Three Mechanical Characteristics Of Muscle

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Previously we discussed the passive tension in an MTU as it is passively stretched. Now it is time to examine the tensile forces the MTU experiences in the wide variety actions, lengths, and other active conditions encountered in movement. The force potential of an MTU varies and can be described by three mechanical characteristics. These characteristics deal with the variations in muscle force because of differences in velocity, length, and the time relative to activation.

Force-Velocity Relationship

The Force-Velocity Relationship explains how the force of fully activated muscle varies with velocity. This may be the most important mechanical characteristic since all three muscle actions (eccentric, isometric, concentric) are reflected in the graph.

We will see that the force or tension a muscle can create is quite different across actions and across the many speeds of movement. The discovery and formula describing this fundamental relationship in concentric conditions is also attributed to A. V. Hill. Hill made careful measurements of the velocity of shortening when a preparation of maximally stimulated frog muscle was released from isometric conditions. These studies of isolated preparations of muscle are performed in what we term in vitro (Latin for "in glass") conditions. Figure 4.7 illustrates the shape of the complete Force-Velocity Relationship of skeletal muscle. The Force-Velocity curve essentially states that the force the muscle can create decreases with increasing velocity of shortening (concentric actions), while the force the muscle can resist increases with increasing velocity of lengthening (eccentric actions). The force in isometric conditions is labeled P0 in Hill's equation. The right side of the graph corresponds to how the tension potential of the muscle rapidly decreases with increases in speed of concentric shortening. Also note, however, that

Muscle Force Relationship

Figure 4.7. The in vitro Force-Velocity Relationship of muscle. Muscle force potential rapidly decreases with increasing velocity of shortening (concentric action), while the force within the muscle increases with increasing velocity of lengthening (eccentric action). The rise in force for eccentric actions is much higher than illustrated.

Figure 4.7. The in vitro Force-Velocity Relationship of muscle. Muscle force potential rapidly decreases with increasing velocity of shortening (concentric action), while the force within the muscle increases with increasing velocity of lengthening (eccentric action). The rise in force for eccentric actions is much higher than illustrated.

increasing negative velocities (to the left of isometric) show how muscle tension rises in faster eccentric muscle actions. In isolated muscle preparations the forces that the muscle can resist in fast eccentric actions can be almost twice the maximum isometric force (Alexander, 2002). It turns out the extent of damage done to a muscle eccentrically overstretched is strongly related to the peak force during the stretch (Stauber, 2004). In athletics they say the sprinter "pulled or strained" his hamstring, but the injury was the results of very large forces (high mechanical stress) from a too intense eccentric muscle action.

If the force capability of an in vitro muscle preparation varies with velocity, can this behavior be generalized to a whole MTU or muscle groups in normal movement? Researchers have been quite interested in this question and the answer is a strong, but qualified "yes." The torque a muscle group can create depends on the previous action, activation, rate of force development, and the combination of the characteristics of the muscles acting at that and nearby joints. Despite these complications, the in vivo (in the living animal) torque-angular velocity relationship of muscle groups usually matches the shape of the in vitro curve. These in vivo torque-angular velocity relationships are established by testing at many angular velocities on isokinetic and specialized dynamometers. These studies tend to show that in repeated isokinetic testing the peak eccentric torques are higher than peak isometric torques, but not to the extent of isolated muscle preparations (Holder-Powell & Rutherford, 1999), while concentric torques decline with varying slopes with increasing speed of shortening (De Koning et al., 1985; Gulch, 1994; Pinniger et al, 2000).

This general shape of a muscle's potential maximum tension has many implications for human movement. First, it is not possible for muscles to create large forces at high speeds of shortening. Muscles can create high tensions to initiate motion, but as the speed of shortening increases their ability to create force (maintain acceleration) decreases. Second, the force potential of muscles at small speeds of motion (in the middle of the graph) depends strongly on isometric muscular strength (Zatsiorsky & Kraemer, 2006). This means that muscular strength will be a factor in most movements, but this influence will vary depending on the speed and direction (moving or braking) the muscles are used. Third, the inverse relationship between muscle force and velocity of shortening means you cannot exert high forces at high speeds of shortening, and this has a direct bearing on muscular power. In chapter 6 we will study mechanical power and look more closely at the right mix of force and velocity that creates peak muscular power output. This also means that isometric strength and muscle speed are really two different muscular abilities. Athletes training to maximize throwing speed will train differently based on the load and speed of the implements in their sport. Athletes putting the shot will do higher weight and low repetition lifting, compared to athletes that throw lighter objects like a javelin, softball, or baseball, who would train with lower weights and higher speeds of movement. One of the best books that integrates the biomechanics of movement and muscle mechanics in strength and conditioning is by Zatsiorsky & Kraemer (2006).

