<P> Research on skeletal muscle properties uses many techniques . Electrical muscle stimulation is used to determine force and contraction speed at different stimulation frequencies, which are related to fiber - type composition and mix within an individual muscle group . In vitro muscle testing is used for more complete characterization of muscle properties . </P> <P> The electrical activity associated with muscle contraction are measured via electromyography (EMG). EMG is a common technique used in many disciplines within the Exercise and Rehab Sciences . Skeletal muscle has two physiological responses: relaxation and contraction . The mechanisms for which these responses occur generate electrical activity measured by EMG . Specifically, EMG can measure the action potential of a skeletal muscle, which occurs from the hyperpolarization of the motor axons from nerve impulses sent to the muscle (1). EMG is used in research for determining if the skeletal muscle of interest is being activated, the amount of force generated, and an indicator of muscle fatigue . The two types of EMG are intra-muscular EMG and the most common, surface EMG . The EMG signals are much greater when a skeletal muscle is contracting verses relaxing . However, for smaller and deeper skeletal muscles the EMG signals are reduced and therefore are viewed as a less valued technique for measuring the activation . In research using EMG, a maximal voluntary contraction (MVC) is commonly performed on the skeletal muscle of interest, to have reference data for the rest of the EMG recordings during the main experimental testing for that same skeletal muscle . </P> <P> B.K. Pedersen and her colleagues have conducted research showing that skeletal muscle functions as an endocrine organ by secreting cytokines and other peptides, now referred to as myokines . Myokines in turn are believed to mediate the health benefits of exercise . </P>

What are three types of skeletal muscle fibers