Relation between Vastus Lateralis Electromyography Activation and VO_2max Values Obtained in Bicycle Ergometry

Abstract

The study aimed to investigate the relationship between maximum oxygen consumption values obtained on a bicycle ergometer and vastus lateralis (VL) muscle EMG activity values during the test. A total of 20 athletes participated in the study. To determine the VO2max, a bicycle ergometer was used with a portable gas analyzer. The discontinuous incremental protocol was used to determine the VO2max values of subjects. The data were gathered from the right side VL muscle via surface electromyography. According to the results obtained, MVIC% values of the VL muscle of the subjects and VO2max (mL min−1) values (p = 0.586, r = 0.130); VO2max (mL min−1 kg−1) values (p = 0.295, r = 0.246); RER values (p = 0.308, r = −0.240) and HRmax (beats min−1) values (p = 0.321, r = 0.234) were not statistically significant and no significant difference was found in the regression analysis between the MVIC% value of the VL muscle obtained and the VO2max (mL min−1 kg−1) values (p = 0.295, R2 = 0.061). There was no significant correlation and regression between the muscle activation involved in the workout during the VO2max test and the cardiovascular response.

1. Introduction

To provide the oxygen necessary for the increased metabolic rate during exercise, an increase in respiratory volume frequency occurs. Maximal exercises can also increase to 200 lt/min. This is achieved by increasing the respiratory volume and frequency. On the other hand, while the respiratory minute volume can reach 200 lt/min in trained athletes during exercises performed with the same intensity, it is 100 lt/min in inactive individuals (sedentary). This is related to the fact that training strengthens the respiratory muscles in trained individuals. There is no significant change in respiratory volume and frequency with training. However, with training, there is an increase in the oxygen consumption rate (VO_2max) in the maximal aerobic metabolism in the tissues. Whether the person is trained or not, if there is no disease, he can always get much more oxygen than the body needs. Therefore, the important factor is to increase the availability of oxygen through training, in other words, to increase the VO_2max level in the tissues.

The most obvious effect of training on athletes is to increase oxygen diffusion capacity. Oxygen diffusion capacity is an indicator of the diffusion rate of oxygen from the alveoli to the blood. With regular training, respiratory volume in athletes does not change much in rest and submaximal exercises, but a significant increase is observed in maximal exercise. This significant increase is also seen in respiratory frequency and respiratory minute volume.

Acute physiological responses to changes in exercise intensity provide comprehensive information about the functional capacity and exercise tolerance of healthy individuals and patient populations. The integration of central and peripheral physiological systems determines the relationship between oxygen demand and allows the individual to adapt to changes in metabolic demand [1]. Among the physiological parameters, the maximum oxygen consumption is an indicator that gives information about the level of cardiovascular fitness and the functionality of the cardiovascular system [2]. The maximal oxygen volume level used by the skeletal muscles during a progressively increased exercise test is defined as maximum oxygen volume. VO_2max is a good indicator of aerobic capacity and is an indicator of the physiological integration of pulmonary, cardiovascular, and neuromuscular functions [3]. Aerobic metabolism during isometric or dynamic activities is estimated by indirect calorimetry, using oxygen and carbon dioxide gas exchange, where the energy used by working muscles reflects changes in pulmonary oxygen uptake [4]. Unfortunately, metabolic power measures based on a gas exchange cannot resolve metabolic costs to a higher resolution than breathing rate. As a result, information about muscle contractions that affect metabolic strength is neglected because these contractions may occur more frequently than breathing rate [5]. The maximum rate of O₂ intake (i.e., VO_2max) measured during large muscle mass exercises such as cycling or running is considered the gold standard measure of integrated cardiopulmonary-muscle oxidative function. The development of fast-responding gas analyzers that allow the measurement of breath-by-breath pulmonary gas exchange and progressive maximum exercise tests that increase rapidly or increase continuously with ramp test protocols are often used in clinical and experimental research. The V̇O_2max estimate obtained after these tests is sometimes not sufficient alone [6]. For this reason, many researchers resort to secondary criteria such as respiratory exchange rate, maximal heart rate, and/or maximal blood lactate concentration, which can result in VO_2max. However, studies examining the relationship between VO_2max values and activations of primary working muscles are limited [7].

Strength changes during muscle contractions are primarily reached by changing the number of active motor units and motor unit ignition rates. These changes can be detected using surface electromyography (EMG), which provides information about the active muscle by measuring the electrical signals of the motor unit action potentials. EMG fluctuates significantly within milliseconds during isotonic movements. Therefore, EMG should contain information about metabolic strength at a higher temporal resolution than gas exchange measures, but it is not known whether EMG can be used to predict (isometric) metabolic power changes during steady-state dynamic activities. However, the most important value that can be obtained from EMG during these isometric activities is muscle fatigue indices [8]. Surface EMG (sEMG) is widely used to measure the magnitude and timing of muscle activation during a variety of physical tasks, which has wide application in sports science research. sEMG’s ability to analyze dynamic situations gives it a special interest in sports. Improving the efficiency of a movement involves the proper use of muscles, both in terms of effort savings and effectiveness, as well as injury prevention. In particular, the performance of a task can be improved in terms of muscle activation and/or muscle fatigue based on analysis of the frequency of electromyographic traces observed. It should be noted that although EMG is an indicator of muscular effort in a particular action, it does not provide us with muscle strength parameters. In this regard, it is important to emphasize that the relationship between EMG activity and effort is only qualitative. Recently, experiments have been carried out on applications for purposes such as the evaluation of muscle fiber type and the characterization of muscles in the field of sports [9].

