A finer and more convincing proof is found when we record by graphic means, the contraction of a muscle or its non-extensible tendon, when the muscle itself is moving the part, and when the same part is being moved by an outside agency.
The following figures are records of the contraction of the third finger tendon of the extensor communis digitorum[1], the muscle responsible for all finger-lift in the hand-knuckle.
The method of recording is shown in Figure 29. A long aluminum lever, with an adjustable setting is so placed that its edge rests upon the tendon,
the contraction of which is to be recorded.
Figure 29: Instrument for recording contractions of finger-extensors.
Its position in any other plane of movement is fixed by its own axis and the two points resting upon the hand near the tendon.
Any contraction of the tendon will then lift the lever, and by making this sufficiently long, and placing the fulcrum sufficiently close
to the tendon, any degree of magnification may be secured, thus recording even extremely slight degrees of contraction. These
are then transferred upon the smoked surface of a revolving drum, passing beneath the free end of the lever .
Figure 30: Active and voluntary muscular contraction
The results are seen in Figure 30. In the upper line, the finger was lifted by another person, in the lower, the subject lifted his own finger by voluntary muscular contraction. An ascent in the line indicates a pull on the tendon resulting from the contraction of the muscle, the height of the ascent furnishing a fair index of the degree of contraction. In the passive movements there is complete absence of contraction. In the active movements the marked contraction for each of three successive finger-lifts is clearly shown. The number, extent, and speed of movement were exactly the same for both experiments.
There are several medical instruments available today to measure muscle contraction, each suited for different types of analysis, from electrical activity to mechanical force output. Below are some of the most common types:
Electromyography (EMG)
Function: Measures the electrical activity generated by muscle contractions.
Types:
Surface EMG (sEMG) – Electrodes are placed on the skin to measure muscle activity non-invasively.
Intramuscular EMG – Needle electrodes are inserted into the muscle for more precise measurements.
Function: Measures the force output of a muscle during contraction.
Types:
Handheld Dynamometers – Measure grip strength or limb muscle strength.
Isokinetic Dynamometers (e.g., Biodex) – Measure torque and force production at controlled speeds.
Applications:
Strength assessment in rehabilitation and physical therapy.
Evaluating muscle imbalances.
Used in sports performance testing.
Mechanomyography (MMG)
Function: Measures muscle vibrations produced during contraction.
How It Works: Uses accelerometers or laser sensors to detect mechanical oscillations of contracting muscle fibers.
Applications:
Research in muscle mechanics and fatigue.
Used as an alternative to EMG in prosthetics and robotics.
Tensiomyography (TMG)
Function: Measures muscle displacement during contraction.
How It Works: A sensor is placed on the skin, and an electrical stimulus is applied to trigger a contraction. The device records the muscle’s response time and contraction speed.
Applications:
Assessing muscle performance and injury risk.
Used in sports science and rehabilitation.
Electroneurography (ENG)
Function: Measures the nerve conduction velocity (NCV), which indirectly reflects muscle contraction ability.
How It Works: Electrodes stimulate nerves, and the resulting signal is analyzed to assess neuromuscular function.
Applications:
Diagnosing nerve damage and neuromuscular diseases.
Ultrasound Elastography
Function: Uses ultrasound to assess muscle stiffness and contraction in real-time.
Applications:
Diagnosing muscle injuries.
Evaluating muscle activation patterns in rehabilitation.
Conclusion
Each instrument provides different insights into muscle contraction:
EMG and MMG analyze electrical and mechanical activity.
Dynamometers and TMG measure force production.
Myotonometry and Ultrasound Elastography assess muscle stiffness.
ENG evaluates nerve-related muscle function.
The best choice depends on the specific clinical or research application.
[1]extensor communis: The extensor digitorum muscle (also known as extensor digitorum communis) is a muscle of the posterior forearm present in humans and other animals. It extends the medial four digits of the hand.
