Conditioning Reflex Protocols

Traditional tenets about the hard-wired nervous system have long been dispelled with mounting evidence for activity-dependent plasticity throughout the central nervous system. Fascinating results from animal, and more recently, human studies have shown that even the “simplest” spinal cord reflex, the stretch reflex pathway or its electrical analog, the H-reflex, can be altered to increase or decrease in size through operant conditioning (Wolpaw 2010). Humans can also learn to increase or decrease the size of the soleus H-reflex (Thompson et al. 2009). Some gait impairments following SCI could be associated with hyperreflexia and abnormal reflex responses in the ankle plantarflexors (Dietz & Sinkjaer 2007). The possibility that H-reflex amplitude could be down-conditioned raises the compelling question of whether such protocols may benefit persons with SCI who present with spastic gait disorder.

This idea was tested in a group of 13 people with chronic (>8 months) motor-incomplete SCI who were all ambulatory and presented with spasticity (e.g., ³1 on Modified Ashworth Score) and weak ankle dorsiflexion (Thompson et al. 2013). Participants were randomly assigned at a 2:1 ratio to the down-conditioning (DC) group (n = 9) or the unconditioned group (n = 4). Each participant completed 6 baseline sessions followed by 30 sessions (3 sessions/week) of control (unconditioned group) or conditioning (DC group). Visual feedback was provided to the DC group to inform them of whether they were successful in reducing their H-reflex amplitude to within the target range. In the unconditioned group, each session involved H-reflex recordings without any visual feedback or instructions about H-reflex amplitude. Note that in this study, no LT was provided; training sessions consisted of only the H-reflex down-conditioning (or control protocol). More recently, a similar RCT performed by Thompson and Wolpaw (2021) assessed a new protocol, in which the H-reflex during the late-swing phase of walking on a treadmill with BWS was elicited (Thompson & Wolpaw 2021). However, the number of conditioning and control sessions, the number of reflex trials per session, and the session schedule were identical in both protocols (Thompson & Wolpaw 2021; Thompson et al. 2013).

Discussion

In the RCT of Thompson and Wolpaw (2021), six of the seven participants in the DC group were able to successfully down-condition (decrease) their H-reflex amplitude; however, there was no reduction in H-reflex amplitude in the unconditioned group. These success rates were similar to those of previous steady-rate operant conditioning studies in people with SCI (Thompson et al. 2013). However, with the swing-phase down-conditioning, the H-reflex decreased much faster and much more than did the H-reflex in previous human studies with the steady-state protocol (Thompson et al. 2013), and the decrease persisted for at least six months after conditioning ended (Thompson & Wolpaw 2021). Across both of these studies, the participants who could successful down-condition their soleus H-reflex amplitude experienced significant increases in their 10MWT speeds (average of 59% (range: 0-123%) in Thompson et al. (2013); 112% ± 9% in Thompson & Wolpaw (2021). For the seven participants in whom H-reflex did not decrease, walking speed also increased, though less and not significantly (Thompson et al. 2013).

Conditioning reflex protocols have been published many years ago for the upper extremity in SCI (Segal & Wolf 1994) to reduce spasticity and are an important neuroscience observation. However, they have not been accepted into practice likely due to the variable results and laborious number of sessions to get a small effect. This one small RCT for the lower extremity shows similar findings as the upper extremity – that soleus spinal reflexes can be down-conditioned in about 2/3 of the participants, although a few of these participants did demonstrate large improvements in gait speed. The success rate of down-conditioning in participants with SCI was comparable to previous studies in participants without SCI. Unfortunately, absolute values were not reported here, making the clinical significance of these results difficult to ascertain. Furthermore, the complexity of this approach may make it inaccessible for most clinicians. Nevertheless, these results are intriguing and point towards another potential approach of directly manipulating spinal cord plasticity to enhance functional recovery.

Conclusions

There is level 1 evidence (from 1 RCT: Thompson et al. 2013) and level 2 evidence (from 1 RCT: Thompson & Wolpaw 2021) that down-conditioning reflex protocols of the soleus could increase walking speed and improve gait symmetry.