The use of near-infrared spectroscopy (NIRS) to investigate muscle oxygenation changes during physical activity has rapidly increased within recent years (8). The technique relies on assessing the relative transparency of near-infrared light through tissue and oxygen-dependent absorption characteristics of hemoglobin and myoglobin (9,15). As the light spectrum cannot distinguish hemoglobin from myoglobin, for convenience (6,20), we will refer to hemoglobin in the manuscript. Using continuous wave NIRS (cw-NIRS), relative or absolute concentrations of oxyhemoglobin (O2Hb), deoxyhemoglobin (HHb), and total hemoglobin (tHb) can be estimated (8). Continuous wave NIRS assumes homogeneity of the detected tissue and due to its simplicity, the devices are suitable for field measurement.
Previously, cw-NIRS–derived parameters during exercise have been shown to be a valid tool for determination of local muscle oxygen consumption, oxygenation threshold, tissue deoxygenation, and O2 recovery rate (3,27,29). The use of cw-NIRS is, therefore, promising as a diagnostic tool in sports where muscular hypoxia is common such as local isometric contractions or high-intensity exercise seen in rock climbing. Although this technique has been extensively used in this population to assess muscle oxygenation (1,11–14,18,19,22), the reliability of cw-NIRS is not known. As such, it is not known whether reported changes are of physiological significance or due to variability with the data collection methods. Previously, reliability studies assessing muscle oxygenation have been limited to laboratory conditions or using frequency domain spectroscopy (5,21,24,26). There is no known study which has aimed to determine reliability within an ecologically valid setting. This may be in part due to the complex nature of determining reliability in sport-specific settings where controlling confounding factors is inherently difficult. However, given the growing use of this technique in both climbing and clinical exercise populations (1,11–14,18,19,22), determining the reliability of cw-NIRS–derived parameters would provide important information for clinicians and couches.
Therefore, the aim of the current study was to determine intersession reliability and minimal detectable change (MDC) of cw-NIRS parameters during intermittent handgrip contractions in rock climbers.
Experimental Approach to the Problem
A repeated-measures design (test-retest method) was used to assess intersession reliability of cw-NIRS–derived parameters. Subjects visited an environmentally controlled laboratory (20.0 ± 0.7° C) on 4 separate occasions during a 3-week period (Figure 1). Before each visit, subjects were asked to restrain from heavy physical activity and alcohol for 24 hours, caffeine for 12 hours, and food 3 hours. The first visit was used to determine anthropometric markers and finger flexor maximal voluntary contraction (MVC) using a climbing-specific handgrip dynamometer (Figure 2). In addition, subjects familiarized themselves with the intermittent handgrip test. During the second, third, and fourth visit (3 separate sessions), subjects performed the intermittent handgrip endurance task until volitional failure occurred (contracting for 8 seconds at 60% MVC and relaxing for 2 seconds). Each of these 3 visits was separated by 3–6 days and completed in the afternoon between 14.00 and 18.00. All testing was conducted during early spring when climbing training is focused on high volume with submaximal intensity.
Sport climbers were chosen, as they are accustomed to the repeated nature of high-intensity contractions. As such, 32 sport climbers (age range 20–44 years) volunteered (15 males: ± SD age 27.7 ± 10.2 years; body mass 71.0 ± 9.3 kg, height 178.3 ± 9.7 cm; 17 females: age 26.3 ± 4.6, body mass 57.7 ± 5.6 kg; 166.4 ± 5.7 cm). Self-reported climbing ability ranged from 11 to 23 on the IRCRA scale which corresponds to intermediate to advanced ability level defined by Draper et al. (7). The mean climbing experience was 9.3 ± 6.2 years and the volume of climbing training 5.8 ± 2.3 hours per week. Subjects were healthy nonsmokers who were not taking any vascular acting medications. The study conformed to the recommendations of the local Ethics Committee in accordance with the Declaration of Helsinki. The study was approved by the Review Board of Charles University, Faculty of Physical Education and Sport. All subjects were informed of the experimental risks and signed an informed consent document before data collection.
