Background and objective: There are many studies that have examined the differences in isokinetic strength performance differences in athletes; however, observing these angular velocity-dependent differences on surface maps is a very practical and useful application. The aim of this study was to determine the angular velocity-dependent isokinetic knee extensor/flexor muscle strength and jump performance and to compare the test results and 3D torque-angle-velocity surface maps of amateur and professional volleyball players.
Materials and methods: We included 25 well-trained male volleyball
players (14 professionals and 11 amateurs) with a mean age of 21.8
Results and conclusions: Significant differences were identified for extensor–flexor
PT between PRO and AT at 300 and 120
The isokinetic dynamometer is one of the most reliable methods for assessing muscle strength in athletes [1,2]. The isokinetic dynamometer is also considered the gold standard for determining the angular velocity-dependent torque, work, and power and agonist–antagonist muscle strength ratio [3,4]. Peak torque (PT) is a good criterion for assessing joint function and muscle strength. The PT angle indicates the angle at which the muscles of the joint reach the PT and the torque when the maximal muscle contraction occurs. These data are useful in designing training prescriptions and rehabilitation programs for athletes [5,6].
Athletes show different strength parameters based on age, training history, and musculoskeletal development . In addition, they may also have a history of acute or chronic injuries characterized by increased and frequent training intensity during their sports career . These injuries are known to be mainly caused by an imbalance in strength between muscle groups or between the dominant and non-dominant leg [9,10]. Therefore, it is critical to determine any agonist–antagonist muscle imbalances to accurately observe the strength parameters and minimize the risk of injury [11-13].
Studies focusing on the balance of lower limb strength in volleyball players show that the hamstring-to-quadriceps ratio (H:Q) varies depending on training history and experience. Additionally, the total work (TW) output of knee flexors seems to be lower in inexperienced and young athletes [13-18]. Lower limb strength is extremely important for athletes. In addition, a detailed analysis of angular velocity-dependent strength is believed to play an important role in training planning.
In volleyball, one of the popular sports, vertical jump skill is critical in blocking, offensive, and defensive moments [19,20]. Strong knee joint muscles are required in the take-off and landing phases of the jump, and high levels of torque and power are required in the concentric/eccentric contractions of these muscles .
Isokinetic dynamometer tests are used to assess the relationship between torque and angle. Isolated analyses of peak torque, mean power, or total work parameters do not provide insights into the torque–angular velocity relationship of muscle strength. Although this relationship represents the dynamic behavior of the muscle, it also allows for a comprehensive analysis of functional capacity by revealing the lengthening–contraction, and lengthening–velocity relationships . The torque and angle relationship is affected by increasing speed in dynamic evaluations . Therefore, a three-dimensional (3D) surface map allows for a visual examination of the torque-velocity relationship and a more comprehensive analysis of the torque output in dynamic evaluations than that of static evaluations [22,23].
The purpose of this study was to determine the angular velocity-dependent knee isokinetic extensor/flexor muscle strength and vertical jump performance in amateur and professional volleyball players and to compare test results and investigate torque-angle-velocity relationships by plotting 3D surface maps. We hypothesized that (1) PRO players have higher isokinetic test results and lower H:Q ratio, (2) Depending on experience PRO players can generate more power and their vertical jump performances are also better. (3) Isokinetic strength evaluations are not only based on torque-angle relationship; therefore, the dynamic torque capabilities can be better represented using 3D maps that are plotted by including the velocity parameter while performing a dynamic assessment.
This was a cross-sectional laboratory study.
This study was conducted with the voluntary participation of 25 right-footed well-trained male volleyball players. Athletes who participated in the Turkish national leagues from different teams were divided into two groups: professionals (PRO) and amateurs (AT). The PRO group comprised the premier league athletes (n = 14) and the AT group comprised local league athletes (n = 11). Inclusion criteria were active volleyball athletes, of amateur or professional status, who had least 5 years’ experience in training and participating in competitions, and performed training regularly for at least 3 d a week (frequency of training was 16 h/wk for PRO and 10 h/wk for AT). Those players who reported a major or moderate lower extremity injury or any injury to the knee or thigh and those taking any supplements were excluded (not meeting inclusion criteria: n = 3; refused to participate: n = 1).
Testing sessions were conducted on 2 d with an interval of 24 h between them. On the first day, the purposes and methodological procedures of the study were presented to the athletes and an informed consent form was signed by the athletes. Body mass and height of the players were measured with a digital device (Seca 769 scale, Seca GMBH, Hamburg, Germany). Later, the CMJ test was performed. On the second day, isokinetic strength test of the knee muscles was performed using an isokinetic dynamometer (Fig. 1).
