Rebound responses to prolonged flexor reflex stimuli in human spinal cord injury

Abstract

The purpose of this study was to examine the reflex effects of electrical stimulation applied to the thigh using skin electrodes, targeting the sensory fibers of the rectus femoris and sartorius, in people with spinal cord injury (SCI). Thirteen individuals with SCI were recruited to participate in experiments using prolonged electrical stimuli on the right medial thigh over the regions of the sartorius and rectus femoris muscles. Three stimuli, spaced 20 s apart, were applied at 30 Hz for 1 s at four different intensities (15–60 mA) while subjects rested in a seated position. Isometric joint torques of the hip, knee and ankle, and electromyograms (EMGs) from six muscles of the leg were recorded during the stimulation. Early in the stimulation, a flexion response was observed at the hip and ankle, analogous to a flexor reflex; however, this response was usually followed by a “rebound” response consisting of hip extension, knee flexion and ankle plantarflexion, occurring in 10/13 subjects. Stimuli applied in a more lateral (mid thigh) electrode position (i.e. over the rectus femoris) were less effective in producing the response than medial placement, despite vigorous quadriceps activation. This complex reflex response is consistent with activation of a coordinating spinal circuit that could play a role in motor function. The reversal of the reflex pattern emphasizes the potential connection between skin/muscle afferents of the thigh, possibly including sartorius muscle afferents and locomotor reflex centers. This knowledge may be helpful in identifying rehabilitation strategies for enhancing gait training in human SCI.

Appendix

• 1. ASIA impairment scale:

  A = Complete.:

  No Sensory or motor function preserved in sacral segments S4-S5.

  B = Incomplete.:

  Sensory but not motor function preserved below neurological level and extends through S4–S5.

  C = Incomplete.:

  Motor function preserved below neurological level and through S4–S5.

   :

  Majority of Key muscles below neurological level have a grade less than 3.

  D = Incomplete.:

  Motor function preserved below neurological level and through S4–S5.

   :

  Majority of Key muscles below neurological level have a grade at least 3.

  E = Normal.:

  Sensory and Motor function is normal.

• 2. The transformation matrix relating the measured torque and force of the 6-DOF load cell to the torques of each joint.

$$ \left[ \begin{gathered} \tau_{\text{h}} \hfill \\ \tau_{\text{k}} \hfill \\ \tau_{\text{a}} \hfill \\ \end{gathered} \right] = \left[ \begin{gathered} a_{11} \quad a_{12} \quad a_{13} \hfill \\ a_{21} \quad a_{22} \quad a_{23} \hfill \\ a_{31} \quad a_{32} \quad a_{33} \hfill \\ \end{gathered} \right] = \left[ \begin{gathered} F_{xc} \hfill \\ F_{yc} \hfill \\ \tau_{zc} \hfill \\ \end{gathered} \right] $$

(1)

where a ₁₁ = l _s sinθ _a + l _t sin(θ _k  − θ _a); a ₁₂ = −l _s cosθ _a + l _t cos(θ _k − θ _a); a ₁₃ = 1; a ₂₁ = −l _s sinθ _a; a ₂₂ = l _s cosθ _a; a ₂₃ = −1; a ₃₁ = 0; a ₃₂ = 0; a ₃₃ = 1. l _t was the length of the femur, and l _s was the length between the ankle and knee axes of rotation (as shown in Fig. 1). θ _k was the angle of knee joint, while θ _a was the angle of ankle joint, which is indicated in Table 2 for each subject. The forces and torqueF _xc , F _yc , τ _zc were measured using the 6-DOF load cell. The calculation using this matrix yielded the joint torques τ _h, τ _k, τ _a for hip, knee and ankle joints respectively.

Table 2 Subject test parameters indicating isometric angles (in degrees) of the hip, knee and ankle joints during test