European Journal of Cardiovascular Medicine

Muscle oxygen saturation (SmO₂) is a key physiological parameter reflecting the balance between oxygen delivery and utilization in skeletal muscles during rest and physical activity. The ability to measure SmO₂ provides valuable insights into muscle metabolism, cardiovascular performance, and the efficiency of oxygen extraction during exercise (1). Traditionally, techniques like venous blood sampling and muscle biopsies were employed to assess muscle oxygenation; however, these methods are invasive and unsuitable for continuous or field-based monitoring.

Near-infrared spectroscopy (NIRS) has emerged as a non-invasive, portable, and real-time method to assess muscle oxygen saturation by detecting the relative concentrations of oxygenated and deoxygenated hemoglobin and myoglobin within the muscle tissue (2). The use of NIRS has gained traction in both clinical and sports settings due to its capacity to monitor dynamic changes in SmO₂ during various intensities of exercise and recovery (3).

The extent to which muscle oxygen saturation fluctuates during exercise is influenced by an individual’s training status. Trained individuals are known to possess enhanced capillary density, mitochondrial efficiency, and cardiovascular adaptations, all of which contribute to improved oxygen delivery and utilization (4). In contrast, untrained individuals typically demonstrate more pronounced SmO₂ desaturation and delayed recovery, indicating less efficient oxygen extraction mechanisms. Comparing these groups provides valuable insights into the physiological impact of training and the potential applications of NIRS in performance evaluation and rehabilitation (5).

This study aims to assess and compare the muscle oxygen saturation profiles in trained and untrained individuals during progressive exercise using NIRS technology, thereby exploring its effectiveness as a tool for physiological assessment and fitness monitoring.

Study Design and Participants
This cross-sectional study involved 30 healthy male participants aged between 18 and 30 years. The participants were divided equally into two groups: trained (n=15), consisting of individuals engaged in structured aerobic training (≥4 sessions/week for at least 6 months), and untrained (n=15), comprising sedentary individuals with no regular physical activity. Exclusion criteria included any history of cardiovascular, pulmonary, or musculoskeletal disorders and the use of performance-enhancing substances.

Exercise Protocol
All participants performed an incremental cycling test using a calibrated cycle ergometer (Monark Ergomedic 828E, Sweden). After a 3-minute warm-up at 50 W, the workload was increased by 25 W every 2 minutes until volitional fatigue or the inability to maintain the cadence above 60 rpm. Participants were continuously monitored for heart rate using a chest-strap monitor (Polar H10, Finland), and rate of perceived exertion (RPE) was recorded at the end of each stage using the Borg 6–20 scale.

Muscle Oxygen Saturation Measurement
SmO₂ was continuously monitored using a portable near-infrared spectroscopy (NIRS) device (Moxy Monitor, USA) placed on the belly of the right vastus lateralis muscle, approximately 15 cm above the patella and 2 cm lateral to the midline. The area was shaved and cleaned with alcohol to ensure proper sensor contact. The NIRS device recorded real-time SmO₂ throughout baseline, each workload increment, and the 5-minute recovery period.

Data Collection and Analysis
Baseline SmO₂ was recorded after 5 minutes of seated rest. Peak exercise SmO₂ was determined at the final completed stage of the exercise test, and recovery SmO₂ was monitored for 5 minutes post-exercise. Time taken to return to 90% of baseline SmO₂ was also noted. Heart rate and RPE values were documented at each workload level.

All data were analyzed using SPSS software version 25.0. Descriptive statistics were presented as mean ± standard deviation. Differences between groups and across time points were assessed using repeated measures ANOVA. A p-value of less than 0.05 was considered statistically significant.

The baseline characteristics of the participants are summarized in Table 1. Both groups were matched for age and BMI, with no statistically significant differences (p>0.05). However, trained individuals had a significantly lower resting heart rate (66.4 ± 5.2 bpm) compared to untrained individuals (74.1 ± 4.7 bpm, p<0.01).

