Which statement regarding muscle contraction is best supported by data in this graph

Isometric Versus Isotonic Contraction

Data for P0 are obtained under isometric conditions (length unchanged). When muscle is allowed to shorten against a steady load, the conditions areisotonic (tonic, “contractile force”).4 Thus the force-velocity curve may be a combination of initial isometric conditions followed by isotonic contraction and then abrupt and total unloading to measure Vmax. Although isometric conditions can be found in the whole heart (e.g., during isovolumic contraction), isotonic conditions are rare because afterload is constantly changing during the ejection period, and complete unloading is impossible. However, as shortening progresses during ejection, the maximal P0 declines. and velocity is lower for any given nonzero load. Therefore the force-velocity relationship is heuristically useful, but measurements in vivo are limited.

Isometric contraction

Isometric contraction occurs when muscle length remains relatively constant as tension is produced. For example, during a biceps curl, holding the dumbbell in a constant/static position rather than actively raising or lowering it is an example of isometric contraction.21,22 Although the forces generated during isometric contractions are potentially greater than during concentric contractions, muscles are seldom injured during this type of contraction. Isometric exercises are often used during the early phases of rehabilitating a musculotendinous injury because the intensity of contraction and the muscle length at which it contracts can be controlled.19

Isometric Contractions Do Not Shorten Muscle, Whereas Isotonic Contractions Shorten Muscle at a Constant Tension

Muscle contraction is said to beisometric when the muscle does not shorten during contraction andisotonic when it shortens but the tension on the muscle remains constant throughout the contraction. Systems for recording the two types of muscle contraction are shown inFigure 6-12.

In the isometric system, the muscle contracts against a force transducer without decreasing the muscle length, as shown in the bottom panel ofFigure 6-12. In the isotonic system, the muscle shortens against a fixed load, which is illustrated in the top panel of the figure, showing a muscle lifting a weight. The characteristics of isotonic contraction depend on the load against which the muscle contracts, as well as the inertia of the load. However, the isometric system records changes in force of muscle contraction independently of load inertia. Therefore, the isometric system is often used when comparing the functional characteristics of different muscle types.

Types of Muscle Contraction

Period of Isovolumic (Isometric) Contraction

Immediately after ventricular contraction begins, the ventricular pressure rises abruptly, as shown inFigure 9-8, causing the A-V valves to close. Then, an additional 0.02 to 0.03 second is required for the ventricle to build up sufficient pressure to push the semilunar (aortic and pulmonary) valves open against the pressures in the aorta and pulmonary artery. Therefore, during this period, contraction is occurring in the ventricles, but no emptying occurs. This period is called the period ofisovolumic orisometric contraction, meaning that cardiac muscle tension is increasing but little or no shortening of the muscle fibers is occurring.

Externally Rotated Tibia (Fig. 6.15)

Isometric contraction is used to correct the mechanical dysfunction.

The patient is seated with the leg passively extended and resting across the physician's knees. The physician should be seated somewhat below the patient, allowing for a slight flexion in the patient's knee.

The tibia is rotated internally to the barrier, without causing any rotation in the thigh.

The patient is asked to gently turn his leg towards external rotation, thereby externally rotating the tibia. The physician resists this movement to the extent that the long restrictors of the knee are engaged. Care is taken that there is no undue stress placed on the ligamentous structures of the knee.

This isometric contraction is maintained for 4–5 seconds, and then the patient is asked to cease contraction while the physician simultaneously reduces her counterforce.

The physician maintains contact and waits for tissue relaxation (approximately 4–5 seconds).

The physician internally rotates the tibia to reposition it to the new barrier. The sequence is repeated twice.

Types of Muscle Contraction

Isometric contractions are contractions in which there is no change in the length of the muscle. No joint or limb motion occurs. Isotonic contractions occur when the muscle changes length, producing limb motion. Concentric contractions occur when the muscle shortens. Eccentric contractions occur when the muscle lengthens. More fast-twitch fibers are recruited during eccentric contractions. Isokinetic contractions occur when muscle contraction is performed at a constant velocity. This can be done only with the assistance of a preset rate-limiting device. This type of exercise does not exist in nature.

