Examining the Relationship Between Changes in Corticospinal Excitability and Motor Learning After Acute Intermittent Hypoxia in Able-bodied Individuals for Subsequent Study in Individuals With Incomplete Spinal Cord Injury.
Phase: NA
Conditions: Incomplete Spinal Cord Injury
Sponsor: University of Colorado, Boulder

The effect of acute intermittent hypoxia on motor learning  
NCT Number NCT05341466  
Protocol Id 21-3980  
Last Updated: 3.14.25 
APPROACH: 
Methodology: We implemented a placebo controlled, two group study design that characterized how changes in 
corticospinal excitability are associated with motor adaptation and walking energetics in 26 persons following repetitive 
exposure to AIH. Each intervention involved 5 consecutive treatment visits. For each treatment visit, participants received 
an exposure [15, 1.5 min episodes at 9% O2 (AIH) or 21% O2 (SHAM) at 1 min intervals]. Split belt adaptation assessments  
and metabolic cost quantification protocols were implemented at the end of each intervention. Delivery of intermittent gas 
mixtures was accomplished using air generators [HYP123, Hypoxico] attached to a non-rebreathing face mask. In order to 
ensure subject safety during interventions, we monitored oxygen saturation (SpO2), heart rate, respiratory rate and blood 
pressure [Root 7, Masimo] at 1 second intervals. 
Transcranial Magnetic Stimulation: TMS was delivered via a custom-built batwing coil for leg stimulation connected 
to a DUOMAG unit [Deyemed]. Single pulse TMS with monophasic current waveform was applied to the contralateral 
motor cortex of the more impaired target TA muscle. Coil position and head position were digitally tracked to ensure 
similar coil placement in repeated measurements using a Visor 2LT optical system [ANT Neuro]. The hot spot was defined 
as the region where largest and most consistent MEPs are evoked using a stimulation intensity of 1.2 x resting motor 
threshold (RMT) intensity. RMT was defined as the stimulator output intensity that elicits MEPs with peak-to-peak 
amplitudes greater than 50 µV in four out of eight trials in the relaxed muscle. MEPs from the tibialis anterior were obtained 
during rest and at static contraction of up to 20% MVC. Real-time visual feedback about EMG contraction level was 
provided. The slope of the recruitment curve reflects gain in MEP amplitude with increasing stimulus intensity, providing 
an indication of gain modulation. TMS intensity was increased from below motor threshold at stimulator intensity intervals 
representing 90% to 120% of RMT. 20 randomized stimuli were given per stimulation intensity, and the MEPs were 
ensemble averaged for analyses. The mean MEP response was plotted against the corresponding stimulation intensity (% 
of RMT) to produce a stimulus-response recruitment curve. We compared recruitment curve parameters (MEPmax, slope) 
before and after AIH. 
Sensorimotor Adaptation: We characterized motor adaptation by quantifying changes in step asymmetry in response to 
split belt treadmill speed perturbations, before and after repetitive AIH exposure. Step asymmetry was quantified as the 
ratio of normalized difference in step lengths of each leg. Temporal asymmetry was computed from the time series of joint 
angles as the lag time at peak cross-correlation of limb angle trajectories. The first 5-10 strides of the ‘adaptation’ and ‘de-
adaptation’ epochs were used to quantify speed perturbation induced effects. Motor adaptation was evaluated relative to 
the baseline epoch by comparing step asymmetries during the first and last 10 steps of the ‘adaptation’ and ‘de-adaptation 
epochs’. A greater difference indicates greater motor adaptation. Improvement of interlimb coordination was estimated 
based on decreases of asymmetry in both spatial and temporal step parameters. We compared changes in step asymmetry 
and interlimb coordination between AIH and SHAM interventions. 
Net Metabolic Power : We quantified the metabolic costs during treadmill gait before and after AIH during the final 2 
minutes of the motor adaptation epochs. We identified spatiotemporal gait parameters that affect the capacity to minimize 
the metabolic cost in participants with iSCI. We measured the net metabolic power during treadmill walking as the rate of 
metabolic energy consumption. Participants wore a nose clip and breathed in and out of a mouthpiece to measure the rates 
of O2 consumption V̇o2 and CO2 production V̇co2 (liter/min) using expired gas analysis [ParvoMedics, TrueMax2400]. 
The measured rates (liter/min) of V̇o2 and V̇co2, which we then converted to metabolic power (J/s) using the Brockway 
equation as the outcome measure representing the rate of metabolic energy consumption. Metabolic power was normalized 
to the subject's body weight (J · s−1 · kg−1). To calculate net metabolic power, we subtracted baseline resting metabolic 
power while quiet standing.  
 
 

 
 
The effect of acute intermittent hypoxia on motor learning  
NCT Number NCT05341466  
Protocol Id 21-3980  
Last Updated: 3.14.15 
STATISTICAL ANALYSES: 
Hypothesis Testing for Corticospinal Excitability:   
We hypothesized that exposure to repetitive, acute intermittent hypoxia enhances CST excitability to the TA (i.e., increase 
MEPmax and recruitment curve slope) after repetitive AIH. TMS testing was completed within this time, as we expected 
this facilitation to persist for up to 1 hour post-AIH based on prior research in our lab. Data was evaluated for normality 
using the Kolmogorov-Smirnov test. If data satisfied the normality test, a repeated measures analyses of variance was used 
to determine if each excitability parameter differed across PRE and POST conditions. Sphericity was assessed using the 
Mauchly test. If sphericity was violated, we used the Huynh-Feldt corrected p value. Post hoc comparisons were used in 
the Tukey-Kramer correction for multiple comparisons. 
Hypothesis Testing for Motor Adaptation:  
Spatiotemporal asymmetry parameters were calculated in each epoch (adaptation, second adaptation) in 3 periods: initial 
(mean of first 5 strides), early (mean of strides 6-20), and the late (mean of last 30 strides). Differences from BL to 
adaptation was compared between and within interventions using a mixed model ANOVA at a 0.05 significance level; the 
between-subjects factor is intervention (AIH vs. SHAM), and the within-subjects factor is time (initial, early change, late), 
with spatiotemporal parameters (e.g., step length asymmetry, step time asymmetry) as the dependent variables. Analysis of 
covariance (ANCOVA) was used if PRE measures are significantly different between interventions. If sphericity was 
violated, we used the Huynh-Feldt corrected p value. Post hoc comparisons used the Tukey-Kramer correction for multiple 
comparisons. 
 
Hypothesis Testing for Metabolic Cost:  
We tested the hypothesis that exposure to repetitive, acute intermittent hypoxia decreased net metabolic power using a 
mixed model ANOVA at a 0.05 significance level; the between subject’s factors is intervention (AIH vs SHAM), and the 
within subjects’ factors is time (initial, early change, plateau), with net metabolic power as dependent variable. A multiple 
linear regression was implemented to determine the spatiotemporal variables associated with capacity to reduce asymmetry 
and net metabolic power for the following independent variables: MEPmax, recruitment curve slope, step length, step time, 
stance duration and baseline metabolic power. Variance inflations factors were calculated to avoid multicollinearity with 
the Akaike information criterion model utilized for model selection.