During the past two decades, the favorable effects of inspiratory muscle training (IMT) in health and disease have become much more apparent (14,15,18,29). A variety of IMT methods exist, but few studies have compared IMT methods. The two most common forms of IMT are resistive and threshold IMT; currently, only two studies comparing these two forms of IMT are available (17,23). The results of these two studies indicate that resistive and threshold IMT elicit similar training effects (17,23). However, other forms of IMT exist and include specific inspiratory breathing exercises and calisthenics, isocapnic/normocapnic hyperventilation, and a form of isokinetic-like IMT referred to as inspiratory flow resistive loading (IFRL). This latter approach has been defined as the Test of Incremental Respiratory Endurance (TIRE), available through the RT2 (DeVilbiss Healthcare, Wollaston, UK) or TRAINAIR (Project Electronics Limited, Kent, UK) IMT devices. Based on the favorable results that we and others have observed as well as the novel outcome measures from the TIRE, the focus of this article will be on the current body of evidence assessing TIRE testing and training (3–8,10–13,20,22,24,30) and the development of several hypotheses related to this approach. The hypotheses include that 1) TIRE testing provides a template from which a variety of outcome measures and IMT methods are made possible; 2) several TIRE tests are directly related to inspiratory muscle physiology as well as cardiorespiratory fitness; and 3) targeted TIRE IMT will significantly alter inspiratory muscle physiology as well as cardiorespiratory function, which will improve exercise performance in both apparently healthy and patient populations.
MEASUREMENTS AVAILABLE FROM TIRE IMT
Figure 1 presents the manner by which TIRE testing and IMT are performed as well as the TIRE testing and training template obtained from the RT2 device in which the maximal inspiratory pressure (MIP), sustained MIP (SMIP), and SMIP inspiratory duration can be seen (3–8,10–13,20,22,24,30). The MIP is the highest pressure measured during inspiration and is measured at residual volume (RV), with the unit of measure being centimeters of water (cm H2O). The MIP shown in Figure 1B is 118 cm H2O. The standardized methods that are used to measure MIP encourage participants to inspire deeply and generate as much pressure as possible within 1 to 2 s of inspiration (3–8,10–15,17,18,20,22–24,29,30).
The SMIP, on the other hand, is measured from RV to total lung capacity (TLC) and represents the work under the curve that is generated from the start of inspiration to MIP to the end of inspiration at 10.25 s on the x axis of Figure 1B. The SMIP unit of measure can be described in both joules and the area under the curve with the area under the curve unit of measure being pressure time units (PTU) (Fig. 1B). In view of this, the sustained maximal inspiratory pressure (SMIP) has been described as single-breath inspiratory work capacity and represents single-breath work/endurance. The SMIP presented in Figure 1B represents 12.67 J and 493.31 PTU. We have found that the SMIP is correlated significantly to a number of physiologic measures (5,13,22,24,30). We also believe that the slope of the SMIP line is important and represents a unique physiologic perspective on inspiratory muscle performance in much the same way as that of the Wingate exercise test or isokinetic dynamometry (19,21). The slope of the SMIP line in Figure 1B is −13.13.
Finally, we also believe that the SMIP inspiratory duration is important and is not simply the measurement of inspiratory duration but rather inspiratory flow during a maximal inspiratory effort through the 2-mm opening in the RT2 or TRAINAIR mouthpiece. A 2-mm leak is commonly used to prevent the generation of pressure from the buccal muscles, however, as used in TIRE testing and training, it also allows IFRL (3–8,10–15,17,18,20,22–24,29,30). Resistance with IFRL is dependent on the velocity or flow of inspiration. A faster airflow correlates with greater inspiratory muscle power and pressure generation capability, which has been corrected for and adjusted with specific software within the TRAINAIR and RT2 devices, providing an accurate workload to the inspiratory muscles (3–8,10–13,20,22,24,30). Thus, the SMIP inspiratory duration reflects the time one is able to inspire from RV to TLC with an isokinetic-like resistance via the 2-mm opening in the mouthpiece and in Figure 1B is shown to be 10.25 s.
