Biomechanical Characteristics and Potential Injury Risks of Reverse Vertical Jumps for Male Firefighters of Different Grades: A Test Based on 86 Firefighters Embedded with IMU Insoles

1. Research objects and methods

1.1. Research Object

This study was a single-group cross-sectional study. GPower3.1 software was used to estimate the sample size in advance. Relevant parameters p=0.30, a=0.05, and power=0.85 were set. The minimum estimated sample size was 77 people. To ensure the reliability of the sample size, the minimum sample size was expanded by 10%, and the final actual included sample size was 86 people, ensuring that the experimental requirements were met. The research samples selected active male firefighters from two fire rescue brigades and three fire rescue stations in Jiangmen City, Guangdong Province.The test period was from September to October 2026. A total of 90 male firefighters' CMJ data were obtained. After screening, 86 people were finally included as the research sample. The inclusion and exclusion criteria for samples are as follows: ① Male firefighters; ② No lower limb injuries in the past half month; ③ Service period of no less than 12 months; ④ There was no situation of staying up late or insomnia on the test day. ⑤ The dominant leg is the right leg. Exclusion criteria: ① Service period is less than 12 months; ② Abnormal gait and lower limb function occur within 72 hours due to tasks or training; ③ There are organic lesions and injuries that affect the motor function of the lower extremities, such as cruciate ligament rupture, Achilles tendon rupture, etc. ④ There was a situation of staying up late or insomnia on the day before the test.

According to the classification standards for the service years and grades of firefighters stipulated in the "Regulations of the People's Republic of China on Fire Rescue Ranks", the tested firefighters can be classified into junior (service years 1-5 years), intermediate (service years 6-12 years), and senior (service years 13 years or more)[11]. At the same time, groups are classified based on team attributes (national teams and professional teams), Position attribute (commanders and firefighters), and veteran attributes (veterans and non-veterans).An in-depth analysis was conducted on the biomechanical characteristics of the CMJ of male firefighters from the perspectives of different levels and groups to explore the biomechanical characteristics and potential injury risks of the CMJ of firefighters. This study has been approved by the Medical Ethics Review Committee of Wuhan Institute of Physical Education (Approval No. : 2026145). The detailed basic information of the tested firefighters is shown in Table 1.

1.2. Research Methods

1.2.1. Testing instruments

The CMJ test was carried out based on the insole embedded with IMU developed by Digitsole SAS of France. This in-pad is equipped with an IMU inductive sensor, integrating a six-axis speedometer, gyroscope and magnetometer. It synchronously collects acceleration and angular velocity data in three-dimensional space at a acquisition frequency of 104 Hertz and outputs the relevant kinematic index results through reverse dynamics algorithm [12]. Research shows that when the IMU insole used in this study was tested for functionality and accuracy against the gold standard motion capture analysis system, the results indicated that the data acquisition results of the two sets of equipment had a high degree of consistency [13,14].It has been confirmed that the insole embedded with IMU has good validity of the calibration label association and reliable data results. Compared with traditional laboratory equipment, this set of equipment not only breaks through the site limitations but also provides portable, efficient and economical technical support for conducting cross-site, multi-dimensional and large-scale CMJ tests in this research.

1.2.2. Test Method

This study adopted the field testing method and the convenient spot check method to conduct tests. Before the test, the tester went to the fire station to explain the test requirements, emphasizing that firefighters should avoid staying up late, engaging in strenuous exercise and ensuring adequate rest to reduce the interference of confounding factors on the CMJ test the next day. On the day of the test, after obtaining the oral consent of the firefighters, the basic information was recorded and the samples were screened according to the inclusion and exclusion criteria. Select IMU insoles according to the firefighters' foot sizes. Firefighters should uniformly wear the strength and conditioning training suits and shoes provided to them to avoid the influence of mixed factors caused by clothing, boots and shoes.The CMJ testing process and detailed requirements refer to previous research results and are implemented[15].First, warm up for 1 to 3 minutes and get used to the IMU insoles; Secondly, firefighters should stand upright and stable, hold their waists with both hands, and are prohibited from swinging their arms. Subsequently, the tester activated the instrument and gave the order. The firefighter quickly squatted and bent his knees about 90 degrees before jumping with all his might. When in the air, his legs were straight. When landing, his legs were together and his knees were straight at 180 degrees. Finally, the firefighters waited on the spot to obtain and follow the instructions to end. If the data is invalid or the test is incorrect, retest.