Types of Muscle Work and Training Effects

Isometric muscle contraction is the production of muscle tension without a change in muscle length or joint angle. The tension in the cross-bridges (the portion of myosin filament that pulls the actin filaments toward the center of the sarcomere during muscle contraction) is equal to the resistive force, thereby maintaining constant muscle length.

Concentric muscle contraction is muscle shortening as the muscle produces tension while the insertion moves toward the origin. Movement occurs in the same direction as the tension and joint motion because the contractile force is greater than the resistive force. Based on the sliding filament theory, the cross-bridges on the myosin filament attach to the active site on the actin filament. When all the muscle cross-bridges shorten in a single cycle, the muscle shortens by approximately 1%. Muscles have the capacity to shorten up to 60% of their resting length; therefore the contraction cycle must be repeated multiple times.17

Eccentric muscle contraction is muscle lengthening as the muscle produces tension and the insertion moves away from the origin. The net muscle movement is in the opposite direction of the force of the muscle because the contractile force is less than the resistive force. Eccentric contractions require less energy than concentric contractions and are thought to be responsible for some aspect of postexercise muscle soreness. The cross-bridges of myosin stay attached to the active sites while the resistance is lowered. It may be the actual “tearing” away of the cross-bridges while resisting the lowering of a heavy resistance that results in the delayed-onset muscle soreness.17,18

Stability Concepts: Type of Exercise

Isometric contractions, small midrange isotonic contractions, oscillations, rhythmic stabilization, or agonist/antagonist reversals are all appropriate methods for enhancing muscle support within midrange (see Table 6.3). Closed-chain positions provide joint compressive forces that result in sensory feedback, which, in turn assists with postural muscle activation and support (Videos 2.24, 6.16, and 6.18).63 Coordinated muscle activation around the joint creates a centralized axis of motion and increases stability of the lever system. Closed-chain exercise involves movement at multiple joints, and motion at the proximal or distal joints can be used to help distribute forces. If motion is distributed to surrounding joints, the forces on the injured or hypermobile area are reduced and there is less chance of reaching the extreme ranges where the tissues are most vulnerable to combinations of stress and strain. Exercising in the midrange and avoiding the end range decreases the excessive shear forces and motion allowed by slack or absent support from the inert structures. Therefore, it is important to include goals to improve range-specific muscle activation, endurance, and strength. Balance activities trigger vestibular and ocular reflexes that automatically stimulate the balance and equilibrium reactions that involve postural muscle activity to bring the body back to a midposition (Videos 6.21 and 6.26). Therefore, using balance responses is one of the quickest and most automatic methods for promoting postural muscle stability. Oscillations or perturbations challenge alternating muscles and also create the same effect (Videos 6.21 and 6.25).64,65 A comparison of the types of exercise and parameters used for hypomobility and hypermobility impairments is illustrated in Fig. 6.5 and found in Table 6.4.

Types of Muscle Contractions:

An isometric contraction is a muscle contraction without motion. Isometric contractions are used to stabilize a joint, such as when a weight is held at waist level neither raising nor lowering it. Dynamic contractions are muscle contractions with a fixed amount of weight. They are divided into concentric and eccentric contractions. A concentric contraction occurs when the muscle length is shortened during a contraction, e.g., a biceps curl. An eccentric contraction occurs when the muscle length is increased during the contraction, i.e., the “negative” contraction. Eccentric contractions are used for decelerating or controlling motions. Isokinetic contractions are activated at a constant velocity and are artificially created by types of exercise equipment. Measurements of these contractions are often used in research settings but little relevance has been proven under real conditions. Plyometrics refers to a contraction sequence when a rapid eccentric contraction precedes a concentric contraction such as during a jump. An example is a jumper lowering the body and eccentrically loading the gluteal muscles prior to the jump which then requires concentric gluteal muscle contraction. Plyometric training can be especially useful in sport-specific rehabilitation. Strength is the maximal force generated during a single contraction while power is the amount of force generated per unit time. Power may be more important to emphasize for a person to return to maximal function. The amount of force generated by muscle contraction type from highest to lowest is: eccentric > isometric > concentric.