In view of the above, the SMIP template provides valuable information that allows a better understanding of inspiratory performance. In fact, inspiratory power, an index available via the TIRE RT2, incorporates flow and pressure as a result of previous work that established a calibration curve for the electronic manometer via the 2-mm leak of the mouthpiece. This provides the volume of air entering the manometer for a given pressure to be determined and enables the conversion of pressure to energy and power via: Power (P) = p × Q where p is pressure expressed as N m-2 and Q is flow rate expressed in m3 s-1 using a calibration constant for the 2-mm leak of Q = 3.226 × 10−6 x √p, with inspiratory power expressed in watts and inspiratory work expressed in joules (7,8,10,24). Furthermore, the measurement of inspiratory work per breath can be obtained from the power curve and can be expressed in joules per breath of inspiratory work (7,8,10,24). These characteristics of the inspiratory power profile provide novel measures that expand the clinical utility of SMIP and IMT.
TIRE IMT TECHNIQUE
TIRE IMT is characterized by the serial presentation of submaximal isokinetic-like profiles based on the MIP/SMIP (3–8,10–13,20,22,24,30). These efforts are presented at an on-screen target of any desired percentage of MIP/SMIP using a progressive work-to-rest ratio, with rest periods decreasing from 60 s at level A to 45, 30, 15, 10, and 5 s at levels B through F, respectively. Each level has six resisted inspiratory efforts, and the rest periods can be modified (increased or decreased) based on desired outcomes. Participants continue to inspire and match the on-screen target that typically is set at 60% to 80% of MIP/SMIP while wearing a nose clip (Fig. 1). Thus, IMT continues until task failure, indicated by an inability to match the on-screen target, or until a maximum of 36 resisted inspirations have been performed (3–8,10–13,20,22,24,30). It is important to note that each subsequent IMT session begins with a testing session providing the same variety of outcome measures described above.
WHY IS TIRE TESTING AND TRAINING ISOKINETIC-LIKE AND IS IT A GOOD THING?
These questions are important as we begin our comparison of IMT methods and an overview of TIRE inspiratory testing and training. Isokinetic exercise is administered by dynamometers that enable the contraction of peripheral skeletal muscle to be performed at a constant velocity with accommodating resistance (9,16,21). The constant velocity and accommodating resistance have been touted as major safety aspects of isokinetic muscle contractions compared with other types of muscle contractions (e.g., isotonic, isometric) (9,16). An additional characteristic of isokinetic exercise is that the dynamometer measures a variety of skeletal muscle outcomes including strength, endurance, range of motion, duration of contraction, peak torque and the degradation of torque, work, velocity, power, and mode of contraction. Also, the degradation of torque during isokinetic skeletal muscle endurance testing has been correlated to muscle fiber type (9,16,21). Furthermore, isokinetic testing and exercise have been suggested to be safe in even the weakest, most frail individuals (9,16,21).
Is TIRE testing and training of the inspiratory muscles via an isokinetic-like IFRL method a good thing? From a safety perspective, the constant velocity and accommodating resistance during peripheral skeletal muscle testing and training can be extrapolated to the inspiratory musculature. Thus, TIRE testing and IMT are appropriate for markedly impaired individuals with the potential to improve functional status and a variety of physiologic measures as previously shown (3,4,6–8,10,20), and it also is appropriate for apparently healthy persons (5,11,12) as well as those with higher levels of fitness and functional status because it seems to have the capacity to further improve work capacity and the physiologic response to exercise (13,22,24,30). Secondly, the biofeedback from isokinetic testing of peripheral skeletal muscle elicits greater testing and training results (9,16,21), and the same seems to be true of TIRE testing (3–8,10–13,20,22,24,30). Third, the numerous outcome measures obtained from isokinetic testing of the peripheral muscles described above are possible from TIRE testing and are listed in Table 1 (3–13,16,20–22,24,30).
