Slow (type 1) and Fast Twitch (Type 2 a and b) Types of muscle fibres Slow (type 1) and Fast Twitch (Type 2 a and b)

Muscle Fibres Skeletal muscle fibres are not all uniform, in fact they can differ in both structure and function. A single muscle such as the bicep brachi will be composed of two principal types of muscle fibres: slow twitch (type 1) and fast twitch (type 2). Slow twitch fibres contract more slowly but are highly resistant to fatigue and are therefore favoured by endurance athletes, whilst fast twitch fibres can contract more rapidly, generating greater forces but are moreliable to fatigue. These fibres are more prevalent in sprinters and power athletes. Fast twitch fibres have been further classified into type a and type b. Type 2a fibres, also known as fast oxidative glycolytic fibres (FOG) are more resistant to fatigue than type 2b fibres which have a greater anaerobic capacity and are termed fast twitch glycolytic fibres (FTG).

STRUCTURAL AND FUNCTIONAL CHARACTERISTICS OF SLOW AND FAST TWITCH TYPE FIBRES Slow twitch (type 1) Fast oxidative glycolytic (FOG) (type 2a) Fast twitch glycolytic (FTG) (type 2b) Speed of contraction (ms) Slow (110) Low Fast (50) Force of contraction Low High Highest Resistance to fatigue Very high Moderate Aerobic capacity Anaerobic capacity Structural Characteristic Fibre Size Small Large Mitochondrial density Capillary density Myoglobin content PC store Glycogen store Triglyceride store Motor neuron size Activity suited Marathon 1500 M Shot Put

KEY TERMS Slow twitch muscle fibre: a type of muscle fibre that uses oxygen to produce energy (high oxidative capacity). They are associated with endurance-based activities Fast twitch muscle fibre: a type of muscle fibre that has a high glycolytic capacity (anaerobic). They are associated with speed and power- based activities

Sports for different fibres

Athletes V Non Athletes

Muscle fibre recruitment The primary function of skeletal muscle is to contract and facilitate movement of the body. Muscle contraction involves the interaction of the muscles with the nervous system. Individual muscles such as the anterior deltoid are connected to the nervous system via a group of motor neurons. Each muscle fibre within the muscle belly is supplied by only one motor neuron, however this neuron can innervate (stimulate) anything from just a few fibres to several hundred. The motor neuron plus the fibres it innervates is known as the motor unit.

The motor unit The motor unit is the basic functional unit of skeletal muscle. Stimulation of one motor neuron causes all the muscle fibres in that motor unit to contract simultaneously. Each individual muscle will be made up of a number of motor units (just like a school is made up of a number of form or tutor groups). The number of motor units that are recruited at anyone time in the muscle varies with the amount of strength required for a given movement. The more strength needed the greater the number of motor units activated. The number of fibres within a particular motor unit is dependent upon the control of movement required in that muscle. A small muscle that is required to perform fine motor control suchas those that enable the eye to focus may only have one fibre per motor neuron; whereas large muscles responsible for gross movements such as the quadriceps group when kicking a ball may be innervated by a motor neuron supplying 500 or more fibres. Motor units are usually made up of the same type of muscle fibre. Consequently we see both fast and slow twitch motor units in a muscle. Fast twitch motor units are generally recruited during high intensity activity such as sprinting or throwing the javelin whilst slow twitch motor units are used during lower intensity exercise such as running a half marathon or cross country skiing.

Innervation The innervation of skeletal muscle is accomplished by a motor neuron transmitting a nerve impulse or action potential to the muscle fibre. Just how the muscle fibre responds is governed by the ‘all or none law’. The all or none law The all or none principle essentially states that individual muscle fibres within a motor unit contract either fully or not at all. In other words individual muscle fibres cannot partially contract! In order to activate these muscle fibres however a minimum amount of stimulation is needed (termed the threshold). If the stimulation equals or exceeds the threshold all the fibres within the motor unit will contract at the same time and to their maximum possible extent. If the stimulus falls short of the threshold, however, the muscle fibres do not respond and muscular contraction fails to occur.

Spatial Summation For a muscle to contract the excitatory postsynaptic potential (EPSP) must be of a certain level of intensity to initiate the sliding filament mechanism of muscle contraction. Spatial Summation describes the progressive increase in size of the excitatory postsynaptic potential (EPSP) as a result of the arrival of a number of impulses at the synaptic cleft of individual muscle fibres Basically an increase in responsiveness of a nerve resulting from the additive effect of numerous stimuli. A certain level of intensity is needed before a muscle fibre responds by contracting

Innervation of a muscle fibre

Role of Aceytlcholine Acetylcholine is a chemical substance that allows the transmission of an impulse across the synaptic cleft and enables the muscle to contract.

