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Comparative Study on Muscle Activity and Torque Value of Quadriceps of Healthy Adults

Purpose: The current study seeks to identify the relationship between torque values of the quadriceps and muscle activity during isometric and isokinetic exercises. Methods: The subjects of the study included 29 healthy individuals 17 men and 12 women) who took part in isometric and isokinetic exercises that utilized the quadriceps. The isometric exercises were performed three times each at 4 different angles (0, 30, 60, and 90 degrees). For the isokinetic exercises, concentric contraction and eccentric contraction were undertaken three times each at two angular velocities (30 and 60 degrees). Results: The muscle activity of the quadriceps during the isometric exercises showed significant differences at the 30, 60, and 90 degree angles. During the concentric contractions and eccentric contractions, muscle activity at the peak torque of the quadriceps indicated significant differences in vastus medialis, vastus lateralis, and rectus femoris at angular velocities of 30 and 60 degrees. Conclusion: When applied clinically based on the biomechanical analysis of the current study, it is possible to anticipate and selectively strengthen muscles with isometric and isokinetic exercises for not only healthy adults and professional athletes, but also for those who experience limited knee movement for long periods following knee surgeries.


INTRODUCTION
Knee joints are some of the joints that are regularly used in physical activity, and they also help provide overall balance for the body. 1 By performing flexion frequently, knee joints help carry out a great deal of activities and undergo substantial stress, which often leads to damage and wear and tear to the joints and causes pain and functional restrictions when walking, climbing stairs, and sitting/standing up. 2 Successful rehabilitation of the knee joints requires restoring muscular strength and thus stabilizing knee joints' functionality. 3 Also critical are exercises that stabilize and strengthen the quadriceps and restore their functions. 4 Weakened quadriceps often cause difficulties in the extension of knee joints when sitting, which also happens when external loads are limited to the weight of the calf. 5 Of the quadriceps, if muscular imbalance of vastus medialis and vastus lateralis occurs, patellofemoral pain syndrome can develop and weaken the functions of the knee joints. There have studied the muscle activity of vastus medialis and vastus lateralis in patellofemoral pain syndrome patients, and other research has found that as the quadriceps constitute the tendon in the knee joints, they can affect dislocation of the knee bones. [7][8][9][10] Out of all the quadriceps, the vastus medialis is the weakest physiologically is the first to atrophy, and is slow to recover; therefore, it can cause imbalance of the quadriceps. 11 While studies on the quadriceps have primarily focused on rehabilitation and preventive interventions, some studies have examined how damage to the quadriceps can change the mechanical stress environment of the knees and cause adjustments to the nerve roots.
In particular, some studies have looked into the causes that undermine the ability of the quadriceps to produce muscular strength by Vol. 28, No. 3, June 2016 pISSN 1229-0475 eISSN 2287-156X J KPT J KPT analyzing electromyograms and walking. These studies have provided basic information related to mechanical stress on the knees caused by injuries. 12,13 For the purpose of strengthening and assessing the muscular strength of the quadriceps, isokinetic devices, such as the Biodex System, are used. 14 Such devices are considered gold standard tools as they enable not only quantitative evaluation of muscular functions, but also assessment of various factors such as torque values, strength, and endurance. 15 In addition, torque values are critical indicators for quantitative evaluation of muscles, and they are regarded as the overall result of tension caused by muscular contractions. 16 Previous studies on torque values include comparisons of peak torque values of the quadriceps and hamstrings with isokinetic exercises, 17 and of the effects of maximum grip strength and wrist torque values during isokinetic exercises with different wrist positions. 18 In addition, there have been several studies on the correlation between muscle activity caused by the movement of joints and torque values, which have proven the existence of a correlation between muscle activity and torque values. 19 While there have been studies on muscle activity and torque values utilizing electromyograms and isokinetic devices, no study has examined the activity of different muscles at different angles in the relationship between the muscle activity of the quadriceps and torque values. Therefore, the current study seeks to identify the relationship between torque values and the muscle activity of the quadriceps with isometric exercises conducted at four angles (0, 30, 60, and 90 degrees) and isokinetic exercises performed at the angle of 30 and 60 degrees.