Several additional aspects directly related to TIRE IMT are worthy of mention, including 1) the fact that interval or incremental training is used in many skeletal muscle training programs that facilitates the implementation of an effective training intensity within a shortened exercise duration; 2) the suggested American College of Sports Medicine training frequency of three to five times per week allows for physiological and morphological adaptation from the imposed training program; 3) muscle training loads and the design of exercise programs are governed by the principles of overload, specificity, and reversibility, highlighting the fact that exercise can be designed specifically to obtain a particular training response. However, if one or more training parameters are set too low or if training ceases, the training effect will either not be reached or lost; and 4) long-term compliance with imposed effective training loads is further facilitated by optimal feedback to participants in exercise programs (1,3–13,16,20–22,24,30). TIRE testing and training are characterized by many, if not all, of the previously mentioned factors of which each will be presented throughout the remainder of this article.
TIRE IMT OUTCOMES
Several of the TIRE inspiratory muscle testing outcomes listed in Table 1 that are available through TIRE RT2, TRAINAIR, and Excel software (Microsoft, Seattle, WA) packages will be highlighted below and include inspiratory work and slope of the SMIP power profile. Several indices of fatigue also will be presented later in the article.
Inspiratory Work: PTU, Joules, and Watts
As shown in Figure 1B, TIRE testing and training provide a template from which additional testing outcomes can be obtained (3–8,10–13,20,22,24,30). Figure 2 provides a new TIRE template that will be used to outline the calculation of inspiratory work, the slope of the SMIP line, and the TIRE fatigue index from the Excel worksheet shown in Table 2. Inspiratory work via the TIRE is obtained from the SMIP profile shown in Figure 2 and the data outlined in Table 2. Inspiratory work measured in PTU is automatically calculated by the TIRE software, whereas other indices require Excel. Inspiratory work measured in Joules is displayed in column five of Table 2 and is calculated using the MIP (shown in column 2) at each second of inspiration multiplied by the constant 98.1 to convert cm H2O to Newton Meters as shown in column three. Thus, the MIP obtained at 1.5 s of inspiration is 158 cm H2O and is equivalent to 15,499.8 Newton Meters (158 x 98.1) and after multiplying 15,499.8 and inspiratory flow shown in column 4 (0.000401631) yields 6.225203 watts (per second) or 6.2 joules which decreases to 0.099122 watts or 0.099 joules at 16 seconds of inspiration as shown in column five of Table 2. Column 5 of Table 2 presents the power output through the full range of inspiration (from RV to TLC) expressed as watts. The total joules expended throughout inspiration is 40.3099, as shown at the bottom of column 5 in Table 2.
Column 4 of Table 2 presents inspiratory flow against the resistance of the 2-mm leak expressed in meters cubed per second, which is the equivalent of milliliters per second and is derived from the orifice flow function for the 2-mm leak (3.226 × 10 -6√p). Thus, the inspiratory flow at 1 s is 0.000402 L and decreases to 0.000101 L at 16 s of inspiration. The total inspiratory flow or resisted inspiratory vital capacity (IVC) is 4.413 L, as shown at the bottom of column 4.