So there are major implications for human movement because of the functional relationship between muscle force and velocity. What about training? Does training alter the relationship between muscle force and velocity or does the Force-Velocity Relationship remain fairly stable and determine how you train muscle? It turns out that we cannot change the nature (shape) of the Force-Velocity Relationship with training, but we can shift the graph upward to

Force Velocity Analysis Strength
Figure 4.8. Training shifts the Force-Velocity curve upward and is specific to the kind of training. Heavy weight training primarily shifts the curve upward for isometric and slow concentric actions, while speed training improves muscle forces at higher concentric speeds.

improve performance (De Koning et al., 1985; Fitts & Widrick, 1996). Weight training with high loads and few repetitions primarily shifts the force-velocity curve up near isometric conditions (Figure 4.8), while fast lifting of light loads shifts the curve up near Vmax, which is the maximum velocity of shortening for a muscle.

Another area where the Force-Velocity Relationship shows dramatic differences in muscle performance is related to muscle fiber types. Skeletal muscle fibers fall on a continuum between slow twitch (Type I) and fast twitch (Type II). Type I are also called Slow-Oxidative (SO) because of their high oxidative glycolysis capacity (considerable mitochrondion, myoglobin, triglycerides, and capillary density). Type II fibers are also called Fast-Glycolytic (FG) because of their greater anaerobic energy capacity (considerable intramuscular ATP and gly-colytic enzymes). Muscle fibers with intermediate levels are usually called FOG (Fast-

Oxidative-Glycolytic) fibers. Muscle fibers type have been classified in many ways (Scott, Stevens, & Binder-Macleod, 2001), but biomechanics often focuses on the twitch response and velocity of shortening characteristics of fiber types. This is because the force potential of fast and slow twitch fibers per given physiological cross-sectional area are about the same. The timing that the muscle fibers create force and speed of shortening, however, are dramatically different. This fact has major implications for high-speed and high-power movements.

The easiest way to illustrate these differences is to look at the twitch response of different fiber types. If an in vitro muscle fiber is stimulated one time, the fiber will respond with a twitch. The rate of tension development and decay of the twitch depends on the fiber type of the fiber. Figure 4.9 illustrates a schematic of the twitch responses of several fiber types. A fiber at the

Fast Twitch Slow Twitch Muscles
Figure 4.9. The twitch response of fast-twitch (FG) and slow-twitch (SO) muscle fibers. Force output is essentially identical for equal cross-sectional areas, but there are dramatic differences in the rise and decay of tension between fiber types that affect the potential speed of movement.

slow end of the fiber type continuum gradually rises to peak tension in between 60 and 120 ms (about a tenth of a second). A fiber at the high end of the continuum (FG) would quickly create a peak tension in 20 to 50 ms. This means that the muscle with a greater percentage of FG fibers can create a greater velocity of shortening than a similar (same number of sarcomeres) one with predominantly SO muscle fibers. Muscles with higher percentages of SO fibers will have a clear advantage in long-duration, endurance-related events.

Human muscles are a mix of fiber types. There are no significant differences in fiber types of muscles across gender, but within the body the antigravity muscles (postural muscles that primarily resist the torque created by gravity) like the soleus, erector spinae, and abdominals tend to have a higher percentage of slow fibers than fast fibers. The fiber type distribution of elite athletes in many sports has been well documented. There is also interest in the trainability and plasticity of fiber types (Fitts & Widrick, 1996; Kraemer, Fleck, & Evans, 1996). Figure 4.10 illustrates the Force-Velocity Relationship in the predominantly slow-twitch soleus and predominantly fast-twitch medial gastrocnemius muscles in a cat. This fiber distribution and mechanical behavior are likely similar to humans. If the gastrocnemius were a more significant contributor to high-speed movements in sport, how might exercise position and technique be used to emphasize the gastrocnemius over the soleus?

