Muscle Architecture, Mechanics and Specific Adaptation to Resistance Training

Thrasivoulos Paxinos, M.Sc., Ph.D. Athens College of Sport Sciences

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Introduction


     Skeletal muscle represents the largest organ of the body. It makes up approximately 40% of the total body weight and it is organized into hundreds of separate entities, or body muscles, each of which has been assigned a specific task to enable the great variety of movements that are essential to normal life:
     Each muscle is composed of a great number of subunits, muscle fibers, that are arranged in parallel and typically extend from one tendon to another. In order to understand the performance of muscle it is essential to know the properties of the individual fibers. With the laboratory techniques now available it is possible to study the contractile behavior of intact single fiber. The single fiber preparation offers the possibility to study the mechanical performance under strict control of sarcomere length. This is of particular importance as the sarcomere length reflects the state of overlap between the two sets of filaments that constitute the main functional elements of the contractile system.


Architecture of Muscle


Gymboss Timers

    The muscle fiber is composed of tightly packed subunits, myofibrils, that fill up most of the fiber volume. They contain the contractile element and are therefore the structures within the muscle that are responsible for force generation and active shortening. The basic functional unit of the myofibril is the sarcomere. Many sarcomeres packed together in series form the myofibril. The principal elements in the myofibrillar structure are two sets of filaments of different thickness that show a highly ordered, segmental arrangement that corresponds to the striated appearance of the myofibril.
1. The thicker filaments are made up of a fibrous protein, myosin (Hanson and Huxley, 1953; Hasselbach, 1953).
2. The second set of filaments are mainly built up of a globular protein, actin.
     

   Interaction of these two filaments is the primary cause of muscle contraction and therefore force production. The degree of interaction is controlled by tropomyosin and troponin which are helicaly based on the actin filament.


Sliding Filaments Theory


     Our knowledge about the structural organization of the contractile system in the form of two sets of filaments, stems from the pioneering work of H. B. Huxley and J. Hanson (Hanson and Huxley, 1953; Huxley, 1953; Huxley and Hanson, 1954). Their observation that the thick and thin filaments remain constant in length during muscle contraction, while the region of overlap between the two filaments changes with the fiber length, led these authors to suggest that muscle contraction is based on a sliding motion of the two sets of filaments. This idea has now gained general acceptance.
     According to this view, the driving force for the sliding motion is generated by the myosin cross-bridges within the region where the thick and thin filaments overlap. The experimental evidence suggests that the myosin bridges make repeated contacts with adjacent thin filaments and that each such contact makes a contribution to the force developed during contraction. This occurs when the fiber is stimulated and calcium is released into the myoplasm from its storage site in the sarcoplasmic reticulum. Adenosine triphosphate offers the energy for the continuous action of the cross-bridges.


Motor Units


     The smallest subunit that can be controlled is called a motor unit because it is separately innervated by a motor axon. Neurologically, the motor unit consists of:

    Under the control of the motor units are as few as three fibers or as many as 2000, depending on the fineness of the control required. Muscles of the fingers, face and eyes have a small number of shorter fibers in a motor unit, while the large muscles of the leg have a large number of long fibers in their motor units.
     Each muscle has a finite number of motor units, each of which is controlled by a separate nerve ending. Excitation of each is an all-or- nothing event. The electrical indication is a motor unit action potential; the mechanical result is a twitch of tension. An increase in tension can therefore be accomplished in two ways:

    1. By an increase in stimulation rate for that motor unit, or
    2. By the excitation (recruitment) of an additional motor unit.

 

   Recruitment of motor units follows the size principle, which states that the size of the newly recruited motor unit increases with the tension level at which it is recruited (Henneman, 1974). This means that the smallest unit is recruited first and the largest unit last. In this manner low tension movements can be achieved in finely graded steps. Conversely, those movements requiring high forces but not needing fine control are accomplished by recruiting the larger motor units. When maximum voluntary contraction is needed, all motor units will be firing at their maximum frequencies. Tension is reduced by the reverse process: successive reduction of firing rates and dropping out of the larger units first (Milner-Brown and Stein, 1975).


     Two types of motor units are present in the muscle.

  1. The smaller, slow twitch motor units have been called tonic units. Histochemically they are the smaller units (type I) and metabolically they have fibers rich in mitochondria, are highly capillarized, and therefore, have high capacity for aerobic metabolism. Mechanically, they produce twitches with a low peak tension and a long time to peak (60 to 120 msec).