Body mass was measured on an electronic scale (Soehnle 7730; Soehnle Industrial Solutions GmbH, Backnang, Germany) with the subjects wearing underwear only. Height was measured using a stadiometer (SECA; Vogel & Halke GmbH, Hamburg, Germany). Forearm skinfold thickness (under the cw-NIRS optode) was measured using a skinfold calliper (Harpenden; Baty, West Sussex, United Kingdom). Adipose fat was not considered to effect the NIR signal, as skinfold measures were below the limits set by van Beekvelt et al. (25) (1.7 ± 0.5 and 2.0 ± 0.5 mm for male and females, respectively).
Finger Strength and Endurance Tests
Each session began with the same warm-up, which consisted of 5-minute stair walking, 5-minute traversing on a climbing wall, and 5-minute individual 1-arm intermittent hanging on a 30- and 23-mm wide rung.
To test for finger flexor strength and endurance, a custom-made dynamometer (3D-SAC; Spacelab, Sofia, Bulgaria) (Figure 2) was used. The dynamometer enabled the subjects to control the intensity and duration of each contraction. Furthermore, the device provided real-time feedback using both visual and acoustic signals. The dynamometer was calibrated for a wooden hold 23-mm deep which maximized activation of the flexor digitorum profundus (FDP) (23).
During the first visit, MVC of the dominant finger flexors was completed twice (separated by 2 minutes). On presentation of an acoustic signal, subjects were asked to progressively pull on the rock climbing hold as hard as possible for 5 seconds. Subjects were verbally encouraged to achieve maximal effort throughout. The highest value from the 2 trials was used to determine their relative workloads (60% MVC) in the following endurance trials. Maximal voluntary contraction was determined while standing with the shoulder flexed to 180° and the elbow fully extended to simulate sport-specific conditions (2), and to prevent blood pooling (25).
The intermittent handgrip endurance test was performed in the same anatomical position as the MVC trials. Each contraction was performed at 60% MVC with a ratio of 8-second contraction to 2-second relief. The test started with an acoustic signal, and subjects were provided with visual feedback to ensure that the correct force was applied. If this force dropped by more than 10% for more than 1 second, the test was automatically terminated. An acoustic signal as well as the visual display marked the start and end of each contraction/relief period. Subjects were required to achieve the desired target force as quickly as possible. Only the time in the target zone was used in the analysis.
Near-infrared spectroscopy optodes were placed in 1/3 of proximal distance between the medial epicondyle of the humerus and carpus. The FDP was located by a chartered physiotherapist using the technique suggested by Schweizer and Hudek (23), where the thumb and the first finger were squeezed together. The flexor was then palpated to locate the middle of the muscle belly. Optodes were placed in a secure holder on the skin using biadhesive tape. The holder was fixed using dark tape to prevent contamination from ambient light (Figure 2). To ensure no variability in optode placement between sessions, pen marks were used on the skin.
Continuous wave NIRS (Oxymon; Artinis Medical System, BV, Elst, Netherlands) was used to assess the oxygenation and blood volume changes. OxyMon uses 2 lengths of NIR light at 765 and 855 nm. The absorption changes of the wavelengths used are converted into concentration changes of O2Hb and HHb using a modified Lambert-Beer law in which a path-length factor is incorporated to correct for scattering of light in the tissue (9). Two emitting optodes were spaced 4 cm and 3.6 cm from 1 receiver, respectively. This configuration enables the calculation of absolute ratios of O2Hb, HHb, and tissue saturation index (TSI) using the spatially resolved spectroscopy approach. Absolute concentration of tHb was estimated from the path-length factor, which was fixed to 4.0 as suggested by van Beekvelt et al. (26) for forearm measurements. Sampling frequency was set to 10 Hz and data were sent through optical cables in personal computer, where online recording and all parameters were undertaken using the Oxysoft software (Artinis Medical System, BV). Only TSI and tHb from spatially resolved spectroscopy were included in the analysis as the measures of tissue oxygenation and blood volume (15). Tissue saturation index is determined as O2Hb/(HHb + O2Hb) and, it is, therefore, independent of actual volume of tHb under the optodes. Resting tHb and TSI were determined using the last minute of 10-minute seated rest. Changes of tissue oxygenation and blood volume during exercise (∆ TSI and ∆ tHb) were calculated from all 8-second contractions and 2-second relief periods, respectively. Moreover, the minimum and mean minimum concentrations of tHb (tHbmin, tHbmean min) and TSI (TSImin, TSImean min) from the contraction periods were used in the analysis (Figure 3). To enable comparisons with other studies (12,22), deoxygenation (∆ TSI during contraction) and reoxygenation (∆ TSI during relief) from the first and last 3 contraction and relief periods were calculated.