Interdisciplinary Issue: Speed

Running speed in an important ability in many sports.The force-velocity relationship suggests that as muscles shorten concentrically faster they can create less tension to continue to increase velocity.What are the main factors that determine sprinting speed? Do muscle mechanical properties dominate sprinting performance or can technique make major improvements in to running speed? Several lines of research suggest that elite sprinting ability may be more related to muscular and structural factors than technique. Near top running speed, stride rate appears to be the limiting factor rather than stride length (Chapman & Caldwell, 1983; Luthanen & Komi, 1978b; Mero, Komi, & Gregor, 1992). In the 100-meter dash running speed is clearly correlated with percentage of fast twitch fibers (Mero, Luthanen, Viitasalo, & Komi, 1981) and the length of muscle fascicles (Abe, Kumagai, & Brechue, 2000; Kumagai, Abe, Brechue, Ryushi, Takano, & Mizuno, 2000) in high-level sprinters. Athletes with longer fascicles are faster. Future research into genetic predisposition to fiber dominance and trainability might be combined with bio-mechanical research to help improve the selection and training of sprinters.

Fast Twitch Biology

Velocity (cm/s)

Figure 4.10. Differences in the Force-Velocity Relationship of the primarily fast-twitch medial gastrocne-mius and primarily slow-twitch soleus of the cat. Reprinted, by permission, from Edgerton, Roy, Gregor, & Rugg, (1986).

Velocity (cm/s)

Figure 4.10. Differences in the Force-Velocity Relationship of the primarily fast-twitch medial gastrocne-mius and primarily slow-twitch soleus of the cat. Reprinted, by permission, from Edgerton, Roy, Gregor, & Rugg, (1986).

Application: Domains of Muscular Strength

Therapists, athletes, and coaches often refer to a functional characteristic called muscular strength.While muscular strength is commonly measured in weight training with one-repetition maxima (IRM is the maximum weight a person can lift only one time), most researchers define muscular strength in isometric conditions at a specific joint angle to eliminate the many mechanical factors affecting muscle force (e.g., Atha, 1981; Knuttgen & Kraemer, 1987). Many fitness test batteries include tests for components called muscular strength and muscular endurance. Early physical education research demonstrated that muscular strength has several domains of functional expression. Statistical analysis of fitness testing demonstrated that muscular strength is expressed as static (isometric), dynamic (slow to moderate movements), and explosive for fast movement (Jackson & Frankiewicz, 1975; Myers et al., I 993). This corresponds closely to the major changes in force capability in the Force-Velocity Relationship. Others experts often include another domain of muscular strength related to eccentric actions: stopping strength (Zatsiorsky & Kraemer, 2006). Functional muscular strength is also complicated by the fact that the force a muscle group can express also depends on the inertia of the resistance. The peak force that can be created in a basketball chest pass is nowhere near peak bench press isometric strength because of the small inertia of the ball. The ball is easily accelerated (because of its low mass), and the force the muscles can create at high shortening velocities rapidly declines, so the peak force that can be applied to the ball is much less than with a more massive object. So the Force-Velocity property of skeletal muscle and other biomechanical factors is manifested in several functional "strengths." Kinesiology professionals need to be aware of how these various "muscular strengths" correspond to the movements of their clients. Professionals should use muscular strength terminology correctly to prevent the spread of inaccurate information and interpret the literature carefully because of the many meanings of the word strength.

cross-bridges between the actin and myosin filaments in the Sliding Filament Theory. Peak muscle force can be generated when there are the most cross-bridges. This is called resting length (L0) and usually corresponds to a point near the middle of the range of motion. Potential active muscle tension decreases for shorter or longer muscle lengths because fewer cross-bridges are available for binding. The passive tension component (solid line) shows that passive tension increases very slowly near L0 but dramatically increases as the muscle is elongated. Passive muscle tension usually does not contribute to movements in the middle portion of the range of motion, but does contribute to motion when muscles are stretched or in various neuromuscular disorders (Salsich, Brown, & Mueller, 2000). The exact shape of the Force-Length Relationship slightly varies between muscles because of differences in active (fiber area, angle of pennation) and passive (tendon) tension components (Gareis, Solo-monow, Baratta, Best, & D'Ambrosia, 1992).

