What do you call on the maximum amount of force that a muscle can exert against an opposing force?

Eccentric Contractions

Eccentric contractions slow down and smooth joint motions. The anterior tibialis muscle contracts eccentrically at initial contact, firing despite plantar flexion of the ankle as the foot is lowered to the ground. In doing so, the foot is gently lowered to the floor and acceptance of body weight can occur gradually. If the anterior tibialis muscle does not fire, the foot “slaps” to the floor at initial contact. The gastrocsoleus contracts eccentrically throughout the second rocker of stance phase, controlling the rate of dorsiflexion of the ankle as the tibia advances forward over the plantigrade foot.53 In the absence of normal gastrocsoleus strength, the ankle dorsiflexes excessively, resulting in poor push-off and calcaneus gait.30,46

A powerful eccentric contraction occurring during weight acceptance in stance phase is that of the hip abductors. The abductors of the stance phase limb fire to limit contralateral pelvic drop as the swing limb comes off the ground. Meanwhile, the stance limb hip adducts slightly. If the gluteal muscles are weak, they cannot generate a sufficient eccentric contraction and the hemipelvis of the swing limb drops, resulting in a Trendelenburg gait. The trunk can compensate for the pelvic drop by swaying over the stance limb. This brings the center of gravity over the affected hip and lessens the pelvic drop. Patients with Trendelenburg gait use more energy to walk.

Eccentric contraction

Eccentric contraction occurs when the total length of the muscle increases as tension is produced. For example, the lowering phase of a biceps curl constitutes an eccentric contraction. Muscles are capable of generating greater forces under eccentric conditions than under either isometric or concentric contractions.17-19 Large tensile forces are generated during sudden eccentric contractions (e.g., a linebacker coming to a rapid stop at the line of scrimmage generates large eccentric quadriceps forces).

Traditional rehabilitation programs have often omitted eccentric training. Although there are no definitive studies to support eccentric training as an absolute prerequisite before returning to athletic play,19 research is emerging to support its use, particularly for the rehabilitation of microtrauma/overuse injuries. For example, Roos and colleagues20 designed a prospective randomized clinical trial to test the hypothesis that eccentric calf muscle exercises reduce pain and improve function in patients with Achilles tendinopathy. At 12 weeks, members of the group who performed eccentric exercises reported significantly less pain, and more patients in that group returned to sports participation after 12 weeks.20

Force-velocity relationship in a muscle–tendon unit

The everyday physical tasks that individuals perform are usually well within the strength capability of the muscle–tendon units used. In such movements, the muscle–tendon units generate just enough tension to overcome the external load acting on them so they can move the external load. The external load may simply be the weight of a limb segment such as the forearm in a movement involving elbow flexion. At other times the external load consists of the weight of the limb segments together with any additional load that is being moved such as something held in the hand.

When the amount of force produced by a muscle (muscle–tendon unit) just matches the external load, the muscle contracts isometrically. The maximum load the muscle can sustain isometrically is called the isometric strength of the muscle. When the external load is less than isometric strength, the muscle is able to contract concentrically. The speed of shortening in a concentric contraction depends on how much force the muscle needs to produce to move the external load. The greater the external load, the greater the muscle force needs to be, and the greater the muscle force (as a proportion of isometric strength), the slower the speed of shortening. A muscle can shorten at maximum speed when the external load on the muscle is zero. When the external load on a muscle is greater than the isometric strength of the muscle, it is forced to lengthen (contract eccentrically).

In an eccentric contraction a muscle resists the stretching load. In so doing, the attached cross bridges are themselves stretched, adding to the overall tension such that the force produced by the muscle is greater than the isometric strength of the muscle. The force produced by a muscle during eccentric contraction depends on the speed of lengthening, which depends on the size of the external load. The greater the external load (in relation to the isometric strength of the muscle), the greater the speed of lengthening. The greater the speed of lengthening, the greater the effect of the stretch reflex, and, therefore, the greater the force produced by the muscle. When the external force exceeds the maximum strength of the muscle, the muscle and its tendon will be damaged. The relationship between muscle force and speed of shortening or lengthening is referred to as the force–velocity relationship (Figure 6.18). Figure 6.19 shows the effect of the force–velocity relationship on the length–tension relationship of a muscle–tendon unit. The figure shows that at any particular length, the greater the speed of shortening, the lower the tension, and the greater the speed of lengthening, the higher the tension.

