Effect of the frequency of weight-free vibration training on the isokinetic strength of knee muscles in juvenile football players

Abstract

BACKGROUND:

Vibration training can affect strength improvement. However, the role of the vibration frequency, in terms of knee muscle strength, is unclear.

OBJECTIVE:

To evaluate the effect of vibration training with the same amplitude and different frequencies on the isokinetic muscle strength of the knee in juvenile football players.

METHODS:

Juvenile football players were divided into four groups: low frequency ( $n=$ 13, 25 Hz), medium frequency ( $n=$ 14, 40 Hz), high frequency ( $n=$ 14, 50 Hz), and control ( $n=$ 13). The frequency groups completed 12 weeks of weight-free vibration training (three times/week) with the same amplitude (3 mm) but different frequency.

RESULTS:

Compared with baseline, the peak extension torque of the knee at 60 ${}^{\circ}$ /s and 240 ${}^{\circ}$ /s increased by 8.4% and 12.9%, respectively, in the medium-frequency group, and by 8.9% and 15.5%, respectively, in the high-frequency group. The extensor endurance (the ability of joint muscle groups to maintain a force output over time) of the knee in the high-frequency group increased by 4.3%. At 12 weeks, the high-frequency group had greater knee extensor endurance than the low- and medium-frequency groups.

CONCLUSION:

In juvenile football players, weight-free vibration training at 40 Hz and 50 Hz improves peak torque of the knee extensors at 60 ${}^{\circ}$ /s and 240 ${}^{\circ}$ /s, while training at 50 Hz improves endurance of the knee extensors.

Keywords

Vibration training juvenile football player knee joint isokinetic muscle strength

Table 1
Basic information of the study participants

Group	$n$	Age (y)	Height (cm)	Weight (kg)	Training years (y)
Low frequency group	13	15.7 $\pm$ 1.3	173.1 $\pm$ 3.4	66.1 $\pm$ 5.8	3.7 $\pm$ 0.5
Medium frequency group	14	16.0 $\pm$ 1.2	172.7 $\pm$ 3.9	66.3 $\pm$ 6.0	3.9 $\pm$ 0.7
High frequency group	14	15.8 $\pm$ 1.8	172.4 $\pm$ 4.5	66.5 $\pm$ 4.9	3.8 $\pm$ 0.8
Control group	13	15.9 $\pm$ 1.3	172.9 $\pm$ 3.9	65.9 $\pm$ 6.2	4.0 $\pm$ 0.6

1. Introduction

Vibration training affects the muscle strength of the knee joint. During the vibrational training, the motor unit of the muscle group at the vibrational site is activated, stimulates the muscle fibers to participate in the contraction, promotes the contraction of the tendon, and facilitates the improvement of the joint strength [1]. Many studies have discussed the effect of vibration training on strength improvement from the perspectives of intervention time [1, 2], vibration frequency [3, 4, 5, 6, 7], and amplitude [8]. However, it is unclear whether the vibration frequency affects the improvement in knee joint muscle strength. One study that evaluated the effects of 24 weeks of vibration training with the same amplitude and different frequencies in older women reported that the high-frequency group (40–45 Hz) achieved better improvements in the muscle strength of the knee joint than the low-frequency group (10–15 Hz) [3]. Furthermore, 8 weeks of vibration training at 25–35 Hz and 35–50 Hz with the same amplitude significantly improved the dynamic testing peak torque (PT) of the knee extensor muscles of softball, swimming, and diving athletes, but vibration at 35–50 Hz achieved a better improvement [5]. In ordinary men who performed 4 weeks of vibration training at different frequencies (20 Hz, 40 Hz, and 60 Hz), the PT of the knee flexor and extensor dynamic muscles significantly increased in the 60 Hz group [6]. In contrast, young women who were not trained achieved better improvements in the PT and maximum isometric contractile force of the knee extensors after 8 weeks of vibration training with the same amplitude (2–4 mm) at 30 Hz than 50 Hz [7]. Good joint muscle strength is the basis of the physical quality of athletes, and effective scientific methods are needed to measure this parameter. The isokinetic muscle strength test method reflects the strength of the measured joint muscle group under different angular velocities, and reflects the endurance level of the muscle group by measuring the attenuation of repeated joint work [9, 10]. A review of the influence of vibration training on human muscle strength found that it remains controversial whether vibration frequency affects knee muscle strength. Under different angular velocity test conditions, the peak torque changes are different [11]. Furthermore, there is a lack of literature on the effect of vibration training on the endurance ratio (ER) in the knee joint of athletes, it is very important for athletes to maintain a good competitive state at the end of the match. The improvement of muscle group endurance plays a positive role in maintaining good sports performance. In addition, the most effective vibration frequency needs further clarification.

