Muscle Strength and Power

The assessment of muscle power is the part of the motor examination that most directly answers the questions a clinician brings to the bedside: is there weakness, where is it, and what does its distribution imply about the level of the lesion? Strength testing is deceptively simple to perform and surprisingly easy to perform badly. Reliable grading depends on a fixed technique — eliciting the patient's maximal effort, isolating the muscle to be tested, stabilizing the limb, and comparing one side with the other — and on a willingness to read the pattern of weakness rather than to record isolated numbers. This chapter sets out the principles of strength testing, the Medical Research Council (MRC) grading scale, a systematic approach to the major muscle groups, the root and nerve supply of selected high-yield muscles, the recognizable patterns of weakness and what they localize, and the common pitfalls that corrupt an otherwise careful examination.

Principles of Strength Testing

Manual muscle testing measures the force a muscle can generate against the examiner's resistance. A few principles distinguish a dependable examination from a haphazard one.

• Test maximal effort against resistance. Ask the patient to contract the muscle as forcefully as possible and to hold the position while you attempt to overcome it. The reading is only as good as the effort behind it; encourage, coach, and re-test when effort seems submaximal.
• Compare side to side. The patient is their own best control. A small but reproducible asymmetry between homologous muscles is more informative than an absolute grade, and it is the asymmetry that most often localizes.
• Isolate the individual muscle. Position the limb so that the muscle of interest is the prime mover and substitution by synergists is minimized. Without correct positioning, a strong neighbor can mask the weakness of its partner.
• Stabilize the proximal joint. Fix the segment proximal to the joint being tested so that the patient cannot recruit trunk or girdle muscles to assist. Stabilization also lets the examiner direct resistance precisely across the intended joint.
• Apply resistance consistently. Use a steady, graded force across the full range and test comparable muscles in the same manner, so that grades are comparable within and between examinations.

The Medical Research Council (MRC) Scale

Power is graded on the six-point ordinal scale codified by the Medical Research Council, running from 0 to 5. The scale anchors each grade to a reproducible bedside observation.

Because grade 4 spans the entire range between movement that barely overcomes added resistance and movement that is almost full, examiners commonly subdivide it into 4−, 4, and 4+ to record, respectively, slight, moderate, and strong resistance below normal. This refinement adds useful granularity, but it should be understood for what it is: the MRC scale is ordinal and its intervals are unequal, so even the subdivided grade 4 remains a semiquantitative judgment rather than a measurement. When precise, reproducible quantification is required — for example, in tracking the course of a neuromuscular disorder — dynamometry is preferable to manual grading.

A Systematic Approach

Strength is examined the same way every time, so that nothing is omitted and findings are comparable across visits. Move through the major muscle groups of the upper limbs, the lower limbs, and, when relevant, the neck and trunk, testing each against resistance and comparing the two sides. In the upper limb this includes shoulder abduction, elbow flexion and extension, wrist extension, finger extension and flexion, and the intrinsic hand muscles; in the lower limb, hip flexion, knee flexion and extension, and ankle dorsiflexion and plantarflexion. Neck flexion and extension and the abdominal and paraspinal muscles are added when a myopathy, a cervical lesion, or a thoracic level is suspected.

Once a deficit is found, the screening survey gives way to focused testing organized by anatomy. Examining muscles according to their myotome (root supply) and their peripheral nerve supply is what converts a description of weakness into a localization: a deficit confined to one myotome points to a root, one confined to a single nerve's territory points to that nerve, and weakness that respects neither pattern points elsewhere.

Selected High-Yield Muscles, Roots, and Nerves

The following pairings are well established and are the ones most often used to separate a radiculopathy from a mononeuropathy at the bedside. They are a working subset, not an exhaustive table.

A practical use of the table: foot drop from weak ankle dorsiflexion and great-toe extension may arise from an L5 radiculopathy or a common peroneal (fibular) mononeuropathy. Both weaken tibialis anterior and extensor hallucis longus, but only the L5 root lesion will also affect muscles outside the peroneal territory — foot inversion (tibialis posterior, also L5 but tibial-innervated) helps make that distinction at the bedside.

Patterns of Weakness and What They Suggest

The distribution of weakness localizes more reliably than its severity. Several recurring patterns map onto distinct levels of the motor system.

• Pyramidal (upper motor neuron) distribution. Weakness is selective rather than uniform: in the arm the extensors and abductors are weaker than the flexors and adductors, and in the leg the flexors are weaker than the extensors. This "pyramidal pattern" accompanies the other upper motor neuron signs — spasticity, hyperreflexia, and an extensor plantar response — and underlies the characteristic hemiplegic posture and pronator drift.
• Proximal symmetric weakness. Symmetric weakness affecting the shoulder and hip girdles, with relatively preserved distal strength, producing difficulty rising from a chair, climbing stairs, and lifting the arms overhead. This is the signature of a myopathy.
• Distal weakness. Weakness predominating in the hands and feet — foot drop, tripping, and loss of fine finger control — commonly with accompanying distal sensory loss in a stocking-glove distribution. This is the pattern of a length-dependent peripheral neuropathy.
• Fatigable weakness. Strength that is reasonable at rest but declines with sustained or repeated effort and recovers after a pause, often with ptosis or diplopia. This pattern points to a disorder of the neuromuscular junction, such as myasthenia gravis.
• Non-organic ("give-way") weakness. Power that is normal momentarily and then collapses abruptly, varies with distraction, or is inconsistent with the patient's functional capacity. This pattern does not conform to any anatomical territory and suggests a functional or effort-dependent cause rather than a structural lesion — though it must never be diagnosed by exclusion of organic disease alone.

Pitfalls in Strength Testing

Several factors degrade the reliability of manual muscle testing, and recognizing them prevents both over- and under-calling weakness.

• Poor or submaximal effort. Inattention, fatigue, depression, or limited cooperation produces apparent weakness that is uniform and ungraded. Coaching, encouragement, and re-testing usually distinguish poor effort from true weakness; effort-limited muscles often yield gradually under steady resistance rather than at a fixed threshold.
• Pain-limited testing. Pain from an arthritic joint, a fracture, or soft-tissue injury inhibits full contraction and mimics weakness. The grade obtained while the patient is guarding against pain is not a true measure of strength, and this limitation should be recorded explicitly rather than scored as a deficit.
• Give-way weakness. The muscle holds briefly at full or near-full strength and then suddenly lets go — a "ratchety," collapsing quality unlike the smooth, sustained yielding of organic weakness. Give-way is characteristic of poor effort or a functional disorder, but pain can also produce it, so it is interpreted in context.
• Faulty technique. Failure to stabilize the proximal joint, to isolate the muscle, or to apply resistance consistently allows substitution by synergists and yields grades that are neither reproducible nor comparable.