Muscle characteristics
9 Sep 2004  Feedback  QL: Muscles10

The two main features that muscles are evaluated on is their ability to produce force and their resistance the fatigue.  Factors affecting the ability to generate force are:

Muscle Size

A prerequisite of muscle performance is its size.  It is important though to see this as a foundation for muscle performance rather than an end goal in itself.  This is because developments in size tend to go unused if no task specific training is done.  The specificity of muscle force training appears to relate to neurological requirements.  Muscle size contributes greatly to a measure of force called maximum voluntary contraction (MVC) which assess the force capability of a muscle in an isometric contraction.  However, as most tasks that are of interest to us concern movement the MVC can be a poor indicator of performance as it does not reflect the required neurological control.  An example of these concepts can be seen with body builders.  They favour training regimes that favour muscle hypertrophy (increase in size) above all else.  Despite their large muscles can can perform relatively poorly in many strength events compared to people who have trained specifically for that event yet have smaller muscles.

Composition and fiber types

Muscles consist of fibers which provide the basic unit of force.  One or more fibers are controlled by a motor neuron.  The motor neuron and the fibers it controls form a motor unit.  Motor units vary in their force generation capabilities and their resistance to fatigue.  Every time they are activated they produce a contraction spike called a twitch.  The motor units are repeatedly activated and their twitches fuse together into a tetanus that provides the force throughout a movement.

Classification of motor units has traditionally been in terms of their resistibility to fatigue and contraction speed during a twitch.  Three classifications have been suggested: slow twitch/fatigue resistant (S), fast twitch/fatigue resistant (FR) and fast twitch/fast fatiguing (FF).  The reason for this classification is that in non-human mammals it appears (S) type units produce the least force, then FR and then FF producing the most force.  Furthermore, this classification parallels biochemical and molecular analysis of the underlying muscle fibers in each motor unit (with classifications such as type I, IIa and IIb based on the amount of myosin ATPase in a fiber).  This led to the possibility of predictive tests of young athletes based on chemical tests of the muscles to determine if they were suited to power sports (type II dominance) or endurance sports (type I dominance) by the percentage of each fiber type present.

However, when applied to humans the situation is a lot more complicated.  Although different muscles within the body roughly follow: the more force the more type II fibers relationship, differences between subjects in a given muscle vary greatly with a weak association with that rule.  In conclusion, low force/high fatigue resistance is associated with type I fibers or (S) motor units and high force/fast fatiguing with type IIa fibers or (FF) motor units, but the association is not very strong in humans.

At the moment it as reasonable to only look at muscles as a whole and in terms of performance and reactions to training.


Vitally important in producing high forces is a well adapted neurological control system.  The action must be well coordinated (motor skills), the central drive (command from the brain) must be high, the transmission and reflex systems must operate well and the firing rate of motor unit twitches must be a high frequency.  These factors can be trained but are subject to fatigue and psychological constraints (e.g. arousal).  As discussed before, neurological adaptation accounts for the specificity of training and performance.  Proper motor unit recruitment must happen to achieve the movement goal.  The more units that are recruited, the stronger the force.

Contraction rate

The faster a muscle contracts, the smaller the maximum amount of force it can produce.  This means that producing faster movements is doubly difficult as a higher force is needed to move quicker but the ability to generate force reduces as the muscle contracts faster.

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