The present study evaluated the influence of vibration training with the same amplitude and different frequencies on the knee joint muscle strength of juvenile football players. The aim was to use isokinetic muscle strength tests of different angular velocities to provide a practical basis for improving the joint muscle strength of athletes. The hypothesis was that vibration training with the same amplitude and different frequencies has different effects on the knee joint muscle strength of juvenile football players, and that high-frequency vibration training achieves better improvements in knee joint muscle strength than training at medium and low frequencies.

2. Materials and methods

2.1 Participants

The study population comprised 54 elite male juvenile (age 14 to 18 years old) football players in Sichuan Province, China, who had been recruited by the provincial football management center as football reserve talents and had been trained intensively. Inclusion criteria: players in the top three teams in the provincial juvenile championship who passed the health examination and questionnaire screening and provided written informed consent after understanding the study purpose. Exclusion criteria: cardiac pacemaker; epilepsy; history of lower limb joint injury or surgery.

The participants were divided into (14 ones, 14 twos, 13 threes, 13 fours) the low-frequency group, medium-frequency group, high-frequency group, and control group by digital random distribution (Table 1). And the participants blinded to their allocation. No participants dropped out during the whole experiment. This study was conducted in accordance with the Declaration of Helsinki and approved by the school ethics committee (NO. 2019023)

2.2 Whole-body vibration training

The resonance frequency of the human body is approximately 5 Hz, the resonance frequency of the viscera and spine is approximately 8 Hz, and the resonance frequency of head is approximately 18 Hz, while frequencies of higher than 60 Hz can cause eyeball resonance [12]. To avoid the resonance causing harm, the vibration frequency in the low-, medium-, and high-frequency groups was set at 25 Hz, 40 Hz, and 50 Hz, respectively, and the amplitude was set at 3 mm [3, 13]. To keep the original training plan (Because the subjects recruited in this study are athletes, they carry out the same training program. In addition to their same original training schedule, additional whole body vibration training is added.) and intensity of all subjects unchanged, the low-, medium-, and high-frequency groups (vibration group) performed vibration training intervention using four Power-Plate vibration training instruments (American model Pro5 AIR™, vibration frequency range 25–50 Hz, amplitude range 1–3 mm, maximum weight 250 kg) three times per week for 12 weeks. Each subject completed five sets of static weight-free squats (There was no external resistance except the body weight of the subjects, 20 seconds/set with an interval of 10 seconds), half squats, deep squats, and left and right lunge squats (10 times/set with a 10-second rest between sets). For the half squats, deep squats, and left and right lunge squats, the subjects completed one complete movement in accordance with rhythmic prompt tones (with intervals of about 2 seconds). The control group completed the same actions as the vibration group without the vibration training instrument. A researcher supervised the whole process to ensure that the training quality was satisfactory.

2.3 Isokinetic muscle strength test

An isokinetic muscle strength tester (IsoMed 2000, Germany) was used to test the isokinetic muscle strength of the bilateral knee joints in the four groups at 0 and 12 weeks; muscle strength was tested during flexion and extension five times at 60 ${}^{\circ}$ /s and 25 times at 240 ${}^{\circ}$ /s, with an interval of 1 min between testing at the two angular velocities. The testing was performed with the subjects in the sitting position (right side of Fig. 1), and the joint motion was 80 ${}^{\circ}$ . Before the formal test, the subjects completed two to three sub-ultimate strength exercises. The researcher used verbal encouragement to make the subjects complete the test as well as possible to ensure the reliability of the measurement data.

Figure 1.

A study participant performing vibration training (left) and knee joint isokinetic muscle strength testing (right).

The isokinetic muscle strength was measured using the PT and ER. PT refers to the maximum output torque (Nm) generated by muscle contraction during the whole joint activity. An athlete’s slow strength is reflected by the results of testing at 60 ${}^{\circ}$ /s, while the fast strength is reflected by testing at 240 ${}^{\circ}$ /s [14]. ER refers to the ability of joint muscle groups to maintain a force output over time. In the 240 ${}^{\circ}$ /s test, the ER was calculated as the ratio of (the sum of the total work done at the 21 ${}^{\text{st}}$ time – the 25 ${}^{\text{th}}$ time) to (the sum of the total work done at the 1 ${}^{\text{st}}$ – the 5 ${}^{\text{th}}$ time), with an ER approaching 1 indicating better endurance of the corresponding muscle groups [15].

