overlaps the actin fibers (Abands). The Aband is where all of the work of the cross-bridging takes place.
The actin filament is a thin fiber with two negatively charged molecules that spiral around each other. The myosin filament is a much thicker filament with “globu- lar heads” on it. These filaments are also negatively charged. In the resting state, these two filaments lie next to each other, mutually repelled by their negative charges. In the 1950s, Huxley8proposed a “ratchet” or sliding filament model that describes the generation of active tension. In this model, each globular head of the myosin fiber has one ATP molecule attached to it, which is negatively charged.
The nerve action potential from the lower motor neu- ron causes a release of acetylcholine (ACh) at the neuro- muscular junction. This sends a charge through the transverse tubules (Figure 2–3) that, when it reaches the sarcoplasmic reticulum, allows pores to be opened and calcium ions to flood the space where the myosin and actin fibers are located. Each calcium ion has a very strong positive charge to it, and it bonds instantly with the actin filament. At this point, the negatively charged myosin filament with its ATP molecule is strongly at- tracted to the now positively charged actin filament (Figure 2–4). As these two filaments are pressed against each other by the chemically induced electromagnetic attraction, the globular heads are forced to flatten out;
the resulting ratchet effect forces the two filaments to move past each other. The force of the bending of the globular heads, however, causes the ATP molecule to be released. The energy associated with this release pro- vides the energy needed to free the calcium ion from the actin fiber and pumps it back to the sarcoplasmic reticulum. Simultaneously, the myosin and actin fila- ments separate from each other, being held slightly apart by the two negative charges. Immediately, an- other ATP molecule attaches to the globular head on the myosin filament and the cycle is ready to begin all over again. Thus, the globular heads of the myosin act as cross-bridges for the actin chains. Through successive activations and cross-bridging, the muscle twitches while it shortens and work is done.
The metabolized ATP molecule, now known as adeno- sine diphosphate (ADP), is reconstituted in the mito- chondria. Using the Krebs cycle, the mitochondrion rebuilds the ADP back into ATP, using the glucose and oxygen provided by the circulation system. The by- products of this process include lactic acid, free hydrogen ions, and carbon dioxide. These by-products need to be pattern along the entire kinetic chain increases dramati-
cally. Such a weight can be held at this height for only a short period of time, because the more distal muscles begin to fatigue much faster than the more proximal postural muscles. Within the first minute, the volley of activity to the middle deltoid has increased and synchro- nized to counter the fatigue factor. Within 4 minutes the failure point is reached, the arm gives way to the gravita- tional pull, and the posture is recalibrated.
Figure 2–2 The composition of muscle cells, muscle fascicles, muscle fiber, myofibril, myofilaments, sarcomere, thick and thin filaments.
Source:Netter Anatomy Illustration Collection, ©Elsevier, Inc. All Rights Reserved.
Strength is found in the middle range, where a number of myosin-actin cross-bridges can be formed.
Muscle fibers can be divided into three broad cate- gories based on appearance, speed of contraction, and fatigability:
• Slow-twitch muscles take more than 35 milliseconds to complete a
depolarization/repolarization cycle and are reddish in appearance. These muscles twitch removed and transported away from the muscle cell via
the circulatory system.
The strength of the contraction is greater when the muscle is elongated to about its midpoint of the fila- ment sliding range. This phenomenon has to do with the structure of the myosin and actin fibers. Figure 2–5 illustrates the relationship between strength (percentage of maximum tension) and degree of myosin–actin over- lap. Strength is lost when there is very little overlap of the two fiber types and when the overlap is complete.
Myofibrils
Triad of the reticulum
Z line
A band
I band Sarcolemma
Transverse tubule
Mitochondrion
Terminal cisternae Sarcoplasmic
reticulum
Sarcotubules
Transverse tubule
Figure 2–3 The system of transverse tubules in a section of muscle cell. Transverse tubules create an intricate, three- dimensional maze, surrounding every bundle of myosin and actin filament.
Source:Adapted from W. Bloom and D.W. Fawcett, A Textbook of Histology, ©1970, Chapman & Hall.
fewer than 25 times per second, usually around 10 to 20 Hertz (Hz—a unit of frequency equal to one cycle per second).
• Fast-twitch, fatigue-resistant muscles are pale in appearance and, like the slow-twitch muscles, have a considerable ability for aerobic
metabolism. These are classified as Type I, fast- twitch fibers.
• Fast-twitch, fatigable muscles take less than 35 milliseconds to complete a contraction or twitch
cycle and are whitish in appearance. These are classified as Type II, fast-twitch fibers. Fast-twitch fibers twitch at a rate faster than 25 twitches per second, which is typically between 30 and 50 Hz.
A classic example in the human body of fast- and slow-twitch muscles is found in the calf of the leg. The soleus consists of predominantly slow-twitch fibers and is reddish in color, while the gastrocnemius consists primarily of fast-twitch fibers and is a paler color. The morphological differentiation of the fiber type is deter- mined by the type of motor nerve that activates the fibers. Small, slow nerve fibers develop slow-twitch muscles; large, fast nerve fibers develop fast-twitch muscles. Most muscles contain a mixture of fast- and slow-twitch fibers.
The distinctions made between the two types of mus- cle fibers—slow, fatigue-resistant versus fast, fatigable—
represent a gross oversimplification; in reality, a gradation along a continuum for the attributes given later in this section would better represent the types of fibers. In general, however, Type I fibers are smaller in size and produce less tension. They are innervated by small, slow-moving neuronal axons. They are fairly re- silient to fatigue and are more amenable to anaerobic glucolysis. They have a low reflex threshold from the muscle spindle and Golgi tendon organ and generally tend to maintain a tonic repetitive discharge. In con- trast, Type II fibers have a low reflex threshold and re- spond reflexively with short burst patterns. In general terms, the Type I fibers appear ideal for postural activi- ties and the Type II fibers seem best suited for phasic movement.
