Muscle – Motor Units Learning Objective: To be able to explain how muscle fibres are recruited in groups according to the movement required.

Single Muscle Contraction (muscle twitch) The period that a muscle contraction is initiated is known as a latent period. This occurs as the result of an electrical impulse being sent down the neuromuscular junction into the muscle.

Wave Summation Partial relaxation TIME MUSCLE TENSION Another stimulus is applied before the motor unit (muscle) relaxes completely, resulting in more tension. These stimuli can be added together to produce a more powerful contraction.

Tetanus When stimuli are at a high frequency there is no time for relaxation between impulses so muscle tension is maintained. It is through tetanic contraction that we can maintain the continuous muscle tension in our core for good posture

Explain wave summation. (6)

Motor Units A motor unit consists of a motor neurone and all of the fibres that are supplied by that motor unit. A neurone does not activate a single muscle fibre, but a group of muscle fibres (the ‘all or none law’). All the muscle fibres in a motor unit will be the same type (e.g Type IIa). Therefore it is more accurate to talk of fast and slow twitch motor units rather than muscle fibres. The number of muscle fibres supplied by a neurone in a motor unit is dependent on the degree of control required by the muscle. In muscles that require a high degree of fine control there may be as few as 5 muscle fibres supplied by the neurone e.g. eyelids. In large muscles that do not require a high degree of control there may be 1000 muscle fibres supplied by a neurone in a motor unit e.g. the hamstrings

Motor Units Motor units vary in size according to how many muscle fibres are served by the neurone in that unit. A larger motor unit, with more fibres, will create a greater force than a smaller motor unit, with less fibres.

Multiple Unit Summation The response of a motor unit follows the ‘all or nothing law’. However, the strength of the response of the whole muscle is determined by the number of motor units involved. For greater force the brain recruits more motor units. This is called multiple unit summation.

Strength of Contraction – Multiple Unit Summation The strength of contraction performed by a muscle is dependent on how many of its motor units it recruits. The diagram below demonstrates a muscle in latent state as no motor units are being recruited, building up to all of the motor units within the muscle being recruited to create maximal force of contraction.

Spacial Summation The recruitment of motor units within a muscle is rotated to avoid fatigue. TIME MUSCLE TENSION Some motor units are used (BLUE), while the other remain unused (GREEN & RED). Motor units are rotated so different units (GREEN) take over the work. Those previously working (BLUE) can rest while others remain unused (RED). Motor units are rotated again (RED). Those previously working (GREEN & BLUE) rest. A whole muscle (e.g anterior deltoid) with many motor units. Motor units vary in size according to the number of fibers attached to the motor neurone. (The different colours shown are only to illustrate “special summation” below)

Motor Unit (Fibre) Types All of the fibres within a motor unit are of the same type (e.g type 1, 2a or 2x). A fast and slow motor unit of the same size will produce different contractions. TIME FORCE Less force shown by lower peak, and slower speed of contraction shown by a more gradual rise following impulse. TIME FORCE Greater force shown by higher peak, and faster speed of contraction shown by a more rapid rise following impulse. A slow twitch motor unit produces less force than a fast twitch motor unit the same size. The slow twitch is also slower to produce the force.

How is our body able to create optimal force for the required skill? How do you know how much force is required for a 30m pass in football? Long term memory – you might have got it wrong first time (and many more times) but you store that experience and each time you perform the skill again you refer to past experience. What about during a contraction? Where do we get feedback about the contraction being performed? Muscle spindles detect change in muscle length over time and, therefore, speed of contraction. This information is passed to the brain for it to alter the signals being sent if necessary.

An example…. Muscle Spindles detect length of muscle changing over time, realise it isn’t produce enough force. This message is relayed to the brain. The brain alters the message being sent to increase force by either: Increase the number of units being recruited. Recruit larger motor units. Recruit motor units of a different (fibre) type.

Using Newton’s Laws of Motion, explain how a player moves towards a ball during a rally and outline how the force of muscle contraction can be varied to ensure that skills are executed correctly. O. Size of motor units recruited/larger for greater contraction/smaller for less powerful P. Multiple unit summation Q. Number of units recruited/more or less R. Type of muscle fibre size determines force of contraction/fast twitch fibres rather than slow twitch fibres for more powerful contractions S. All or none law/All or nothing law/or explanation/action potential or threshold is reached / contraction occurs in all of the fibres in the motor unit T. Wave summation U. frequency of impulse/innervations V. Motor unit (muscle) unable to relax/increase the force W. Tetanus/tetanic for powerful contraction X. Muscle spindles detect changes in muscle length/speed of contraction Y. Send information to brain/CNS Z. Compares information to long term memory to ensure correct force applied/past experiences AA.Spatial summation BB.rotating the frequency of the impulse to motor units to delay fatigue/some work while others rest or equiv.

Fibre Types - Revision Name Speed of contraction Force of contraction Fatigue Structure

Fibre Types - Revision Name Type I Type IIa Type IIx Fast Oxidative Glycolytic (FOG) Fast Glycolytic (FG) Speed of contraction Slow Fast Fastest Force of contraction Less High Highest Fatigue Slow to fatigue Quick to fatigue Quickest to fatigue Structure Smaller in size, more blood capillaries for carrying oxygen. Bigger in size – more myosin filaments which are also thicker. More developed sarcoplasmic reticulum – quicker release of calcium. Different myosin ATPase – releases energy quicker for the powerstroke.