Length

Figure 4.11. The Force-Length Relationship of human skeletal muscle. The active component follows an inverted "U" pattern according to the number of potential cross-bridges as muscle length changes. Passive tension increases as the muscle is stretched beyond its resting length (L0). The total tension potential of the muscle is the sum of the active and passive components of tension.

Length

Figure 4.11. The Force-Length Relationship of human skeletal muscle. The active component follows an inverted "U" pattern according to the number of potential cross-bridges as muscle length changes. Passive tension increases as the muscle is stretched beyond its resting length (L0). The total tension potential of the muscle is the sum of the active and passive components of tension.

Force-Length Relationship

The length of a muscle also affects the ability of the muscle to create tension. The Force-Length Relationship documents how muscle tension varies at different muscle lengths. The variation in potential muscle tension at different muscle lengths, like the Force-Velocity Relationship, also has a dramatic effect on how movement is created. We will see that the Force-Length Relationship is just as influential on the torque a muscle group can make as the geometry (moment arm) of the muscles and joint (Rassier, Macintosh, & Herzog, 1999).

Remember that the tension a muscle can create has both active and passive sources, so the length-tension graph of muscle will have both of these components. Figure 4.11 illustrates the Force-Length Relationship for a skeletal muscle fiber. The active component of the Force-Length Relationship (dashed line) has a logical association with the potential numbers of

Qualitative Features Frictional Force
Figure 4.12. The three regions of the active component of the Length-Tension Relationship. Differences in the work (W = F • d) the muscle can do in the ascending limb (WA) versus the plateau region (WP) are illustrated. Work can be visualized as the area under a force-displacement graph.

The active tension component of the Force-Length Relationship has three regions (Figure 4.12). The ascending limb represents the decreasing force output of the muscle as it is shortened beyond resting length. Movements that require a muscle group to shorten considerably will not be able to create maximal muscle forces. The plateau region represents the high muscle force region, typically in the midrange of the anatomical range of motion. Movements initiated near the plateau region will have the potential to create maximal muscle forces. The descending limb represents the decreasing active tension a muscle can make as it is elongated beyond resting length. At extremes of the descending limb the dramatic increases in passive tension provide the muscle force to bring a stretched muscle back to shorter lengths, even though there are virtually no potential cross-bridge attachment sites. Biomechani-cal research has begun to demonstrate that muscles adapt to chronic locomotor movement demands and the coordination of muscles may be organized around muscles suited to work on the ascending, plateau, or descending limb of the force-length curve (Maganaris, 2001; Rassier et al., 1999). It is clear that the length of muscles influences how the central nervous system coordinates their actions (Nichols, 1994).

Activity: Force-Length Relationship

Active insufficiency is the decreased tension of a multiarticular muscle when it is shortened across one or more of its joints.Vigorously shake the hand of a partner. Fully flex your wrist, and try to create a large grip force.What happened to your strength? Which limb of the Force-Length Relationship creates this phenomenon?

The implications for a muscle working on the ascending limb versus the plateau region of the force-length curve are dramatic. The mechanical work that a muscle fiber can create for a given range of motion can be visualized as the area under the graph because work is force times displacement. Note the difference in work (area) created if the muscle fiber works in the ascending limb instead of near the plateau region (Figure 4.12). These effects also interact with the force-velocity relationship to determine how muscle forces create movement throughout the range of motion. These mechanical characteristics also interact with the time delay in the rise and fall of muscle tension, the force-time relationship.

Force-Time Relationship

Another important mechanical characteristic of muscle is related to the temporal delay in the development of tension. The Force-Time Relationship refers to the delay in the development of muscle tension of the whole MTU and can be expressed as the time from the motor action potential (electrical signal of depolarization of the fiber that makes of the electromyo-graphic or EMG signal) to the rise or peak in muscle tension.

