Early loading after a muscle strain injury is important because immobilization can swiftly and adversely affect muscle and tendon structure and function.
Intramuscular Tendon Strains in Hamstrings and Quadriceps
A muscle strain may tear the myofibrillar attachments from the intramuscular tendon, with resultant bleeding and edema.
Straight Leg Raise and Manual Resistance Tests for Strains and Sprains
Active SLR test assesses both sprains and strains. Passive SLR test assesses only sprains… however, this is a bit of an oversimplification.