Eccentric Contractions Explained

Eccentric Contractions

• Muscle physiology involves the study of tension/force, which equals the number of crossbridges.
• Isometric muscle contraction occurs when the muscle is stimulated but doesn't shorten; force is produced, but the distance is zero, resulting in zero work (W = 0).
• Concentric muscle contraction results in the muscle shortening while activated; force is produced during the shortening, leading to positive work (W = +), also work is done BY the muscle.
• Eccentric muscle contraction occurs when the muscle is activated but lengthening; force is produced, and the stretch is considered a negative distance, resulting in negative work (W = -); work is done ON the muscle.

Eccentric Contraction Mechanisms and Metabolic Cost

• During eccentric contractions, lengthening occurs while the muscle is activated.
• Force response during eccentric contractions is much greater than the peak isometric force, suggesting resistance to lengthening.
• Velocity has almost no effect on force during eccentric contractions.
• Submaximal responses are important, daily movements combine concentric and eccentric contractions.
• Concentric F/V is due to the time needed to move to the active state and attach myosin heads.
• Faster concentric contractions result in fewer attachments.
• Eccentric F/V results from breaking a number of cross-bridges
• Metabolic cost differs between eccentric and concentric contractions, with VO2 concentric being 5 times VO2 eccentric.

Force Development Factors

• At high concentric velocities, force is low and independent of length.
• At low concentric velocities, force depends strongly on length.
• During eccentric velocities, force depends mostly on length and very little on velocity.
• Isometric tension in a sub-maximally activated muscle depends on average overlap of actin strands and myosin heads, free Ca concentration in the cytosol, number of troponin binding sites occupied, and frequency/oscillations of Ca reflecting the rate of muscle stimulation.

Length-Dependent Activation

• "Length-dependent activation" refers to a greater force measured relative to what is predicted via the length-tension curve for maximal contractions.
• Modification of force production occurs through the change in force relative to Ca concentration (Ca sensitivity), defining force produced at a given Ca concentration (pCa2+).
• The proposed mechanism involves changes in myosin-actin filament spacing, where increased stretch brings myosin heads closer to actin binding sites.
• This effect is seen in CM, FSM, and SSM muscles.

Sarcomere Length and Motor Units

• When different motor units have different mean sarcomere lengths at a given muscle length, the effect is that different motor units have different mean sarcomere lengths on different parts of the length-tension curve.
• The results of varied sarcomere lengths include decreased peak force, extended length span for available force, and smooth muscle performance through greater range of motion.

Muscle Architecture: Sarcomeres in Series vs. Parallel

• Sarcomeres in series (anatomical muscle length) affect contraction velocity; force is independent of anatomical length.
• Contraction velocity depends on muscle length; each sarcomere shortens a small amount, and total shortening equals the sum of all sarcomeres in the series (velocity = displacement/time).
• Increasing muscle fiber contraction velocity requires more sarcomeres in a series, not faster contraction by each sarcomere
• Sarcomeres in parallel (cross-sectional area CSA) affect force; forces increase on z-lines, greater CSA increases the force.
• Velocity is independent of CSA.
• Force tension is independent of anatomical length.
• Each plane (z-disk) of sarcomeres will only shorten to the same amount per unit of time.

Power and Velocity

• Power is the product of force and velocity.
• Peak or maximum power is similar, arrangement A (series) can sustain power over a greater range of velocities, whereas peak power is about 30% max.

Semitendinosus and Muscle Fiber Length

• Semitendinosus muscle demonstrates concepts with regards to velocity, force, and power.
• Length-tension and force-velocity curves are such that force equals CSA and velocity equals muscle length.

Length/Tension and Force/Velocity Curves

• With length tension, longer muscles/fibers can generate force; however, peak force is the same for short or long fibers when cross-sectional area is the same.
• With force velocity, peak force is the same when CSA is the same; however, velocity is greater with longer fibers relative to short fibers.
• Adding sarcomeres gives a larger range of velocities.
• With different CSA and the same length, force will be greater, but muscle length will have the same range because the fiber is the same length.
• With force-velocity, velocity is the same because the length is the same, but a small CSA will generate less force than large CSA.

Muscle Architecture

• Pennation describes the relationship between fiber length and velocity of contraction, and it is more likely to occur in muscles generating higher forces.
• Orienting fibers in a penniform fashion enables the ability to pack more fibers into a muscle volume, does not cost the muscle much loss of force, and results in more fibers equaling more PCSA, thus more force for the same volume.
• Anatomic cross-sections are "straight across," while physiologic cross-sections turn with the muscle.
• When increasing ankle joint angle, as muscle length is stretched, fascicle length and pennation angle both diminish. At any ankle joint angle, muscle contraction shortens fibers and increases pennation angle; fiber length affects Vmax, but pennation doesn't affect velocity.
• Muscle thickness and pennation angle increase with increasing muscle thickness during maturation. Bodybuilders exhibit this in comparison to untrained subjects, with no sex differences.
• With resistance training, strength, muscle volume, CSA, and pennation angle all increase.
• Sprinters have greater muscle thickness, smaller pennation angles, and greater muscle fascicle length compared to distance runners and controls, giving them an advantage because longer muscle fascicles allow the muscle to shorten faster and allow the angle of pennation to be less for the same muscle volume. Furthermore, less potential force is lost, and the same tendon excursion sarcomeres could shorten at a slower velocity, therefore, on a stronger portion of the sarcomere force-velocity curve in a faster person.

