Acute Effects of Functional Inspiratory Muscle Training in Elite Male Volleyball Players

This prospective, randomized, controlled, parallel-group interventional study is designed to investigate the acute effects of a single session of functional inspiratory muscle training on muscle oxygenation, functional exercise capacity, dynamic balance, and agility in elite male volleyball players aged 18 to 25 years. The study is conducted at the Cardiopulmonary Rehabilitation Unit, Department of Physiotherapy and Rehabilitation, Faculty of Health Sciences, Gazi University. A total of 30 participants are planned, with 15 participants allocated to the functional inspiratory muscle training group and 15 participants allocated to the no-intervention control group.

After eligibility screening and written informed consent, participants are randomly assigned to the study groups in a 1:1 ratio. The computer-generated allocation sequence is created using the Sealed Envelope online randomization service. Outcome assessments are performed by physiotherapists who are not informed of group allocation. Participants and the personnel administering the training cannot be masked because participants in the intervention group use a POWERbreathe inspiratory muscle training device, whereas participants in the control group receive no sham inspiratory muscle training. Therefore, the study uses single masking at the level of the outcome assessor. When feasible, the statistician will receive the dataset with the treatment groups coded.

Before the training session, maximal inspiratory pressure is measured in the intervention group using a mouth pressure meter to determine the individual training resistance. Participants are assessed in a seated position while wearing a nose clip and maintaining a complete seal around the mouthpiece. After a complete expiration to residual volume, participants are instructed to perform a maximal inspiratory effort. At least three acceptable maneuvers are performed, with a 30-second rest period between maneuvers. When the difference between repeated measurements is less than 10%, the highest value is recorded as maximal inspiratory pressure in cmH2O. The training resistance is set at 50% of the individual maximal inspiratory pressure. Because maximal inspiratory pressure is measured only to determine the intervention dose and is not assessed before and after the study period in both groups, it will not be treated as a comparative study outcome.

Following warm-up, participants in the intervention group perform functional inspiratory muscle training using a POWERbreathe pressure-threshold device while completing a sport-specific fixed-ball throwing drill. The session consists of two sets of 30 resisted inspirations, corresponding to a total of 60 breaths. The session lasts approximately 10 to 12 minutes. Inspiration is performed against a resistance equivalent to 50% of the participant's maximal inspiratory pressure, whereas expiration is unrestricted. The training is integrated into the throwing drill without intentionally interrupting the participant's sport-specific movement pattern. Participants in the control group receive no inspiratory muscle training or sham respiratory intervention and undergo the same pre-intervention and post-intervention outcome assessments.

All outcome assessments are conducted before and immediately after the single-session intervention or corresponding control period on the same day. The post-intervention assessments are intended to determine the immediate acute response to the training session. The main muscle oxygenation outcomes are quadriceps muscle oxygen saturation and total hemoglobin. Functional exercise capacity, dynamic balance, and agility are evaluated as additional outcomes.

Local muscle oxygenation is monitored noninvasively using a portable Moxy near-infrared spectroscopy device positioned over the midpoint of the right quadriceps femoris muscle. The placement site will be kept consistent between the pre-intervention and post-intervention assessments. The Moxy device continuously records muscle oxygen saturation, expressed as a percentage, and total hemoglobin, expressed in g/dL. Recordings are obtained during three prespecified phases: rest before the Incremental Shuttle Walk Test, exercise during the Incremental Shuttle Walk Test, and recovery after test termination. For each assessment, muscle oxygen saturation and total hemoglobin will be summarized over predefined rest, exercise, and recovery analysis epochs. The beginning and end of each analysis epoch and the summary metric used for that epoch will be determined in the statistical analysis plan before outcome analysis. Pre-to-post changes in these muscle oxygenation measures will be compared between the intervention and control groups.

Functional exercise capacity is assessed using the Incremental Shuttle Walk Test on a 10-meter course. Participants walk back and forth between two markers in accordance with externally generated audio signals. Walking speed increases by approximately 0.17 m/s every minute. The test is terminated when the participant is unable to maintain the required pace, cannot follow the audio signals, or stops because of limiting fatigue or symptoms. The total distance completed is recorded in meters. A greater distance represents better functional exercise capacity. The pre-intervention to post-intervention change in total distance will be compared between groups.

