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Curtin University
School of Physiotherapy

Does Muscle Structure Determine Function?

Brief Review - by Chiara von Perger

Contents

Learning Objectives

  • Understand the sliding filament theory and the crossbridge cycle.
  • Describe the different architecture of skeletal muscles and how this may affect force production.
  • Describe physiological cross-sectional area and how this relates to force production.
  • Understand fibre length to muscle length ratio and what this implies clinically.
  • Understand the process of excitation-contraction coupling.

Introduction

Skeletal muscle is a highly organised tissue with two main functions:

  1. the generation of force or tension
  2. the production of movement

The function of skeletal muscle and its relationship to its structural architecture has been the subject of study for hundreds of years. Aristotle in 384 B.C. first discovered muscles as organs of force and movement production. With the aid of electron microscopy, histochemical staining, X-ray diffraction and more lately MRI scanning, the microscopic and macroscopic properties of skeletal muscle structure, and changes to this in muscle contraction, has been more accurately revealed. However, despite centuries of research on muscles and their behaviour, some aspects of muscle force production have still not been resolved. For example, the precise mechanism of cross-bridge attachment and movement that are believed to cause relative movements of the myofilaments, and so produce force, are not clearly understood (Nigg & Herzog, 1994).

Microscopic Structure of Muscles

Skeletal muscle may be thought of in structural units of decreasing size. The entire muscle is surrounded by a connective tissue sheath called the epimysium. The next smaller structure is the muscle bundle or fascicle which consists of a number of muscle fibres surrounded by a connective tissue sheath called the perimysium. The muscle fibre is next, which is an individual muscle cell surrounded by the endomysium, a thin sheath of connective tissue which binds the individual fibres together within a fascicle. Muscle fibres are composed of smaller functional units called myofibrils. These units are approximately one micrometer in diameter and are composed of smaller subunits called sarcomeres. Muscle fibres are basically thousands of myofibrils arranged in parallel (side by side). These myofibrils are arranged in a woven rope-like formation. This has important functional implications as various myofibrils may act together rather than independently during normal muscle contraction (Lieber, 1992).

Sarcomeres

The functional unit of muscle contraction is the sarcomere. Sarcomeres are arranged in series (end to end) within the myofibril. The length of each sarcomere in its resting state is approximately 2.5 micrometres. The total number of sarcomeres within a fibre depends on the muscle fibre length and diameter. Because they are arranged in series, the total distance of myofibrillar shortening is equal to the sum of the individual shortening distances of the individual sarcomeres. Therefore although a muscle may shorten by several centimetres, the actual shortening of the sarcomere unit is a mere one micrometre (Lieber, 1992).

Myofilaments

The sarcomere is composed of contractile filaments called myofilaments. These filaments are primarily involved in the mechanical process of muscle action. The myofilaments consist mainly of two large polymers of protein actin and myosin. These proteins make up 85% of the myofibrillar complex (McArdle, Katch & Katch, 1996).

Thick filaments are typically located at the centre of the sarcomere and are primarily composed of myosin molecules. A myosin molecule consists of a long tail portion and a globular head attached to the tail. The head portion extends outward from the thick filament. It contain a binding site for actin and an enzymatic site that catalyses the hydrolysis of ATP that releases the energy needed for muscular contraction (Nigg & Herzog, 1994). At rest, the protein elastic filament titin links the M-lines of the thick filaments to the Z-bands and keeps the myosin filaments centred within the sarcomere (McArdle et al. 1996).

The thin filaments are located on either side of the Z-bands within the sarcomere. The backbone of the thin filaments is composed of two chains of serially-linked actin globules. The other main proteins that interact with the actin and myosin filaments during muscle action are tropomyosin, troponin, alpha-actinin and beta-actinin (McArdle et al. 1996). The thicker myosin filaments and the thinner actin filaments interdigitate to form an hexagonal lattice. There are various bands and zones within the structure of the sarcomere unit that correspond to the positions of these proteins. It is this interdigitating pattern that gives skeletal muscle its striated appearance.

Sarcomere length is defined as the distance from one Z-band to the next. This is an important variable in the generation of force.

