Muscle Injuries

Muscle injuries account for about a third of sports injuries and are a major cause of time lost from training and competition.

They are structural (macroscopic fibre disruption) or functional (pain without macroscopic disruption, such as delayed onset muscle soreness (DOMS) and cramp). Most structural injuries affect the myotendinous junction (MTJ) of bi-articular muscles during eccentric contraction. Management centres on early loading, and return to sport is based on criteria rather than time.

The MTJ is the commonest site of structural injury because the stiffness mismatch between muscle and tendon concentrates stress there. Injuries extending into an intramuscular tendon or aponeurosis (hamstrings, rectus femoris, gastrocnemius) recover more slowly and recur more often.

• •Bi-articular muscles (hamstrings, rectus femoris, gastrocnemius) are stretched across 2 joints at once and are the most vulnerable in sport
• •Type II (fast-twitch) fibres generate high eccentric forces and are most often damaged in acute strains

Classification:

• •Clinical grades 1 to 3 (minor partial, moderate partial, extensive or complete tear) are still widely used
• •The British Athletics Muscle Injury Classification (BAMIC) grades injuries 0 to 4 on MRI. Grade 0 has a normal MRI (0a focal neuromuscular injury, 0b generalised soreness such as DOMS); grades 1 to 3 are small, moderate and extensive by MRI extent; grade 4 is a complete tear
• •BAMIC grades 1 to 4 add the site: a myofascial, b musculotendinous, c intratendinous. 'c' injuries take longer to return to full training and recur more often
• •The Munich consensus separates functional disorders without fibre tear (type 1 overexertion-related, type 2 neuromuscular) from structural injuries (type 3 partial tears, type 4 subtotal or total tears and tendinous avulsions)

Mechanisms:

• •Sprinting type: high-speed eccentric loading, in the hamstrings usually biceps femoris long head at the proximal MTJ in late swing
• •Stretching type: end-range positions in dance and kicking, usually the proximal semimembranosus free tendon, with a longer recovery
• •Tennis leg is a medial gastrocnemius MTJ tear

Risk factors: for hamstrings, a 2020 meta-analysis found older age and previous hamstring injury the strongest; fatigue and eccentric weakness also contribute.