Torque

• Torque is the angular force that causes rotation.
• Muscle force (F) is the actual force generated between tendons.
• Moment arm (r) is the distance between the pivot point and the point of force on the bone.
• Torque can be increased by increasing force, moment arm length, or changing the angle.
• Baboons have greater elbow extensor strength relative to humans due to a longer movement arm in baboons (olecranon process of ulna) where the triceps attach to the ulna, allowing it to create more force at the end of the moment arm.
• Sine helps to determine effective force of a muscle that only can generate the muscle characteristics as well as joint angle and the effects of torque.
• The mountain is shaped with 90 being the greatest which decreases more or less after.
• A correction factor of 90 degrees sin is 1 so all the force is applied, going past that the effective sign decreases.

Torque and Muscle Length-Tension Relationship

• Muscle force is on the right of the LT curve, and moment arm reaches a peak at 90 degrees.
• Peak torque occurs at a knee joint angle that is neither the peak force of the muscle or angle where sin is peak.
• Contractions that the quadriceps perform during running and cycling differ, where running leads the muscle to operate on the ascending portion of the L-T curve and cycling operates on the descending portion.
• Runners have a greater number of sarcomeres in a series relative to cyclists.
• At a given joint angle/muscle length, the length of each sarcomere would be smaller, which further prevents the muscle/sarcomere from going over the top of the length-tension curve.
• Cyclists have fewer sarcomeres in a series, allowing the sarcomeres to remain in the best range of lengths for force development when they shorten.
• Muscles can adapt to chronic concentric or eccentric contractions through the deletion or addition of sarcomeres.
• Passive force affects total force relative to muscle length, differing based on the muscle.
• Onset of passive force depends on the position relative to the L-T curve where Gastrocnemius exhibits higher passive and total, Sartorius exhibits middle passive and total, and Semitendinosus exhibits low passive where the total hits peak/keeps going down.

Motor Units

• The motor unit is considered the smallest functional unit because all the fibers contract together, thus all the fibers in a motor unit are recruited together based on an all-or-nothing principle.
• Each muscle fiber is innervated by only one motoneuron, and each motoneuron innervates several dozen to hundreds of fibers (not a 1:1 ratio).
• The three primary types of motor units differ by fast-twitch fatigable (FF; fast contraction, high peak tension, quick fatigue), fast-twitch fatigue resistant (FR; fast contraction, intermediate tension, fatigue resistant), and slow-twitch (S; slow contraction, low peak tension, fatigue resistant).
• Tension/force relates to the number of crossbridges, EDL are a fast twitch type 2a and 2b, and Soleus are slow twitch type 1. EDL (fast twitch) has 3x faster contraction time relative to SOL, and motoneuron cell body and axon size is IIb>IIa>I, as well as Myosin ATPase activity, which relates to IIB > IIA > I.
• In a leg of ham, type 1 fibers are closer to the bone, then type 2a, then 2x; in humans, slow twitch fibers are deeper/closer to the bone while fast-twitch fibers are closer to the surface.
• Fast-twitch muscles show a 3x faster contraction time and are 2x faster at relaxing relative to slow-twitch muscles; twitch force in EDL is 2x than twitch force/CSA, thus the velocity for fiber types shows 2b>2a>1.
• Greater velocity allows for greater power, where power equals force times velocity, therefore power type shows 2b>2a>1.

Muscle Contraction and Fiber Type

• Peak force developed relates to the enzyme (myosin ATPase); ATP is sped up, thus velocity is quicker relates to the rate AM crossbridges cycle through. limiting steps are related to the steps of reaction.

Fiber Type Differences

• Contractile properties are determined by myosin heads/actin strands and myofibrils in vitro motility assay (IVMA; step size for each myosin head), though compared to MHC2B, MHC1 only has a slightly greater step size yet not enough to account for Vo in MHC1.
• Low twitch tensions MU with longer (Slower) contraction times are recruited first (type 1 motor units), as twitch tension goes up it trends towards faster MU (Fta and FTb), threshold force added for each motor unit, CSA for either bigger and more fibers determines fiber.

Motor Unit Recruitment

• Small CSA Mus are recruited before larger MUS, leads to a non-linear increase of the MU.

Muscle Strength

• Muscle strength is achieved by describing two ways to use increasing force by a larger or smaller muscle.
• Muscle fiber percent is determined by needle biopsy and gel electrophoresis. Needle sample for healthy subjects

Histochemistry and Gel Electrophoresis

• Histochemistry shows composition is based on amounts of various proteins of all based on myosin. Mount thin slides of muscle on microscope Slides-thin slices of muscle on microscope slides, pre-incubate at pH 4.3, 4.5, 9.6 which denatures myosin ATPase in fiber specific manner
• Gel electrophoresis test includes stains on muscles in proteins and other molecules which can be altered in a electric field according to the total charge/ mass of the molecule.

Muscle Structures

• Muscle structures in type I and type II are different proteins. Muscle fiber recruitment involves use of different types of muscle.