Inspiratory Muscle Training Changed HRV Signals, but Evidence Remains Very Uncertain

A new systematic review found that low-to-moderate inspiratory muscle training was associated with shifts in heart-rate variability often interpreted as greater parasympathetic activity—but the evidence was very uncertain, and breathing mechanics may explain part of the signal.

Inspiratory muscle training asks the breathing muscles to work against resistance, usually through a handheld device. It is already studied for respiratory performance and exercise tolerance, but a review published September 4 in Frontiers in Cardiovascular Medicine examined a narrower question: does training the inspiratory muscles change cardiac autonomic function in healthy adults?

The review included 13 reports representing 10 independent cohorts and 214 participants from ages 18 to 80. Six reports studied chronic training lasting four to 11 weeks, while seven examined acute responses to a single inspiratory-loading session.

At lower-to-moderate loads—generally around 30% to 60% of maximal inspiratory pressure—the studies often reported increases in vagal-related heart-rate-variability measures. But the reviewers graded the certainty of evidence as very low across outcomes because samples were small, protocols differed substantially and respiratory pattern itself can change HRV even when underlying autonomic control has not meaningfully adapted.

What Inspiratory Muscle Training Actually Is

Inspiratory muscle training, often shortened to IMT, typically uses a pressure-threshold device that makes inhalation harder. The diaphragm and other inspiratory muscles must generate enough pressure to open a valve, creating a repeatable form of resistance training for breathing.

Researchers often set the resistance as a percentage of maximal inspiratory pressure, or MIP. That makes 30% MIP a lower load than 60% MIP in the same person, while allowing the training to be scaled to individual respiratory strength.

The new review separated acute experiments from multiweek training programs because they answer different questions. An immediate HRV change during or after a breathing session may reflect the mechanics of breathing, while a change after several weeks could represent adaptation—or simply a more persistent effect of altered respiratory behavior.

What the Review Found at Lower Loads

Several studies using low-to-moderate inspiratory resistance reported increases in measures such as RMSSD and high-frequency HRV, which are commonly associated with cardiac vagal modulation. In some chronic studies, changes appeared after roughly two weeks of training and diminished after training stopped.

That pattern is interesting, but it needs careful interpretation. Heart-rate variability is shaped by breathing rate and depth as well as autonomic signaling. Slow or more pronounced breathing can increase respiratory sinus arrhythmia, which may make vagal-related HRV indices rise even if the person’s baseline autonomic state has not fundamentally changed.

The review therefore warns against treating a higher HRV number as automatic proof of a healthier nervous system. This is especially relevant for fitness enthusiasts who track HRV with wearables and may assume that any intervention that raises the metric is necessarily improving recovery.

Why Higher Resistance Looked Different

Studies using higher or sustained inspiratory loads—often above about 60% of maximal inspiratory pressure or taken toward fatigue—tended to show a different autonomic pattern. Some reported relatively greater low-frequency activity and lower high-frequency activity during the loading period.

The reviewers discuss the respiratory muscle metaboreflex as one possible explanation. When breathing muscles work hard enough to accumulate metabolites and fatigue, the body can increase sympathetic vasoconstrictor activity to help preserve blood flow to the respiratory muscles.

That does not mean higher-load inspiratory training is inherently harmful, nor does it mean lower loads are always superior. It means intensity changes the physiological problem. A gentle breathing-resistance session and a fatiguing respiratory workout should not be expected to produce identical cardiovascular responses.

Why the Evidence Is Still Very Uncertain

The biggest limitation is scale. Across 13 reports there were only 214 participants, and individual studies commonly enrolled a few dozen people or fewer. That makes results more vulnerable to random variation and makes it difficult to identify who is most likely to respond.

The evidence base was also concentrated. Nine of the 13 reports came from three Brazilian research groups. Participants were often older women or younger male cyclists, while middle-aged adults and broader recreational populations were less represented.

Methods varied substantially. Studies used different inspiratory loads, session durations, breathing frequencies, HRV recording periods and analysis methods. Because of that heterogeneity, the reviewers did not pool the results in a conventional meta-analysis.

Another limitation is measurement context. Short resting HRV recordings can be strongly influenced by breathing pattern, posture and recent activity. The review found little use of 24-hour ambulatory HRV, which would provide a broader picture of autonomic behavior away from the training session.

Most importantly, the studies did not establish that an HRV change translated into better performance, fewer cardiovascular events, improved recovery between workouts or meaningful long-term health benefits.

What Fitness Enthusiasts Can Take From It

The review supports curiosity more than prescription. Inspiratory muscle training appears capable of changing HRV signals, and the direction of that response may depend on load. But a wearable HRV number should not become the reason to add breathing resistance to a program.

If someone is already using IMT for a specific sport, respiratory limitation or clinician-guided goal, the new review provides useful context: autonomic measures may change alongside respiratory training, but those changes are not yet established as independent health benefits.

The finding also complements recent Fitness Living coverage of heart-rate-variability recovery after resistance training. Both stories show why short-term autonomic measurements require restraint: they can reveal physiology without automatically predicting long-term training outcomes.

For cardiorespiratory fitness itself, our report on the new VO₂ max fitness-test framework highlights a more established performance outcome. HRV can add context, but it is not a substitute for measuring the capacities a training program is intended to improve.

A Practical Way to Use This Research

Fitness enthusiasts do not need to buy an inspiratory trainer solely to improve HRV. The evidence is too uncertain to support that conclusion, and standard aerobic exercise, resistance training, adequate sleep and appropriate recovery remain much better established foundations.

If you do use an IMT device, follow the device instructions and any professional guidance relevant to your health or sport. Avoid assuming that more resistance is better: the review suggests that higher loads create a distinctly different physiological response.

And if you track HRV, interpret trends alongside sleep, training load, resting heart rate, symptoms and performance rather than treating a single metric as a scorecard for nervous-system health.