Muscle oxygen saturation (SmO₂) values at rest, during peak exercise, and recovery are shown in Table 2. At rest, the trained group had a significantly higher SmO₂ (78.6% ± 3.1) than the untrained group (72.3% ± 4.2, p=0.001). During peak exercise, SmO₂ dropped in both groups, but the decline was less in trained participants (42.5% ± 5.0) compared to the untrained group (35.8% ± 6.4, p=0.008).

Recovery data revealed that the time required to return to 90% of baseline SmO₂ was significantly shorter in the trained group (90 ± 11 seconds) compared to untrained individuals (131 ± 16 seconds, p<0.001) (Table 3).

Heart rate and RPE responses during incremental exercise also differed significantly between the groups. At a workload of 100 W, the mean heart rate in trained participants was 135 ± 8 bpm, while it reached 150 ± 9 bpm in the untrained group (p=0.005). Similarly, RPE values were consistently lower in the trained group at each workload stage (Table 4).

Table 1. Baseline Characteristics of Participants

Table 2. Muscle Oxygen Saturation (%) During Different Phases

Table 3. Time to Return to 90% Baseline SmO₂ During Recovery

Table 4. Heart Rate and RPE at 100 W Workload

These results demonstrate that trained individuals maintained higher SmO₂ levels at rest and during exercise, experienced less desaturation under load, and recovered faster post-exercise than untrained individuals (Tables 2 and 3). Furthermore, trained participants exhibited a lower physiological load at matched exercise intensities (Table 4), reflecting superior cardiovascular and muscular efficiency.

This study assessed muscle oxygen saturation (SmO₂) responses during progressive exercise using near-infrared spectroscopy (NIRS) in trained and untrained individuals. The findings highlight distinct physiological adaptations in trained participants, evidenced by higher baseline SmO₂, less pronounced desaturation during exercise, and quicker recovery post-exercise. These observations underscore the role of aerobic conditioning in enhancing oxygen delivery and utilization mechanisms at the muscular level.

The significantly higher resting SmO₂ values observed in trained individuals align with previous findings, suggesting enhanced capillary density, mitochondrial content, and improved oxidative capacity in response to chronic endurance training (1,2). These physiological adaptations contribute to increased oxygen extraction efficiency, allowing muscles to maintain better oxygenation even during escalating exercise intensities (3,4).

During peak exercise, both groups demonstrated a decline in SmO₂, reflecting an increased metabolic demand and oxygen extraction by active muscles. However, the trained group experienced a less pronounced decrease, indicating a more effective oxygen delivery system and better vascular autoregulation (5,6). Previous studies have shown that trained athletes can delay the onset of muscular hypoxia due to more efficient cardiovascular dynamics and peripheral adaptations (7,8).

Post-exercise recovery of SmO₂ was markedly faster in trained individuals. This can be attributed to enhanced mitochondrial oxidative capacity, faster phosphocreatine resynthesis, and more efficient lactate clearance (9,10). Such findings are consistent with the work of Ryan et al., who reported quicker SmO₂ normalization following high-intensity interval training in aerobically trained individuals (11).

NIRS has proven to be a reliable, non-invasive tool for monitoring SmO₂ during exercise, offering valuable insights into real-time muscular oxygenation dynamics (12). Compared to traditional oxygenation assessments, NIRS is advantageous in terms of portability, safety, and real-time application in both clinical and athletic settings (13). Moreover, it has been validated as a surrogate for muscle oxidative capacity, especially in endurance-trained populations (14).

Despite its strengths, the study has limitations. The sample size was relatively small and limited to young males, which may reduce generalizability to other populations. Additionally, variations in subcutaneous fat thickness could influence NIRS signal accuracy (15). Future research should explore larger, more diverse populations and consider integrating other performance markers like lactate threshold or VO₂max to provide a comprehensive physiological profile.

In conclusion, trained individuals exhibit superior muscle oxygen saturation responses during and after exercise compared to their untrained counterparts. NIRS offers a valuable tool for assessing muscle oxygen dynamics, training adaptations, and potential fatigue or performance deficits. These findings support the incorporation of NIRS monitoring in athletic training and rehabilitation programs for targeted physiological feedback.

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