Flexor reflex responses triggered by imposed knee extension in chronic human spinal cord injury

Ming Wu*, T. George Hornby, Jennifer H. Kahn, Brian D. Schmit

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

11 Scopus citations


Hypersensitivity of the flexor reflex pathways to input from force-sensitive muscle afferents may contribute to the prevalence and severity of muscle spasms in patients with spinal cord injury (SCI). In this study, we triggered flexor reflexes with constant velocity knee movements in 15 subjects with SCI. Ramp and hold knee extension perturbations were imposed on one leg while the hip and ankle were held in an isometric position using an instrumented leg brace. Knee, ankle and hip torque responses and electromyograms from six muscles of the leg were recorded following controlled knee extension at four different velocities. Tests were conducted with the hip in both flexed and extended positions. During the movement into knee extension, a velocity-dependent stretch reflex, represented by a progressively increasing knee flexion torque, was observed. In addition, another type of reflex that resembled a flexor reflex (flexion of the hip and ankle) was also triggered by the imposed knee extension. The magnitude of the ankle dorsiflexion torque responses was significantly correlated to the stretch reflex torque at the knee in 9 of the 15 subjects. We concluded that stretch reflexes initiate a muscle contraction that then can contribute to a flexor reflex response, possibly through muscle group III/IV afferent pathways. These results suggest that spasticity in SCI consists of a myriad of complex reflex responses that extend beyond stretch reflexes.

Original languageEnglish (US)
Pages (from-to)566-576
Number of pages11
JournalExperimental Brain Research
Issue number4
StatePublished - Jan 2006


  • Flexor reflex
  • Knee extension
  • Spinal cord injury
  • Stretch reflex

ASJC Scopus subject areas

  • General Neuroscience


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