Key Concepts

The amount of force generated by a muscle–tendon unit depends on the length of the muscle–tendon unit at the time of stimulation (length–tension relationship) and the speed with which it changes length in the ensuing contraction (force–velocity relationship)

PATHOLOGY

PATHOLOGY

2.1 Muscle Contraction

The axons of the neuromuscular junctions originate from the motor neurons located in the ventral horn of the spinal cord. The motor neuron and all the muscle fibers to which it connects is a motor unit. The neuromuscular synapse comprises of postsynaptic and presynaptic membranes. The presynaptic membrane is the axon terminal containing acetylcholine vesicles. Upon stimulation, the stored acetylcholine pool is released and binds its receptor on the postsynaptic membrane of the sarcolemma, causing ion channels to open, and allows sodium ions to flow across the membrane into the muscle cell. This generates an action potential which travels to the myofibril through the transverse tubule system, and the sarcoplasmic reticulum; consequently Ca-ions release, which results in muscle contraction. Ca-ions bind to troponin C, resulting in conformational changes, which allow myosin to bind to actin, producing muscle contraction. In a resting state, troponin-I of the troponin complex covers the actin-myosin contact area. Actin-myosin binding requires ATP, which binds to the myosin-heavy chains (head) (Fig. 2.6).

In the resting state of a muscle, myosin has ADP and Pi (inorganic phosphate) bound to its nucleotide binding pocket. In the first step of the actin-myosin binding process, inorganic phosphate is released, followed by a power stroke and the release of ADP. This will pull the Z-lines toward each other, thereby shortening the sarcomere, approximately 10–12 nm/stroke. In the next step, ATP binds to the myosin, which weakens the attachment between actin-myosin filaments, in turn allowing the release of actin and the break of the cross bridge. If the level of ATP is low, it may result in a contracture.

Myosin heads, which are involved in the formation of the cross bridges, have different characteristics in slow-twitch and fast-twitch muscle fibers. In fast-twitch muscle fibers, formation of the cross bridges between actin and myosin filaments occurs faster because of the higher ATPase activity of myosin, and also the binding capacity of their binding sites are significantly high. Following contraction, Ca-ions are transported back to the sarcoplasmic reticulum by active transport, and troponin C returns to its resting state, so the muscles are able to relax.

Fast strokes demand a great deal of ATP molecules, which challenges the metabolic capacity of the body. To create significant force, more cross bridges need to be formed, whereas fast movements require cyclic changes of cross bridges and high amounts of ATP. Thus, fast-twitch muscle fibers show high ATPase activity, which was described for the first time by a Hungarian scientist, Mihály Bárány. Slow muscle movements require fewer cross bridges to be formed in a given time frame, reducing the ATP demand. This explains why huge amounts of ATP are required by maximal velocity movements, and why lower amounts of ATP are necessary for maximal force movements and much less for endurance movements. This topic will be further discussed in the next chapters.

2.1.1 Types of Contractions

There are three types of muscle contraction: concentric, isometric, and eccentric. Labeling eccentric contraction as “contraction” may be a little misleading, since the length of the sarcomere increases during this type of contraction. Thus in this context contraction does not necessary imply shortening (Table 2.1).

Table 2.1. Types of contraction

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In a concentric contraction, the force generated by the muscle is less than the muscle’s maximum, and the muscle begins to shorten. This type of contraction is widely known as muscle contraction. It requires more energy compared to the other two types, but this contraction generates the least force.

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An isometric contraction generates force without changing the length of the muscle, and no mechanical work is done since the muscle does not shorten. However, this type of contraction requires high amounts of energy because of the force generated by the muscle. This force is equal to the external load, thus the length of the muscle does not change.

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In an eccentric contraction, the external force on the muscle is greater than the force that the muscle can generate, thus the muscle is forced to lengthen due to the high external load. The maximal force generated by the muscle is the highest; however, the energy consumption is the lowest.

Comparison of maximal force generation in concentric, isometric, and eccentric contractions show the following ranking: eccentric > isometric > concentric. This ranking can be explained by muscle-tendon characteristics.