In this context, this study aimed to investigate the EMG activation of the VL muscle, which is the primary muscle involved in the workout with VO_2max values a cardiovascular response during cycling exercise.

The hypothesis of the study is that there is a relation between individuals’ VO₂ values and actively working muscle EMG values.

2. Material and methods

2.1 Study participants

Twenty active athletes’ volunteer subjects (10 females, 10 males) participated in the study (Table 1). Volunteers participate in any sportive activity at least 3 days a week and have similar performance levels. The inclusion criteria of the volunteers participating in the study were determined as follows:

• Having dominant legs right,

• Participating in any sporting activity at least 3 days a week,

• Not having any joint injury in the last 6 months,

• Does not interfere with exercising for cycling,

• Not using caffeine-type stimulants on the day of the study,

• Not smoking,

• Being a volunteer.

Variables	Female (n = 10)	Male (n = 10)	Total (n = 20)
Age (years)	19.7 ± 1.15	19.9 ± 0.87	19.8 ± 0.22
Height (cm)	166.6 ± 3.02	174.7 ± 9.33	170.6 ± 1.77
Weight (kg)	56.7 ± 4.69	72.7 ± 7.00	64.7 ± 2.24
BMI (kg/m^2)	20.4 ± 1.6	23.7 ± 1.64	22.0 ± 0.53

Table 1.

General characteristics of subjects.

Abbreviations: BMI; body mass index.

Values are presented as mean ± SD.

All participants approved the volunteering form. The study was conducted by the principles of the latest Helsinki Declaration upon receiving the necessary permissions.

2.2 VO_2max measurement

To determine the VO_2max, a bicycle ergometer (Monark 839E, Monark Ltd., Varberg, Sweden) was used with a portable gas analyzer (K5, Cosmed, Rome, Italy) and heart rate monitor (Cosmed, Rome, Italy). Discontinuous incremental protocol (DP) was used to determine the VO_2max values of subjects [10]. DP protocol concerned five workloads of 5 min each, interspersed by at least 5 minutes. Basal measurements were recorded with the participants standing on the bicycle ergometer. The first two workloads were set at 50 W and 100 W for all participants. The following three workloads were tailored for each participant according to the individual cardiorespiratory responses to the first two workloads and considering the theoretical maximum heart rate determined. Firstly, based on the VO₂ and the heart rate recorded during the first two stages, a submaximal linear regression was determined up to the predicted peak heart rate, to predict the speed corresponding to possible exhaustion. Then, the third, fourth, and fifth workloads corresponded to approximately 80%, 90%, and 105% of the predicted peak workload, respectively. The fourth and fifth workloads were recalculated using the heart rate and VO₂ recorded during the third and fourth stages, respectively [11]. The last stage was tailored to let the participants maintain the task for at least 4 min.

2.3 EMG measurement

In the study, the vastus lateralis (VL) muscle, which is one of the most active muscles during cycling exercise [12], was studied. EMG measurements of the VL muscles were recorded during the VO_2max test. EMG signals of the right leg were recorded. A Noraxon DTS wireless system (Noraxon Telemyo DTS System, Scottsdale, USA) was used for EMG recordings. Dual Ag/AgCl EMG electrodes (spacing—2.0 cm) were placed on the central points of the VL muscle in a parallel fashion to their muscle fibrils according to SENIAM (Surface Electromyography for the Noninvasive Assessment of Muscle) recommendations. Before the electrodes were placed, they were cleaned with an alcohol solution to prevent artifacts, and the skin was shaved to make it smooth [13, 14, 15]. After placing the electrodes, the impedance was observed to be within the acceptable range (<50 kOhms).

Raw EMG signals were processed via the Noraxon MyoResearch XP Master Edition software (Noraxon, Scottsdale, USA). All EMG signals were filtered using a 500 Hz low-pass filter. The signals were rectified and smoothed using a root mean square (RMS) algorithm (150 ms window).

The maximum value of three Maximal Voluntary Isometric Contraction (MVIC) trials for every 5 s was used for normalization of the EMG data obtained during the exercises. For the VO2_max test condition, a peak signal amplitude for VL was determined and divided by the MVIC value for VL. A normalized EMG (nEMG) as MVIC% was used for statistical analysis.

