Human Lumbosacral Spinal Cord Interprets Loading During Stepping

Abstract

Studies suggest that the human lumbosacral spinal cord can generate steplike oscillating electromyographic (EMG) patterns, but it remains unclear to what degree these efferent patterns depend on the phasic peripheral sensory information associated with bilateral limb movements and loading. We examined the role of sensory information related to lower-extremity weight bearing in modulating the efferent motor patterns of spinal-cord-injured (SCI) subjects during manually assisted stepping on a treadmill. Four nonambulatory subjects, each with a chronic thoracic spinal cord injury, and two nondisabled subjects were studied. The level of loading, EMG patterns, and kinematics of the lower limbs were studied during manually assisted or unassisted stepping on a treadmill with body weight support. The relationships among lumbosacral motor pool activity [soleus (SOL), medial gastrocnemius (MG), and tibialis anterior (TA)], limb load, muscle-tendon length, and velocity of muscle-tendon length change were examined. The EMG mean amplitude of the SOL, MG, and TA was directly related to the peak load per step on the lower limb during locomotion. The effects on the EMG amplitude were qualitatively similar in subjects with normal, partial, or no detectable supraspinal input. Responses were most consistent in the SOL and MG at load levels of < 50% of a subject's body weight. The modulation of the EMG amplitude from the SOL and MG, both across steps and within a step, was more closely associated with limb peak load than muscle-tendon stretch or the velocity of muscle-tendon stretch. Thus stretch reflexes were not the sole source of the phasic EMG activity in flexors and extensors during manually assisted stepping in SCI subjects. The EMG amplitude within a step was highly dependent on the phase of the step cycle regardless of level of load. These data suggest that level of loading on the lower limbs provides cues that enable the human lumbosacral spinal cord to modulate efferent output in a manner that may facilitate the generation of stepping. These data provide a rationale for gait rehabilitation strategies that utilize the level of load-bearing stepping to enhance the locomotor capability of SCI subjects.

Modulation of ankle muscle postural reflexes in stroke: influence of weight-bearing load

“…Thus, supraspinal input may not be required for integrating load information. For example, weight-bearing load has been shown to modulate lower limb muscle activity in individuals with spinal cord injury (Harkema et al, 1997). This supports the notion that afferent input regarding weight-bearing load is integrated within the spinal cord.…”

“…Harkema et al (1997) found that EMG amplitude during treadmill induced stepping in individuals with spinal cord injury was less coupled to muscle length (i.e. muscle spindle input) factors than to peak limb load.…”

“…Application of weights near the centre of mass has resulted in increased ankle extensor burst amplitude during human locomotion (Stephens and Yang, 1999). Stepping in selected spinal cord injured patients can be facilitated through body-weight supported treadmill training, presumably due to the fact that the increased load results in greater activation of ankle musculature (Harkema et al, 1997). Studies in individuals with stroke utilizing this form of rehabilitation have shown benefits (Hassid et al, 1997;Trueblood, 2001).…”

Maintenance of locomotor abilities following Laufband (treadmill) therapy in para- and tetraplegic persons: follow-up studies

“…One of these features, limb loading during stance, has recently been shown to dramatically enhance motor output from muscles of this limb in persons with complete (and incomplete) spinal cord transections, supporting the idea that the isolated spinal cord correctly interpretes proprioceptive a erent information in the sense of maintaining upright body position. 9,14 Applying these rules, elements of stepping on the treadmill short of complete step cycles, could recently be evoked in a 13-year old girl completely paralysed below spinal segment Th 6 12 (detailed manuscript in preparation; see also Wernig and MuÈ ller 5 and Wernig et al 6 for further reports on elements of stepping after complete spinal cord transection, and Dobkin et al 16 and Dietz et al 17 for phasic and alternating¯exor and extensor muscle activity during aided stepping).…”

Coordination training in individuals with incomplete spinal cord injury: consideration of motor hierarchical structures

“…Instead of focusing on the corticospinal tract and trying to train the phylogenetically youngest part of the motor system first, it may be beneficial to adopt an approach where we try to influence the motor system at the lowest level initially, namely, by influencing the CPGs, the phylogenetically older system. From several group studies, 15,21,25 it was highlighted that circuitry in the human species is able to learn, especially when this training is combined with loadbearing. We should consider the fact that the corticospinal tract is not the only tract to project to the motoneurons in the spine.…”