Muscle Physiology - Types of Contractions
In these studies, the investigators usually recorded the amount of force Using EMG monitoring and controlled displacement velocities, Thilmann et al. and then smoothing (averaging) them to obtain a curve that indicates the amount of terms, including tension headache, muscle contraction headache, psychomyogenic. Looking for online definition of muscle contraction in the Medical Dictionary? See also excitation-contraction coupling, force-velocity relationship, myofibrils. Muscle contractions can be described based on two variables: force and length. .. Length-tension relationship relates the strength of an isometric contraction to the Tension headache, also known as tension-type headache, is the most.
Force-velocity Relationship The force generated by a muscle is a function of its velocity. Historically, the force-velocity relationship has been used to define the dynamic properties of the cross-bridges which cycle during muscle contraction. The force-velocity relationship, like the length-tension relationship, is a curve that actually represents the results of many experiments plotted on the same graph. Experimentally, a muscle is allowed to shorten against a constant load.
The muscle velocity during shortening is measured and then plotted against the resistive force. The general form of this relationship is shown in the graph below. On the horizontal axis is plotted muscle velocity relative to maximum velocity Vmax while on the vertical axis is plotted muscle force relative to maximum isometric force Po. What is the physiologic basis of the force-velocity relationship? The force generated by a muscle depends on the total number of cross-bridges attached.
Because it takes a finite amount of time for cross-bridges to attach, as filaments slide past one another faster and faster i. This can occur involuntarily e. Over the short-term, strength training involving both eccentric and concentric contractions appear to increase muscular strength more than training with concentric contractions alone. During an eccentric contraction of the triceps musclethe elbow starts the movement straight and then bends as the hand moves towards the shoulder.
Desmintitinand other z-line proteins are involved in eccentric contractions, but their mechanism is poorly understood in comparison to crossbridge cycling in concentric contractions.
For example, one expends more energy going up a flight of stairs than going down the same flight. Muscles undergoing heavy eccentric loading suffer greater damage when overloaded such as during muscle building or strength training exercise as compared to concentric loading.
When eccentric contractions are used in weight training, they are normally called negatives.
- Force velocity relationship
During a concentric contraction, muscle myofilaments slide past each other, pulling the Z-lines together. During an eccentric contraction, the myofilaments slide past each other the opposite way, though the actual movement of the myosin heads during an eccentric contraction is not known.
Exercise that incorporates both eccentric and concentric muscular contractions i. During virtually any routine movement, eccentric contractions assist in keeping motions smooth, but can also slow rapid movements such as a punch or throw.
Part of training for rapid movements such as pitching during baseball involves reducing eccentric braking allowing a greater power to be developed throughout the movement. Eccentric contractions are being researched for their ability to speed rehabilitation of weak or injured tendons. Achilles tendinitis  and patellar tendonitis  also known as jumper's knee or patellar tendonosis have been shown to benefit from high-load eccentric contractions.
Vertebrates In vertebrate animals, there are three types of muscle tissues: Skeletal muscle constitutes the majority of muscle mass in the body and is responsible for locomotor activity.
Smooth muscle forms blood vesselsgastrointestinal tractand other areas in the body that produce sustained contractions. Cardiac muscle make up the heart, which pumps blood. Skeletal and cardiac muscles are called striated muscle because of their striped appearance under a microscope, which is due to the highly organized alternating pattern of A bands and I bands. Skeletal muscle Organization of skeletal muscle Excluding reflexes, all skeletal muscles contractions occur as a result of conscious effort originating in the brain.
The brain sends electrochemical signals through the nervous system to the motor neuron that innervates several muscle fibers. Other actions such as locomotion, breathing, and chewing have a reflex aspect to them: Neuromuscular junction Structure of neuromuscular junction.
A neuromuscular junction is a chemical synapse formed by the contact between a motor neuron and a muscle fiber. The sequence of events that results in the depolarization of the muscle fiber at the neuromuscular junction begins when an action potential is initiated in the cell body of a motor neuron, which is then propagated by saltatory conduction along its axon toward the neuromuscular junction.
Acetylcholine diffuses across the synapse and binds to and activates nicotinic acetylcholine receptors on the neuromuscular junction. The membrane potential then becomes hyperpolarized when potassium exits and is then adjusted back to the resting membrane potential. This rapid fluctuation is called the end-plate potential The voltage-gated ion channels of the sarcolemma next to the end plate open in response to the end plate potential.
These voltage-gated channels are sodium and potassium specific and only allow one through. This wave of ion movements creates the action potential that spreads from the motor end plate in all directions. The remaining acetylcholine in the synaptic cleft is either degraded by active acetylcholine esterase or reabsorbed by the synaptic knob and none is left to replace the degraded acetylcholine. Excitation-contraction coupling Excitation—contraction coupling is the process by which a muscular action potential in the muscle fiber causes the myofibrils to contract.
DHPRs are located on the sarcolemma which includes the surface sarcolemma and the transverse tubuleswhile the RyRs reside across the SR membrane. The close apposition of a transverse tubule and two SR regions containing RyRs is described as a triad and is predominantly where excitation—contraction coupling takes place.
