Causes of Muscle Imbalance and Assessment Methods
Disruption of normal muscle balance in any body segment often serves as the starting point for the development of persistent deformities. There are two main groups of causes for such disorders:
- Neurogenic disorders: consequences of poliomyelitis, cerebral palsy (CP), spinal cord or peripheral nerve injuries.
- Prolonged functional inactivity: occurs due to extended bed rest, plaster cast immobilization, and other conditions that limit activity.
Two approaches are used to assess muscle strength. The objective method involves using a mechanical dynamometer. However, manual muscle testing (MMT) scoring is the most common in practice, where the physician manually determines the patient's ability to overcome applied resistance.
Motion Testing as a Functional Indicator
Muscle strength is inextricably linked to joint mobility. Movements are divided into:
- Active: performed independently by the patient.
- Passive: performed by the examiner while the patient's muscles are relaxed.
It is the passive range of motion that serves as an indicator of the true range of motion. All measurements are taken from a strictly defined initial position ("zero") for each specific joint. During the examination, it is important to avoid sudden movements—forceful manipulations are not only hazardous to the patient but also provoke muscle tension, preventing an objective assessment.
Goniometry Rules and Goniometer Placement Points
Range of motion is measured using a specialized goniometer. The arms of the instrument are always set strictly along the axes of the limb segments, and its fulcrum (center) is placed over the axis of motion in the joint.
Key points for fulcrum placement:
- Shoulder joint: projection of the humeral head.
- Elbow joint: 1 cm below the lateral humeral epicondyle.
- Wrist joint (flexion/extension): projection of the radial styloid process.
- Knee joint: projection of the outer joint line (corresponds to the level of the lower pole of the patella).
- Ankle joint: projection of the medial malleolus.
- Hip joint: level of the greater trochanter.
Planes of Motion and Special Terminology
Evaluating muscle function requires a precise understanding of types of movement:
- In the frontal plane: standard abduction (abductio) and adduction (adductio). The exception is the wrist joint, which features ulnar and radial deviation.
- In the sagittal plane: flexion (flexio) and extension (extensio). In the ankle joint, the terms change to plantarflexion and dorsiflexion, and in the wrist joint—to palmar flexion and dorsiflexion.
- Around the longitudinal axis: internal and external rotation.
For the forearm, external rotation is termed supination (supinatio), and internal rotation is termed pronation (pronatio). In the subtalar joint, supination means turning the sole inward from the horizontal plane, and pronation means turning it outward.
Initial Positions ("Zero") for Large Joints
Standardized initial positions are used to unify measurement results:
| Joint | Initial Position ("Zero") |
|---|---|
| Shoulder | Arm hanging alongside the trunk, greater tubercle facing anteriorly, epicondyles in the frontal plane. |
| Elbow | Full extension. |
| Wrist | Forearm in supination, wrist and hand in line with the forearm. |
| Hip, knee | Anterior superior iliac spines at the same level (forming a right angle with the legs), patella facing strictly anteriorly. |
| Ankle | Foot at a right angle to the axis of the lower leg. |
Additional Assessment Methods
In addition to dynamometry and goniometry, comparative limb girth measurements are used. These must be performed at strictly symmetrical levels. For example, the circumference of both lower legs is measured in the upper third—10 cm below the joint line.
Stabilography is used to study balance—recording the oscillations of the human center of mass in the frontal and sagittal planes while standing on a force platform.
When assessing spinal movements, it should be kept in mind that the cervical spine is the most mobile segment, and the thoracic spine is the least mobile (restricted by the rib cage). During spinal range of motion measurements, the involvement of the hip and knee joints must be strictly excluded.
Briefly: answers to topical questions
What are the main grades of manual muscle testing and their evaluation criteria according to the Medical Research Council scale?
- 5 points (5 points — Normal strength) — Muscle strength is fully preserved, the examiner cannot overcome the patient's resistance.
- 4 points (4 points — Slight strength reduction) — Active movement is possible in full range against the resistance of the examiner's force.
- 3 points (3 points — Moderate strength reduction) — The patient holds the limb against gravity but cannot overcome additional load.
- 2 points (2 points — Marked weakness) — Full range of motion in the horizontal plane, incomplete range against gravity, no resistance can be overcome.
- 1 point (1 point — Trace movements) — Minor contractions are detected without joint movement.
- 0 points (0 points — Plegia) — Complete absence of muscle movement.
Class and exam questions
What is the difference between active and passive movements?
Active movements are performed by the patient independently, while passive movements are produced by the physician with the patient's muscles relaxed. Passive movements show the true range of motion of the joint and usually exceed active movements.
How to properly position a patient with a flexion contracture of the hip joint?
Such a contracture causes compensatory lumbar lordosis, which hinders proper positioning. To eliminate it, the patient's healthy leg must be maximally flexed before measurement.
Why are sharp movements prohibited during joint examination?
Rough manipulations cause pain and reflex muscle tension. This is not only dangerous but also prevents obtaining an objective picture of joint condition.
What else to study on this topic
- Features of wrist joint goniometry in the frontal plane.
- Measurement of rotational movements in the elbow joint (pronation and supination).
- Specifics of subtalar joint biomechanics of the foot.
- Methodology of stabilography using a dynamometric platform.
- Assessment of mobility in various segments of the spine.
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