Assessing Muscle Strength

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Introduction

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Factors Determining Muscle Strength

Type of Muscle Contraction

The three main types of muscle contraction are:

1. Isometric Contraction
   • Isometric contractions produce a static contraction with a variable and accommodating resistance without producing any appreciable change in muscle length.
   • Tension is being generated in the muscle but the distance between the muscle attachments remains the same. In an isometric contraction the cross bridges form, disengage and reform.
   • There is no movement and no external work is done by the muscle.
2. Isotonic Contraction
   • In an isotonic contraction, tension remains the same, whilst the muscle's length changes. There are two types of isotonic contractions: (1) concentric and (2) eccentric.

1. 1. Concentric Contraction
      • A concentric contraction produce a shortening of the muscle such that the origin and insertion of the muscle move closer together. A muscle performs a concentric contraction when it lifts a load or weight that is less than the maximum tetanic tension it can generate.
      • Cross bridges form and the myosin head swings towards the M-line. This pulls the Z-lines together and the overlap between the actin and myosin filaments increases as the sarcomere shortens. Inertia and slack taken up first on tendons. Energy costs are high as cross bridges have to recycle rapidly resulting in heat production. Actin binding sites are also moving past the myosin cross bridges and it takes a certain amount of time for cross bridges to attach to, and detach from the actin binding sites. Therefore the number of attached cross bridges and thus force generation is less than during an isometric contraction.
      • The muscle shortens, movement occurs and external work is done.
   2. Eccentric Contraction
      • An eccentric contraction occurs when a muscle lengthens as it gives in to an external force that is greater than the contractile force it is exerting. In reality, the muscle does not lengthen; it merely returns from its shortened position to its normal length.
      • During eccentric work cross bridges form and the myosin head swings towards the Z line, which is opposite to what happens during a concentric contraction. The overlap between the actin and myosin decreases and the sarcomere increases in length. Muscle is actively lengthening, in the direction of the force of gravity. The extrinsic forces acting on the muscle are greater than those generated intrinsically.
      • The muscle lengthens, movement occurs and external work is done.

Integrity of Connective Tissue

Pain

Inflammation

Disease Process

Length of Muscle

The length of a muscle is an important factor in governing the force and tension it can generate. When a muscle starts from a fully stretched position and contracts to its shortest length it has worked through FULL RANGE. This is divided into three parts;

1. Muscle Working in Outer Range:
   • Muscle is working in a maximally stretched position (moves between the longest length and the mid-point of range).
   • Actin and myosin have least overlap, fewer cross bridges can form and so less tension is produced.
2. Muscle Working in Inner Range:
   • Muscle is working in a maximally shortened position (moves between the shortest length and the mid-point of range).
   • Actin and Myosin overlap, which decreases the number of sites available for cross bridge formation and therefore less force is generated
3. Muscle Working in Mid Range:
   • Muscle is working between between the mid point of the outer range and the midpoint of the inner range (muscle changes length from the middle positions of outer and middle ranges).
   • Actin and Myosin filaments have optimal overlap, which provides the optimum number of sites for cross bridge formation, and therefore maximum tension is generated in this range.

Speed of Contraction

Differs for Concentric and Eccentric Work. The higher the velocity of shortening the shorter the time for myosin cross bridges to attach to actin. The number of cross bridges that attach decreases as velocity increases.

Concentric - Increased Velocity = Decreased Cross Bridges = Decreased Force

Eccentric = Increased Velocity = Increased Force

• Number and Size of Motor Units Activated

Each muscle fibres generates a force so small as to be impractical for even the most delicate movement. Therefore the system is designed such that a group of muscle fibres share common innervation from a single alpha motor neuron.

• Motor Unit Recruitment - Hennemann Size Principle

The force generated can be increased by activating more motor units and this is termed motor unit recruitment. The smaller motor units have the most excitable motor neurones and therefore are recruited first. As more force is required the larger and progressively less excitable motor neurones are recruited in an orderly fashion. This has become known as the Hennemann Size Principle.

• Rate Coding - Firing Frequency of Motor Units

Rate Coding is a term used to describe the firing frequency or discharge rates of motor units. The force of active motor units can also be varied by the frequency of stimulation of the motor neurone and by utilising the force frequency characteristics. Motor units alter firing rates.

The fibres belonging to a motor unit may be scattered throughout a muscle thus adjacent muscle fibres are unlikely to belong to the same motor unit. A muscle fibre will contract when the nerve stimulus is of sufficient intensity to release calcium. This is called threshold stimulus.

• If there are less than 30 twitches per second then the fibre will contract but the cross bridges will detach before the next stimulus. This allows the muscle to   relax back to its former length.
• However if the frequency of successive impulses is greater than 30 each impulse will produce the twitch of the fibre before there is time to relax. This   means there is a progressive shortening of the fibre causing an increase in tension between the muscle attachments so a stronger contraction is produced.   This is called a tetanic contraction

• Muscle Fibre Type

Each muscle will contain all three types of muscle fibres but there are wide variations in their exact composition. Genetics determines some of this, main function of muscle will also determine this. ​​Colour differences exist because of the presence of myoglobin which is red

Type I Slow Twitch Oxidative

Type IIa Fast Twitch Oxidative-Glycolytic

Type IIb Fast Twitch Glycolytic

Type I;

Able to carry out sustained activity and so are called stabilising or tonic muscle, Have a high aerobic capacity, efficient at working isometrically. Generally Deep, Penniform, Cross One Joint - Extension, Abduction, External Rotation

Type II

Tend to be involved with mobility - also called phasic, produce large ranges of motion. They respond quickly but fatigue rapidly and have a relatively slow recovery rate. Generally Superficial, Parallel - Flexion, Adduction, Internal Rotation.

ON average, most fibres are composed of roughly 50% slow twitch and 25% FTa Fibres.

• Fast twitch fibres have more energy available and quicker for muscle contraction due to the  different forms of myosin ATPase.
• Fast twitch fibres have a more highly developed sarcoplasmic reticulum than do ST fibres. And are more adept at delivering calcium into the muscle cell

ST Small Motor Neruon  10 -180 Muscle Fibres

FT Large Cell Body  300 - 800 Muscle Fibres

The difference in force development is due to this number of fibres per motor unit not the force generated by each fibre.

• Slow twitch fibres have a high level of aerobic endurance very efficient at producing ATP from the oxidation of carbohydrate and fat
• Fast twitch fibres have relatively poor aerobic endurance, better suited to perform anaerobically than the slow twitch fibres. ATP is formed through an anaerobic pathway

• Cross Sectional Area of Muscle

The greater the CSA the greater the force a muscle can produce. The CSA is proportional to the force that is produces. Force is independent of fibre length. The only forces transmitted though the muscle attachments are those generated by the sarcomeres at the end of the muscle. In a pennate structure more fibres can be packed in and so they tend to be stronger/greater CSA.

• Age and Fitness of Subject
• Psychological Factors
• Neural Factors

Manual Muscle Testing

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Test Scales

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Oxford Scale

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Kendall Scale

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Daniels and Worthing

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Test Procedures

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Break Test

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Active Resistance

Dynamometry

Clinical Significance

Resources

References