The time delay that represents the Force-Time Relationship can be split into two parts. The first part of the delay is related to the rise in muscle stimulation some-

Application: Strength Curves

The torque-generating capacity of a muscle primarily depends on its physiological cross-sectional area, moment arm, and muscle length (Murray, Buchanan, & Delp, 2000).The maximum torque that can be created by a muscle group through the range of motion does not always have a shape that matches the in vitro force-length relationship of muscle fibers.This is because muscles with different areas, moment arms, and length properties are summed and often overcome some antagonistic muscle activity in maximal exertions (Kellis & Baltzopoulos, l997).There is also some evidence that the number of sarcomeres in muscle fibers may adapt to strength training and interact with muscle moment arms to affect were the peak torque occurs in the range of motion (Koh, 1995). "Strength curves" of muscle are often documented by multiple measurements of the isometric or isokinetic torque capability of a muscle group throughout the range of motion (Kulig, Andrews, & Hay, l984).The torque-angle graphs created in isokinetic testing also can be interpreted as strength curves for muscle groups. The peak torque created by a muscle group tends to shift later in the range of motion as the speed of rotation increases, and this shift may be related to the interaction of active and passive sources of tension (Kawakami, Ichinose, Kubo, Ito, Imai, & Fukunaga, 2002). How the shape of these strength curves indicates various musculoskeletal pathologies is controversial (Perrin, 1993). Knowledge of the angles where muscle groups create peak torques or where torque output is very low is useful in studying movement. Postures and stances that put muscle groups near their peak torque point in the range of motion maximizes their potential contribution to motion or stability (Zatsiorsky & Kraemer, 2006). In combative sports an opponent put in an extreme joint position may be easily immobilized (active insufficiency, poor leverage, or pain from the stretched position).Accommodating resistance exercise machines (Nautilus™ was one of the first) are usually designed to match the average strength curve of the muscle group or movement. These machines are designed to stay near maximal resistance (match the strength curve) throughout the range of motion (Smith, 1982), but this is a difficult objective because of individual differences (stone, Plisk, & collins, 2002). There will be more discussion of strength curves and their application in chapter 7.

times called active state or excitation dynamics. In fast and high-force movements the neuromuscular system can be trained to rapidly increase (down to about 20 ms) muscle stimulation. The second part of the delay involves the actual build-up of tension that is sometimes called contraction dynamics. Recall that the contraction dynamics of different fiber types was about 20 ms for FG and 120 ms for SO fibers. When many muscle fibers are repeatedly stimulated, the fusion of many twitches means the rise in tension takes even longer. The length of time depends strongly on the cognitive effort of the subject, training, kind of muscle action, and the activation history of the muscle group. Figure 4.13 shows a schematic of rectified electromyography (measure the electrical activation of muscle) and the force of an isometric grip force. Note that peak isometric force took about 500 ms. Revisit Figure 3.14 for another example of the electromechanical delay (delay from raw EMG to whole muscle force). Typical delays in peak tension of whole muscle groups (the Force-Time Relationship) can be quite variable. Peak force can be developed in as little as 100 ms and up to over a second for maximal muscular strength efforts. The Force-Time Relationship is often referred to as the electromechanical delay in electromyographic (EMG) studies. This delay is an important thing to keep in mind when looking at EMG plots and trying to relate the timing of muscle forces to the movement. Recall that the rise in muscle tension is also affected by the stiffness of the connective tissue components of muscle (passive tension from SEC and PEC), so the size of the electromechanical delay is affected by the slack or tension in the connective tissue (Muraoka et al., 2004). In chapter 5 we will see that kinematics provides a precise description of how motion builds to a peak velocity and where this occurs relative to the accelerations that make it occur.

This delay in the development of muscle tension has implications for the coordination and regulation of movement. It

Human Physiology With Lab Tutor Emg

Figure 4.13. The rectified electromyographic (REMG) signal from the quadriceps and the force of knee extension in an isometric action. The delay between the activation (REMG) and the build-up of force in the whole muscle is the electromechanical delay and represents the Force-Time Relationship of the muscle. It takes 250 to 400 ms for peak force to be achieved after initial activation of the muscle group.

Figure 4.13. The rectified electromyographic (REMG) signal from the quadriceps and the force of knee extension in an isometric action. The delay between the activation (REMG) and the build-up of force in the whole muscle is the electromechanical delay and represents the Force-Time Relationship of the muscle. It takes 250 to 400 ms for peak force to be achieved after initial activation of the muscle group.

turns out that deactivation of muscle (timing of the decay of muscle force) also affects the coordination of movements (Neptune & Kautz, 2001), although this section will limit the discussion of the Force-Time Relationship to a rise in muscle tension.

