Stretch reflex changes in ACL‐deficient individuals and healthy controls during normal and surprise landings

<jats:p>No studies in ACL‐D individuals have examined neuromuscular adaptations during landing from a jump where an unexpected mechanical event changes the pre‐programmed course of movement. The purpose of this study was to compare pre‐ and post‐landing muscle activation in ACL‐D individuals and uninjured controls during normal and surprise landings. Nineteen ACL‐D and 17 uninjured volunteered. Participants performed repeated single leg landings from 30 and 15 cm heights. During 15 cm landings, a single surprise landing was performed where participants unexpectedly fell through a false surface at 15 cm to the solid floor a further 15 cm below. Electromyography (EMG) amplitude from vastus lateralis (VL), lateral hamstrings (LH), and soleus (Sol) was recorded. Pre‐landing (−60 to 0 ms), post‐landing short latency (31‐60 ms), and post‐landing medium latency (61‐90 ms) periods were examined. Comparisons in EMG amplitudes were made across limbs (ACL‐D, ACL intact, and control) in 30 cm landings. Additionally, the ratio of EMG amplitude in surprise:30 cm normal landings was analyzed. Post‐landing LH EMG was reduced in the ACL‐D compared to control limbs at short latencies (<jats:italic>P</jats:italic> < 0.05). Post‐landing VL EMG was reduced in the ACL‐D and ACL intact compared to the control limb at both latencies (<jats:italic>P</jats:italic> < 0.05). Surprise landings notably increased post‐landing EMG in all muscles, across all limbs (<jats:italic>P</jats:italic> < 0.001). However, the gain in VL EMG was significantly greater in ACL‐D and ACL intact limbs (<jats:italic>P</jats:italic> < 0.05). These changes in neuromuscular control of ACL‐D individuals during expected and surprise landings may have important implications for rehabilitation, instability, and the risk of secondary injury.</jats:p>