Introduction
External tissue compression has been used extensively in both clinical and sports-related applications. Since its first documented uses, one of the most widespread applications of compression was to improve blood flow in patients with reduced limb circulation and tissue swelling (14,21,23). More recently, tissue compression has also been employed to manipulate the concentration of various metabolites in muscle tissue during exercise and during the recovery phase after exercise with the goal to improve exercise performance and postexercise recovery (12,13,17,22).
Compression may enhance performance by altering muscle force, muscle power, or muscle contraction efficiency (8,12,13). The results of these studies have suggested that, in general, compression improved muscle function. A number of mechanisms for the observed effects of compression have been proposed, including increased blood flow, decreased blood and lymph accumulation, increased proprioception and decreased soft tissue vibrations (8,12,13).
Another important factor influencing exercise performance is oxygen availability to the muscles. Muscle tissue oxygenation is directly related to the balance between oxygen consumption rate and the rate of oxygen supply. Although muscle oxygen use is generally constant for a given task, the oxygen-replenishing rate can vary and is closely related to the tissue blood flow. A number of studies have investigated the effect of compression on muscle tissue oxygenation. For instance, measurements of gastrocnemius muscle oxygenation during exercise have shown that tissue oxygenation was higher at the end of the exercise when using compression (4). In addition, compression induced an increase in long-term (>5 minutes) tissue oxygenation during both continuous and intermittent exercise (1,22). In contrast, other studies found no significant increase of deep venous blood flow with compression (18). However, these studies did not exclude the possibility of mechanisms other than increased venous flow (e.g., increased tissue perfusion) (18).
Most of the aforementioned studies focused on the relatively long-term (>5 minutes) effects of compression on tissue oxygenation and blood flow (1,2,18). In comparison, to date, little is known about the short-term effects (minutes after the start of exercise) before reaching a steady state between oxygen supply and oxygen use. However, during the first 2 minutes after the beginning of an exercise oxygen use is greater than oxygen supply. Subsequently, additional oxygen is transported to the muscle resulting in a period of steady state between oxygen supply and oxygen use. In addition, the period immediately after the onset of exercise is important in sports that are characterized by sporadic bursts of activity followed by relatively long periods of low-moderate activity (e.g., during soccer, football). It is conceivable that an increase in muscle tissue oxygenation because of compression could have a positive effect on sports performance by means of increasing the oxygen availability to the muscles. However, to date, the dynamics of tissue oxygenation during this critical period has not been addressed.
Indirect evidence suggests that compression might also affect muscle contraction efficiency (5). It is assumed that compression induced vasodilation could affect the aerobic vs. anaerobic energy production balance at the onset of exercise. Therefore, for a given task, a lower energy use rate with compression would be expected compared with that without compression.
Because tissue oxygenation can be quantified noninvasively using near infrared spectroscopy (NIRS), one could design a study that would quantify the effect of compression at the onset of exercise while applying compression on a specific muscle.
Under certain experimental conditions, changes in tissue oxygenation can be associated with energy use (3,9). However, NIRS does not allow for distinguishing between changes in tissue blood flow (and implicitly oxygen supply) and changes in oxygen use, and hence, in the absence of arterial occlusion (AO), any energetic considerations based on muscle tissue oxygen use are inappropriate. It is imperative that any statements relating the muscle contraction efficiency using NIRS measurements should include arterially occluded testing sessions.
From a practical perspective, the importance of understanding the effects of compression on muscle contraction efficiency and oxygenation is evident given that a higher efficiency and tissue oxygenation at the onset of exercise can potentially lead to increased sports performance. Even a relatively small effect for each exercise bout—rest cycle could result in a significant performance improvement over a large number of exercise—rest cycles.
Therefore, the goals of this study were as follows: (a) to quantify the effects of externally applied compression on the gastrocnemius medialis muscle energy use during short-term activity and (b) to quantify the effects of compression on the dynamics of gastrocnemius medialis tissue oxygenation at the beginning of exercise.
Specifically, we tested the hypotheses that (a) muscle energy consumption will be altered during the first 2 minutes of exercise and (b) muscle tissue oxygenation dynamics will be increased with compression compared with control (without compression) at the beginning of exercise.
Methods
Experimental Approach to the Problem
To test the 2 hypotheses, a cross-sectional intervention study was designed that quantified the specific response of a muscle (Gastrocnemius Medialis) oxygenation to different levels of externally applied compression during short bouts of controlled exercise. The muscle tissue oxygenation was quantified using an NIRS device (NIRO 200, Hamamatsu Photonics, Japan). Two NIRS optodes were placed on the subject's gastrocnemius muscles. Each subject performed 2 trials. In each trial, the subjects stood on a platform for 2 minutes. Subsequently, the subjects performed 40 heel raises per minute for 2 minutes with either compression applied to the lower leg or no compression. A 10-minute resting period between trials was imposed to eliminate the effects of fatigue. During each trial, the blood flow in one of the lower legs was occluded at the beginning of the exercise while 1 leg was occlusion free (Figure 1).