1.2.3. Collection Indicators

In this study, the CMJ index system is composed of spatiotemporal parameters, kinematic parameters, dynamic parameters and their related extended indicators. The spatiotemporal parameters include the time to lift off and the height to lift off. The kinematic parameters include the flexion Angle of the ankle joint upon lifting off the ground, the flexion Angle upon landing, and the pronation/supination Angle upon landing. The dynamic parameters include the propulsion velocity, the maximum acceleration upon landing, and the stability upon landing. This system analyzes the CMJ movement performance of firefighters from multiple perspectives. It reveals the CMJ performance characteristics and potential injury analysis of firefighters of different grades from three aspects: movement performance, movement ability and structural function, providing quantitative research results for subsequent related studies.

12.4. Statistical Methods

The CMJ biomechanical data of male firefighters were rinsed and organized, and then input into SPSS29.0 software for statistical analysis. Firstly, the Shapiro-Wilk test was used to examine the normality of the data, and the Levene test was used to examine the homogeneity of variance of the data. For data that conform to a normal distribution and have homogeneity of variance, Bonferroni in ANOVA one-way analysis of variance is used for post hoc pairwise comparisons. If the data does not conform to the normal distribution index data,the Kruskal-Wallis H test was used for statistical comparison among firefighters of different grades (junior, intermediate and senior firefighters), and the statistical results were marked with H value and P value. The Mann-Whitney U test was used for statistical comparisons among different types of firefighters (such as those in positions belonging to middle commanders and firefighters), and the statistical results were labeled with U values and P values. Among them, P < 0.05 was considered statistically significant, and P < 0.01 was considered statistically significant.

2. Research Results

2.1. Basic Information Statistical Results

As can be seen from the results in Table 1, there are statistical differences in the age and years of service of firefighters at different levels and Position attribute. In terms of the age index, there is a significant difference between junior firefighters and intermediate and senior firefighters (p < 0.01), and there is a difference between intermediate and senior firefighters (P < 0.05). In terms of the length of service index, there was a significant difference (p < 0.01) between junior firefighters and intermediate and senior firefighters, while no statistical significance was shown between intermediate and senior firefighters.In terms of Position attribute, there were statistically significant differences between commanders and firefighters in terms of age and years of service (P < 0.05). In terms of team attributes, there was only a statistical difference between the national team and full-time firefighters in the indicator of years of service (P < 0.05), and no statistical difference was shown in the indicator of age. It can be seen that the length of service is an effective indicator for distinguishing and classifying firefighters of different grades. Therefore, this article classifies firefighters of different grades based on their length of service. The relevant statistical results are shown in Table 1.

2.2. Comparison Results of CMJ Indicators among Firefighters of Different Grades

Among the spatio-temporal parameters, there was a statistically significant difference in the right leg lift height between junior firefighters and senior firefighters (H=6.270, P < 0.05), and the right leg lift height of junior firefighters was higher than that of senior firefighters. In the kinematic parameters, there was a statistically significant difference in the left ankle landing flexion Angle between senior and junior, intermediate firefighters (P < 0.05), but no statistically significant difference was observed between junior and intermediate firefighters. From the trend perspective, the landing flexion angles of senior firefighters are all smaller than those of junior and intermediate firefighters.Among the dynamic parameters, there are statistically significant differences in the leg propulsion speeds of firefighters of different grades. There was a significant difference in the left leg propulsion speed between senior firefighters and junior firefighters (P < 0.01), and there was also a difference between senior firefighters and intermediate firefighters (P < 0.05). There is a difference in the right leg propulsion speed between senior firefighters and junior firefighters (P < 0.05). It can be seen from the results that with the promotion of firefighters' grades and the increase of their service years, various indicators show a downward trend, indicating that the CMJ ability and athletic performance level of senior firefighters tend to deteriorate. The detailed statistical results are shown in Table 2.

2.2. Comparison Results of CMJ Indicators among Different Types of Firefighters

In terms of the team attribute dimension, there is a difference in the right-leg landing stability index between national team firefighters and professional team firefighters (P < 0.05). The average landing stability of national team firefighters is higher than that of professional team firefighters, that is, the right-leg landing stability of national team firefighters is lower than that of professional team firefighters. In terms of the dimension of Position attribute, there were statistically significant differences between commanders and firefighters in four indicators: the time it took for both legs to be in the air, the height of the air, the Angle of the left foot upon landing, and the speed of the left leg's advancement (P < 0.05),the characteristic trend is that the average values of the four indicators of the commander are all lower than those of the firefighters, that is, the commander is lower than the firefighters in terms of CMJ height, flight time, joint range of motion during the landing stage of the left ankle, and propulsion speed. In terms of the dimension of veterans, veterans and non-veterans all show statistical differences in the three first-level indicators of spatio-temporal parameters, kinematic parameters and dynamic parameters. The detailed results are shown in Table 3.