2.4 Statistical analysis

The sample size was reached as a result of power analysis. The data were normally distributed. Obtained VO_2max values (VO_2max (mL min⁻¹), VO_2max (mL min⁻¹ kg⁻¹), RER (respiratory exchange ratio), HR_max (beats min⁻¹), MVIC%) were tabulated and the arithmetic mean was found and, correlation and regression analysis of all values obtained using SPSS 22 statistics program with MVIC% values of the subjects participating in the test were performed.

3. Results

The values obtained during the VO_2max test of the male and female athletes participating in the study and the VL muscle activation values obtained during the test are shown in Tables 2 and 3.

Values	N	Min	Max	X	SD
VO2max (mL min^−1)	10	2371.35	3838.89	3074.59	420.81
VO2max (mL min^−1 kg)	10	45.60	65.09	54.57	6.93
RER	10	0.98	1.26	1.12	0.08
HRmax (beats min^−1)	10	159.00	187.00	176.34	11.77
MVIC%	10	59.12	132.40	85.76	22.64

Table 2.

Values of female athletes participating in the study.

Values	N	Min	Max	X	SD
VO2max (mL min^−1)	10	4419.77	5993.39	5348.50	461.35
VO2max (mL min^−1 kg)	10	67.78	78.33	72.02	3.75
RER	10	1.13	1.26	1.16	0.04
HRmax (beats min^−1)	10	162.00	194.00	184.10	12.40
MVIC%	10	78.71	121.60	93.08	12.09

Table 3.

Values of male athletes participating in the study.

Correlation and regression analysis results for the relationship between the values obtained from VO_2max measurement of all subjects and EMG muscle activation values obtained from VL muscle are given below.

According to the results obtained, MVIC% values of the VL muscle of the subjects and VO_2max (mL min⁻¹) values (r₍₁₈₎ = 0.130, p = 0.586); VO_2max (mL min⁻¹ kg⁻¹) values (r₍₁₈₎ = 0.246, p = 0.295); RER values (r₍₁₈₎ = −0.240, p = 0.308) and HRmax (beats min⁻¹) values (r₍₁₈₎ = 0.234, p = 0.321) were not statistically significant in correlation analysis.

According to the result obtained, MVIC% values of the VL muscle of the subjects and VO_2max (mL min⁻¹) values (R² = 0.017, p = 0.586); VO_2max (mL min⁻¹ kg⁻¹) values (R² = 0.061, p = 0.295); RER values (R² = 0.058, p = 0.308); HRmax (beats min⁻¹) values (R² = 0.055, p = 0.321) were not statistically significant in the regression analysis.

4. Discussion

In this study, muscle activation of VL and VO_2max values were compared during the VO_2max test on the bicycle. During cycling exercises, the VL muscle is one of the most involved muscles. VL muscle, which is one of the quadriceps muscles, cannot be neglected, especially during cycling exercise [16]. As a result of the tests and statistical data, no significant correlation was found between VO_2max and VL muscle activation values. In the VO_2max test performed with the discontinuous incremental protocol, cardiovascular responses and increases in muscle activation were observed at each loading but with this VO₂, muscle activation increased linear rupture. Lactic acid may play a dominant role in a linear rupture in the relationship between muscle activation and VO₂. Exercise-induced intracellular acidosis reduces the capacity of muscle fibers involved in producing work. Acidosis affects muscle activation, with a decrease in pH level. Alternatively, a disruption in myocyte membrane potential due to insufficient Na⁺/K⁺ pump activity might lead to impairment of excitation-contraction coupling regardless of pH [17]. In a study by Bearden and Moffatt [18], they could not find a linear relationship between VO₂ and power functions. This result is like our study. In a study by Sasaki et al. [19], a significant decrease was observed in the regression slope for VL muscle during cycling exercise, while an increase in the regression slope was observed in the biceps femoris and gastrocnemius muscles. VO₂ is not a linear function of power. During an incremental test, neuromuscular activity and VO₂ increase faster during heavy exercise. Both VO₂ and neuromuscular activity can show a break that can point to an upper limit for sustainable exercise at a very high-power output. Therefore, a similar study can be tried in different VO_2max test protocols.

5. Conclusions

According to the results obtained in the study with the hypothesis that there may be a relationship between muscular strain and oxygen consumption, there was no relationship between the oxygen consumption test used to determine the cardiorespiratory endurance of the individual in sports sciences and the EMG test used to determine muscle activation during movement. It can be suggested that future studies should be done on different exercise types and with more subjects.

Conflict of interest

No conflict of interest was declared by the author.

Financial disclosure

The author declared that this study has received no financial support.

Notes/thanks/other declarations

The author thanks all the participants involved in the study, for their patience and committed involvement.

Acronyms and abbreviations

EMG	electromyography
sEMG	surface electromyography
nEMG	normalized electromyography
SENIAM	surface electromyography for the noninvasive assessment of muscle
VL	vastus lateralis
DP	discontinuous incremental protocol
MVIC	maximal voluntary isometric contraction
VO2max	maximal oxygen consumption
RER	respiratory exchange ratio
HR	heart rate