Figure 1. Actin and myosin arrangement within the sarcomere (McArdle et al.1996)

Sliding Filament Theory

The sliding filament theory was first proposed nearly 40 years ago to describe the molecular movements that define muscle action. It proposes that a muscle shortens or lengthens because of overlapping of myosin and actin filaments without a change in their length. It is the action of the myosin cross-bridges which cyclically bind or attach, rotate, and detach from the actin filaments which acts as the molecular motor that drives the shortening process. The energy is provided by the hydrolysis of ATP (McArdle et al. 1996).

During the contractile process, thick and thin filaments are believed to remain at a constant length. Sarcomere, and thus fibre shortening, is associated with relative movements of the myofilaments. The relative movements of the thick and thin filaments during contraction causes shortening of the I-band of the sarcomere whereas the A-band remains at a constant length (Nigg & Herzog, 1994).

The Cross-Bridge Cycle

Myosin plays both an enzymatic and a structural role in muscle action. As described earlier, the globular head of the myosin cross-bridge contains an actin-activated ATPase in its actin binding site and provides the mechanical power for the actin and myosin filaments to slide past each other through the hydrolysis of ATP. All cross-bridges are functionally identical, act independently and the probability of attachment and detachment is only influenced by the local environmental conditions, e.g. calcium ion concentration. During muscle shortening the cross-bridges undergo many repetitive cycles of asynchronous attachment and detachment involving subsequent attachment sites on the actin filament within reach of the crossbridge (Huijing, 1992).The asynchronicity of cross-bridge formation allows for smooth, finely tuned movement to occur (McArdle et al. 1996). As the myosin interacts with the adjacent actin filament it splits a phosphate from the ATP, and releases its stored energy as it straightens. This causes the sliding motion that generates muscle tension. At any one time, only about 50% of the crossbridges are in contact with the actin filaments to form actomyosin, with its contractile properties, the other crossbridges are at some other position in their vibrating cycle. The actin and myosin filaments can slide past each other at a speed of up to 15 micrometres (McArdle et al. 1996). As the velocity of shortening of the muscle fibres increases, fewer crossbridges are able to form and thus less force can be produced. This can be seen from the force-velocity curve.

Thus it can be seen that the myosin filament is responsible for the generation of tension during muscle contraction. The actin filament on the other hand is responsible for the regulation of the tension generated (Lieber, 1992).

Motor Units

Skeletal muscle is organised into motor units. A motor unit is defined as a set of muscle fibres that are all innervated by the same motor neuron. A motor unit of a large human skeletal muscle may contain more than 2000 individual muscle fibres. When a motor neuron is stimulated strongly enough to cause a contraction of the muscle, all the fibres of the motor unit will contract. Since the force of the contraction is, to a large extent, dependent on the number of fibres activated, a large motor unit can exert more force than a small motor unit with only a few muscle fibres (Nigg & Herzog, 1994).

Muscle Architecture

Skeletal muscle is not only highly specialised and organised at a microscopic level. The arrangement of muscle fibres at the macroscopic level is also highly organised. This arrangement of muscle fibres within whole muscles is known as muscle architecture and plays an important role in determining a muscle's contractile capabilities (Lieber, 1992). This arrangement varies in relation to the long axis of the muscle. The difference of alignment of the sarcomeres plays an important role in the muscle's overall capacity to generate force (McArdle et al. 1996).

There are three main types of fibre architecture:

  1. Fusiform or parallel muscles
  2. Unipennate muscles
  3. Multipennate muscles
Figure 2. Pennation forms (McArdle et al. 1996)

Fusiform Muscles

These muscles have fibres that extend parallel to the muscle force-generatingaxis. An example of this is the biceps brachii. In a fusiform muscle the cross-sectional area, (CSA), of the fibre is representative of the true anatomical cross section. Because of the parallel orientation of the fibres, this arrangement facilitates a rapid rate of muscle

shortening.

Unipennate Muscles

These muscles have fibres which are orientated at a single angle to the force generating axis, (for example, tibialis anterior). These fibres have a larger CSA as more sarcomeres can be packed into a given volume of muscle. The angle between the fibre and the force-generating axis is termed the angle of pennation and varies between 0 and 30 degrees (Lieber, 1992).

