Arthrogenic muscle inhibition and quadriceps activation failure: Reconciling terminology and clinical perspectives across recovery phases.

Quadriceps activation failure, defined as the inability to fully activate the quadriceps during a maximal voluntary contraction, is a common consequence of anterior cruciate ligament injury and reconstruction. Although often attributed to arthrogenic muscle inhibition, activation deficits are likely influenced by a broader range of factors, including pain, disuse, fatigue and psychological influences. This commentary argues that the term quadriceps activation failure (or deficit) is preferable to arthrogenic muscle inhibition because it describes the clinical phenomenon without presuming a specific underlying mechanism and better reflects the multifactorial nature of impaired quadriceps activation. This commentary also proposes a clinically relevant framework that recognises three manifestations of quadriceps activation failure based on its magnitude and temporal evolution following anterior cruciate ligament injury and reconstruction: severe, substantial and persistent. Severe quadriceps activation failure typically occurs during the acute postinjury and postoperative phases, substantial activation failure characterises much of the rehabilitation period and persistent failure may remain detectable despite recovery of strength and return to sport. Recognising these distinct forms of quadriceps activation failure may facilitate communication among clinicians and researchers, support individualised rehabilitation strategies and enhance monitoring of recovery. Although illustrated in the context of anterior cruciate ligament injury, this framework may also be relevant to other musculoskeletal conditions associated with activation deficits. LEVEL OF EVIDENCE: N/A.