3. Discussion

3.1. Discussion of Results and Potential Damage Analysis of Firefighters of Different Grades

In the spatio-temporal parameter dimension, there was a statistically significant difference in the right leg lift time index between junior and senior firefighters (P < 0.05). All the firefighters tested in this study had the right leg as their dominant leg. Compared with the non-dominant leg, the dominant leg is prone to fatigue accumulation and muscle tension, resulting in a decrease in the lifting height of the dominant leg. This phenomenon is more pronounced among senior firefighters. Research has shown that the intervention method combining static and dynamic stretching not only increases the vertical jump height of the subjects but also helps absorb the ground reaction force during the landing stage [16].Therefore, targeted combined stretching training not only enhances the lower limb movement performance of senior firefighters and prevents sports injuries, but also has the additional effect of extending their service life. In addition, as age and years of service increase, junior and intermediate firefighters are at risk of lower limb motor ability decline and sports injuries. Therefore, while providing benign intervention for the right dominant leg of senior firefighters, attention and vigilance should be paid to the right dominant leg of junior and intermediate firefighters.

In the kinematic parameter dimension, senior firefighters and junior and intermediate firefighters showed different results in the left ankle landing flexion Angle index (P < 0.05), and the left ankle landing flexion Angle of senior firefighters was smaller than that of junior and intermediate firefighters. Firefighters use their right leg as the dominant leg and their left leg as the supporting leg, which causes their left ankle joint to bear the dual effects of body weight and ground force for a long time, resulting in a decrease in the functional range of motion of the left ankle joint. Abnormal ankle joint functional range of motion is the main cause of the risk of "secondary injury"[17].Ankle sports injuries are limited by internal factors (weight, past medical history, proprioception, joint range of motion, muscle strength and psychological factors) and external risk factors (venue, weather, season, type of exercise, warm-up level, shoes and equipment, rehabilitation training level). At the same time, they are also affected by excessive fatigue, impact, uneven ground or missing steps, etc. It is extremely likely to cause ankle sprains[18].Based on the above model analysis, due to the complexity and high risk of fire rescue tasks and the environment, firefighters are a high-risk group for ankle injuries. ankle sprain (AS) is the most common sports injury, accounting for 10% to 30% of all sports injuries [19]. Approximately 40% of patients will develop chronic ankle instability (CAI). Severe cases can lead to ankle osteoarthritis[20].Compared with normal people, firefighters have a higher risk of developing AS and CAI. In addition, senior firefighters have served for a longer period of time, and the adverse effects on the left ankle joint last for a longer time, resulting in abnormal functional range of motion and increasing their potential injury risk.

In the dimension of dynamic parameters, there is a significant difference in the left leg propulsion speed between senior firefighters and junior firefighters (P < 0.001), and a difference between senior firefighters and intermediate firefighters (P < 0.05). There is also a difference in the right leg propulsion speed between senior firefighters and junior firefighters (P < 0.05). From the perspective of dynamic parameter indicators, as the firefighter's rank increases, the left leg propulsion speed, the maximum acceleration upon landing increase, and the landing stability tends to decline. This indicates that while the firefighter's left leg propulsion ability is deteriorating, the impact force on the lower left leg upon landing is continuously increasing, and the ability to absorb ground impact force and maintain joint stability is continuously decreasing.

Overall, compared with the dynamic parameters of the right leg, the degeneration trend of the left leg of firefighters is more significant and the risk of potential injury is higher. Research has found that the propulsion speed is influenced by the strength level and the coordinated ability of muscle groups[21]. Therefore, training should be conducted to enhance the strength and athletic performance of firefighters based on their bilateral lower limb propulsion speed. Training should also be carried out to prevent sports injuries, focusing on ground absorption braking and maintaining the stability of lower limb joints. This is to curb the adverse trend of strength decline and increased potential injury risks among firefighters as their service years and grades increase.

3.2. Discussion of Results and Potential Damage Analysis for Different Types of Firefighters

In terms of team attributes, there are differences in the right-side landing stability index between national team and professional team firefighters (P < 0.05), and the average index of national team firefighters is higher. The value of the landing stability index is inversely proportional to the stability, indicating that the landing stability of the right lower limb of the national team firefighters is worse. Studies have shown that the stability of lower extremity landing is highly correlated with the level of strength quality, past injuries and rehabilitation[22,23]. Firefighters have a high frequency and intensity of daily training. If the treatment is not timely or the treatment plan is inappropriate, some patients may develop chronic ankle instability[24].Studies have shown that the vertical jump height of patients with ankle instability is lower than that of the normal group, and the recruitment ability of the calf muscles on the affected side is reduced, the RMS peak of the thigh muscles lags behind, and the muscle activation sequence changes, which also has an impact on the healthy side[25]. To sum up, national team firefighters have a higher risk of chronic ankle instability and sports injuries, which leads to their aerial height indicators being lower than those of professional team firefighters. Therefore, for the right side stability problem of national team firefighters, exercise therapy is carried out. Exercise therapy is an effective way to improve ankle instability and enhance athletic performance[26].To sum up, the shock absorption capacity and stability of the right ankle joint of national team firefighters need more attention and improvement than those of professional team firefighters. During the execution of tasks and strength and conditioning training, the probability of right ankle injury for national team firefighters is higher.