Dynamic balance and postural control are assessed using the Star Excursion Balance Test. The participant stands on the dominant leg and reaches as far as possible with the opposite leg in eight directions separated by 45-degree intervals. Three trials are performed in each direction. The longest valid reach distance in each direction is recorded in centimeters. Reach distance is normalized to limb length using the following calculation: normalized reach distance (%) = reach distance (cm) divided by limb length (cm), multiplied by 100. Higher normalized values indicate better dynamic balance. Direction-specific pre-to-post changes will be compared between the intervention and control groups.

Agility is assessed using the T-Test. The participant completes a 40-meter movement sequence involving forward running, lateral shuffling to both sides, and backward running while remaining oriented in the same direction. The course includes two 90-degree changes of direction and one 180-degree change of direction. Completion time is recorded in seconds using the same timing method at the pre-intervention and post-intervention assessments. The fastest valid completion time obtained under the study protocol is used for analysis. A shorter completion time indicates better agility performance. The pre-to-post change in completion time will be compared between groups.

Statistical analyses will be performed using IBM SPSS Statistics version 25 with a reproducible syntax file. Participant characteristics and baseline measurements will be summarized by randomized group. Normally distributed continuous variables will be presented as mean and standard deviation, non-normally distributed variables as median and interquartile range or minimum-maximum values, and categorical variables as frequencies and percentages. Baseline significance testing will not be used to determine whether randomization was successful.

Participants will be analyzed according to their randomized groups. The principal treatment effect will be the difference in pre-intervention to post-intervention change between the intervention and control groups. This effect will be tested using the group-by-time interaction rather than separate within-group significance tests.

For Incremental Shuttle Walk Test distance and T-Test completion time, the primary analysis will use a 2 × 2 mixed-design repeated-measures analysis of variance, with group as the between-participant factor and time as the within-participant factor. The group-by-time interaction will represent the acute effect of functional inspiratory muscle training.

Muscle oxygen saturation and total hemoglobin will be analyzed separately using models that include group, time, and measurement phase. Group will include the intervention and control groups, time will include the pre-intervention and post-intervention assessments, and phase will include rest, exercise, and recovery. The group-by-time interaction will determine whether the overall pre-to-post response differs between the groups. The group-by-time-by-phase interaction will determine whether the training response differs across the rest, exercise, and recovery phases. The primary muscle oxygenation contrast will be the between-group difference in pre-to-post change during the prespecified exercise phase. Because muscle oxygen saturation and total hemoglobin are treated as two related primary oxygenation outcomes, their primary probability values will be adjusted using the Holm procedure.

Normalized Star Excursion Balance Test reach distances will be analyzed using a model that includes group, time, and reach direction. The group-by-time interaction will evaluate the overall balance response, and the group-by-time-by-direction interaction will determine whether the response differs among the eight reach directions. Direction-specific follow-up comparisons will be performed only when supported by the corresponding interaction and will be adjusted for multiple comparisons.

When a statistically significant interaction is identified, prespecified simple-effects comparisons of estimated marginal means will be performed with Bonferroni adjustment, as appropriate. Results will be reported using pre-intervention and post-intervention values, mean within-group changes, the between-group difference in change, 95% confidence intervals, exact two-sided p values, and partial eta-squared effect sizes. Statistical significance will be set at a two-sided p value below 0.05 after any applicable multiplicity adjustment.

Model residuals will be evaluated using graphical methods, including Q-Q plots, and the Shapiro-Wilk test when appropriate. Homogeneity of variance will be assessed using Levene's test. For models containing more than two repeated levels, sphericity will be evaluated using Mauchly's test, and Greenhouse-Geisser-corrected results will be reported when the sphericity assumption is violated. If substantial deviations from the model assumptions are identified, appropriately transformed outcomes, robust analyses, or rank-based sensitivity analyses will be considered.

A baseline-adjusted analysis of covariance, with the post-intervention value as the dependent variable and the corresponding baseline value as a covariate, will be performed as a sensitivity analysis for the principal outcomes. If repeated outcome measurements are incomplete, a linear mixed-effects model using all available observations will be used as an additional sensitivity analysis under a missing-at-random assumption. Because of the limited sample size, no data-driven subgroup analysis or extensive covariate selection is planned.

The study was designed and conducted as a prospective randomized interventional study. However, registration occurred after participant enrollment had begun. Therefore, the registration is retrospective in relation to the beginning of enrollment, although the study design itself is not retrospective. The date of first participant enrollment and the reason for delayed registration will be reported transparently.