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Figure 1: Test time diagram for both the occluded (AO) and nonoccluded (N/O) leg for one subject. Key testing elements and the corresponding times are marked on the diagram.
Subjects
Sixteen male subjects (mean ± SD; age 26.3 ± 5.1 years; mass 71.2 ± 4.8 kg) participated in this study. All the subjects signed an informed, written consent according to the guidelines of the University Ethics Committee. All the subjects were free of any injuries at the time of the experiments. All the participants were physically fit, having a history of consistent moderate daily exercise (e.g., running, interval training).
Procedures
Exercise
The subjects were asked to stand on a flat platform with the center of their feet 35 cm apart. To ensure consistent foot placement, the foot positions were marked on the platform with adhesive tape. An NIRS optode was placed on the belly of each of the right and left gastrocnemius medialis muscles. The distance between the source and the detector was fixed (4.5 cm). The NIRS data were collected at a frequency of 2 samples per second.
The test movement task was rising from normal bipedal stance onto the toes by lifting the heel to a maximal height and returning to the normal stance. Each subject performed this task 40 times per minute, guided by a metronome, for 2 minutes. The movement was checked visually to ensure consistency. The subjects used a short warm-up period of 1 minute to familiarize with the movement.
Before each test trial (compression and control), the subjects were standing relaxed for 2 minutes to allow for quantifying steady-state pretest blood flow and oxygenation. Steady state was achieved when the slopes of the tissue oxygen index (TOI) and the normalized total hemoglobin index (nTHI) were <±3%. During postprocessing, data sets were rejected when steady state was not achieved. Each subject performed 80 repetitions per trial, with 10 minutes between the 2 trials to minimize the effects of fatigue.
Arterial Occlusion
A pressure cuff was placed randomly on either the right or the left thigh and was inflated rapidly to 300 mm Hg immediately (<4 seconds) before the start of the exercise. The opposite limb was free of compression.
Compression Application
Gastrocnemius muscle compression was achieved using an elastic compression sleeve (custommade, one size, 75% polyester 25% spandex) that was pulled over both calves. Each testing session consisted of 2 trials: 1 without compression sleeves and 1 with compression sleeves on both legs. The setup and the exercise performed were identical for both trials. The order of the trials was randomized.
Before testing, the subjects were verbally asked to describe their physical and mental status (e.g., fatigue, lack of sleep). The subjects that did not show a consistent pattern of physical and mental preparedness for the exercise were discarded and asked to return for testing at a few days later. All the trials for a given subject were performed in the morning during 1 (2 hours long) session. The subjects were asked to hydrate properly before the test, and there was no food or fluid intake during the testing session.
Calf circumference was measured at the widest section of the calf for each subject while wearing the compression sleeve. The pressure (P) exerted by the compression sleeve was estimated using the stretch values characteristic for each subject and the elastic properties of the elastic sleeve using the equation:
where k is the elastic modulus of the sleeve fabric (determined in a pretest mechanical stress–strain test), dL is the relative stretch of the sleeve in respect with its initial length, and A the unit area covered by the sleeve. The estimated pressure was in agreement with the pressure measured using a calibrated force-sensing resistor (402FSR, Interlink Electronics, CA, USA) inserted between the compression sleeve and the calf. Because identical elastic sleeves were used for all the subjects, the exerted pressure depended on the calf circumference.
Near-Infrared Spectroscopy
Near-infrared spectroscopy is a noninvasive technique designed to quantify the change in concentration of oxygenated and deoxygenated forms of hemoglobin (O2Hb, HHb) and myoglobin (O2Mb, HMb), respectively (9). The spectrometer used in this study provided tissue oxygen saturation data in the form of TOI. The TOI was expressed as a percentage of tissue oxygen saturation. In addition, this device provided the normalized nTHI. The nTHI is a measure of the total hemoglobin in the tissue and directly related to changes in tissue blood flow (16).