Descriptive statistics (mean ± SD) were used to characterize time of the test, TSI, and tHb during rest and exercise. The differences between trials were assessed using repeated-measures analysis of variance (ANOVA). To assess the intersession reliability, several coefficients were used as recommended by Hopkins (17) and Weir (28).
Intraclass correlation coefficient (ICC) was calculated from the equation:where MSB and MSW correspond to mean squares between and within subjects from repeated-measure ANOVA, respectively, and k is the number of trials (3 in this case). This equation encompasses both the variability because of systematic changes between trials and error variability. Intraclass correlation coefficient was expressed with 95% confidence interval.
Coefficient of variation (CV) was calculated as:where μ is the mean of the 3 trials.
SEM was computed from the equation . This equation is to be preferred, as it is not dependent on the type of ICC (28).
The MDC often referred to as the “smallest detectable difference” was calculated from the SEM as (28). Minimal detectable change determines the magnitude of change that must be observed before the change can be considered to exceed the measurement error and variability at the 95% confidence level (16).
Statistical significance was set to p ≤ 0.05. Partial eta squared () was calculated to evaluate the effect size of differences between trials. As Weir (28) showed that the ICC values are influenced by between-subject variability and, therefore, higher ICC values can be found in heterogeneous samples even if the reliability is poor and vice versa, the reliability was not scaled as high, moderate, or low. In addition, CV is often criticized as it determines the error variability related to a common mean. When the common mean approaches zero, the CV increases even for low SDs. Because of these delimitations, MDC was introduced. Minimal detectable change is an absolute measure of reliability (measurement error) and is being increasingly used to assist in interpreting results and determining whether a change between repeated tests is random variation or a true change (16). Statistical computations were performed using IBM SPSS for Windows (version 22; Chicago, IL, USA).
There were no differences between trials for time to failure or cw-NIRS–derived parameters, which indicates no systematic error between trials (Table 1). The ICC for cw-NIRS–derived parameters ranged from 0.294 to 0.692 and CV from 8.3% to 41.8%. Mean ∆ TSI from all contractions and relief periods (Table 2) provided the highest intersession reliability with respect to other parameters (ICCs = ∼0.7, CV <20%, and MDC = ∼4.5%). ∆ TSI from the first and last 3 contractions showed higher error variability and measurement error (ICCs = 0.48–0.65, CV = 21–24%, MDC = 5.3–6.1%) than mean ∆ TSI from all contractions. Scores of ∆ tHb, tHbmin, and TSImin provided the lowest intersession reliability, and high measurement error (Table 2). Individual responses are depicted in Figure 4 where dark lines show climbers with the variability between trials exceeding the MDC.
To date, there is no known research determining reliability of cw-NIRS during high-intensity intermittent contractions during an ecologically valid sport performance setting. As such, the main aim of the study was to determine the intersession reliability and MDC of cw-NIRS parameters during intermittent handgrip contractions to failure in rock climbers. The main finding of the study was that (a) cw-NIRS provides a reliable measure of forearm muscle oxygenation, and (b) during both rest and exercise, MDC was determined for indicators of muscle oxygenation and blood volume.
To the best of our knowledge, the reliability of NIRS in forearm muscles during exercise was only assessed using frequency domain NIRS or while completing protocols without exhaustion in very standardized laboratory settings (4,5,26). Frequency domain spectroscopy provides absolute concentrations of HHb + O2Hb and accounts for the variability of many biological factors which cw-NIRS does not (8). Using incremental handgrip exercise, Celie et al. (4) reported ICCs between 0.025 and 0.873 for HHb and 0.22–0.774 for TSI. From similar handgrip exercise, van Beekvelt et al. (26) found CVs for muscle oxygen consumption ranging from 13.3% to 23.2%. No other reliability parameters were reported and we can only estimate that the high variability in the ICCs might be due to low reproducibility of the measurements or low between-subject variability. Nevertheless, the authors suggested that their protocol was highly reliable for determination of HHb and TSI. These findings cannot be compared with our protocol because of different methodology and parameters used. Parameters in the current study were chosen because of the extensive amount of sport rock climbing literature which has focused on them over the past decade, with no known measure of reliability (1,11,12,19,22).