How does it affect muscle contraction On arrival at the neuromuscular junction the transmitter substance acetylcholine is released which enables the impulse to cross the synaptic cleft and create electrical potentials (EPSPs) in the muscle fibre (the potential to contract). However, if the EPSP is not of sufficient intensity the muscle fibre will not contract, consequently the additive effect of a number of stimuli arriving can be used which ensures that the excitatory threshold is reached and the muscle fibres contract – all or nothing!

Variations of muscle strength Sporting performance requires variations in strength or muscular force from very weak efforts such as a short putt in golf to all out maximal efforts such as a shot put. How then does the body cope with these different requirements? The strength of a muscle can be graded in several ways: Multiple unit summation Wave summation Synchronicity of motor unit stimulation.

Multiple unit summation The strength of a muscle contraction can be increased by recruiting more motor units Maximal contractions will recruit all motor units within a particular muscle whilst weaker contractions will recruit fewer units Fast twitch motor units will be recruited ahead of slow twitch units for more powerful contractions

Wave Summation Wave summation considers the frequency with which impulses arrive at the motor unit. Typically the motor unit will respond to an impulse (innervation) by giving a twitch-a very short period of contraction followed by relaxation. If a second impulse arrives at the motor unit before it had time to completely relax from the first twitch the motor unit responds with stronger contractions since the effect of the second stimulus is added to the first. They summate creating greater tension within the motor unit. When a motor unit is stimulated many times in quick succession there is little or no time relaxation. This produces the highest level of sustained tension referred to as tetanus or tetanic contraction and will continue until fatigue ensues.

VArying the strength of muscle contraction with wave summation

Neuromuscular adadptations to resistance training Recruitment of more motor units – More motor units may be trained to act synchronously (together) so that greater forces can be generated therefore resulting in greater strength gains. Muscle hypertrophy – The size of the muscle belly will increase due to an increase in the size of individual muscle fibres (hypertrophy) and the possible splitting of fibres (hyperplasia). Hypertrophy of fast twitch muscle fibres – As fast twitch fibres are predominantly recruited during resistance training, these fibres in particular will enlarge. Hyperplasia of fast twitch muscle fibres – There is some evidence to suggest that muscle fibres split, particularly with heavy resistance training. This splitting will contribute to the general hypertrophy of the muscle. Conversion of type 2b fibres to type 2a fibres – Some studies have shown that the percentage of type 2b fibres within a trained muscle actually decrease in favour of type 2a fibres. This could account for the delay in muscular fatigue associated with prolonged training.

Recap A motor neuron together with the fibres it innervates is known as the motor unit. The motor unit is the basic functional unit of skeletal muscle. When a motor unit is called upon to contract all the muscle fibres under its control will contract simultaneously. The all or none law is the principle that explains that individual muscle fibres within a motor unit will either contract fully or not at all. In order to cause the muscle fibres within the motor unit to contract fully the level of stimulation must reach a certain ‘critical’ threshold. A muscle can vary the strength of its contractile response through multiple unit summation and wave summation. There are two principal types of muscle fibre, each having a different structure and function. Slow twitch fibres (type 1) contract more slowly and are highly resistant to fatigue, so are favoured more by endurance athletes who use the aerobic system to supply the majority of their energy during the exercise period. Fast twitch fibres (Type 2) can contract more rapidly and generate greater forces so occur more widely in sprinters and power athletes.

RECAP Type 2a fibres (fast twitch oxidative) have a slightly better resistance to fatigue than type 2b fibres and so are prominent in athletes who use the lactic acid pathway to supply energy such as a 400m hurdler. Type 2b fibres (Fast twitch glycolytic) have the highest anaerobic capacity. Although they produce the greatest forces they fatigue the most rapidly. Type 2b fibres are most widely found in athletes who predominantly use the ATP-PC system during the exercise period. There are a number of neuro-muscular adaptations to resistance training which include: Muscle hypertrophy, hypertrophy of fast twitch muscle fibres, hyperplasia of fast twitch muscle fibres, recruitment of more motor units and possible conversion of type 2b fibres to type 2a fibres.

Homework 1. Name the three types of muscle fibre and give a sporting example where each of these fibres prevail. 2. Give two structural and two functional characteristics of each. 3. Explain the role of motor units in controlling the strength of muscular contractions in sporting activity. 4. Name a muscle of the body that will contain motor units composed of several hundred fibres and a muscle which will contain motor units of relatively few fibres. 5. What types of training would cause hypertrophy (enlargement) of a) slow twitch fibres b) fast twitch (type 2b) fibres? 6. Outline the stages of the sliding filament process of muscle contraction. 7. Explain how the strength of muscle contraction can be varied in relation to a high jumper and a distance runner. 8. Give three neuromuscular adaptations to training.