Slope of the SMIP Power Profile
The slope of the SMIP power profile can be calculated easily using the slope function of Excel, and we hypothesize that it provides valuable information regarding inspiratory muscle fiber type or changes associated with maladaptations caused by disuse and/or disease or, conversely, positive adaptations caused by training. The degradation of torque during isokinetic peripheral skeletal muscle exercise has been found to be significantly correlated to muscle fiber type, with individuals having a greater proportion of fast-twitch muscle fibers generating significantly greater peak power, rate of power production, and work compared with persons with predominantly slow-twitch muscle fibers (21). The degradation of pressure in the SMIP profile is practically identical to the degradation of torque during isokinetic testing of peripheral skeletal muscle (Fig. 3A). The SMIP profile also is similar to the Wingate exercise test power output shown in Figure 3B that also has been found to be related to muscle fiber type (19). We recently examined the slope of the SMIP power profile (watts) in patients with chronic obstructive pulmonary disease (COPD) and cystic fibrosis (CF) and found that that slope of the SMIP profile in patients with CF was significantly greater than that of patients with COPD (Fig. 4A). We believe that this difference is caused by the adaptation of the inspiratory muscles to chronic COPD in which a known disease-specific adaptation from Type II muscle fibers to Type 1 muscle fibers occurs to maintain the efficiency of breathing (26). Conversely, although patients with CF have an obstructive lung disease, the need to generate greater inspiratory and expiratory pressures to overcome the restrictive pathophysiology from recurrent pneumonias and pulmonary fibrosis as well as facilitate secretion removal may maintain a greater proportion of Type II inspiratory muscle fibers, yielding a steeper slope of the SMIP profile (26). It also is possible that fiber type transitions may differ between the diaphragm and accessory muscles of inspiration (26).
In addition, the time frame of disease progression is different between the two patient groups, and these observations may change as longevity increases in CF. However, it may be observed simply that there is a basic endurance profile in COPD and a power profile in CF. These profiles can be changed by IMT and, therefore, do seem to reflect adaptation to disease as well as training (10,26). Such adaptation, however, does not imply that maximal “self-training” has occurred. It may be that, during an exacerbation, both individuals with CF and COPD need more endurance and power, respectively, to cope with the acute change in loading consequent on the increased burden of acute infection or inflammation (10,26). Effective IMT may provide a reserve capacity created by planned adaptive response after targeted IMT programs (10,26). Finally, the slope of the SMIP power profiles shown in Figure 4A can be compared with the slope of the SMIP profiles in Figures 1B, 2, and 4B, which are SMIP profiles of healthy adults and demonstrate the above differences in SMIP in health and disease.
The slope of the SMIP profile in Figure 2 is -8.99, which is less steep than the slope of the SMIP profile in Figure 1B (−13.13) and has a longer SMIP inspiratory duration. The slope of the SMIP power profile presented in watts in Figure 4A of patients with COPD is - 0.08, and the slope of the power profile for patients with CF is - 0.20. Finally, Figure 4B shows the effects of IMT and detraining (8 wk after IMT) on the SMIP power profile in Division 1 college ice hockey players. The slope of the SMIP power profile after IMT was significantly greater than the profile before IMT but not significantly different from the detraining SMIP power profile (30). Further investigation of the SMIP power profile is warranted based on the previously mentioned findings.
The Tension-Time Index (TTi) was first described by Bellemare and Grassino (2) and provides an index of the likelihood of respiratory muscle fatigue. Previously, Roussos and Macklem (28) suggested that respiratory failure will occur when the work demanded of the ventilatory pump exceeds its energy supply. This also may be described as the point at which the work required to breathe exceeds the individual’s capacity to perform that work (2,28). The TTi is a dimensionless measure that relates the strength of the inspiratory muscles to the time that they are used and is the product of two quotients: the average transdiaphragmatic pressure or the pressure generated during the act of tidal breathing (Pdi) divided by the maximum transdiaphragmatic pressure (PdiMax; thus, Pdi/PdiMax) as well as the time that the inspiratory muscles are actively contracting (Ti) divided by the total respiratory cycle time (Ttot; thus, Ti/Ttot). The TTi, therefore, is Pdi/PdiMax * Ti/Ttot (2,28).
