Heather Brant Hayes

Daily intermittent hypoxia enhances walking after chronic spinal cord injury A randomized trial

Objectives: To test the hypothesis that daily acute intermittent hypoxia (dAIH) and dAIH combined with overground walking improve walking speed and endurance in persons with chronic incomplete spinal cord injury (iSCI). Methods: Nineteen subjects completed the randomized, double-blind, placebo-controlled, crossover study. Participants received 15, 90-second hypoxic exposures (dAIH, fraction of inspired oxygen [Fio2] = 0.09) or daily normoxia (dSHAM, Fio2 = 0.21) at 60-second normoxic intervals on 5 consecutive days; dAIH was given alone or combined with 30 minutes of overground walking 1 hour later. Walking speed and endurance were quantified using 10-Meter and 6-Minute Walk Tests. The trial is registered at ClinicalTrials.gov (NCT01272349). Results: dAIH improved walking speed and endurance. Ten-Meter Walk time improved with dAIH vs dSHAM after 1 day (mean difference [MD] 3.8 seconds, 95% confidence interval [CI] 1.1–6.5 seconds, p = 0.006) and 2 weeks (MD 3.8 seconds, 95% CI 0.9–6.7 seconds, p = 0.010). Six-Minute Walk distance increased with combined dAIH + walking vs dSHAM + walking after 5 days (MD 94.4 m, 95% CI 17.5–171.3 m, p = 0.017) and 1-week follow-up (MD 97.0 m, 95% CI 20.1–173.9 m, p = 0.014). dAIH + walking increased walking distance more than dAIH after 1 day (MD 67.7 m, 95% CI 1.3–134.1 m, p = 0.046), 5 days (MD 107.0 m, 95% CI 40.6–173.4 m, p = 0.002), and 1-week follow-up (MD 136.0 m, 95% CI 65.3–206.6 m, p < 0.001). Conclusions: dAIH ± walking improved walking speed and distance in persons with chronic iSCI. The impact of dAIH is enhanced by combination with walking, demonstrating that combinatorial therapies may promote greater functional benefits in persons with iSCI. Classification of evidence: This study provides Class I evidence that transient hypoxia (through measured breathing treatments), along with overground walking training, improves walking speed and endurance after iSCI.

Neuromuscular constraints on muscle coordination during overground walking in persons with chronic incomplete spinal cord injury

OBJECTIVE Incomplete spinal cord injury (iSCI) disrupts motor control and limits the ability to coordinate muscles for overground walking. Inappropriate muscle activity has been proposed as a source of clinically observed walking deficits after iSCI. We hypothesized that persons with iSCI exhibit lower locomotor complexity compared to able-body (AB) controls as reflected by fewer motor modules, as well as, altered module composition and activation. METHODS Eight persons with iSCI and eight age-matched AB controls walked overground at prescribed cadences. Electromyograms of fourteen single leg muscles were recorded. Non-negative matrix factorization was used to identify the composition and activation of motor modules, which represent groups of consistently co-activated muscles that accounted for 90% of variability in muscle activity. RESULTS Motor module number, composition, and activation were significantly altered in persons with iSCI as compared to AB controls during overground walking at self-selected cadences. However, there was no significant difference in module number between persons with iSCI and AB controls when cadence and assistive device were matched. CONCLUSIONS Muscle coordination during overground walking is impaired after chronic iSCI. SIGNIFICANCE Our results are indicative of neuromuscular constraints on muscle coordination after iSCI. Altered muscle coordination contributes to person-specific gait deficits during overground walking.

An In Vitro Spinal Cord–Hindlimb Preparation for Studying Behaviorally Relevant Rat Locomotor Function

Although the spinal cord contains the pattern-generating circuitry for producing locomotion, sensory feedback reinforces and refines the spatiotemporal features of motor output to match environmental demands. In vitro preparations, such as the isolated rodent spinal cord, offer many advantages for investigating locomotor circuitry, but they lack the natural afferent feedback provided by ongoing locomotor movements. We developed a novel preparation consisting of an isolated in vitro neonatal rat spinal cord oriented dorsal-up with intact hindlimbs free to step on a custom-built treadmill. This preparation combines the neural accessibility of in vitro preparations with the modulatory influence of sensory feedback from physiological hindlimb movement. Locomotion induced by N-methyl D-aspartate and serotonin showed kinematics similar to that of normal adult rat locomotion. Changing orientation and ground interaction (dorsal-up locomotion vs ventral-up air-stepping) resulted in significant kinematic and electromyographic changes that were comparable to those reported under similar mechanical conditions in vivo. We then used two mechanosensory perturbations to demonstrate the influence of sensory feedback on in vitro motor output patterns. First, swing assistive forces induced more regular, robust muscle activation patterns. Second, altering treadmill speed induced corresponding changes in stride frequency, confirming that changes in sensory feedback can alter stride timing in vitro. In summary, intact hindlimbs in vitro can generate behaviorally appropriate locomotor kinematics and responses to sensory perturbations. Future studies combining the neural and chemical accessibility of the in vitro spinal cord with the influence of behaviorally appropriate hindlimb movements will provide further insight into the operation of spinal motor pattern-generating circuits.

Stance-phase force on the opposite limb dictates swing-phase afferent presynaptic inhibition during locomotion.