Kinesiology professionals need to know about these temporal limitations so they understand the creation of fast movements and can provide instruction or cues consistent with what the mover's body does. For example, it is important for coaches to remember that when they see high-speed movement in the body, the forces and torques that created that movement preceded the peak speeds of motion they observed. The coach that provides urging to increase effort late in the movement is missing the greater potential for acceleration earlier in the movement and is asking the performer to increase effort when it will not be able to have an effect. Muscles are often preactivated before to prepare for a forceful event, like the activation of plantar flexors and knee extensors before a person lands from a jump. A delay in the rise of muscle forces is even more critical in movements that cannot be preprogrammed due to uncertain environmental conditions. Motor learning research shows that a couple more tenths of a second are necessary for reaction and processing time even before any delays for increases in activation and the electromechanical delay. To make the largest muscle forces at the initiation of an intended movement, the neu-romuscular system must use a carefully timed movement and muscle activation strategy. This strategy is called the stretch-shortening cycle and will be discussed in the following section.

STRETCH-SHORTENING CYCLE (SSC)

The mechanical characteristics of skeletal muscle have such a major effect on the force

Application: Rate of Force Development

Biomechanists often measure force or torque output of muscle groups or movements with dynamometers. One variable derived from these force-time graphs is the rate of force development (F/t), which measures how quickly the force rises.A high rate of force development is necessary for fast and high-power movements. The vertical ground reaction forces of the vertical jump of two athletes are illustrated in Figure 4.14. Note that both athletes create the same peak vertical ground reaction force, but athlete A (dashed line) has a higher rate of force development (steeper slope) than athlete B (solid line). This allows athlete A to create a larger vertical impulse and jump higher. The ability to rapidly increase the active state and consequently muscle force has been demonstrated to contribute strongly to vertical jump performance (Bobbert & van Zandwijk, 1999). Rate of force development is even more important in running jumps, where muscle actions and ground contact times are much shorter (50 to 200 ms) than in a vertical jump or MVC (Figure 4.13). Training the neuromuscular system to rapidly recruit motor units is very important in these kinds of movements (Aagaard, 2003). What kinds of muscle fibers help athletes create a quick rise in muscle force? What kinds of muscle actions allow muscles to create the largest tensions? The next two sections will show how the neuromuscular system activates muscles and coordinates movements to make high rates of muscle force development possible.

and speed of muscle actions that the central nervous system has a preferred muscle action strategy to maximize performance in most fast movements. This strategy is most beneficial in high-effort events but is also usually selected in submaximal movements. Most normal movements unconsciously begin a stretch-shortening cycle (SSC): a countermovement away from the intended direction of motion that is slowed down (braked) with eccentric muscle action

Muscle Force Time Eccentric Concentric

Figure 4.14. Vertical jump ground reaction forces in units of bodyweight (BW) for two athletes. Athlete A (dashed line) has a greater rate of force development compared to athlete B (solid line). The rate of force development is the slope of the graphs when vertical forces are building above 1BW. Do not interpret the up and down motion of the graph as motion of the jumper's body; it represents the sum of the vertical forces the athlete makes against the ground.

Figure 4.14. Vertical jump ground reaction forces in units of bodyweight (BW) for two athletes. Athlete A (dashed line) has a greater rate of force development compared to athlete B (solid line). The rate of force development is the slope of the graphs when vertical forces are building above 1BW. Do not interpret the up and down motion of the graph as motion of the jumper's body; it represents the sum of the vertical forces the athlete makes against the ground.

that is immediately followed by concentric action in the direction of interest. This bounce out of an eccentric results in potentiation (increase) of force in the following concentric action if there is minimal delay between the two muscle actions (Elliott, Baxter, & Besier, 1999; Wilson, Elliott, & Wood, 1991). In normal movements muscles are also used in shortening-stretch cycles where the muscle undergoes concentric shortening followed by eccentric elongation as the muscle torque decreases below the resistance torque (Rassier et al., 1999).