2.4 Statistics analysis

The PT and ER of the knee joint of the four groups (the left and right data of each subject were averaged) were processed by SPSS Statistics 20.0. The PT and ER were presented as mean $\pm$ standard deviation. Two-way analysis of variance (ANOVA) tests were initially performed to determine whether there was an interaction between group (4) and time (2) [16]. If there was an interaction, a simple effect analysis was carried out. If there was no interaction, one-way ANOVA and the least significant difference test were performed to compare the differences at the same timepoint between groups, and the paired sample $t$ test was performed to compare the differences at different timepoints within groups. The significant level was set as $\alpha=$ 0.05.

3. Results

One-way analysis of variance showed that there were no significant differences between the four groups in age, height, weight, and training years ( $P>$ 0.05, Table 1). Figure 2 summarizes the isokinetic muscle strength of the knee joint in the four groups at the two angular velocities. Two-way ANOVA showed that time and group had no interaction for the peak flexion torque at 60 ${}^{\circ}$ /s ( $F_{(3{,}100)}=$ 0.063, $P=$ 0.979), peak extension torque at 60 ${}^{\circ}$ /s ( $F_{(3{,}100)}=$ 0.775, $P=$ 0.511), peak flexion torque at 240 ${}^{\circ}$ /s ( $F_{(3{,}100)}=$ 0.062, $P=$ 0.980), peak extension torque at 240 ${}^{\circ}$ /s ( $F_{(3{,}100)}=$ 2.036, $P=$ 0.114), flexor endurance ( $F_{(3{,}100)}=$ 0.013, $P=$ 0.998), and extensor endurance ( $F_{(3{,}100)}=$ 0.921, $P=$ 0.434). Therefore, one-way ANOVA and the least significant difference test were used to compare the differences at the same timepoint between groups, and the paired sample t test was used to compare the differences at different timepoints within groups.

Figure 2.

Knee Joint Isokinetic Muscle Strength Test Results at Different Angular Velocities. ${}^{\rm A}P<$ 0.01 compared with different timepoints within the same group ${}^{\rm b}P<$ 0.05, ${}^{\rm B}P<$ 0.01 compared with the control group at the same timepoint ${}^{\rm c}P<$ 0.05 for comparisons between the low- or medium-frequency groups and the high-frequency group at the same timepoint.

At week 0, there were no significant differences between groups in the peak flexion torque at 60 ${}^{\circ}$ /s ( $F_{(3,50)}=$ 0.160, $P=$ 0.923), peak extension torque at 60 ${}^{\circ}$ /s ( $F_{(3,50)}=$ 0.054, $P=$ 0.983), peak flexion torque at 240 ${}^{\circ}$ /s ( $F_{(3,50)}=$ 0.155, $P=$ 0.926), peak extension torque at 240 ${}^{\circ}$ /s ( $F_{(3,50)}=$ 0.071, $P=$ 0.975), flexor endurance ( $F_{(3,50)}=$ 0.050, $P=$ 0.985), and extensor endurance ( $F_{(3,50)}=$ 0.164, $P=$ 0.920), indicating that the four groups were homogeneous.

At 12 weeks, there were no significant differences between the four groups in the peak flexion torque at 60 ${}^{\circ}$ /s ( $F_{(3,50)}=$ 0.535, $P=$ 0.660), peak flexion torque at 240 ${}^{\circ}$ /s ( $F_{(3,50)}=$ 0.037, $P=$ 0.990), and flexor ER ( $F_{(3,50)}=$ 0.034, $P=$ 0.991). Compared with the control group, the high-frequency group had a significantly higher peak extension torque at 60 ${}^{\circ}$ /s ( $P=$ 0.049, 95% confidence interval: $-$ 28.85–0.01), peak extension torque at 240 ${}^{\circ}$ /s ( $P=$ 0.007, 95% confidence interval: $-$ 31.59–5.14), and extensor endurance ( $P=$ 0.042, 95% confidence interval: 0.0023–0.1204). The high-frequency group had a significantly greater extensor endurance than the low-frequency group ( $P=$ 0.015, 95% confidence interval: 0.0153–0.1335) and the medium-frequency group ( $P=$ 0.034, 95% confidence interval: 0.0049–0.1208). The medium-frequency group had a significantly greater peak extension torque of the knee joint at 240 ${}^{\circ}$ /s than the control group ( $P=$ 0.022, 95% confidence interval: 2.35–28.80).

