Intermittent Hypoxia and Inspiratory Threshold Loading to Enhance Inspiratory Muscle Function

Rehabilitation approaches introduce a stimulus to a motor system, with the goal to enhance motor function to patients. For example, exposure to brief and intermittent episodes of mild hypoxia has shown to strengthen synaptic pathways to respiratory and skeletal muscle motor neurons. In humans with spinal cord injury, exposure to intermittent hypoxia (IH) alone or in combination with rehabilitative strategies has shown enhanced motor function. Another strategy known as inspiratory threshold loading, which involves breathing against pressure threshold loads, results in improved inspiratory muscle strength. Although there is evidence supporting the use of IH alone or in combination with other rehabilitative strategies in improving motor function in humans, the impact of exposure to IH or IH with inspiratory threshold loading on inspiratory muscle function and ventilation in humans is unknown.

Investigators will test whether exposure to IH alone and in combination with inspiratory threshold loading (ITL) will result in increased inspiratory muscle strength and ventilation.

Investigators will use a Latin square, repeated measures design which will consist of 12 subjects participating in four experimental conditions.There will be an interval of 1 week between each of the four conditions, which will serve as a washout period.

At the end of each of the 5 sessions, the subjects will rest in a reclining chair for 15 minutes while their heart rate, blood pressure, oxygen saturation and breathing are monitored. Subjects that have a blood systolic blood pressure \> 140 or diastolic pressure \> 90 or SPO2 \< 92% will be asked to remain in the laboratory for 15 additional minutes and the vital signs will be repeated. If the blood pressure and/or peripheral capillary oxygen saturation (SPO2) remain outside of the criteria mentioned above, the subject will be instructed to seek assessment by their health provider and the incident will be reported to the Institutional Review Board. Subjects will also be asked to report any discomfort.

Eligibility Criteria:
Inclusion Criteria:

* Participating in three or fewer musculoskeletal strength training sessions per week at an intensity equivalent to 7-10 on the Modified Borg's scale
* Participating in three or fewer cardiovascular endurance training sessions per week at an intensity equivalent to 7-10 on the Modified Borg's scale

Exclusion Criteria:

* Diagnosis of cardiovascular disease (Hypertension, arrhythmias, coronary artery disease, congenital and valvar heart diseases)
* Diagnosis of neuromuscular disease
* Diagnosis of any neurological disease
* Presence of concurrent medical illness including infection, fractures
* Diagnosis of obstructive sleep apnea
* Diagnosis of obstructive/restrictive lung disease
* Diagnosis of exercise induced asthma
* Forced expiratory volume at one second/forced vital capacity (FVC) \<80% and/or FVC\<80% of predicted value indicating airway obstruction
* Subjects on prednisolone therapy or selective serotonin reuptake inhibitor (SSRI) therapy will be excluded from the study.
* Diagnosis of epilepsy or history of seizures and attention deficit disorders
* Pregnancy
* Diabetes
* History of coagulation disorders
* History of chronic pain
* Body mass index(BMI)\> 35kg/m2
* Subjects on prescription medicines such as beta blockers and other drugs that are prescribed in any of the exclusionary disorders listed above.
* Any other factor that in the investigators' opinions would prevent response to training or create an unsafe condition for the subject.