To avoid invasive testing of transdiaphragmatic pressures using esophageal balloons, the TTi also has been measured using mouth pressures including the standard MIP from RV and incorporates the average inspiratory pressure during tidal breathing (Pi) divided by the MIP (Pi/MIP) as well as the time the inspiratory muscles are contracting (Ti) divided by the total respiratory cycle time (Ttot; thus, Ti/Ttot) (27). This noninvasive measure of inspiratory muscle fatigue has been referred to as the TTi of the respiratory muscles (TTmus) and is therefore, Pi/MIP * Ti/Ttot. Higher TTmus and TTi values are associated with increased inspiratory loads, decreased inspiratory strength, or an imbalance between the two. Furthermore, increased TTmus and TTi are indicative of inspiratory muscle fatigue and decreased inspiratory endurance (2,25,27,28).
The TTi and TTmus both have a critical value of approximately 0.15 above which respiratory muscle fatigue is deemed likely to occur (2,25,27,28). Higher values will occur when Pdi is decreased because of weak inspiratory muscles and diminished force generation because of mechanical disadvantage or Pi is increased because the work of breathing is higher (2,25,27,28). Calculation of TTmus via the TIRE software is possible and requires a subject to perform a MIP and tidal volume breath (against the constant resistance of the 2-mm leak) to obtain the necessary variables to input into the equation (TTmus = Pi/MIP * Ti/Ttot) of which the only variable not readily apparent is Ttot. However, inputting the rest period between breaths at each level of TIRE IMT provides a unique surrogate measure of Ttot during TIRE IMT. Thus, at level A, there is a 60-s rest between the six level A breaths and, if the inspiratory duration is 15 s, Ttot would be 75 s (60 s of rest at TIRE level A + 15 s inspiratory duration), yielding a Ti/Ttot value of 15/75.
As apparent from the previous discussion, TIRE IMT offers measurements of power output and work capacity through a range of lung volumes from RV to TLC. It also acts as a method of fixed-load high-intensity IMT, which reflects the premise that an effective training load also is a fatiguing load. The default setting used in most published studies of TIRE IMT is 80% of SMIP (3–8,10–13,20,22,24,30). As previously described, rest periods during the six levels (A to F) of TIRE IMT decrease from 60 s at level A to 5 s at level F. Each of the six levels of TIRE IMT has six breaths resulting in a total of 36 breaths if the six levels of TIRE IMT are completed. Importantly, the 80% load reflects the ratio of Pi/MIP, and the changing work-to-rest ratios indicate that TIRE IMT eventually will lead to fatigue, an example of which is presented below using the TTmus discussed above (2–8,10–13,20,22,24,25,27,28,30).
Example of TIRE IMT at Levels A and D and the Calculation of TTmus
Level A IMT (60 s of rest between the six level A breaths): 0.8 (80% load) × Ti/Ttot. The Ti is 15 because, in this example, we have assumed an inspiratory time of 15 s and Ttot is 75 s, which is the sum of inspiratory time and the 60-s rest period between breaths. Thus, the Ti/Ttot is 15/75 and the quotient is 0.2.
Level A IMT TTmus = Pi/MIP * Ti/Ttot = 0.8 × 0.2 = 0.16 (a value slightly above the value 0.15 associated with inspiratory fatigue) (2,25,27,28)
Level D (15 s rest between the six level D breaths): 0.8 (80% load) × Ti/Ttot. The same inspiratory time of 15 s is assumed so Ti remains 15 and Ttot is now 30 s, which is the sum of inspiratory time (15 s) and the 15-s rest period between breaths. Thus, the Ti/Ttot is 15/30 and the quotient is 0.5.
Level D IMT TTmus = Pi/MIP * Ti/Ttot = 0.8 × 0.5 = 0.4 (a value representing a greater level of fatigue) (2,25,27,28)
This example suggests that the TIRE provides effective training up to the point of fatigue when at failure to match the on-screen target (Fig. 1A) the inspiratory muscles are unloaded and IMT ceases. The IMT imposed in this manner is quite different to the loading of disease, where it has been suggested that patients “self-train.” In the latter case, the load is constantly applied and, if fatigue and respiratory failure are to be avoided, the system has to be unloaded. Without these changes, the only option is to adapt the respiratory muscles at the fiber level with a proportionate increase in endurance fibers or to change breathing patterns to avoid longer Ti (2,10–12,25,27,28). Unloading the inspiratory muscles during the longer term may result in disuse atrophy and it may be of interest to either institute IMT at an earlier point in the disease process or to add periods of IMT to those individuals undergoing invasive or noninvasive ventilatory support (2,25,27,28). In view of this, TIRE testing provides a comprehensive assessment of respiratory performance, which can be incorporated easily into IMT efforts.