There were no significant differences between the results at 0 versus 12 weeks in the flexion torque at 60 ${}^{\circ}$ /s ( $t=$ $-$ 1.444, $t=$ 2.164), extension torque at 60 ${}^{\circ}$ /s ( $t=$ $-$ 1.342, $t=$ $-$ 1.534), flexion torque at 240 ${}^{\circ}$ /s ( $t=$ $-$ 0.450, $t=$ 0.621), extension torque at 240 ${}^{\circ}$ /s ( $t=$ $-$ 1.385, $t=$ $-$ 2.004), flexion endurance ( $t=$ $-$ 1.104, $t=$ $-$ 0.415), and extension endurance ( $t=$ 0.063, $t=$ $-$ 1.307). In the medium-frequency group, the peak extension torque of the knee joint at 60 ${}^{\circ}$ /s ( $t=$ $-$ 6.095) and 240 ${}^{\circ}$ /s ( $t=$ $-$ 6.095) significantly increased from week 0 to week 12 by 8.4% and 12.9%, respectively ( $P<$ 0.01); however, there were no significant changes in the peak flexion torque at 60 ${}^{\circ}$ /s ( $t=$ $-$ 0.707), peak flexion torque at 240 ${}^{\circ}$ /s ( $t$ $=$ $-$ 0.705), flexor endurance ( $t=$ 0.074), and extensor endurance ( $t=$ $-$ 1.791). In the high-frequency group, the peak extension torque at 60 ${}^{\circ}$ /s ( $t=$ $-$ 4.813), peak extension torque at 240 ${}^{\circ}$ /s ( $t=$ $-$ 7.682), and extensor endurance ( $t=$ $-$ 4.125) of the knee joint significantly increased from week 0 to week 12 by 8.9%, 15.5%, and 4.3%, respectively ( $P<$ 0.01); however, there were no significant changes in the peak flexion torque at 60 ${}^{\circ}$ /s ( $t=$ $-$ 0.360), peak flexion torque at 240 ${}^{\circ}$ /s ( $t=$ $-$ 1.536), and flexor endurance ( $t=$ $-$ 0.319).

4. Discussion

In the present study, juvenile football players completed 12 weeks of vibration training with the same amplitude and different frequencies (25 Hz, 40 Hz, and 50 Hz) to explore the changes in the PT and ER of the knee flexor and extensor muscles at 60 ${}^{\circ}$ /s and 240 ${}^{\circ}$ /s to provide a basis for improving the joint muscle strength of athletes.

The main findings of the present study were that the PT of the knee joint at 60 ${}^{\circ}$ /s and 240 ${}^{\circ}$ /s significantly increased by 7.9% and 9.3%, respectively, in the medium-frequency group, and significantly increased by 6.4% and 8.7%, respectively, in the high-frequency group. The PT at 240 ${}^{\circ}$ /s of the extensor muscles in the high-frequency group was significantly greater than that in the low-frequency group. The results show that vibration training at both 40 Hz and 50 Hz under the same amplitude improves the slow and fast strength of the knee joint extensor group of juvenile football players, while a frequency of 25 Hz does not cause significant changes.

During vibration training, the amplitude determines the centrifugal force borne by the muscle, frequency determines the acceleration force borne by the muscle, and additional load determines the tension of the muscle [17]. When the amplitude and additional load are unchanged, the acceleration force borne by the muscles increases with the increase in the vibration frequency. Previous studies have found that vibration training at higher frequencies achieves greater increases in isokinetic knee flexor and extensor muscle strength than training at lower frequencies in ordinary female college students (evaluated frequency range: 25–50 Hz) [5], older women (evaluated frequency range: 15–45 Hz) [13], and ordinary male college students (evaluated frequency range: 15–45 Hz) [18]. The present study found that vibration training caused similar changes in the knee extensor muscles in juvenile football players [5, 13, 18]. However, in contrast with previous studies [5, 13, 18], the present study found that vibration training had no effect on the knee flexor muscles of juvenile football players. This difference between studies may be related to the fact that no additional load was added in the present study, while Ren et al. [5]. added 30% of the maximum squat load. The interstudy differences in the age and occupation of the subjects may also have contributed to the inconsistent conclusions. The present study assessed juvenile football players who had been training regularly for a long time, which may be why the increase in the knee flexor muscle strength after vibration training was smaller than that in ordinary people who were not training regularly. However, a study of older adults who seldom carried out regular strength training found that the strength of the knee flexor muscles significantly increased after vibration training [13].

