Overview
Skeletal muscles are voluntary and attach to bones through tendons. Cardiac muscle is found only in the heart and contracts on its own rhythm. Smooth muscle lines hollow organs such as the intestines, bladder and blood vessels and works without conscious control.
A skeletal muscle contracts when a motor neuron releases acetylcholine at the neuromuscular junction. Calcium is released inside the fiber, and actin and myosin filaments slide past each other, shortening the muscle. This process uses ATP, which is why muscles produce heat.
Main functions
- Movement of the skeleton and body parts
- Posture and joint stability
- Heat production, which rises sharply during shivering
- Movement of blood, food and urine through smooth and cardiac muscle
- Protection of abdominal organs by the abdominal wall
Key structures
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Diaphragm
The dome-shaped muscle under the lungs and the main muscle of breathing.
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Deltoid
The triangular muscle that caps the shoulder and lifts the arm, and a common vaccine injection site.
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Gluteus medius (ventrogluteal site)
A hip muscle under the gluteus maximus and the location of the ventrogluteal injection site.
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Vastus lateralis
The largest part of the quadriceps, on the outer thigh, and the preferred injection site for infants.
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Sternocleidomastoid
The strap muscle on each side of the neck that turns and flexes the head.
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Gastrocnemius (calf)
The calf muscle that points the foot and pumps venous blood back toward the heart.
How a muscle contracts: the sliding filament model
Inside each muscle fiber, repeating units called sarcomeres contain thin actin filaments and thick myosin filaments. When calcium binds to troponin, binding sites on actin are uncovered and myosin heads pull the actin inward.
Each pull uses ATP. When nerve stimulation stops, calcium is pumped back into storage and the muscle relaxes. Without ATP, myosin cannot detach, which explains rigor mortis.
Nursing assessment
- Grade muscle strength from 0 (no contraction) to 5 (full strength against resistance) and compare both sides.
- Inspect for atrophy, fasciculations and asymmetry.
- Check muscle tone: flaccid, normal or spastic.
- Ask about pain, cramps and weakness; note the effect on daily activities.
- Before an intramuscular injection, assess muscle mass and landmarks at the chosen site.
Common conditions
| Condition | What happens | Key signs | Nursing priorities |
|---|---|---|---|
| Rhabdomyolysis | Breakdown of muscle releases myoglobin and potassium into the blood | Muscle pain, weakness, tea-colored urine, very high CK | Aggressive IV fluids, strict intake and output, monitor potassium and kidney function |
| Muscular dystrophy | Inherited progressive muscle weakness | Gowers' sign, falls, calf pseudohypertrophy (Duchenne) | Maintain mobility, respiratory support, family support and genetic counseling |
| Myasthenia gravis | Antibodies block acetylcholine receptors | Weakness that worsens with activity, ptosis, diplopia, difficulty swallowing | Give anticholinesterase drugs on time, aspiration precautions, watch for crisis |
| Strains and sprains | Overstretched muscle (strain) or ligament (sprain) | Pain, swelling, bruising, reduced function | RICE: rest, ice, compression, elevation for the first 24–48 hours |
| Disuse atrophy | Loss of muscle mass from immobility | Reduced muscle size and strength, contractures | Early mobilization, range-of-motion exercises, repositioning every 2 hours |
Labs and normal values
| Test | Typical adult range | Why it matters |
|---|---|---|
| Creatine kinase (CK) | Males about 55–170 U/L; females about 30–135 U/L | Very high in rhabdomyolysis and muscle injury; reference ranges vary by lab |
| Potassium | 3.5–5.0 mEq/L | Released from damaged muscle; affects muscle and heart function |
| Myoglobin (urine) | Negative | Positive in rhabdomyolysis; risk of acute kidney injury |
| Calcium | 9.0–10.5 mg/dL | Low calcium causes tetany, muscle cramps, Chvostek's and Trousseau's signs |
Reference ranges differ between laboratories and textbooks. Use the values your program and facility use.
Exam tips
- The ventrogluteal site is preferred for most adult intramuscular injections: it has no large nerves or vessels nearby.
- Use the vastus lateralis for intramuscular injections in infants.
- The deltoid holds a small volume (generally up to 1 mL): use it for vaccines, not for large or irritating injections.
Practice questions
8 NCLEX-style questions on the muscular system. Pick an answer to see the rationale.
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Which site is preferred for an intramuscular injection in a 6-month-old infant?
Show the answer and rationale
Answer C: Vastus lateralis. The vastus lateralis is well developed in infants and has no major nerves or vessels near the injection area.
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A client with a C3 spinal cord injury is at greatest risk for which problem?
Show the answer and rationale
Answer B: Respiratory failure. The phrenic nerve (C3–C5) controls the diaphragm. Injuries at or above C4 can stop spontaneous breathing.
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A client is admitted with muscle pain after a crush injury and has tea-colored urine. Which lab result is most expected?
Show the answer and rationale
Answer B: Very high creatine kinase. Rhabdomyolysis releases CK and myoglobin from damaged muscle. Potassium tends to rise, not fall.
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When locating the ventrogluteal site, where does the nurse place the palm?
Show the answer and rationale
Answer B: On the greater trochanter. The palm rests on the greater trochanter, the index finger on the anterior superior iliac spine, and the injection is given in the V formed by the fingers.
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Which finding suggests respiratory distress?
Show the answer and rationale
Answer A: Use of the sternocleidomastoid muscles to breathe. Accessory muscle use shows the client is working hard to breathe.
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Which action best prevents deep vein thrombosis in a postoperative client?
Show the answer and rationale
Answer C: Encourage ankle pumps and early ambulation. Calf muscle contraction pumps venous blood. Massaging can dislodge a clot, and pillows under the knees and crossed legs impede flow.
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Muscle strength graded 3/5 means the client can:
Show the answer and rationale
Answer B: Move against gravity but not against resistance. Grade 3 is full range of motion against gravity only. Grade 4 adds some resistance and grade 5 is normal strength.
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Which neurotransmitter triggers skeletal muscle contraction at the neuromuscular junction?
Show the answer and rationale
Answer B: Acetylcholine. Motor neurons release acetylcholine, which depolarizes the muscle fiber. Myasthenia gravis blocks its receptors.