Archibald V. Hill is the only scientist who received a Noble-prize for his work done on sport-related research: the mechanical work in muscles. Hill’s three-element model is a representation of the muscle mechanical response. The model is constituted by a contractile element, a series element, and a parallel element (Fig. 2.7). The contractile element comes from the force generated by the actin and myosin myofibrils cross bridges. The series element represents the tendons, which are extensile but as much as muscles. The parallel element represents the passive force of the connective tissues such as fascia, membranes (epimysium, perimysium, endomysium), titin, and nebulin. In concentric muscle the force is generated collectively by the contractile element and at less extent the parallel element, while isometric muscle contraction involves the full contribution of the parallel element.

Fig. 2.7. Hill’s three-element model of muscle contraction. Force is generated by the contractile element, while the elastic elements store the energy.

In an eccentric contraction, the external force on the muscle is greater than the force that the muscle can generate, thus the series elastic elements are also forced to lengthen due to the high external load. This mechanical energy can be reclaimed during contraction, which provides a higher force. The model in Fig. 2.7 is a simplified explanation of why the different muscle contraction types produce different amounts of maximal force (Fig. 2.8).

Fig. 2.8. Force-velocity relationship. A.V. Hill’s force-velocity curve shows that the speed at which a muscle changes length also affects the force it can generate. The shortening velocity increases as the force declines, and so increasing force results in the decline of shortening velocity. If the force further increases the muscle is not able to shorten further; it contracts isometrically. If the external force on the muscle is greater than the force that the muscle can generate, the velocity turns to negative, in the case of eccentric muscle contraction.

Prone hip extension – knee flexed

To increase loading in concentric and eccentric contractions, the test position described in Chapter 8 (Fig. 8.65c) can be modified so that the patient performs the bent-knee hip extension movement over the end of a bed. This allows increased range of motion. Again, as for the prone gluteus medius exercise, this progression requires significant lumbopelvic stability. It is not always a necessary progression if successful activation of the muscle is occurring during functional integration exercises and their progressions. However, in cases where marked atrophy is present, the prone hip extension exercise may be indicated.

Muscle Strains (Partial or Complete Tears)

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Sudden onset of pain occurs during activity

Occurs during eccentric muscle contraction (i.e., muscle lengthens as it contracts, such as the biceps while lowering a weight)

Often affects muscles crossing two joints (rectus femoris, biceps femoris, gastrocnemius) and affects myotendinous junction

Three grades

Grade I—few muscle fibers torn, no functional loss, interstitial blood

Grade II—more fibers torn, some loss of strength, focal defect and interstitial blood in muscle, blood surrounding tendon from myotendinous junction injury

Grade III—muscle completely torn, loss of strength, focal and interstitial blood

Mechanical loading-induced muscle adaptation in vivo

The active force a muscle is able to exert at different lengths depends on the number of sarcomeres arranged in series (within myofibers) and in parallel (within the muscle). The more sarcomeres in series (i.e., longer optimum fiber length), the higher is the length range of active force exertion. However, this has no effect on the maximum force a myofiber can exert (at its optimum length). Therefore, optimal muscle force is determined by both the number of the myofibers and the number and size of myofibrils arranged in parallel within myofibers. The muscle cross-sectional area (Af) perpendicular to the fiber direction at a standardized mean sarcomere length (e.g., optimum length) provides an estimate of the maximum force a muscle is able to exert.

Both prime parameters, Af and serial sarcomere number, are highly adaptable in response to changes in mechanical loading of muscle. In myology, for adaptation of Af the terms atrophy and hypertrophy are used specifically, whereas adaptation of the serial sarcomere number refers to changes along the myofiber.

How does mechanical loading stimulate adaptation of muscle size? Several in-vivo experiments indicate that mechanical loading-induced adaptation of muscle size is determined by both type and intensity of active contractile activity, as well as the strain applied onto the muscle.

Coactivation Exercise

Coactivation exercises (using slow balance and control activities) utilize isometric, eccentric, and concentric contractions to increase proprioception and CNS awareness – thereby improving wrist stability.35 A simple method of coactivation training is to perform balance ball exercises (Fig. 22.4). The client’s hand(s) are placed on a weighted ball. The client is instructed to slowly move the ball around the table, which allows for simultaneous activation of extensors, flexors, and deviators of the wrist. Coactivation retraining with vision and vision occluded improves wrist stability by increasing proprioceptive awareness during motion, with greater control of the muscles. Exercises through range and with resistance using free weights or elastic bands will increase sensorimotor feedback and reciprocal and recurrent muscle action. Limited plane and range activities (e.g., toy hammer or miming a dart throw) will activate stabilizers with graded loading.49