  2. The larger, fast twitch motor units are called phasic (type II) units. They have little mitochondria, are poorly capillarized, and therefore, rely on anaerobic metabolism. Their twitches have larger peak tensions in a shorter time (10 to 50 msec). Type II motor units can be subdivided in type Ila and type lib (Burke and Edgerton, 1975)

    It must be added that there have been many criteria and varying terminologies associated with the types of motor units present in any muscle. Biochemists have used metabolic or staining measures to categorize the fiber types. Biomechanics researchers have used force (twitch) measures (Milner-Brown et, al., 1973), and electrophysiologists have used electromyographic indicators (Warmolts and Engel, 1973; Milner- Brown et, al., 1975).
     Furthermore, a muscle with a higher percent- age of type II fibers reacts with shol1er electro- mechanical delay, time to peak tension and relaxation. Persons having this type of muscles, produce higher maximal speeds and a higher level of force for a cel1ain speed of contraction.


Muscle Cross-Sectional Area


     Muscle force is defined as the maximum tension produced during one contraction. This is related to the number of myosin cross-bridges, in parallel formation, which are able to interact with actin and produce tension. Each cross-bridge is a separate factor of force production. When air is present and calcium passes into the fiber, the cross-bridges stal1 a cyclic procedure of attachment to actin, tension production and relaxation. This cyclic procedure is not the same for the different types of muscles (it depends on the type of the heavy meromyosin of the cross-bridges). Scientific results also show that different types of cross-bridges produce various levels of tension. Also, even during maximum contraction, only a pal1 of the whole of cross-bridges is active.
     The maximum force that a muscle can generate is directly related to its cross-sectional area (Morris, 1949; Tricker and Tricker, 1967; Ikai and Fukunaga, 1970; Norman, 1977). Hypothesizing that the number of myofibrils of muscle fibers are not significantly different, cross-sectional area is an accurate way to foresee the maximum tension of the muscle.


Mechanical Model of the Muscle


     Three elements compose the mechanical model of the muscle influencing its mechanical behavior and effecting contraction:

  1. The contractile element

  2. The series elastic element, and

  3. The parallel elastic element

    This model could be useful in order to explain the dynamic properties of the muscle and to understand its mechanical behavior.
     The contractile element represents the muscle fibers, which are the active part of the muscle and are competent to produce tension. The parallel elastic element represents the connective tissue surrounding each muscle fiber, groups of fibers and the whole of the muscle. Furthermore, the elastic element represents the elasticity of cross- bridges (Huxley, 1974). They lengthen and respond like a spring. The series elastic element refers mainly to the tendons of the muscle which are placed "in series" with the contractile and parallel elastic elements. Finally, friction is represented on the model by a viscous piston used to explain the passive viscoelastic characteristics of muscle influenced by intracellular and extracellular fluids of muscle fibers.


Types of Muscle Contraction


     The term contraction can be thought of as the state of muscle when tension is generated across a number of actin and myosin filaments. Depending on the external load, its direction of action, and its magnitude, contraction has been given different names.

  1. Concentric Contraction refers to the situation in the muscle when the muscle shortens its length during contraction: at a joint the term describes the situation in which the net muscle movement is in the same direction as the change in joint angle. Utilizing concentric exercises, mechanical work is positive.

  2. In Eccentric Contraction muscle is lengthened while it is contracting. The net muscle movement is now in the opposite direction from the change in the joint angle. In eccentric exercises mechanical work is negative.

  3. Isometric Contraction refers to the condition where neither the muscle nor the joint angle changes. The corresponding mechanical work is zero.

   For a muscle, eccentric contraction produces the highest tension while concentric the lowest with isometric in between.

 

Force-Length Relationship of the Muscle


     This relationship refers to force production from the muscle depending on its initial length before contraction. According to this, the muscle produces the highest force when it starts contraction from its resting length and possibly with a small elongation. The key to the shape of the force-length curve is the changes of the structure of the myofibril at the sarcomere level (Gordon et. al., 1966). At resting length, there are a maximum number of cross-bridges between the filaments, and therefore, maximum tension is possible. As the muscle lengthens the filaments are pulled apart and the number of cross-bridges and the tension reduces to zero. As the muscle shortens to less than resting length there is an overlapping of the cross-bridges and an interference takes place. This results in a reduction of tension that continues until a full overlap occurs. The tension never drops to zero, but is drastically reduced by these interfering elements.
     In the human body the starting length of the muscle is effected by the joint angles. When the joint is in full extension, the extensors are shortened while the flexors are extended. Intermediate joint angles produce different muscle lengths and different force production according to force- length relationship. This has an effect on resistance training programs especially when free weights are used.
     The connective tissue that surrounds the contractile element also influences the force- length curve. It is called the parallel elastic component, and it acts much like an elastic band. When the muscle is at resting length or less, the parallel elastic component is in a slack state with no tension. As the muscle lengthens, the parallel element is no longer loose, so tension begins to build up, slowly at first, and then more rapidly. Unlike most springs, which have a linear force- length relationship, the parallel element is quite nonlinear. The passive force of the parallel element is always present, but the amount of active tension in the contractile element at any given length is under voluntary control. Thus the overall force-Iength characteristics is a function of percent of excitation.