How Does the TIRE Provide a Variety of IMT Methods?
The results of TIRE testing enable a variety of IMT methods to be provided and include endurance, strength, power, and combinations of endurance, strength, and power IMT throughout the full range of inspiration. Specifically, all of the SMIP patterns previously shown (Figs. 1, 2, & 4) provide a template from which to target IMT efforts. Thus, not only does the SMIP template provide biofeedback but also IMT efforts can be focused on the initial, mid-range, or terminal aspects of inspiration given baseline SMIP characteristics (i.e., SMIP duration, slope of the SMIP power profile, oscillations in the SMIP) and the specific needs or tasks required of individuals. In fact, in an attempt to facilitate breathing during sport-specific tasks, we have targeted IMT at different locations of the SMIP profile including focusing on early versus mid-range IMT and using leaks larger and smaller than the standard 2-mm mouthpiece opening. For example, because of the body position changes associated with rowing (specifically the catch and late recovery), we targeted IMT efforts on the initial ¼ of the SMIP using power IMT methods to facilitate an optimal powerful early inspiration to train to the task of rowing in an attempt to promote rapid and more complete inspiration before the full catch was achieved, resulting in greater IVC, ventilation, and rowing recovery capacity (22).
Figure 5 displays a comparison of the pressure generated by the same person (y axis) during TIRE and Threshold IMT (Healthscan, Inc.) throughout inspiration (x axis). The relatively square graph in the lower left-hand corner of Figure 5 shows the threshold IMT template, which is accomplished with a calibrated spring that provides resistance to inspiration and allows inhalation to occur when the set threshold resistance is overcome as well as the relatively abrupt termination of inhalation when the set threshold resistance can no longer be overcome. Thus, a characteristic square-like IMT template is produced. Threshold IMT fixes the generation of pressure for each breath so that the spring valve opens only when a preset pressure is reached and closes when the preset pressure is no longer achieved. The pressure load generally is related to the MIP measured at RV at the optimal length of muscle contraction capability. It is clear from both skeletal muscle training and increasingly in IMT that the fixation of workload to fulfill the principle of overload is vital to the training outcome (1,14,15,18,29) for which reason threshold IMT may be suboptimal compared with TIRE IMT.
The IFRL of TIRE IMT provides a constant and accommodating resistance throughout all of inspiration and enables a greater MIP and inspiratory duration to be achieved compared with the square IMT template associated with Threshold IMT (Fig. 5). The MIP and inspiratory duration obtained during TIRE IMT is 117 cm H2O and 10.62 s, respectively (Fig. 5). The MIP and inspiratory duration obtained during threshold IMT using the Threshold IMT device is 49 cm H2O and 6.12 s, respectively (Fig. 5). Thus, the major differences between these two forms of IMT include a lower MIP, a shorter inspiratory duration, and a relatively square IMT template with Threshold IMT. A limitation of Threshold IMT using the Threshold IMT device is that the maximal resistance is approximately 42 cm H2O, which is the Threshold calibrated spring pressure and may provide inadequate workloads for healthier individuals. However, other threshold IMT devices exist, which provide greater workloads via larger springs (e.g., PowerBreathe, PowerLung). Finally, the threshold square-like IMT template signifies an abrupt increase and decrease in pressure during IMT efforts, which provides a very different form of IMT compared with TIRE IMT, which enables a greater initial workload and greater generation of pressure throughout a longer inspiratory duration as shown in Figure 5, as well as a greater capacity for targeted IMT.