Experimental design flowchart. CMJ, countermovement jump; km/h, kilometer per; s, second; HR, heart rate; bpm, beats per minute.
Participants performed a warm-up protocol composed of walking/running for 3 min
at 7 km/h on a treadmill before the beginning the test. CMJ vertical jump test
was performed to assess muscle strength performance of the lower limb and jump
height . For the CMJ performance, the participant started from the standing
position with their hands on the hips. After an audible command, the participant
performed hip and knee flexion (approximately 90
The isokinetic tests were conducted using the Humac NORM dynamometer and data
were collected using Humac2009 v10 software (CSMI, Stoughton, MA, USA). Before
the tests, the isokinetic strength test protocol was introduced and for
adaptation, the participants were asked to do 15 maximal repetitions at 300 and
The 3D torque-angle-velocity surface maps were created with the MATLAB (r2014b, Math Works, Natick, MA, USA) algorithm using raw data obtained from the isokinetic strength test result. The “surf” mathematical function was used to create surface maps. All reps were used for the analysis and interpolated for all three angular velocities, in accordance with the duration and values of the average curves for each participant. To better describe surface maps, the entire ROM of extension–flexion motion was considered and polynomial regression curve was plotted for acceleration, constant speed (load range), and deceleration stages of the motion [15,21]. 3D surface maps used Z-axis for the torque value, Y-axis for the angle value, and X-axis for the velocity value. The map used red color for high torque values and blue for low torque values.
The results were analyzed using IBM SPSS Statistics (Versions 22 for macOS; IBM,
Armonk, NY, USA). The data were assessed for skewness and kurtosis with the
skewness–kurtosis test. Data distribution was analyzed using the Shapiro–Wilk
test, followed by Mann–Whitney test for cross-group comparisons. The effect size
Participants’ descriptive statistics (14 professionals, 11 amateurs; average
|PRO (n = 14)||AT (n = 11)||p|
|Mean (min–max)||SD||Mean (min–max)||SD|
|Age (year)||22.2 (19–26)||2.2||21.2 (18–27)||3.1||0.349|
|Height (cm)||187.57 (181–193)||4.18||183.18 (172–197)||8.19||0.95|
|Body mass (kg)||82.07 (66–103)||9.77||80.72 (63–110)||12.48||0.765|
||23.35 (18.87–30.09)||2.95||24.01 (20.68–30.47)||2.92||0.586|
A statistically significant difference was found in the extensor and flexor PT
values in favor of PRO volleyball players at angular velocities of 300 and
3D surface maps showed high torque values of extension and flexion in the red field for PRO and AT groups. Both groups achieved higher extensor torque values at low and medium angular velocities. High extensor torque values were detected in the first ROM angles following the start of a movement for both groups (Fig. 2). Flexor torque data showed less variation between angular velocities. High torque values were recorded in the terminal phase of the flexion movement for both groups (Fig. 3).
3D surface maps of extension torque-angle-velocity
relationship. (A) Knee extension of the PRO players. (B) Knee extensions of the
3D surface maps of flexion torque-angle-velocity relationship.
(C) Knee flexion of the PRO players. (D) Knee flexion of the AT players.
|Peak torque (Nm)||191.42
|Total work (Nm)||1017.14
|Mean power (W)||125.42
|Angle at peak torque (°)||68.71
|* p |
|Peak torque (Nm)||120.07
|Total work (Nm)||751.42
|Mean power (W)||92.21
|Angle at peak torque (°)||32.51
|* p |
The hypotheses were: (1) PRO players have higher isokinetic test results and
lower H:Q ratio, (2) depending on experience PRO players can generate more power
and their vertical jump performances are also better. (3) Isokinetic strength
evaluations are not solely based on torque-angle relationship; therefore, the
dynamic torque capabilities will be better represented by three-dimensional maps
that can be plotted by including the velocity parameter while performing a
dynamic assessment. The main finding of this study is more experienced athletes
had higher peak torque, mean power, angle at peak torque in both extension and
flexion movements at angular velocities of 300 and 120
Similar results have been obtained in other studies which were attributed to the difference in the training frequency and intensity of experienced high-level athletes and different levels of musculoskeletal development [7,13,15-18,31].
