S-Index stands for Strength-Index. It is one of the POWERbreathe K-Series’ test modes. Its purpose is to calculate inspiratory muscle strength based upon peak inspiratory flow.
Peak Inspiratory Flow (PIF)
We can evaluate improvements in inspiratory muscle strength by monitoring changes in a person's peak inspiratory flow.
Peak Inspiratory Flow is a measure which reflects the ability of the inspiratory muscles (the muscles we use to breathe in) to contract rapidly and overcome the inherent resistance and elastance of the respiratory system.
Another of the K-Series’ test modes is the measurement of maximal inspiratory pressure (MIP). In fact, MIP is the most common measure in use for gauging inspiratory muscle strength. It is used as a diagnostic tool and an independent predictor of all-cause mortality.
Purpose of study
Currently, the most common test for assessing inspiratory muscle performance is the maximum ‘quasi-static’ inspiratory pressure (PImax).
However, the K-Series’ S-Index test has since become available for ‘dynamically’ evaluating the maximum inspiratory pressure.
In fact, it is suggested that the S-Index might be more appropriate for measuring inspiratory muscle performance than PImax.
Therefore, this study investigates this premise. It also assesses its reliability and whether an inspiratory muscle warm-up effects strength—index assessment.
Measurement validity of the K-Series
There are, in fact, current studies that have independently verified the measurement validity of the K-Series. Consequently, findings from these previous studies demonstrate its accuracy to measure dynamic inspiratory muscle pressure 1,2.
Therefore, this study feels that a proper assessment and the reliability of the S-Index should be addressed. In particular, it investigates the variability in response to repeated measurements. Furthermore, it evaluates whether an inspiratory muscle warm-up effects strength-index assessment. It is investigating whether using strength-index assessment improves clinical outcomes by reducing the bias effect.
Reliable values of the S-Index
What this study demonstrates is that at least 8 inspiratory manoeuvres are necessary to reach maximum and reliable values of the S-Index. Moreover, it also shows that specific inspiratory muscle warm-up could improve inspiratory muscle performance.
The authors believe this to be the first study to evaluate S-Index reliability in healthy subjects. Furthermore, they believe it to be the first study to investigate the effect of inspiratory warm-up in strength-index assessment.
In conclusion, inspiratory muscle warm-up should be used for detecting the true maximum values of the S-Index to evaluate the performance of inspiratory muscles for any intervention.