Early research on frog muscle by Cavagna, Saibene, & Margaria (1965) demonstrated that concentric muscle work was potentiated (increased) when preceded by active stretch (eccentric action). This phenomenon is know as the stretch-shortening cycle or stretch-shorten cycle and has been extensively studied by Paavo Komi (1984, 1986). The use of fiberoptic tendon force sensors and estimates of MTU length has allowed Komi and his colleagues to cre ate approximate in vivo torque-angular velocity diagrams (Figure 4.15). The loop in the initial concentric motion shows the higher concentric tensions that are created following the rather less-than-maximal eccentric tensions. The performance benefit of SSC coordination over purely concentric actions is usually between 10 and 20% (see "Stretch-Shortening Cycle" activity), but can be even higher, and the biomechanical origin of these functional benefits is still unclear. Many biomechanical variables have been examined to study the mechanism of the SSC, and the benefits of the SSC are dependent on when these variables are calculated (Bird & Hudson, 1998) and the resistance moved (Cronin, McNair, & Marshall, 2001b).

The mechanisms of the beneficial effects of SSC coordination is of considerable interest to biomechanics scholars. There are four potential sources of the greater muscle force in the concentric phase of an SSC: contractile potentiation, reflex potentiation,

Force

Potentiation

Mechanical Qualitative Test

Figure 4.15. Schematic of the in-vivo muscle force-velocity behavior during an SSC movement, and force-velocity behavior derived from multiple isoki-netic tests (see Barclay, 1997). Initial concentric shortening in an SSC creates higher forces than the classic Force-Velocity Curve. Muscle length is not measured directly but inferred from joint angle changes, so the true behavior of the muscle in human SSC motions is not known.

Velocity

Figure 4.15. Schematic of the in-vivo muscle force-velocity behavior during an SSC movement, and force-velocity behavior derived from multiple isoki-netic tests (see Barclay, 1997). Initial concentric shortening in an SSC creates higher forces than the classic Force-Velocity Curve. Muscle length is not measured directly but inferred from joint angle changes, so the true behavior of the muscle in human SSC motions is not known.

Activity: Stretch-Shortening Cycle

The benefits of stretch-shortening cycle muscle actions are most apparent in vigorous, full-effort movements.To see the size of the benefit for performance, execute several overarm throws for distance on a flat, smooth field. Measure the distance of your maximal-effort throw with your feet still and body facing the direction of your throw. Have someone help you determine about how far your trunk and arm backswing was in the normal throw. Measure the distance of a primarily concentric action beginning from a static position that matches your reversal trunk and arm position in the normal throw. Calculate the benefit of the SSC (prestretch augmentation) in the throw as (Normal - Concentric)/Concentric. Compare your results with those of others, the lab activity, and research on vertical jumping (Walshe, Wilson, & Murphy, 1996; Kubo et al., 1999). The benefit of the SSC to other faster movements is likely to be even higher than in vertical jumping (Komi & Gollhofer, l997).What factors might affect amount of prestretch augmentation? What might be the prestretch augmentation in other movements with different loads?

storage and reutilization of elastic energy, and the time available for force development (Komi, 1986; van Ingen Schenau, Bobbert, & de Haan, 1997). Contractile po-tentiation of muscle force is one of several variations in muscle force potential based on previous muscles actions. These phenomena are called history-dependent behaviors (see Herzog, Koh, Hasler, & Leonard, 2000; Sale, 2002). Shortening actions tend to depress force output of subsequent muscle actions, while eccentric actions tend to increase concentric actions that immediately follow. Force potentiation of muscle is also dependent on muscle length (Edman et al., 1997).

Another mechanism for the beneficial effect of an SSC is a greater contribution from the myotatic or stretch reflex (see the section on "Proprioception of Muscle Action and Movement"). Muscle spindles are proprioceptors of muscle length and are particularly sensitive to fast stretch. When muscles are rapidly stretched, like in an SSC movement, muscle spindles activate a short reflex loop that strongly activates the muscle being stretched. Studies of athletes have shown greater activation of muscles in the concentric phase of an SSC movement compared to untrained subjects (Komi & Golhoffer, 1997). The lack of a precise value for the electromechanical delay (the Force-Time Relationship) makes it unclear if stretch reflexes contribute to greater muscle forces in the late eccentric phase or the following concentric phase. The contribution of reflexes to the SSC remains controversial and is an important area of study.

