Skeletal muscle structure and function
2.3 Muscle fibre types
and myosin remains bound to actin until the next supply of ATP binds to myosin. Upon ATP binding, myosin then becomes released and the contraction cycle can begin again when ATPase hydrolyzes ATP.
In order for the above events to occur, it is important to appreciate that there needs to be a continual supply of ATP and Ca2+. Depending on the particular contractile conditions (e.g. intensity and duration), the continual production of ATP can be provided from high-energy phosphates and/or the metabolism of carbohydrate, fat or protein, as introduced in Chapter 1. Part 2 will outline the basic biochemical pathways involved in these processes and, in Part 3 we will examine the regu- lation of these pathways during different types of sports and exercise.
Nerve impulse arrives at axon terminal of motor neuron and triggers release of acetylcholine (ACh).
ACh diffuses across synaptic cleft, binds to its receptors in the motor end plate, and triggers a muscle action potential (AP).
Transverse tubule
Muscle AP travelling along transverse tubule opens Ca2+
release channels in the sarcoplasmic reticulum (SR) membrane, which allows calcium ions to flood into the sarcoplasm.
Ca2+ binds to troponin on the thin filament, exposing the binding sites for myosin.
Ca2+ active transport pumps
Ca2+ release channels in SR close and Ca2+ active transport pumps use ATP to restore low level of Ca2+ in sarcoplasm
Contraction: power strokes use ATP; myosin heads bind to actin, swivel, and release;
thin filaments are pulled toward center of sarcomere.
SR
Muscle relaxes.
Troponin–tropomyosin complex slides back into position where it blocks the myosin binding sites on actin.
Ca2+
Elevated Ca2+
Muscle action potential
Acetylcholinesterase in synaptic cleft destorys ACh so another muscle action potential does not arise unless more ACh is released from motor neuron ACh receptor
Nerve impulse
1
2
3
9
8 5
6 7
4
Synaptic vesicle filled with ACh
Figure 2.8 Initiation and propagation of the action potential, excitation-contraction coupling. (From Tortora and Derrickson,Principles of Anatomy and Physiology, Twelfth Edition, 2009, reproduced by permission of John Wiley &
Sons Inc.)
Key:
= Ca 2+
Myosin heads hydrolyze ATP and become reoriented and energized 1
2
3 4
ATP
ADP
ADP
ADP Myosin heads bind to actin, forming crossbridges
Myosin crossbridges rotate toward center of the sarcomere (power stroke) As myosin heads
bind ATP, the crossbridges detach from actin
ATP
Contraction cycle continues if ATP is available and Ca2+ level in the sarcoplasm is high
P
P
Figure 2.9 The contraction-relaxation cycle. (From Tortora and Derrickson,Principles of Anatomy and Physiology, Twelfth Edition, 2009, reproduced by permission of John Wiley & Sons Inc.)
weightlifting. ATP production is mostly supported through anaerobic metabolism via the breakdown of carbohydrates or through phosphocreatine metabolism. These fibres also have a large capac- ity to hypertrophy (i.e. get bigger) during strength training programmes, via the accumulation of more myofibrillar proteins which ultimately result in greater force production.
2.3.2 Muscle fibre distribution
Most of our muscles contain a mixture of all three fibre types. In sedentary men and women, the ratio is typically around 45–55% slow twitch, with the remainder equal proportions of the type II sub-fibre types. There are instances, however, in which muscles have a greater proportion of certain fibre types, depending on their function or the individual’s training history, genetics, etc. For example, postural muscles such as those in the neck, back and legs tend to have a high proportion of slow twitch fibres due to the continual need for tension.
Given the distinct differences between muscle fibres’ structural, functional and biochemical char- acteristics, it is tempting to speculate that elite athletes have a greater proportion of specific fibre types, depending on their chosen sport. There are, indeed, numerous data sets available to confirm this hypothesis.
For example, endurance-based athletes such as elite runners and cyclists tend to have a greater percentage proportion of type I fibres, and these fibres tend to be slightly larger than type II fibres (Costillet al., 1976). Conversely, athletes involved in power-based sports, such as weightlifters and sprinters, have a greater proportion of type II fibres, which are also hypertrophied. A com- parison of the percentage of type I and type II fibres from male sprinters, distance runners and non-athletes is shown in Table 2.2.
