Assessing Muscle Strength

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Introduction

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Factors Determining Muscle Strength

Type of Muscle Contraction

Muscle contraction is the activation of tension-generating sites within the muscle cells, and are defined by changes in the length of the muscle during contraction.

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: concentric and eccentric contractions.
   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.

Length of Muscle

The length of a muscle is an important factor in governing the force and tension it can generate.

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

The speed of muscle contraction is dependent on how quickly myosin’s ATPase hydrolyses ATP to produce cross-bridge action. Fast fibres hydrolyse ATP approximately twice as quickly as slow fibres, resulting in much quicker cross-bridge cycling, which pulls the thin filaments toward the center of the sarcomeres at a faster rate.

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 with the amount of force generated based on the number of motor units recruited and the firing frequency of the motor units.

1. 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.
2. 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.
   • 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

There are three types of muscle fibres, which can be classified based on how fast the fibres contract relative to other fibres and how the fibres regenerate adenosine triphosphate (ATP), which is muscles source of energy. Each muscle will contains a mix of these muscle fibres with wide variations in their exact composition, based on genetics and the main function of muscle, which can be influenced by training. People who do well at endurance sports tend to have a higher number of slow-twitch fibres, whereas people who are better at sprint events tend to have higher numbers of fast-twitch muscle fibres. ​​

1. Type I Slow Twitch Oxidative
   • Slow oxidative fibres contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP.
   • They produce low power contractions over long periods and are slow to fatigue.
   • High aerobic capacity, efficient at working isometrically.
   • Generally Deep, Penniform, Cross One Joint - Extension, Abduction, External Rotation
2. Type IIa Fast Twitch Oxidative-Glycolytic
   • Fast oxidative fibres have fast contractions and primarily use aerobic respiration.
   • Respond quicker than Type I but fatigue more quickly because they may switch to anaerobic respiration (glycolysis).
3. Type IIb Fast Twitch Glycolytic
   • Fast glycolytic fibres have fast contractions and primarily use anaerobic glycolysis.
   • Quickest response but fatigue rapidly and have a relatively slow recovery rate.
   • Generally Superficial, Parallel - Flexion, Adduction, Internal Rotation.

Cross Sectional Area of Muscle

The greater the cross sectional area of the muscle the greater the force a muscle can produce. The cross sectional area is proportional to the force that is produces with force 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 and have a greater cross sectional area.

Integrity of Connective Tissue

Pain

Inflammation

Disease Process

Age

Aging effects all body organs and systems in the skeletal muscle. As we age our muscles undergo progressive changes, primarily involving loss of muscle mass and strength. Muscle mass decreases approximately 3–8% per decade after the age of 30 and this rate of decline is even higher after the age of 60. Total number of muscle fibres reduces with age, beginning at about 25 years and progressing at an accelerated rate thereafter, with reduced muscle cross-sectional area and as results reduced muscle power. There is also a decrease in the number of functional motor units associated with enlargement of the cross sectional area of the remaining units in the aging motor unit. Overall these changes in the muscle mass, muscle fibre and cross sectional area of the muscle during the aging process is important clinically as it reduces muscle strength.

Fitness

Psychological Factors

Neural Factors

Measuring Muscle Strength

Manual Muscle Testing

Manual muscle testing helps to determine the extent and degree of muscular weakness resulting from disease, injury or disuse to provide a basis for planning therapeutic procedures and periodic re-testing.

It is a procedure used to evaluate the strength of an individual muscle or muscle group, based on the performance of a movement in relation to the forces of gravity or manual resistance through the available range of motion.

Medical Research Council Scale (Oxford Scale)
0	No Contraction
1	Flickering Contraction
2	Full Range of Motion with Gravity Eliminated
3	Full Range of Motion Against Gravity
4	Full Range of Motion Against Gravity with Minimal Resistance
5	Full Range of Motion Against Gravity with Maximal Resistance

Dynamometry

Dynamometry is a more precise and objective measurement of the force that a muscle can exert and can allow for comparison across extremities or as a measure of progress in strengthening during rehabilitation.

Principles of Measuring Muscle Strength

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Positioning

Stabilisation

Demonstration

Application of Resistance

Application of Grades

Comparison (if applicable)

Objectivity

Documentation

Dynamometry

Clinical Significance

Resources

References