One of the most controversial issues is the role of elastic energy stored in the eccentric phase, which can be subsequently recovered in the concentric phase of an SSC. There has been considerable interest in the potential metabolic energy savings in the reutilization of stored elastic energy in SSC movements. It may be more accurate to say elastic mechanisms in the SSC are preventing energy loss or maintaining muscle efficiency (Ettema, 2001), rather than an energy-saving mechanism. Animal studies (e.g., wallabies and kangaroo rats) have been used to look at the extremes of evolutionary adaptation in muscletendon units related to SSC movement economy (Biewener, 1998; Biewener & Roberts, 2000; Griffiths, 1989; 1991). A special issue of the Journal of Applied Biomechanics was devoted to the role of stored elastic energy in the human vertical jump (Gregor, 1997). Recent in vivo studies of the human gastrocnemius muscle in SSC movements has shown that the compliant tendon allows the muscle fibers to act in near isometric conditions at joint reversal and while the whole muscle shortens to allow elastic recoil of the tendinous structures to do more positive work (Kubo, Kanehisa, Takeshita, Kawakami, Fukashiro, & Fukunaga, 2000b; Kurokawa, Fukunaga, & Fukashiro, 2001). The interaction of tendon and muscle must be documented to fully understand the benefits of the SSC action of muscles (Finni et al., 2000).

Another mechanism for the beneficial effect of SSC coordination is related to the timing of force development. Recall that the rate of force development and the Force-Time Relationship have dramatic effect on high-speed and high-power movements. The idea is that if the concentric movement can begin with near-maximal force and the slack taken out of the elastic elements of the MTU, the initial acceleration and eventual velocity of the movement will be maximized. While this is logical, the interaction of other biomechanical factors (Force-Length Relationship, architecture, and leverage) makes it difficult to examine this hypothesis. Interested students should see papers on this issue in the vertical jump (Bobbert, Gerritsen, Litjens, & van Soest, 1996; Bobbert & van Zandwijk, 1999) and sprint starts (Kraan, van Veen, Snijders, & Storm, 2001).

The most influential mechanism for the beneficial effect of an SSC will likely depend on the movement. Some events like the foot strike in sprinting or running jump (100 to 200 ms) require high rates of force development that are not possible from rest due to the Force-Time Relationship. These high-speed events require a well-trained SSC technique and likely have a different mix of the four factors than a standing vertical jump.

Plyometric (plyo=more metric=length) training will likely increase the athlete's ability to tolerate higher eccentric muscle forces and increase the potentiation of initial concentric forces (Komi, 1986). Plyo-metrics are most beneficial for athletes in high-speed and power activities. There has been considerable research on the biome-chanics of lower-body drop jumping plyo-metrics (Bobbert, 1990). Early studies showed that jumpers tend to spontaneously adopt one of two techniques (Bobbert, Makay, Schinkelshoek, Huijing, & van Ingen Schenau, 1986) in drop jumping exercises. Recent research has focused on technique adaptations due to the compliance of the landing surface (Sanders & Allen, 1993), the effect of landing position (Kovacs et al., 1999), and what might be the optimal drop height (Lees & Fahmi, 1994). Less research has been conducted on the biomechanics of upper body plyometrics (Newton, Krae-mer, Hakkinen, Humphries, & Murphy, 1996; Knudson, 2001c). Loads for plyomet-ric exercises are controversial, with loads ranging between 30 and 70% of isometric muscular strength because maximum power output varies with technique and the movement (Cronin, McNair, & Marshall, 2001a; Izquierdo, Ibanez, Gorostiaga, Gaurrues, Zuniga, Anton, Larrion, & Hakkinen, 1999; Kaneko, Fuchimoto, Toji, & Suei, 1983; Newton, Murphy, Humphries, Wilson, Kraemer, & Hakkinen, 1997; Wilson, Newton, Murphy, & Humphries, 1993).

Plyometrics are not usually recommended for untrained subjects. Even though eccentric muscle actions are normal, intense unaccustomed eccentric activity is clearly associated with muscle damage. Eccentric-induced muscle fiber damage has been extensively studied and appears to be related to excessive strain in sarcomeres (see the review by Lieber & Friden, 1999) rather than the high forces of eccentric actions. Kinesiology professionals should carefully monitor plyometric technique and exercise intensity to minimize the risk of injury.

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  • CAIN
    What is mechanical characteristices of muscle?
    2 years ago

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