The NIRS method has been successfully employed to measure brain and muscle tissue oxygenation in previous studies (3,9). This method has 2 major limitations: (a) skin and fat tissue may influence the measurement and (b) the use of relative vs. absolute oxygen saturation (16). The 2 limitations can be overcome by studying the same tissue volume while varying the testing conditions. In addition, relative and absolute changes in tissue oxygenation can be caused by both oxygen depletion and changes in tissue blood flow and perfusion. Therefore, when ‘true’ tissue oxygen depletion rates are required; tissue blood flow must be monitored and suppressed. The most common method used in this situation is complete AO, which ensures a constant tissue blood concentration, and it was used in this study.
Tissue Oxygenation and Energy Use
Tissue oxygenation depletion rate was computed from the total oxygenation index data measured in the AO leg by linearly fitting the linear section of the TOI concentration decay (Figure 2B). The rate of change of tissue oxygenation was computed as the slope of the regression line. Tissue oxygenation recovery rate was computed from the TOI data recorded from the muscle without AO (N/O). Total oxygenation index data between the point of minimum concentration and the last recorded data point were fitted linearly. The recovery rate was defined as the slope of the regression line (Figure 2A).
Figure 2: A) Time course of total oxygenation index (TOI) during 1 exercise session for 1 subject. Two groups are shown corresponding to the compression (gray line) and control (black line) conditions without arterial occlusion (AO). The TOI recovery rate (r) was computed from the slopes of the regression lines (doted lines). B) Time course of TOI during AO. Two groups are shown corresponding to the compression (gray line) and control (black line) conditions without AO. The TOI depletion rate (D) was computed from the slopes of the regression lines (doted lines). Vertical dotted line represents the beginning of exercise. C) Time course of normalized tissue hemoglobin index (nTHI) during 1 exercise session without occlusion. Compression (gray line) and control (black line) conditions are shown.
Statistical Analyses
Statistical analyses were performed with commercial SPSS software, version 16.0 (SPSS Inc., Chicago, IL, USA). A paired t-test was used to determine the statistical significance of the relative difference between the mean values of the measured variables (tissue oxygenation index and normalized tissue haemoglobin index) corresponding to the control and compression condition for each of the occluded and nonoccluded legs. The Pearson correlation coefficient was computed for the applied compression and change in tissue oxygenation data for the nonoccluded leg. The significance level was set a priori to 0.05.
Results
Mean short-term TOI recovery rate was 24% higher (p = 0.04) for the compression condition compared with control in the N/O leg (Figure 2A). The normalized total hemoglobin Index (nTHI) showed a depletion recovery curve similar to the one observed for the TOI index (Figures 2A, C). A strong correlation (r = 0.83, p = 0.026) between the TOI and nTHI index corresponding to the N/O leg was observed. The rate of tissue oxygenation decay during the depletion phase for the AO leg was 2% lower (p = 0.34, not significant) for the compression condition compared to the control condition (Figure 2B).
The estimated applied compression correlated positively with the relative change in total tissue oxygenation recovery rate (r = 0.61, p = 0.012; Figure 3).
Figure 3: Relationship between the externally applied compression and the individual average relative change of total oxygenation index (TOI) recovery rate with compression as compared with control for all the 16 subjects. Regression line (solid line) and 95% confidence interval (doted lines) are shown.
Discussion
The purpose of this study was to test the hypotheses that (a) muscle energy consumption will be altered during the first 2 minutes of exercise and (b) muscle tissue oxygenation dynamics will be altered with compression compared with without compression during the first 2 minutes of exercise. The results of this study suggest that externally applied compression does not have a significant impact on the rate of oxygen utilization during a dynamic task; therefore, the first hypothesis of this study was rejected. Although the effective mechanical work was not measured in this experiment, it can be speculated that muscle contraction efficiency does not seem to be affected by compression. Although a number of studies have shown that compression can have an effect on muscle force and power generation (8,12,13), the fact that the oxygen use rate does not seem to be affected by compression might lead to the conclusion that the observed effects might be attributed to factors other than increased muscle contraction efficiency. However, in this experiment, measurements were performed only at a submaximal level. Therefore, the above conclusion is only valid to the extent that the measured muscle oxygen use rate at submaximal levels is also relevant for maximal muscle force and power production.
