where the muscle's tedious insertion and origin do not approach each other, even
though the muscle sarcomeres shorten slightly to support or attempt to move the
load. Holding a lighter weight stationary, and trying to lift an immovable
object are examples of isometric contractions.
PLYOMETRIC MUSCLE ACTION
Plyometric muscle action is the stretch-shortening cycle of a muscle.
Plyometric
action features the combination of eccentric and concentric contractions when a
load is lifted. Most natural and sporting movements make use of the
stretch-shortening cycle. In any sporting movement, if you intend to move in one
direction, you unconsciously move in the opposite direction first and
pre-stretch the muscle before pushing off in the intended direction. A side step
in rugby or football is a good example. The term stretch-shortening cycle
perfectly describes it: first you stretch the muscle and then you shorten it.
You use a quick eccentric stretch of the involved muscle to enhance the
concentric contraction to follow. In resistance training, this is a fast and
explosiveness and power.
Summary
-
Concentric contraction occurs when the muscles generate sufficient force to
overcome inertia and lift the weight.
-
Eccentric contraction occurs when the weights are lowered (the muscles
resist lengthening).
-
In an isometric contraction no movement is visible, while the muscles try to
overcome an immovable load or hold a submaximal load in a static position. Here
the top stoppers resist movement.
Dated 29 November 2011
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