Standard TIRE IMT
As described previously, standard TIRE IMT commonly consists of inspiring against a workload set at 80% of MIP/SMIP, which can be changed easily via RT2 software to accommodate particular training regimens (<80% or >80%). Thus, TIRE IMT is not provided solely on MIP but on the entire SMIP. Each of the six TIRE levels consists of six breaths resulting in a total of 36 breaths if the standard TIRE protocol is completed. Also, the work-to-rest ratio changes from level A to level F with a 60-s rest period during level A to a 5-s rest period during level F IMT. Each session of TIRE IMT is preceded with TIRE testing, and the best of three SMIP templates are generated on the computer screen. This ensures that a true 80% (or nondefault chosen percentage) target is provided and that the load is always at the true percentage level for that day. Thus, the training is individualized and progressive in nature. It is important to note that the SMIP template provides key biofeedback for individuals to observe and improve each inspiratory effort such that greater inspiratory efforts during baseline testing will yield greater MIP, SMIP, and inspiratory duration resulting in greater IMT efforts (3–8,10–13,20,22,24,30).
TIRE training begins after the best of the three SMIP templates is chosen and a new SMIP profile is presented on the computer screen representing the chosen workload (the default workload is set at 80% of SMIP). Participants must match or exceed the SMIP template and generate at least 90% of the area under the curve during each inspiratory effort. If 90% of the area under the curve is not achieved, the individual will receive a prompt that they have failed the inspiratory effort and can terminate or continue IMT. We typically allow for two to three failed inspiratory efforts and assess subject signs and symptoms before terminating TIRE IMT. An important outcome of TIRE IMT is the accumulated SMIP, which is the summation of the total Joules or total area of each SMIP generated during a TIRE IMT session (3–8,10–13,20,22,24,30).
Novel Methods of TIRE IMT
Several novel methods of TIRE IMT have been examined recently and are presented in Figure 6. The use of different sized leaks through which to inspire to facilitate different forms of IMT (power vs strength vs endurance) is presented in Figure 6A, with a 1.5-mm leak yielding the greatest MIP, SMIP, and inspiratory duration — characteristics associated with strength and endurance IMT. Figure 6A also shows that inspiration through a 5.5-mm leak generated the lowest MIP, SMIP, and inspiratory duration — characteristics associated with power IMT. The effect of different sized leaks (1.5, 4.0, and 5.5 mm) on the TIRE template shown in Figure 6A can be compared with the standard 2.0-mm leak used with most testing and IMT methods.
Figure 6B shows IMT targeted at ½, ¾, and the terminal portion of the SMIP via the standard 2-mm leak, yielding lower MIP values but relatively similar SMIP values. Of note is that IMT targeted at the terminal portion of the SMIP substantially lengthened the inspiratory duration. Figure 6C shows IMT targeted at different locations of the SMIP with a 5.5-mm leak focusing on power IMT during early inspiration and a 1.5-mm leak focusing on strength and endurance IMT at mid- and terminal inspiration. It also should be noted that all forms of novel IMT shown in Figure 6 are contrasted to standard TIRE IMT through the standard 2-mm leak.
TIRE testing and IMT provide a template from which a variety of outcomes and training regimens are possible. Several key relevant outcomes include the MIP, SMIP, accumulated SMIP, slope of the SMIP, and TTmus. TIRE IMT provides a training stimulus throughout the full range of inspiration with the capacity to alter the form of training (endurance, strength, power, and combinations of each) at various points in the SMIP to elicit wanted effects for specific tasks or sports. Future research is warranted to further examine novel forms of TIRE IMT as well as the clinical utility of TIRE testing outcomes in various healthy and diseased populations in whom improved breathing could result in improved symptoms and functional/exercise performance. Furthermore, the methods and outcomes of TIRE testing and training should be examined and applied to the expiratory muscles.
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