In the absence of AO, muscle tissue oxygenation in this experiment showed a pattern similar to those observed in previous studies (22,24). A relatively rapid decrease in muscle tissue oxygenation was observed—immediately after the start of the exercise—followed by a slightly slower linear increase in tissue oxygenation (Figure 2A). The linear increase in oxygenation was significantly higher for the compression condition than for the control condition; confirming the second hypothesis of this study. Furthermore, the relative slope of the tissue oxygenation increase seems to depend on the magnitude of the applied compression (Figure 3). Tissue oxygenation for a given task is dictated by 2 main factors: oxygen-replenishing rate and oxygen use rate. In the first part of this study, we showed that the oxygen use rate does not seem to be affected by compression. Therefore, it was assumed that in this experiment, oxygen-replenishing rate was altered by compression, hence the observed increase in tissue oxygenation rate. Because tissue oxygen replenishing is directly related to the arterial blood flowing into the tissue, it is expected that compression induced changes in blood flow might be responsible for the observed effects. Indeed, nTHI data showed that total hemoglobin in the tissue shows a pattern similar to the TOI pattern. Because nTHI is a measure of changes in blood flow, it can be concluded that compression increases blood flow immediately after the start of the exercise, and consequently, tissue oxygenation increases more rapidly in the compression situation compared with control. A strong correlation between the increase in TOI and nTHI was noticed. When corroborating this observation with the finding that oxygen use rate does not seem to be affected by compression one could conclude that the observed differences between compression and control are almost exclusively caused by compression induced blood flow changes.
The effects of compression on blood flow are well documented, and it is generally agreed that compression has a potentiating effect on absolute values of arterial and venous blood flow and on tissue perfusion level (14,21,23). Locally applied pressures between 8 and 30 mm Hg have been shown to significantly increase tissue blood flow in the lower and upper limbs (15,25). In some experiments, the blood flow increase because of compression was quite substantial, the actual values corresponding to the compression situation increasing by 20–100% compared with control levels (2,11). Although these previous findings addressed only the long-term-equilibrium response to compression, they indirectly support the findings of this study that compression increases tissue oxygenation.
Previous studies have shown a rapid increase in muscle blood flow at the onset of exercise (6). Although the exact mechanism responsible for the increase in blood flow has not been completely elucidated, most studies point toward the muscle compressive effects on blood vessels as the main precursor of the observed hyperemic response (19,20). Based on these observations, one could speculate that externally applied compression might have a cumulative effect on the compression exerted by the contracting muscles on the blood vessels. This could potentially lead to an enhanced hyperemic response because of increased pressure on blood vessels during muscle contraction. However, several other possible mechanisms for the myogenic response could be responsible for the observed effects of compression, and, therefore, the proposed mechanism should be considered cautiously (7).
Continuous or periodic intramuscular pressure may also increase tissue perfusion because of release of endothelium vasoactive factors (10). Increase tissue perfusion could lead to increased tissue oxygen availability and may explain the observed increase in tissue oxygenation while wearing the compression sleeves. However, because the method used cannot distinguish between increased arterial-venous blood flow and increased (capillary) tissue perfusion, this experiment does not provide sufficient information to explain a specific mechanism.
To the best of our knowledge, this is the first study that systematically analyzed tissue oxygenation dynamics during the first 2 minutes after the onset of exercise. The results of this study showed that compression can have a significant impact on muscle oxygenation recovery and that, for the analyzed pressure range, the tissue oxygenation recovery rate correlates with the magnitude of the applied pressure. Because muscle contraction efficiency did not appear to be affected by compression, it was concluded that the observed changes were primarily caused by a compression induced muscle myogenic response.
Practical Applications
Compression seems to have an effect on various physiological and biomechanical variables during exercise and recovery although the magnitude of the effects and the mechanisms are still under debate. A number of studies have shown the medium- and long-term effects of compression on blood flow, tissue perfusion, and muscle oxygenation. In this study, we showed that compression has a significant effect on tissue oxygenation immediately after the onset of the exercise. Although increased tissue oxygenation does not automatically imply increased performance, it is possible that a faster increase in tissue oxygenation might improve muscle functionality during the first 2 minutes after the start of the exercise. Therefore, for activities comprising intermittent bouts of exercise, compression might provide an advantage in the sense that it could more rapidly increase the tissue oxygenation level immediately after the beginning of the exercise, thus increasing the oxygen availability in the muscle.
It is conceivable that externally applied compression during a physical activity that involves repeated bouts of exercise would allow the muscles to operate in the aerobic domain for longer periods of time. This has the potential to increase the muscle energy output during a game and indirectly increase performance. One could test this assumption during a test involving repeated bouts of exercise, the expected outcome being a longer time to exhaustion with compression as compared with control.
Although one of the main results of this study suggests that the increase in oxygenation may be related to the intensity of the applied compression, this conclusion should be regarded cautiously. In addition, this conclusion is presumably only valid for the studied tissue and exercise combination and does not automatically mean that a direct, linear relationship exists between the level of applied compression and the short-term tissue oxygenation recovery for all possible compression levels.
Acknowledgments
This study was financially supported by the Natural Sciences and Engineering Research Council of Canada, the da Vinci Foundation, Calgary, and adidas AG.The authors have no conflict of interest to declare.
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