Subjects
The subjects of the current study were comprised of 29 healthy individuals (17 men and 12 women). Before beginning the experiments, the subjects were briefed on the purpose and methods of the study, and the experiments proceeded with those who voluntarily agreed to participate. The subjects were asked to fill out and submit a statement of consent in accordance with the ethical principles for medical research involving human subjects of the Declaration of Helsinki. In terms of protocol, the current study obtained approval from the Ethics Committee of Catholic University of Daegu to conduct the experiment.
The average age of the subjects was 23.55, the average height was 171.2 cm, the average weight was 64.9 kg and the average BMI (Korean Dietetic Association, 2008) was 22.0 kg/m 2 . The inclusion criteria were; 1. Subject without orthopedic or neurologic problems within 1year, 2. Subject without functional limitations or pain on the knee joints, 3. Subject without diseases causing autonomic nerve system problem, 4. Subject without cognitive limitation causing difficulties to participate in the study, 5. Subject with the distance between ankles or knees no more than 4 cm. 14  For the vastus lateralis, the electrodes were placed at 15 degrees of the long axis of the thigh, 6 to 8 cm outwards above the knee bones.

1) Measurement
Ground electrodes were placed on the surface of the knee bone of the opposite leg, and the spaces between the electrodes for each channel were kept at 2.2 cm. 22 The sampling rate collected through electromyographic signals was set at 1,024 Hz, and 10-450 Hz bandpass filters and 60 Hz notch filters were used. EMG data were rectifiedand smoothed using a Root Mean Square (RMS) algorithm.
%MVIC was used for normalization of the EMG data and %MVIC were measured prior to data collection in manual muscle testing positions. 23 2) Statistical analysis SPSS version 18.0 was used for computing statistics. To compare differences in muscle activity during the isometric exercises at four different angles, a repeated one-way ANOVA was used, while a oneway ANOVA was used to compare differences in muscle activity during isokinetic exercises at two different angle. For the post-hoc test, Bonferroni was used. The level of statistical significance was set at 0.05.

RESULTS
The muscle activity of the quadriceps measured at different angles of isometric exercises showed significantly higher across the three  Table 2).

DISCUSSION
The current study aims to explore and compare the relationship between torque values and muscle activity of the quadriceps by having healthy men and women in their twenties conduct isometric exer-

J KPT
cises at different angles of knee joints and isokinetic exercises at different angle.
The quadriceps is a very important muscle that provides stability for the knee joints when standing or walking. However, frequent use of the quadriceps can lead to various illnesses that cause knee pain, such as degenerative arthritis and patellofemoral pain syndrome. 2,24,25 In the case of patellofemoral pain syndrome, the strengthening of vastus medialis of the quadriceps is suggested among various rehabilitation strategies. In addition, resistance exercises for increasing muscular strength include isometric, isotonic, and isokinetic exercises, out of which isokinetic exercises are widely used for rehabilitation including those conducted within an individual's range of motion. 26 In this study, muscle activities of rectus femoris, vastus medialis, and vastus lateralis during isometric contractions at 0, 30, 60, and of the study indicate significant differences in all three muscles (the vastus medialis, vastus lateralis, and rectus femoris) during the con-centric and eccentric contractions. Also, it was shown that the more the angle increased in all muscles except for the vastus lateralis, the more the muscle activity increased during both concentric and eccentric contractions. This is in line with the results of past study, which contend that the lower the angular velocity fell during isokinetic exercises, the lower the muscle activity dropped. 27  It means vastus medialis is contributing more in torque generation during movement with fast angular velocity, while vastus lateralis contributes less. These results were consistent with the study by Na et al. 28 in which vastus medialis showed increased muscle activities with increased angle like 60˚/sec, 120˚/sec, and 180˚/sec during eccentric contractions, while the muscle activities of vastus lateralis was decreased. In their study, it was also found that the activity of vastus lateralis was increased mostly during eccentric contraction at 60˚/sec and decreased as angular velocity slowed. A review of the analysis of isometric and isokinetic exercises shows that muscle activity and the ratios of different muscles that contribute to torque values are more or less in proportion. It indicates that muscle activity and torque values are related, which means that information on the value of one of the two could help estimate the value of the other during rehabilitation exercises.
The results of this study showed the correlation between muscle activity levels and the ability of torque generation, in which higher muscle activities is, more torque is generated. Therefore, if the patient activated muscles more after rehabilitation, it is inferred that the muscle generates more torque. Clinical application of the biomechanical analysis of the current study could help selectively anticipate and strengthen muscles through isometric and isokinetic exercises for adults, professional athletes, and people who experience limited knee movement for long periods following knee surgery. Moreover, for those who are in need of knee rehabilitation exercises, the measurement of torque values alone could provide a basis for anticipating the muscle activity of the quadriceps. It can en-