Not only do different fiber types perform different types of work, but the same muscle fiber types can do work in different ways. Three clearly identifiable types of muscle contractions are distinguished: isometric, con- centric, and eccentric. The SEMG patterns observed during dynamic protocols may differ, depending upon which type of contraction one is studying.
Isometric contractionsare muscle contractions in which a constant muscle length is maintained. Technically, the contractile force does not exceed the force of resistance and, therefore, there is no change in muscle length. These contractions are used in postural control and for stabiliza- tion of axial body parts during extreme movements. They are also used during manual muscle testing. Surface EMG recordings are typically greatest under isometric testing conditions.
Z line A
B
C
D
Figure 2–4 The ratchet effect. (A) Prior to the calcium bonding, the myosin heads with their negative ATP molecule are held away from the negative actin filament.
(B) After calcium bonding, the negative ATP myosin heads attach to the now positive actin. This positive bond is also attracted to the negative myosin shaft. (C) The myosin head folds downward, pulling the actin chain forward one notch. (D) The ATP myosin bond breaks under the stress of the folding and releases the myosin filament from the actin chain, allowing the globular head to stand upright again. Immediately, another ATP molecule attaches to it.
As the muscle fibers depolarize, ratchet themselves to a shorter resting length, and repolarize, they undergo one motor unit action potential (MUAP) or twitch.
concentric contraction, because 20% of the energy effi- ciency of the movement is lost during concentric con- traction due to the shortening of the muscle. Thus, the greatest load that a concentric muscle contraction can carry is only 80% of the load carried by maximum iso- metric contraction.
Eccentric contractionsoccur when the muscle lengthens during a contraction. Technically, eccentric contractions occur in an already shortened muscle where the external force is greater than the tension created by the muscle contraction. Here, the muscle acts as a braking agent as a load is manipulated. The classic example of eccentric Concentric contractionsoccur when the muscle short-
ens during the contraction. Technically, a concentric contraction is defined as a contraction with enough force to overcome the external resistance, thereby al- lowing the muscle to shorten. During concentric con- tractions, the moving body part usually accelerates.
This type of contraction is the action taken by a prime mover during the active phase of a movement pattern.
A classic example of concentric contractions is biceps ac- tivity during elbow flexion associated with lifting a weight. The amount of muscular energy available is greater during an isometric contraction than during a
100 80 60 40 20 0
1.0 1.5
E 1.27 1.67 2.0 2.25 A 3.65
6 5 4 3 2
1.05 µ 1.65 µ 2.05 µ
1.0 µ 1.6 µ
3.65 µ
1.0 µ 0.05 µ
1.85 – 1.90 µ 2.20 – 2.25 µ
0.15 – .20 µ 1.
5.
4.
3.
2.
6.
B C D 0.84
2.0
Tension (% of maximum)
2.5 Striation spacing (µ)
3.0 3.5
1
4.0
Figure 2–5 Length-tension curve of frog muscle fiber related to overlap of myosin and actin filaments.
Source:Reprinted with permission from the Journal of Physiology, Vol. 184, pp. 170–192, ©1966, The Physiological Society.
contraction is biceps activity as the weight is slowly low- ered during elbow extension. Eccentric contractions are extremely common. In fact, every movement in the di- rection of gravity is controlled by an eccentric contrac- tion. Other examples include sitting, squatting, lying down, bending forward or sideways, and going down stairs. The amount of energy expended during an ec- centric contraction is always less than that observed during a concentric contraction for the same muscle.
The amount of metabolic work associated with eccen- tric contractions is one-third to one-thirteenth the work of concentric contractions. In SEMG recordings, the mi- crovolt amplitude of a concentric contraction is always larger than it is for the eccentric contraction, given the same amount of weight. For example, the erector spinae of the low back work harder and show higher levels of recruitment when returning from a flexed posi- tion than when going down into forward flexion from the neutral position. Researchers believe that the eccen- tric contractions require less SEMG activity because much of the work has to do with “breaking” existing cross-bridges rather than building new cross-bridges.
Another form of contraction is isotonic contraction, a subset (special class) of the concentric and eccentric contractions. Isotonic contractions occur when constant muscle force is employed as the muscle either shortens or lengthens. This type of force is studied most com- monly on instruments in which the force is controlled over the range of motion.
Surface EMG recruitment patterns may differ when they are observed in an open versus closed kinetic chain. In an open kinetic chain, the distal segment is free to move—as in the case where a person is not bearing weight in the lower extremities. The open kinetic chain entails a movement that is not resisted manually or through weight bearing. Movement of one joint does not necessarily cause movement in other joints. In a closed kinetic chain, the distal segment is fixed, as in weight bearing for the lower extremities. Movement at one joint induces movement at another joint. For exam- ple, in sitting with the leg hanging over the edge of the table (open kinetic chain), a person can move the ankle without moving the knee or hip. But when the person stands (closed kinetic chain), he or she cannot move the ankle without affecting other joints. Surface EMG re- cruitment patterns are typically greater under the con- ditions of a closed kinetic chain compared to those observed under the conditions of an open kinetic chain.
For example, the level of SEMG activity from rectus femoris during a squat (closed kinetic chain) is greater
than when the muscle is contracted from the seated po- sition and the knee extends without resistance (open ki- netic chain).