Previous rock climbing literature has shown that ∆ TSI (rate of deoxygenation) during intermittent handgrip contractions at 40–60% MVC was greater in experienced climbers than in nonclimbers or lower grade climbers (group differences for ∆ TSI ranging from 3% to 9.4%) (1,12,19,22). As such, it has been suggested that rock climbers may have vascular adaptations occur during training. The current study found that MDC for ∆ TSI during contractions is 4.2%; this exceeds the previously reported differences between less and more experienced athletes in numerous previous studies (1,12,22). As such, all between-group differences, which are less than 4.2% may be due to error variability or low reproducibility of measurement, and so, caution should be used when interpreting such data. It seems plausible that climbing ability level is not solely related to hemodynamic changes in forearms, and other factors such as muscle fiber types, training preferences (endurance or strength), or general fitness may also play a substantial role in TSI responses. Data from the current study are in agreement with recent research into the population which suggested that the unknown variability in NIRS methods should mean that data should be interpreted with caution (1,10).
TSImin during handgrip exercise has previously been associated with a greater endurance performance and oxygen utilization in the muscle (1,13). In the current study, the MDC for TSImean min was 20%. This high variability may be caused by the high variability in TSIrest (TSImean min depends on the baseline value of TSIrest). At rest, TSI had poor intersession reliability (ICC = 0.420; SEM = 4.9%; CV = 8.3%). It is likely that this was affected by the previous warm-up, which was needed for safety reasons. To increase intersession reliability, it seems advisable that future research provides (a) a prolonged rest time after a warm-up, and (b) as previously suggested by Celie et al. (4); use the relative decrease in TSI (from baseline/rest).
There were a similar number of men and women used; separate reliability analyses for each sex were calculated. No substantial changes between males and females were found and consequently, the results were not reported. We are in agreement with Crenshaw et al. (5) who did not find any systematic sex differences in forearm and shoulder oxygenation after isometric contractions and blood occlusion.
Individual responses in 3 trials (Figure 4) suggest that the intersession reliability was decreased by the high intraindividual variability of few subjects. Several reasons may have led to these individual responses such as small shift of optode placements, tissue movement during contractions, different dietary uptake and hydration statuses, and the state of fatigue or recovery. Although the climbers were instructed to abstain from previous heavy exercise, caffeine and food intake (see Methods); hemodynamic responses are very complex and the control of all factors during testing in an ecologically valid setting is difficult.
To fully interpret data from the current study, it is important to highlight a number of strengths and weaknesses. This was the first study to determine reliability of high-intensity contractions in a sport-specific setting, thus increasing ecological validity while potentially compromising reliability. The current study had a relatively large sample size; however, data are limited to athletic populations. The use of this experienced group and the inclusion of a familiarization session would have meant that the learning effect was likely negligible. Because of the inherent interpretation problems of ICC's and CV's, several reliability coefficients were calculated to better describe the reproducibility of the measurements.
Continuous wave NIRS provided a reliable measure of tissue oxygenation changes in rock climbing–specific conditions. The most reliable parameters were mean TSI changes during the contraction and relief periods. Mean changes in TSI from all contractions and relief periods were more reliable than those from the first and last 3 contractions. ∆ tHb, tHbmin, and TSImin provided the lowest reliability and should be interpreted with caution. Consequently, we recommend that differences during contraction and relief periods should exceed ∼4.5% for ∆ TSI and ∼18.5 μmol for ∆ tHb to be considered meaningful (based on MDS). Moreover, previous studies which investigated the oxygenation responses in rock climbers may have not been accurate and should be interpreted with caution.
The authors acknowledge Michail Michailov from National Sports Academy, Sofia, Bulgaria, for providing the climbing specific dynamometer 3D-SAC. This study was supported by Charles University program PROGRES, No. Q41: Biological aspects of the investigation of human movement. The authors declare no conflicts of interest.
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