 

Series Elastic Element and Electromechanical Delay


     The relative speed of elongation of the series elastic element seems to be the most important factor for the electromechanical delay observed in the muscle. It is defined as the time lag between the onset of electromyographic activity and tension in the muscle. Other factors associated with electromechanical delay are conduction of the action potential in the t-tubulus system, release of calcium from the sarcoplasmic reticu- lum and the subsequent formation of the cross- bridges between actin and myosin filaments. These events are likely to be short when com- pared to the rate of lengthening of the series elastic element, which might be the primary cause for the value of electromechanical delay in a given muscle.
     In isometric contraction the force is generated through the action of contractile element on the series elastic element, which is stretched (Braun- wald et. al., 1967). Concentric contraction, where the load is attached to the end of the muscle, is always preceded by an isometric type of contraction with rearrangements of lengths of contractile and series elastic elements. The final movement begins when the pulling force of contractile element on the series elastic element equals, or slightly exceeds, that of the load.
     Electromechanical delay is shorter during eccentric contraction in comparison to concentric (Komi, 1973; Komi and Cavanagh, 1977). This can partially be explained by the fact that during eccentric contraction the direction of lengthening of series elastic element is the same with the action of the contractile element. The reverse is the case for concentric contraction. This is also one of the factors for greater tension production with eccentric contraction.

 

Force-Velocity Relationship of the Muscle


     This relationship refers to force production from the muscle according its speed of contraction. The tension in a muscle decreases as it shortens under load (concentric contraction) while the reverse is true in eccentric contraction (muscle lengthens under load).
     During concentric contraction, the decrease of tension as the shortening velocity increases has been attributed to two main causes:

  1. A major reason appears to be the loss in tension as the cross-bridges in the contractile element break and then reform in a shortened condition.

  2. A second cause appears to be the fluid viscosity in both the contractile element and the connective tissue.

    Such viscosity requires internal force to overcome and therefore results in a lower tendon force. There is relatively little knowledge about the details of the force-velocity curve as the muscle lengthens (eccentric contraction). Experimentally, it is somewhat more difficult to conduct experiments involving eccentric work because an external device must be available to do the work on the human muscle. The reasons given for the forces increasing as the velocity of lengthening increases are similar to those that account for the drop of tension during concentric contractions. Within the contractile element it is understood that the force required to break the cross-bridges protein links is greater than that required to hold it at its isometric length, and that this force in- creases as the rate of breaking increases. Furthermore, the viscous friction of shortening is still very much present. However, because the direction of shortening has reversed, the tendon force must now be higher in order to overcome the damping friction.

 

Specific Adaptation to Resistance Training


     Most of the studies exploring this area use isokinetic contraction. One of the unique features of isokinetic training is that the speed of movement may be controlled during the exercise. This is perhaps the most important feature of isokinetic training as related to sports training since, in most sports activities, muscular force is applied during movement at various speeds. The force-velocity relationship is shifted upward and to the right in athletes, particularly those whose muscles contain a high percentage of fast twitch fibers. In view of the fact that fiber type distribution cannot be changed through training, studies try to answer the question if the force-velocity curve can be shifted upward and to the right following isokinetic resistance training.
     The results of these studies present the following conclusions:

  1. Isokinetic training at low speeds of movement (low reps-high intensity) produces substantial increases in strength only at slow movement speeds.

  2. Isokinetic training at fast speeds of movement (i.e" 8-15 reps) produces increases in strength at all speeds of movement (at rates at and below the training speed).

  3. Isokinetic training at fast speeds of movement increases muscular endurance at fast speeds more than slow-speed training increases endurance at slow speeds of movement.

Following the above conclusions, specific needs can be tackled accordingly. However, in order to shift the entire curve, fast-speed isokinetic training must be used.

 

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