In terms of Position attribute, there were differences (P < 0.05) between commanders and firefighters in terms of the time of leg flight, flight height, flexion Angle of the left ankle upon landing, and the propulsion speed of the left leg. Moreover, the results of all indicators for commanders were lower than those for firefighters. Age and sports injuries are the main reasons for the above differences between commanders and firefighters. From the perspective of age, a 47-year longitudinal follow-up study based on the physical capabilities of 427 ordinary people (222 men and 204 women) found that: The maximum aerobic endurance and muscular endurance of both men and women peak between the ages of 26 and 36. Male muscle strength reaches its peak at 27 and tends to decline from the peak age to 63. The overall physical fitness decline ranges from 30% to 48% [27].Compared with the age of the commander (34.57±5.24 years), the age of the firefighter (28.44±4.78 years) was in the optimal age range for the peak of physical fitness. Therefore, due to the age advantage of firefighters, their performance in terms of flight time, flight height and propulsion speed is higher than that of commanders. From the perspective of injury analysis, sports injuries of firefighters are more common in three types of tasks: strength and conditioning training (66.43%), operation training (37.91%), and fire-fighting tasks (31.65%). Among the three types of tasks, lumbar muscle strain, knee joint and ankle joint injuries account for a relatively high proportion [28]. The flexion range of motion of the ankle joint is crucial for rehabilitation and performance. A limited range of motion can lead to biomechanical changes and an increased risk of injury [29].In conclusion, due to the influence of age and past injury factors, the CMJ index level of commanders is lower than that of firefighters, and their potential injury risk is higher in the future when performing tasks and strength and conditioning training.

Among the attributes of veterans, although the various indicators of veterans and non-veterans did not show differences, the P values of the left ankle landing heel pronation/supination Angle index (P=0.067) and the left leg maximum landing acceleration index (P=0.069) were both close to the pre-difference level (P=0.05), showing a critical difference trend. From the perspective of sports biomechanics, the pronation/supination Angle of the landing heel is directly proportional to the variation of the ankle joint's movement Angle, and the maximum landing acceleration is directly proportional to the landing impact absorption efficiency. From the results, it can be seen that the average values of the two indicators of retired military personnel are both lower than those of non-retired military personnel, indicating that retired military personnel have better functional lower limb movement capabilities.Thanks to the fact that veterans have received systematic and high-intensity military strength and conditioning training, a solid foundation of lower limb muscle group strength and neural control ability has been formed. In contrast, non-veterans lack relevant training, which puts their lower limb motor ability at a relatively disadvantage. Therefore, in the landing stage of the CMJ test, the left lower limb of veterans has better performance in responding to ankle Angle variation and shock absorption than that of non-veterans. In conclusion, enhancing the functional motor ability of firefighters' lower limbs is an important way to strengthen ankle stability and landing buffering capacity, and it is also an effective method to prevent and curb potential sports injuries of firefighters.

4. Research Limitations

First, the research samples are mainly concentrated in South China, and the geographical scope of the research samples will be expanded in the future. Secondly, this study is a cross-sectional one. In the future, it will be combined with longitudinal tracking studies to explore the temporal characteristics of various indicators of firefighters' CMJ. Thirdly, the movement data collection is based on IMU insoles. The subsequent research introduces video motion capture and epidermal electromyography technology. While expanding the correlation research indicators, it deeply explores the biomechanical mechanism of firefighters' CMJ capabilities.

5. Conclusion

As the rank of firefighters increases, their vertical jumping ability shows a declining trend. The decline of various indicators of senior firefighters CMJ is more obvious compared with that of junior firefighters, indicating that they have regressed in center of gravity control, strength level and joint range of motion, and have increased the potential risk of injury. The shock absorption capacity and stability of the right ankle joint of national team firefighters need to be improved more compared with those of professional team firefighters. Commanders have lower levels of center of gravity control, ankle range of motion and pushing force than firefighters, indicating a higher potential risk of injury for them during mission execution and strength and conditioning training.

It is suggested that personalized strength and conditioning training and rehabilitation training plans be designed for firefighters of different levels. Basic training should be carried out around strength quality to enhance the vertical jumping ability of firefighters, and corrective training should be conducted to improve the limited range of motion of the lower limb joints of firefighters. Improve the lower limb landing stability of national team firefighters through small muscle group strength training and proprioceptive training. Enhance the lower limb motor chain movement ability of commanders through functional training methods, and promote their functional stability, functional strength and joint flexion and extension range of motion.