Multipennate Muscles

These muscles are composed of fibres that are orientated at several angles to the axis of force generation and comprise the largest group of muscles (Lieber, 1992). An example would be the deltoid muscle. Because of their structure they allow for packing of larger number of fibres into a smaller CSA and are usually shorter than the fusiform muscles.

Multipennate muscles, although slower in contractile velocity than fusiform muscles, are capable of generating much greater tension. This is due to the larger number of sarcomeres that can be arranged in parallel at the expense of those arranged in series, thus enhancing the maximum force capabilities of the muscle (Roy & Edgerton, 1992). Another consequence of fibre pennation is that the range of excursion through which a pennated muscle acts at an efficient sarcomere length is greater than for a fusiform muscle.

The pennated muscle can thus utilise the length-tension relationship more effectively in the production of force than can a muscle whose fibres are arranged in parallel.

Physiological Cross-Sectional Area (PCSA)

This refers to the sum of the cross-sectional areas of all the fibres within a particular muscle. The maximum force that a muscle can generate is directly related to its physiological cross-sectional area (PCSA) (Roy & Edgerton ???). This value is rarely the same as the anatomical CSA. Under in vivo conditions the PCSA of an individual muscle or muscle group can be estimated non-invasively and with reasonable accuracy using the following formula:

PCSA = muscle volume/fibre length

Muscle volume is determined non-invasively using MRI techniques to reconstruct 3-D models of individual muscles. Since muscle density is relatively constant as is the muscle fibre to muscle length ratios, determining the actual muscle volume is a more accurate and reliable technique to determine the CSA. To estimate the fibre length we know that the length of the muscle fibre, i.e. the number of half sarcomeres in series, determines the rate of shortening of that fibre. There is some controversy though, as to how to determine the length of the individual fibres within the muscle (Lieber, 1992).

For the same reason that fibre length increases the active muscle range of the length-tension relationship, it causes an increase in the muscle's absolute maximum contraction velocity (Vmax). Again, while the fibre length increase causes an increase in these extrinsic properties, it has no effect on the intrinsic properties of the muscle (Lieber, 1992).

Fibre Length to Muscle Length Ratio

The different arrangements of fibres within a muscle influence some of the muscle's functional characteristics significantly. Thus an index of architecture was proposed to quantify the structure of a muscle. This index of architecture is defined as the ratio of muscle fibre to muscle belly length at (an assumed) optimal length of all fibres (Woittiez et al. 1984, as cited in Nigg & Herzog, 1994). The ratio of the individual fibre length to the muscle's total length usually varies between 0.2 and 0.6. This implies that even in the longest muscles, the individual fibre length is less than that of the overall muscle length. An example of this is the quadriceps and hamstring muscles of the lower limb. The average pennation angle of the quadriceps muscle is 4.6 degrees, PCSA,s of approximately 21.7 square cms and an average fibre length of 68 mm. In contrast, the biceps femoris muscle of the hamstrings has relatively long fibres (111 mm) and intermediate PCSA's (11.7 square cms).

The quadriceps muscles therefore have approximately 50% greater capacity than the hamstrings muscles to generate force, the latter being designed more for a faster shortening velocity (McArdle et al. 1996).

Figure 3. Properties of muscle architecture (Lieber 1992)

Excitation-Contraction Coupling

Skeletal muscle contracts in response to electrochemical stimuli.

Excitation-contraction coupling is the physiological mechanism whereby an electrical discharge at the muscle initiates the chemical events at the cell surface that lead to the release of intracellular calcium ions and ultimately cause a muscle action (McArdle et al. 1996).