The areas in which the high torque was shown in red in extension movements
corresponded to a larger numerical value in PRO athletes, whereas the equivalents
of these colors were smaller in AT athletes. By angular velocity, joint angle and
high torque values at low and medium velocities (60 and
Regular and well-designed training programs, nutrition, and ideal recovery times can allow professional athletes to reduce injury rates. Furthermore, by attending regular training, the athletes can adapt to increased training load and to carry their biomotor abilities to a higher level thanks to a targeted performance increase. Professional athletes have trained for a longer duration than amateur athletes and the resulting increased strength parameters at different levels are known to enable professional athletes to produce higher torque. These differences support the findings of the study. Age and sports history are important criteria; however, as participants were not divided into age groups in this study, a comparative analysis based on age and maturity levels was not performed. Several studies have shown age-related changes in maximal and rapid strength that were reported in antagonist muscle groups [32-34]. This may also have affected the difference in performance between groups as an additional factor.
Jumping is a skill of specific importance in volleyball. Knee joint muscles are responsible for 49% of the TW produced during a vertical jump . CMJ test results of PRO and AT athletes were found to cause cross-group differences in the study. This may be attributed to different torque output levels. Previous investigations have shown that reduced H:Q ratios in male volleyball players are related to better CMJ performances . Prilutsky and Zatsiorasky  stated that reduced H:Q ratios may be related to the need of producing more mechanical work for a jump, which is due to the lower dissipation of the rotational energy between the knee muscles. In addition, it is known that isokinetic knee strength parameters are associated with vertical jumps rather than spike jumps and are related to quadriceps concentric values [10,37].
According to the literature review, an ideal hamstring-to-quadriceps muscle
strength balance and a healthy joint mean that this ratio is approximately 60%
in healthy individuals [9,18]. Similar results were obtained from our study; the
H:Q ratio was very close to recommended reference values in both PRO and AT
athletes. According to our findings, the dominant leg H:Q ratios of the PRO and
AT groups were 57% and 63%, respectively. Ön et al.  compared
volleyball players of the same age and with different years of training, found
H:Q ratios for both limbs at 60 and 180
This study has certain limitations, particularly including the fact that it was conducted through the analysis of unilateral lower limb performance and in athletes with no history of injury. In the future, studies should be conducted in other sports branches through bilateral lower limb assessments to generate a more comprehensive analysis. In addition, study results including athletes with a history of injury may contribute to the rehabilitation process. In future studies, the research can be further detailed by examining the co-contraction of the extensor and flexor muscles at different angular velocities with electromyography.
We recommend future research to investigate the strength differences, 3D torque-angle-velocity using surface maps plotted using the data obtained from the isokinetic strength test to facilitate the investigation by a simultaneous display of test results at different angular velocities to contribute in designing strength training programs. The results of the study show that professional training and competition programs of professional volleyball players affect the the knee joint muscle strength. These results indicate the critical importance of designing customized strength training programs for athletes at different levels, even in the same sports and age group. These findings may help amateur athletes and coaches working with them to gain awareness about the isokinetic strength of the lower limb and pay attention to strength development by incorporating it into strength training programs. Conducting isokinetic tests for athletes before the competition season and repeating these tests after certain periods by considering macro/microcycles in annual training planning can be highly useful. In investigating athletes’ performance parameters, 3D torque-angle-velocity surface maps plotted with the data obtained can simultaneously show test results at different angular velocities and can be useful in dynamic performance analyses, provide a comprehensive perspective, and facilitate the work of physicians examining the data in non-sports sciences settings who make frequent use of a dynamometer, e.g., in physical therapy, orthopedic rehabilitation, and athlete rehabilitation units.
According to the experience of the athletes, AT and PRO male volleyball player groups showed different values of concentric knee strength depending on the angular velocity. Also, their CMJ heights were significantly different, whereas H:Q ratios demonstrated no differences between groups.
3D surface maps can play an important role in the comparative analysis of athletes with different sports backgrounds or performance analyses when tracking individual development and would provide a detailed and understandable perspective in revealing differences. In addition, it can also help to observe the possible deficits in muscle strength and load range before and after potential injuries.
AS designed the data collection method, collected the data, analyzed the data, prepared tables and wrote the paper.
This study was conducted according to the Declaration of Helsinki. Before the tests, all participants read and signed the informed consent form. The research protocol was approved by Trakya University Ethics Committee (TÜTF-BAEK 2019/473).
We thank all the subjects for participating in this study.
This research received no external funding.
The author declare no conflict of interest.