(4) DOES INTERMITTENT HYPOXIA ELICIT DOSE-DEPENDENT STRENGTH AND NEUROMUSCULAR RESPONSES IN HEALTHY ADULTS?

Purpose: Examine the dose-dependent influence of a Low Dose (LD) and High Dose (HD) acute intermittent hypoxia (AIH) protocol on hand grip strength and neuromuscular function.

Methods: Twelve healthy subjects (mean±SD age: 25.5±7.4 yrs) were randomized to perform three AIH protocols consisting of a Sham [SH] (21% Fraction of Inspired Oxygen [FiO₂]), a LD (15% FiO₂), and HD (9% FiO₂) protocol. The AIH protocol consisted of 15 brief (60s) periods of hypoxic air exposure interspersed with 60s of normoxic air exposure delivered at a 1:1 ratio for 30-min using a hypoxic generator. Maximal isometric handgrip force (HG_MAX) was recorded from the dominant hand, and surface electromyographic amplitude (EMG_AMP) and frequency (EMG_FC) were recorded from the flexor digitorum superficialis muscle of the same dominant arm. In addition, isometric hand grip strength (HG_MAX), and neuromuscular activity were assessed before the AIH protocol (Pre), immediately following the AIH protocol (Post), and 20-min following the AIH protocol (Recovery [Rec]). All independent variables were normalized to the Pre time point during the SH condition. To assess changes in normalized force and normalized neuromuscular activity, four 3 (Condition: SH, LD, and HD) x 3 (Time: Pre, Post, and Rec) 2-way repeated measure ANOVAs (RM-ANOVAs) were performed.

Results: There were significant Protocol x Time interactions for HG_MAX (p< 0.01), and EMG_RMS (p=0.02), whereas, for EMG_MPF there were no significant interactions or main effects for Protocol or Time. Follow-up 1-way RM-ANOVAs were performed for HG_MAX, and EMG_RMS across Protocols and Times. For HG_MAX, there was a 10% Pre-Post increase following LD treatment (p=0.01), whereas, for the HD treatment, there was a 14.2% Pre-Post increase (p=0.02) and a 6.2% Post-Rec increase (p=0.03) observed. In addition, there were no significant differences in HG_MAX across protocols during the Pre and Rec time points (p>0.05), however, during the Post time point, HG_MAX was on average 13.2% greater in the LD and HD protocols compared to the SH (p< 0.01), with no differences observed between the LD and HD protocols. For EMG_AMP there was an 11% Pre-Post increase (p=0.03) followed by an 8% Post-Rec decrease (p=0.01) during the HD protocol, with no differences observed during the LD or SH protocols. In addition, the HD protocol elicited a 14.3% increase in EMG_AMP relative to the SH condition (p=0.02), with no detectable differences observed during the LD protocol or throughout the Pre or Rec time points.

Conclusion: These results suggest a dose-dependent increase in grip strength (HG_MAX) from LD to HD compared to SH, with sustained strength increases remaining for 20-min following the HD protocol. In addition, EMG_AMP, which reflects muscle activation, was sensitive to the HD protocol where muscle activation and grip strength tracked similarly following the HD protocol. However, grip strength remained elevated with a concomitant decrease in muscle activation following 20-min of Rec. PRACTICAL APPLICATIONS: These findings suggest that a greater hypoxic dose (lower FiO₂) used during an AIH protocol can provide sympathoexcitatory increases in isometric grip strength and muscle activation, with functional strength remaining for 20-min following the hypoxic stimulus. These results may be important to strength and conditioning specialists who may want to implement AIH as a pre-competition stimulus to improve short-term strength and power performance.

Acknowledgements: None