Neuromuscular Anatomy & Adaptations to Conditioning-2

Neuromuscular Anatomy & Adaptations to Conditioning-2 -...

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Unformatted text preview: Essentials of Strength Training & Conditioning Neuromuscular Anatomy and Adaptations to Conditioning Chapter 2 Morgan 200 Neuromuscular Anatomy & Physiology Motor Unit – a motor neuron and all of the muscle fibers that it innervates ; the basic functional entity of muscular activity Motor Neuron – nerve that travels from the spinal cord to the target muscle; generally has numerous terminal branches to innervate many muscle fibers; when a motor neuron fires, all fibers it innervates become activated • Precision muscles may have a ratio of muscle fibers to motor neuron that approaches 1:1 • Large muscle group muscle fibers may have hundreds of fibers controlled by one motor neuron Motor neuron Action Potential arrives at all of the motor end plates innervated by neuron; this causes a release of neurotransmitter Acetylcholine (Ach); Ach diffuses across the neuromuscular synapses and electrochemically excites the sarcolemma, causing contraction of all of the muscle fibers All or None Principle – when an action potential has enough strength to activate a muscle fiber, it activates all muscle fibers supplied by the motor neuron Morgan 200 Neuromuscular Anatomy & Physiology (cont.) Twitch – a short period of activation of the muscle fibers within the motor unit, stimulated by an action potential • If a second twitch occurs before the muscle fiber relaxes, the Force generated summates • Decreasing the time interval between twitches results in greater Force summation Tetanus – when continued stimulation of a muscle fiber results in twitches occurring simultaneously without relaxation; this is the largest amount of Force that the muscle fiber and motor unit can generate • “Russian Stimulation” is an electrical stimulation training technique that utilizes an outside electrical charge to generate large amounts of Force within a muscle to increase lifting potential, and therefore increasing the training effect Morgan 200 Muscle Twitch – Summation - Tetanus Major Characteristics of Muscle Fiber Types Characteristic Type I Type IIa Type IIb Contraction Speed Slow Fast Fast Force production Low Intermediate High Power Output Low High High Endurance High Intermediate / Low Low Aerobic enzymes High Intermediate / Low Low Anaerobic enzymes Low High High Fatigability Low Intermediate / High High Capillary density High Intermediate Low Fiber diameter Small Intermediate Large Mitochondria density High Intermediate Low ATPase activity Low High High Myoglobin High Low Low Color Red White (intermediate) White Morgan 2 Motor Unit Recruitment Patterns During Exercise Muscle Force output varies over a wide gradation in order to allow for appropriate use of force to carry out activity in a smooth, controlled manner Frequency of Activation – (Rate Coding) by decreasing the time interval between muscle twitches, greater force production is attained; most commonly used in precision muscles (i.e. hand, eye), but also as a means to increase force generation from a single muscle fiber Recruitment – increasing the number of motor units activated to produce greater Force or carry out a particular movement; most commonly used in large muscle groups (thigh, gluteals, arms, trunk) Increasing frequency of contraction and increasing recruitment occur simultaneously when great Force production is required Depending on the physiological need, different types of muscle fibers are recruited, based on the fibers characteristics Morgan 200 RFD – Rate of Force Development As demonstrated in the graphs to the left, RFD diminishes with age. This drop in rate is coincident with a decrease in cross sectional area of Type II (fast twitch) muscle fibers. The cross sectional area of Type I (slow twitch) fibers tends to remain constant with aging. This fiber type cross sectional change and therefore RFD can be minimized by continued strength training. Force-time curves (up to 2,000 N) and maximal rate of force development (RFD) calculated for absolute values (A), and normalized force-time curves and RFD calculated as a percentage of maximal force (Fmax) developed (B) in fast isometric contraction in diff... Korhonen M T et al. J Appl Physiol 2006;101:906-917. J Appl Physiol 2002. RFD – Rate of Force Development Changes in efferent neural drive. Increases in efferent motor neuron output (“ efferent neural drive” ), as evidenced by changes in EMG signal amplitude, have been reported after heavy-resistance strength training” “ Increased motor neuron firing frequency at the onset of contraction was mainly responsible for the training-induced rise in contractile RFD” “ This change in relative RFD properties indicates that qualitative changes may have occurred with training, i.e., potentially involving alterations in motor neuron recruitment and firing frequency, increased incidence of discharge doublets, and changes in MHC isoform composition and sarcoplasmic reticulum Ca2+ kinetics” “ Contractile impulse is perhaps the single most important strength parameter because it incorporates the aspect of contraction time” “ The involvement of an intended ballistic effort appears to be more important for inducing increases in RFD than the type of contraction actually performed.” (comparing isometric and max ballistic training) Aagaard, Simonsen, Andersen, Magnusson, Dyhre-Poulsen. “ Increased rate of force development and neural drive of human skeletal muscle following resistance training” : J Apl Physiol 93: 1318-1326, 2002 Relative Involvement of Muscle Fiber Types in Sport Events Event Type I Type II 100-m sprint Low High 800-m run High High Marathon High Low Olympic Weightlifting Low High Barbell squat High High Soccer High High Field Hockey High High Football wide receiver Low High Football lineman High High Basketball Low High Distance cycling High Low see also: Morgan 2 Proprioception Proprioceptors – specialized sensory receptors located within joints, muscles, and tendons; sensitive to pressure and tension; relay info regarding muscle dynamics to the central nervous system • Provide the nervous system with info to allow for performance of complex coordinated movements; maintains muscle tone Kinesthetic Sense – spatial awareness; conscious appreciation of the body in 3 dimensions; once learned, most proprioception is subconsciously controlled Two most common proprioceptors are the Muscle Spindle and the Golgi Tendon Organ Morgan 200 Muscle Spindle Muscle Spindle – (intrafusal fiber) • Specialized muscle fibers that run parallel to normal muscle fibers (extrafusal fiber) within a fasiculus; provide info regarding muscle length and rate of length change • Myotactic Reflex - Muscle stretch activates the spindle which sends a signal to the spinal cord via an afferent (sensory) neuron; at the cord the afferent transfers the signal to an efferent (motor) neuron, which sends an action potential to the muscle, causing contraction (if AP is great enough) of the muscle fibers • Spindles indicate the degree to which the muscle must be activated to overcome the given resistance • Precision muscles have a greater concentration of muscle spindles than larger muscles See Link: neuroscience - reflexes Morgan 200 Golgi Tendon Organ Golgi Tendon Organs – (GTO) proprioceptors located within the tendon of a muscle and in contact with extrafusal muscle fibers; activated by stretch of tendon • Autogenic Inhibition - Stretch activation of the GTO results in the same type of neuronal pathway activation as muscle spindle activation, however the neurons activated are inhibitory, that is, they inhibit muscular contraction, reducing tension within the muscle • GTO’s provide protection from excess loads Morgan 200 Major Adaptations to Resistance vs. Aerobic Endurance Training Variable Resistance Training Aerobic Endurance Training Size of muscle fibers Increase No change Number of muscle fibers No change No change Movement speed Increase No change Strength Increase No change Aerobic capacity No change Increase Anaerobic capacity Increase No change Morgan 200 Specific Muscular Development S.A.I.D. Principle ...
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  • Spring '17
  • Timothy Morgan

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