Effects of acute intermittent hypoxia on muscle strength in individuals with spinal cord injury: A systematic review of randomized trials

Introduction

Muscle weakness is among the most common motor deficits in individuals with spinal cord injury (SCI). Acute intermittent hypoxia (AIH) has been used to improve motor function by facilitating neuroplasticity. The purpose of this systematic review is to explore the impacts of AIH on muscle strength in individuals with SCI, identify who would most likely respond well to the intervention, and determine the optimal therapeutic protocol.

Methods

Relevant literature was explored in “PubMed, MEDLINE, The Cochrane Library, Scopus, PEDro, and Web of Science” databases until October 2024. Randomized trials that involved SCI patients who underwent AIH, compared with controls, and assessed muscle strength were included in this review. The methodological quality was assessed using the “Physiotherapy Evidence Database (PEDro)” scale. The effect sizes were calculated using Cohen's d.

Results

Of 502 studies, seven studies met the eligibility criteria, and the sample sizes ranged from 12 to 28 participants across the included studies. In total, 146 SCI patients (mean age 46.76 years; 88 % male) were included in this systematic review. The PEDro scores of the studies included varied between 5 and 8, with a median score of 8.

Conclusions

AIH is a promising therapeutic modality for enhancing muscle strength post-SCI, specifically in patients with motor-incomplete injuries. Based on good quality studies, delivering AIH independently or in combination with other treatments for 15 short (60–90 s) episodes of hypoxic exposure (Oxygen = 9 %) alternating with 15 (60–90 s) normoxic episodes (Oxygen = 21 %), across one or more sessions, could yield meaningful outcomes. Nevertheless, the evidence is limited by treatment protocol variations, small sample sizes, and a lack of standardization in combining AIH with other treatments. Therefore, further studies with larger sample sizes, more diverse populations, and standardized treatment protocols are strongly needed to verify our findings. Future studies should also address the potential bias, examine the long-term effects, and investigate underlying mechanisms to provide more generalized evidence.