How can I differentiate tendonitis from a primary muscle strain?

Differentiating Tendinitis from Muscle Strain

Tendinopathy presents with well-localized tenderness directly over the tendon that reproduces activity-related pain, while muscle strains cause diffuse pain within the muscle belly during acute strenuous activity. 1, 2

Key Clinical Distinctions

History and Onset Pattern

Tendinopathy characteristics:

• Insidious onset of load-related pain that gradually increases with repetitive activity 1, 3
• Patients typically report starting a new activity or increasing intensity of current activity before symptom onset 3
• Early stages show pain during activity that may subside after warm-up; later stages demonstrate increased pain intensity and duration, possibly present at rest 3
• Pain described as "sharp" or "stabbing" localized to the tendon 3

Muscle strain characteristics:

• Acute onset during strenuous activity, not gradual 2
• Occurs suddenly during eccentric contraction or overstretching 2
• Immediate pain at time of injury 2

Physical Examination Findings

For tendinopathy, perform these specific maneuvers:

• Palpate for well-localized tenderness directly over the tendon that precisely reproduces the patient's activity-related pain 1, 3
• Perform physical maneuvers that simulate tendon loading to reproduce pain 1, 3
• Inspect for swelling, asymmetry, and erythema of involved tendons 1, 3
• Assess range of motion, which is often limited on the symptomatic side 3
• Look for muscle atrophy, which suggests chronicity 3, 4

For muscle strain, look for:

• Diffuse tenderness within the muscle belly, not precisely localized to a tendon 2
• Palpable defect or gap in severe strains 2
• Pain with passive stretching of the affected muscle 2
• Pain and weakness with resisted contraction 2

Critical Diagnostic Pitfall

Joint effusions are uncommon with tendinopathy and suggest intra-articular pathology instead 3, 4. If you detect joint effusion, reconsider the diagnosis and evaluate for arthritis, loose bodies, or other intra-articular problems 4.

Location of Tenderness

The single most reliable distinguishing feature is anatomic precision of tenderness:

• Tendinopathy: Tenderness is precisely localized to the tendon insertion or along the tendon course 1, 3
• Muscle strain: Tenderness is diffuse within the muscle belly, away from tendon insertions 2

Imaging When Diagnosis Unclear

Reserve imaging for unclear diagnoses or recalcitrant pain despite adequate conservative management 3:

• Plain radiography: Initial choice for acute injuries; shows osteophyte formation, degenerative joint disease, loose bodies, or fractures but has limited ability to demonstrate soft-tissue changes 3
• Ultrasonography: Shows thickening and heterogeneous echogenicity in tendinopathy 1, 3; preferred for visualizing overall tendon structure 1
• MRI: Shows degenerative thickening of tendons, fibrovascular proliferation, and mucoid degeneration in tendinopathy 3; recommended for muscle strains only when radiologic evaluation is necessary for diagnosis 2

Additional Red Flags

Multiple symptomatic tendons should prompt evaluation for rheumatic disease 3, as this pattern is inconsistent with simple overuse tendinopathy.

Avoid the common error of labeling chronic tendon problems as "tendinitis" 1, 5. Most patients in primary care have chronic degenerative conditions that should be labeled "tendinosis" or "tendinopathy," not inflammatory "tendinitis" 1.