Fig. 1 — Sprain versus strain: a ligament joins bone to bone; a tendon joins muscle to bone.
Two tissues, two injuries
The distinction is structural, and it runs through every medical record:
- Sprain — a stretch or tear of a ligament, the band that joins bone to bone across a joint. Ankle sprains are the everyday example; in a crash, the neck and low back take the same class of injury.
- Strain — a stretch or tear of a muscle or tendon, the cord that attaches muscle to bone. The "pulled muscle" of a seatbelt-loaded shoulder is a strain.
The American Academy of Orthopaedic Surgeons defines both as soft-tissue injuries and grades them the same way — the grading scale the grading file explains.
Where crash loading lands
The direction of force decides which soft tissue takes the load. Common sites follow the restraint geometry:
- Neck — the whiplash mechanism loads the cervical ligaments and muscles; see the whiplash file.
- Low back — the lumbar spine takes the seat-to-spine transfer in a rear impact.
- Shoulder and chest wall — the seatbelt diagonal loads the shoulder girdle and the muscles it crosses (seatbelt use is required by Vehicle Code §27315, and the belt's load path is part of the crash record).
- Knee and wrist — bracing against the dashboard or wheel loads the joints and their ligaments.
Why severity is graded, not guessed
Grade I (mild stretch), Grade II (partial tear), and Grade III (complete tear) describe tissue state, not pain level — a Grade I injury can hurt considerably while a Grade III tear may stiffen and ache differently. The grade comes from the clinical exam and, where ordered, imaging; it is what lets the recovery timeline speak in ranges instead of guesses.
Documentation note: the exact words in the record — "cervical strain," "lumbar sprain," "grade II" — matter downstream, because the claims file is read from them. The record, not the memory of pain, carries the claim.