Sequence of Events in Muscle Contraction and Relaxation

  1. An action potential of motor neuron reaches the pre-synaptic terminal and causes the release of acetylcholine (ACh) from synaptic vesicles. ACh diffuses across the synaptic cleft and attaches to ACh receptors on the sarcolemma of the muscle cell.
  2. The muscle action potential depolarises the transverse tubule at the A-I junction of the sarcomere.
  3. Depolarisation causes the release of calcium ions from the sarcoplasmic reticulum of the sarcoplasm surrounding the myofibrils.
  4. Calcium ions bind to troponin-tropomyosin in the actin filaments. This releases the inhibition that normally prevents actin from binding with myosin to form cross-bridges in the relaxed state.
  5. Cross-bridges then attach to the active sites of the thin actin filaments, and through the breakdown of ATP into ADP plus a phosphate ion, the necessary energy is provided to cause the cross-bridge head to move and so pull the thin filaments past the thick filaments causing muscle shortening.
  6. At the end of the cross-bridge movement, an ATP molecule attaches to the myosin portion of the cross-bridge thus releasing the myosin from its attachment site to go back to its original configuration, and be ready for a new cycle of attachment.
  7. Cross-bridge activation continues as long as the muscle fibre is stimulated.
  8. When stimulation stops, calcium ions are actively transported back into the sarcoplasmic reticulum resulting in a decrease in calcium ions in the sarcoplasm.
  9. The removal of calcium ions restores the inhibitory action of the troponin-tropomyosin and cross-bridge action is not possible in this state (McArdle et al. 1996, Nigg and Herzog, 1994).

Clinical Implications

Fusiform muscles with their longer fibres have a larger range of active force production, a higher contractile velocity and a lower maximal force output compared to pennated muscles with their shorter fibres. This is because for a given change in muscle length, the individual sarcomeres lengthen less, with the change in muscle length being distributed over more sarcomeres (Lieber, 1992).

Quadriceps and plantarflexor muscles are designed for high force production because of their low fibre length (FL) to muscle length (ML) ratios and relatively large cross-sectional areas (Lieber, 1992).

Hamstring and dorsiflexor muscles are designed for high contractile velocity due to their relatively high fibre length to muscle length ratio and long muscle fibres (Lieber, 1992).

Hamstring muscles are more susceptible to tearing if a rapid imbalance in force output between the quadriceps and the hamstrings should occur during, for example, forward sprint running (McArdle et al. 1996).

Conclusion

The myosin filament is responsible for the generation of tension during muscle contraction.

The actin filament on the other hand is responsible for the regulation of the tension generated (Lieber, 1992).

In summarising the important points relating to the architecture of skeletal muscle:

  • Muscle force is directly proportional to the physiological cross-sectional area (PCSA).
  • Muscle velocity is proportional to muscle fibre length.

It is implicit in this statement that the total excursion (active range) of the muscle is also proportional to the muscle fibre length (Lieber, 1992).

Fusiform muscles with their longer fibres have a larger range of active force production, a higher contractile velocity and a lower maximal force output compared to a pennated muscles with their shorter fibres.

Multipennate muscles, although slower in contractile velocity than fusiform muscles, are capable of generating much greater tension. This is due to the large numbers of sarcomeres that can be arranged in parallel and therefore increased fibre packing. Also the range of excursion through which a pennated muscle acts at an efficient sarcomere length is greater than for a fusiform muscle.

The pennated muscle can utilise the length-tension relationship more effectively in the production of force than can a muscle whose fibres are arranged in parallel.

References

Huijing PA (1992)
Chapter 6C, Mechanical muscle models. In Komi PV (Ed): Strength and Power in Sport.
Lieber R (1992)
Skeletal Muscle and Function: Implications for Rehabilitation and Sports Medicine. Baltimore: Williams and Wilkins.
McArdle WD, Katch FI and Katch VL (1996)
Exercise Physiology: Energy, Nutrition and Human Performance. (4th ed.). Philadelphia: Lea and Febiger.
Nigg BM and Herzog W (Eds) (1994)
Biomechanics of the Musculo-skeletal System. West Sussex: John Wiley & Sons Ltd.
Roy RR, and Edgerton VR (1992)
Chapter 6B, Skeletal muscle architecture and performance. In Komi PV (Ed): Strength and Power in Sport. Oxford.

Short Answer Review Questions

  1. Describe the microscopic structure of the muscle.
  2. Describe the sliding filament theory and how this relates to the cross-bridge cycle.
  3. What is muscle architecture? How is this related to force production and contractile velocity?
  4. What is the PCSA? How does this relate to the index of architecture (i.e. muscle length to fibre length ratio)?
  5. Draw a schematic length-tension relationship for muscles with (i) short fibres and large PCSA's and for (ii) long fibres and small PCSA's.
  6. Describe the process of excitation-contraction coupling.

Exercise Physiology Educational Resources 1997