Excitation—contraction coupling occurs when depolarization of skeletal muscle cell results in a muscle action potential, which spreads across the cell surface and into the muscle fiber's network of T-tubulesthereby depolarizing the inner portion of the muscle fiber.
Depolarization of the inner portions activates dihydropyridine receptors in the terminal cisternae, which are in close proximity to ryanodine receptors in the adjacent sarcoplasmic reticulum.
The activated dihydropyridine receptors physically interact with ryanodine receptors to activate them via foot processes involving conformational changes that allosterically activates the ryanodine receptors.
Note that the sarcoplasmic reticulum has a large calcium buffering capacity partially due to a calcium-binding protein called calsequestrin. The near synchronous activation of thousands of calcium sparks by the action potential causes a cell-wide increase in calcium giving rise to the upstroke of the calcium transient.
Sliding filament theory Sliding filament theory: A sarcomere in relaxed above and contracted below positions The sliding filament theory describes a process used by muscles to contract. It is a cycle of repetitive events that cause a thin filament to slide over a thick filament and generate tension in the muscle.
However the actions of elastic proteins such as titin are hypothesised to maintain uniform tension across the sarcomere and pull the thick filament into a central position. A crossbridge is a myosin projection, consisting of two myosin heads, that extends from the thick filaments.
The binding of ATP to a myosin head detaches myosin from actinthereby allowing myosin to bind to another actin molecule. Once attached, the ATP is hydrolyzed by myosin, which uses the released energy to move into the "cocked position" whereby it binds weakly to a part of the actin binding site. The remainder of the actin binding site is blocked by tropomyosin.
Unblocking the rest of the actin binding sites allows the two myosin heads to close and myosin to bind strongly to actin. The power stroke moves the actin filament inwards, thereby shortening the sarcomere. Myosin then releases ADP but still remains tightly bound to actin. At the end of the power stroke, ADP is released from the myosin head, leaving myosin attached to actin in a rigor state until another ATP binds to myosin.
A lack of ATP would result in the rigor state characteristic of rigor mortis. Once another ATP binds to myosin, the myosin head will again detach from actin and another crossbridges cycle occurs. The myosin ceases binding to the thin filament, and the muscle relaxes. Thus, the tropomyosin-troponin complex again covers the binding sites on the actin filaments and contraction ceases. Gradation of skeletal muscle contractions Twitch Summation and tetanus Three types of skeletal muscle contractions The strength of skeletal muscle contractions can be broadly separated into twitch, summation, and tetanus.
A twitch is a single contraction and relaxation cycle produced by an action potential within the muscle fiber itself. Summation can be achieved in two ways: In frequency summation, the force exerted by the skeletal muscle is controlled by varying the frequency at which action potentials are sent to muscle fibers. Action potentials do not arrive at muscles synchronously, and, during a contraction, some fraction of the fibers in the muscle will be firing at any given time.
In multiple fiber summation, if the central nervous system sends a weak signal to contract a muscle, the smaller motor units, being more excitable than the larger ones, are stimulated first. As the strength of the signal increases, more motor units are excited in addition to larger ones, with the largest motor units having as much as 50 times the contractile strength as the smaller ones. As more and larger motor units are activated, the force of muscle contraction becomes progressively stronger.
A concept known as the size principle, allows for a gradation of muscle force during weak contraction to occur in small steps, which then become progressively larger when greater amounts of force are required. Finally, if the frequency of muscle action potentials increases such that the muscle contraction reaches its peak force and plateaus at this level, then the contraction is a tetanus.
Length-tension relationship Muscle length versus isometric force Length-tension relationship relates the strength of an isometric contraction to the length of the muscle at which the contraction occurs. Muscles operate with greatest active tension when close to an ideal length often their resting length. When stretched or shortened beyond this whether due to the action of the muscle itself or by an outside forcethe maximum active tension generated decreases.
Due to the presence of elastic proteins within a muscle cell such as titin and extracellular matrix, as the muscle is stretched beyond a given length, there is an entirely passive tension, which opposes lengthening. Combined together, there is a strong resistance to lengthening an active muscle far beyond the peak of active tension. Force-velocity relationships Force—velocity relationship: Since power is equal to force times velocity, the muscle generates no power at either isometric force due to zero velocity or maximal velocity due to zero force.
The optimal shortening velocity for power generation is approximately one-third of maximum shortening velocity.
Force velocity relationship | S&C Research
Force—velocity relationship relates the speed at which a muscle changes its length usually regulated by external forces, such as load or other muscles to the amount of force that it generates. Force declines in a hyperbolic fashion relative to the isometric force as the shortening velocity increases, eventually reaching zero at some maximum velocity. The reverse holds true for when the muscle is stretched — force increases above isometric maximum, until finally reaching an absolute maximum.
This intrinsic property of active muscle tissue plays a role in the active damping of joints which are actuated by simultaneously-active opposing muscles.