While the variation between fibre types can be highly distinguishable between athletes from very different sporting backgrounds, it should be noted that variation also exists between fibre type propor- tions within elite athletes in a given sport (Costill et al., 1976). For this reason, fibre type alone is
2 Sarcomeres I band H zone
Thick filament Thin filament
Z disc Z disc
(a) Relaxed muscle
(b) Partially contracted muscle
(c) Maximally contracted muscle
M line Z disc
M line
Z disc Z disc
A band
I band A band I band
H zone
Figure 2.10 Sarcomere shortening during contraction. (From Tortora and Derrickson,Principles of Anatomy and Physiology, Twelfth Edition, 2009, reproduced by permission of John Wiley & Sons Inc.)
not the only determinant of sporting performance, and it is the complex interplay between a host of physiological, biochemical, biomechanical and psychological factors which underpin human per- formance (Joyner & Coyle, 2008).
2.3.3 Muscle fibre recruitment
Whereas most skeletal muscles contain a mixture of fibre types, the muscle fibres within a given motor unit are all of the same type. We have already outlined in Table 2.1 that there are more muscle fibres present in type IIa and type IIx motor units compared with type I motor units.
For this reason, when a type I α-motor neuron is activated, there are fewer absolute muscle fibres activated compared with when a type II α-motor neuron is activated. As a result, type II fibres reach peak tension quicker and collectively they generate more force than type I fibres.
When our muscles contract, the contribution of specific fibre types towards the contractile force generated is dependent on the intensity and dura- tion of the contraction. For example, if only small amounts of force are required, such as in walking or in light jogging, then slow twitch fibres will be most active. If the pace of jogging increases to higher-intensity running, then type IIa fibres will become active. Finally, if the exercises progresses to an ‘all-out’ sprint, type IIx fibres are recruited.
Muscle fibres are therefore recruited in a sequential manner (i.e. type I, type IIa and type IIx), accord- ing to the increase in exercise intensity.
This sequential recruitment of fibre types is known as the principle of orderly recruitment, which may be explained by the size principle which states that the recruitment of motor units is directly related to motor neuron size. Because the type I motor units have smaller motor neurons, it is these that are recruited first as exercise
Table 2.1 Qualitative characteristics of human muscle fibre types. Description of activity of key biochemical enzymes are based on review of data cited by Spurway & Wackerhage (2006)
Type I Type IIa Type IIx
Structural
Colour dark red red white
Type of myosin ATPase slow fast fastest
Myoglobin content large medium low
Mitochondrial density high medium low
Fibre diameter small medium large
Capillaries density high medium low
Motor neuron size small medium large
Fibres per neuron ≤300 ≥300 ≥300
Functional/contractile
Fatigue resistance high moderate low
Oxidative capacity high moderate low
Glycolytic capacity low high highest
Vmax (speed of shortening) low intermediate highest
Specific tension moderate high high
Recruitment order first second third
Biochemical
Metabolism oxidative oxidative/
glycolytic
glycolytic
Glycogen stores low medium high
Creatine kinase activity low medium high
Phosphorylase activity low medium high
Phosphofructokinase activity low medium high
Lactate dehydrogenase activity low high highest
Citrate synthase activity high medium low
Succinate dehydrogenase activity high medium low
3-hydroxyacyl dehydrogenase activity high medium low
intensity progresses from rest to low to moderate, after which type II motor units with larger motor neurons also contribute to force production (see Figure 2.11).
Much of what we know about muscle fibre recruitment patterns during exercise comes from research investigating muscle glycogen depletion patterns in different muscle fibre types. Typically, such research has demonstrated that glycogen depletion is greatest in type I fibres during low to moderate intensity exercise, whereas depletion is greatest in type IIa/x fibres during high-intensity activity (Gollnick et al., 1974).
It can therefore be concluded that type I fibres are preferentially recruited during prolonged endurance type events where absolute exercise intensity (e.g. running speed) is relatively low, whereas type II fibres are recruited more in higher- intensity activity such as fast-paced running and sprinting, etc.
In sports characterized by alternating periods of sprinting and running/jogging/walking (e.g.
soccer), it is no surprise to see glycogen depletion in all fibre types during exercise (Krustrup et al., 2006). These sports are characterized by intermittent activity profiles, and we will examine
Table 2.2 Typical muscle fibre composition in elite male athletes and non-athletes. Data taken from Costillet al. (1976)
Per cent slow Per cent fast (Type I) (Type IIa/x) Distance runners 65–75 25–35
Sprinters 20–30 70–80
Non-athletes 45–55 45–55
Exercise Intensity
Fibre Recruitment
I II A
II X
Figure 2.11 Muscle fibre recruitment as a function of exercise intensity
the bioenergetics of this type of activity in detail in Chapter 10.