Presynaptic inhibition is a powerful mechanism for selectively and dynamically gating sensory inputs entering the spinal cord. We investigated how hindlimb mechanics influence presynaptic inhibition during locomotion using pioneering approaches in an in vitro spinal cord-hindlimb preparation. We recorded lumbar dorsal root potentials to measure primary afferent depolarization-mediated presynaptic inhibition and compared their dependence on hindlimb endpoint forces, motor output, and joint kinematics. We found that stance-phase force on the opposite limb, particularly at toe contact, strongly influenced the magnitude and timing of afferent presynaptic inhibition in the swinging limb. Presynaptic inhibition increased in proportion to opposite limb force, as well as locomotor frequency. This form of presynaptic inhibition binds the sensorimotor states of the two limbs, adjusting sensory inflow to the swing limb based on forces generated by the stance limb. Functionally, it may serve to adjust swing-phase sensory transmission based on locomotor task, speed, and step-to-step environmental perturbations.

Effects of acute intermittent hypoxia on hand use after spinal cord trauma: A preliminary study

Objective: To test the hypothesis that daily acute intermittent hypoxia (AIH) combined with hand opening practice improves hand dexterity, function, and maximum hand opening in persons with chronic, motor-incomplete, cervical spinal cord injury. Methods: Six participants completed the double-blind, crossover study. Participants received daily (5 consecutive days) AIH (15 episodes per day: 1.5 minutes of fraction of inspired oxygen [FIo2] = 0.09, 1-minute normoxic intervals) followed by 20 repetitions of hand opening practice and normoxia (sham, FIo2 = 0.21) + hand opening practice. Hand dexterity and function were quantified with Box and Block and Jebsen-Taylor hand function tests. We also recorded maximum hand opening using motion analyses and coactivity of extensor digitorum and flexor digitorum superficialis muscles using surface EMG. Results: Daily AIH + hand opening practice improved hand dexterity, function, and maximum hand opening in all participants. AIH + hand opening practice improved Box and Block Test scores vs baseline in 5 participants (p = 0.057) and vs sham + hand opening practice in all 6 participants (p = 0.016). All participants reduced Jebsen-Taylor Hand Function Test (JTHF) time after daily AIH + hand opening practice (−7.2 ± 1.4 seconds) vs baseline; 4 of 6 reduced JTHF time vs sham + hand opening practice (p = 0.078). AIH + hand opening practice improved maximum hand aperture in 5 of 6 participants (8.1 ± 2.7 mm) vs baseline (p = 0.018) and sham + hand opening practice (p = 0.030). In 5 participants, daily AIH–induced changes in hand opening were accompanied by improved EMG coactivity (p = 0.029). Conclusions: This report suggests the need for further study of AIH as a plasticity “primer” for task-specific training in spinal cord injury rehabilitation. Important clinical questions remain concerning optimal AIH dosage, patient screening, safety, and effect persistence. ClinicalTrials.gov identifier: NCT01272336.

Modulation of hand aperture during reaching in persons with incomplete cervical spinal cord injury

The intact neuromotor system prepares for object grasp by first opening the hand to an aperture that is scaled according to object size and then closing the hand around the object. After cervical spinal cord injury (SCI), hand function is significantly impaired, but the degree to which object-specific hand aperture scaling is affected remains unknown. Here, we hypothesized that persons with incomplete cervical SCI have a reduced maximum hand opening capacity but exhibit novel neuromuscular coordination strategies that permit object-specific hand aperture scaling during reaching. To test this hypothesis, we measured hand kinematics and surface electromyography from seven muscles of the hand and wrist during attempts at maximum hand opening as well as reaching for four balls of different diameters. Our results showed that persons with SCI exhibited significantly reduced maximum hand aperture compared to able-bodied (AB) controls. However, persons with SCI preserved the ability to scale peak hand aperture with ball size during reaching. Persons with SCI also used distinct muscle coordination patterns that included increased co-activity of flexors and extensors at the wrist and hand compared to AB controls. These results suggest that motor planning for aperture modulation is preserved even though execution is limited by constraints on hand opening capacity and altered muscle co-activity. Thus, persons with incomplete cervical SCI may benefit from rehabilitation aimed at increasing hand opening capacity and reducing flexor–extensor co-activity at the wrist and hand.

Force-sensitive afferents recruited during stance encode sensory depression in the contralateral swinging limb during locomotion

Afferent feedback alters muscle activity during locomotion and must be tightly controlled. As primary afferent depolarization‐induced presynaptic inhibition (PAD‐PSI) regulates afferent signaling, we investigated hindlimb PAD‐PSI during locomotion in an in vitro rat spinal cord–hindlimb preparation. We compared the relation of PAD‐PSI, measured as dorsal root potentials (DRPs), to observed ipsilateral and contralateral limb endpoint forces. Afferents activated during stance‐phase force strongly and proportionately influenced DRP magnitude in the swinging limb. Responses increased with locomotor frequency. Electrical stimulation of contralateral afferents also preferentially evoked DRPs in the opposite limb during swing (flexion). Nerve lesioning, in conjunction with kinematic results, support a prominent contribution from toe Golgi tendon organ afferents. Thus, force‐dependent afferent feedback during stance binds interlimb sensorimotor state to a proportional PAD‐PSI in the swinging limb, presumably to optimize interlimb coordination. These results complement known actions of ipsilateral afferents on PAD‐PSI during locomotion.