Heightened flexor withdrawal responses following ACL rupture are enhanced by passive tibial translation

Abstract

Objective Hyperexcitability of nociceptive pathways has been demonstrated with several musculoskeletal conditions but not anterior cruciate ligament (ACL) injury. The purpose was to investigate flexor withdrawal reflex (FWR) excitability following ACL rupture and determine if painless stretch of knee joint structures enhanced reflexive responses. Methods Ten subjects with and 10 subjects without unilateral ACL rupture were compared. FWRs were induced through sural nerve stimulus in symmetrical stance and recumbent positions, with the knee in relaxed and stressed condition. Latencies and amplitudes of hamstring electromyographic activity were analyzed. Results FWR thresholds were significantly diminished ( p = 0.05) on the injured limb (11.8 ± 8 mA) compared to non-injured limb (18.6 ± 13 mA) and controls (22.5 ± 3 mA). Anterior tibial translation resulted in increased ( p = 0.001) amplitude of EMG hamstring response on the injured limb (70 ± 50%) versus control (−1 ± 20%) and decreased latency ( p = 0.01) of hamstring activation (82.0 ± 13 ms). Conclusions Individuals with ACL rupture demonstrated increased excitability of FWR responses indicated by decreased FWR threshold and reduced hamstring muscle latency. Responses were enhanced by passive stretch of the knee joint. Significance Subjects with ACL rupture demonstrated hyperexcitability of nociceptive pathways on the injured limb which may trigger the FWR more readily and promote the sensation of instability at the knee.