The mechanism of vibration training in improving joint muscle strength has been analyzed. During vibration training, the motion units of the vibrated muscle groups are further activated and the contraction function of the tendons is further increased compared with vibration-free training. Compared with vibration-free training, vibration training stimulates more muscle fibers to participate in contraction, and muscle strength is improved after repeated stimulation [19, 20]. Furthermore, vibration training increases the tension reflex of muscle temperature, blood flow, and vibration resistance at the vibrated part, and improves neuromuscular performance [21], which also improves muscle strength. Under the same vibration amplitude, the acceleration generated by higher-frequency vibration stimulation is higher than that generated by lower-frequency vibration stimulation, which affects the vibration intensity borne by muscles during vibration exercise. However, for relatively high-level athletes, the muscle performance is only improved when the vibration stimulation is large enough [17]. This may be the reason why the vibration training only had a marked effect in the medium- and high-frequency groups in the present study.

In the present study, the knee extensor endurance in the high-frequency group increased by 10.8%, and the knee extensor endurance in the high-frequency group was significantly greater than that in the low- and medium-frequency groups. However, the endurance of the knee flexor and extensor muscles did not significantly differ between the low- and medium-frequency groups. This shows that high- frequency vibration training under the same amplitude is conducive to improving the endurance of the knee extensor muscles of elite juvenile football players, while low- and medium-frequency vibration training is insufficient to improve the endurance of lower limb muscles.

During vibration training, the muscle fibers with a low threshold value at the vibrated part are caused to participate in contraction, thus relieving the fatigue of the muscle fibers with a high threshold value [22, 23]. In the present study, the degree of attenuation of the muscle work of the knee joint in the high-frequency group was weakened when 25 extensions were performed, while there was no similar finding in the low- and medium-frequency groups. We speculate that 12 weeks of vibration training at 50 Hz forced the low threshold muscle fibers of the knee joint of juvenile football players to participate in contraction, which was manifested in the improvement of ER in the high-frequency group. However, there was no obvious change in knee flexor endurance, which may be related to the lack of special flexor training and the particularity of the research subjects. A football match lasts for 90 minutes, and it is very important for football players to maintain a good competitive state at the end of the match. The improvement of muscle group endurance plays a positive role in maintaining good sports performance. The present results suggest that 50 Hz vibration training is beneficial in improving knee extensor muscle endurance for juvenile football players performing normal strength training.

The present study had certain limitations. As a limited number of juvenile football players could be recruited, the sample size was small (power is 0.8). In addition, no other indexes were analyzed to determine whether the results were influenced by the changes in muscle strength, amplitude, and single intervention time.

5. Conclusions

Weight-free vibration training at 40 Hz and 50 Hz improves the PT at 60 ${}^{\circ}$ /s and 240 ${}^{\circ}$ /s of the extensor muscles of juvenile football players to different degrees, while training at 25 Hz has no effect. At the same amplitude, vibration training at a frequency of 50 Hz best improves the endurance of the knee extensor muscles of juvenile football players.

Author contributions

CONCEPTION: Liang Cheng, Hanxiao Xu, Benxiang He and Jianan Zhou.

PERFORMANCE OF WORK: Liang Cheng, Hanxiao Xu, Benxiang He and Jianan Zhou.

INTERPRETATION OR ANALYSIS OF DATA: Liang Cheng and Hanxiao Xu.

PREPARATION OF THE MANUSCRIPT: Liang Cheng, Hanxiao Xu, Benxiang He and Jianan Zhou.

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Liang Cheng, Hanxiao Xu, Benxiang He and Jianan Zhou.

SUPERVISION: Liang Cheng, Hanxiao Xu, Benxiang He and Jianan Zhou.

Ethical considerations

This study was approved by the school ethics committee (No. 2019023). Before the assessment, every participant received the same detailed information about the testing procedure and signed the informed consent.

Funding

This research was funded by a grant from the China’s key research and development plan (2019YFF0301704).

Footnotes

Acknowledgments

Thanks to the support of Sichuan elite male juvenile football players for this research.

Conflict of interest

All authors declare no financial or personal conflict of interest.

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Effect of the frequency of weight-free vibration training on the isokinetic strength of knee muscles in juvenile football players

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSION:

Keywords

Table 1 Basic information of the study participants

2. Materials and methods

2.1 Participants

2.2 Whole-body vibration training

2.3 Isokinetic muscle strength test

3. Results

5. Conclusions

Author contributions

Ethical considerations

Funding

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Basic information of the study participants