Purpose: The study aimed to investigate the effect of initial knee flexion angle on squat-jump performance. The best squat-jump performance was observed at 60º, which was the smallest knee flexion angle in this study.
General Introduction
Scope and Limitation of the Research
Problem Statement
Aim and Objectives
Overview of the Structure
The chapter describes the design of the method as well as the method modification that improved its acceptance and feasibility. The chapter leaves readers with a clear understanding of the main findings of the research and the ideas for future research.
Introduction
Key Phases of Squat-Jump
Squat Depth
Biomechanical Parameters Interpretation
- Displacement
- Force
- Velocity
- Power
However, other parameters will be evaluated to obtain a better analysis of the result (Markovic et al., 2004). At the end of the landing phase, the velocity returned to zero again as the subject returned to the static position.
Factors that Influence Squat-Jump Performance
Footwears
This means that the effect of muscle strength on jumping performance cannot be overlooked (Suchmel, Nimphius, and Stone, 2016). Suchomel, Nimphius, and Stone (2016) also reviewed previous studies on the effect of maximal force on jumping performance.
Gender
Harvard Health Publishing (2016) stated that after 30 years of age, humans begin to lose up to three percent to five percent of muscle each decade. Doherty (2001) also mentioned that from about 30 years of age, strength decreases by 10% to 15% every decade without training. There are significant impacts of muscle strength on the force-time relationship of sports performance.
Based on the results, 78% of the correlation magnitudes reported (n = 91) reported a positive effect of strength on jumping performance. In 2010, Laffaye and Choukou discovered that men achieved higher jumps than women and that both men and women had different jumping techniques. In the following year, Magnúsdóttir, Sveinsson and Árnason (2011) also found that males jumped higher than females and that females tend to show more knee valgus on landing.
Body Mass Index
In the function of the musculoskeletal system, it is the combination of physiological, biomechanical and anatomical stresses that ultimately cause an overload of forces in the musculotendinous unit (Kibler, Chandler and Stracener, 1992). One of the possible negative effects resulting from overtraining is the long-term decline in physical performance (Halson and Jeukendrup, 2004). According to BTS Bioengineering (2019), SJ is one of the jump tests included in the BTS G-Walk® sensor.
To investigate the acceptability and feasibility of the experimental design, pre-tests and a pilot study were performed, as reported in Chapter 4. With the manipulation of the initial knee flexion angle, quantitative data (i.e., flight time, peak velocity, peak propulsive force, maximum concentric power, and flight altitude) were collected from 15 healthy subjects using the BTS G-Walk® sensor. Subjects' sex, age, and BMI were controlled, and subjects were restricted to wearing athletic shoes and sportswear during the study.
Experimental Design .1 Independent Variable
Dependent Variable
- Flight Time
- Propulsive Peak Force
- Maximum Concentric Power
- Flight Height
The data collected by the BTS G-Walk® sensor was viewed from BTS-G studio as shown in Figure 3.3. The flight time is the time between take-off and landing, as shown in Figure 3.5. Since it is an important kinematic variable related to peak speed and flight altitude, it was measured by the BTS-G-Walk® sensor during the SJ and data was retrieved from the summary data export.
The propulsive peak force is the largest ground reaction force at the start phase. Maximum concentric power is the maximum power at the start phase as shown in figure 3.7. The flight height is the vertical displacement from the start to the top of the flight as shown in figure 3.8.
Considerations on the Factors Affecting Reliability of the Results .1 Warm-Up
- Experimental Time of Day
- Subjects’ Footwear and Clothing
- Age, Gender, and Body Mass Index of Subjects
Therefore, the subjects were allowed to choose the shoes that were appropriate and comfortable for their perceptions. However, the shoes chosen should be sports shoes, but not limited to the specific type of sport. Similarly, clothing during the experiment was limited to sports clothing, but subjects were allowed to wear any sports clothing as long as they felt the clothing suited them and made them feel comfortable.
Since muscle strength is related to dance performance (Suchomel, Nimphius, & Stone, 2016), subjects' age was controlled to minimize differences in muscle strength. 2010) had already conducted a similar study with men, the current study aimed to study women who were in their twenties. In addition, one of the requirements in this study was that the BMI of the subjects fell within the normal range. This is because dance performance is also affected by BMI (Acar and Eler, 2019), as evidenced by Nikolaidis et al. 2015) who found that being overweight resulted in a negative effect on dance performance.
Subject Recruitment
They were suggested to choose the morning session if they are an early riser or to choose the evening session if they are a late riser. This is because different types of sports shoes will be worn during training for different sports and actual games. The subject did not suffer any joint injury of the lower extremities within the last 12 months before the time of the study.
The subject was asked not to undergo any physical activity of high intensity, i.e. an activity resulting in 77% of maximum heart rate 24 hours before the time of the trial to ensure that jumping performance will not be affected by any muscle damage condition. The subject was asked to refrain from caffeine or alcohol consumption three hours before the time of the experiment. The subject was asked to eat a full meal two to three hours before the time of the experiment to promote longer digestion and sustained energy.
Data Collection
Equipment and Instrument
Using one of the screws on a boss head, the body of the two vertical rods was attached to two boss heads, separately. Both ends of the horizontal bar were then attached to another screw on the boss heads, forming an 'H'. To adjust the height of the horizontal bar, the screws holding the vertical bars were loosened as shown in Figure 3.11 so that the horizontal bar could move freely against the vertical bar.
Data Collection Protocol
Then, the first to fifth desired squat depths were identified based on the unique sequence of squat depths assigned to each individual as in Table 3.1. Before performing SJ at each desired squat depth, the screws on the boss heads of the self-assembling jump rack were loosened and the horizontal bar was placed at the first desired height (eg, 30.0 cm), then the screws were tightened to fix the position. The subjects were instructed to begin each movement with an upright torso and chest out, as well as inhale just before the beginning of each squat, and exhale at the beginning of the jumps.
Using the timer provided by BTS-G-Studio, the subjects were instructed to perform two SJ with five seconds rest between each jump (the gap can help distinguish the data collected in both jumps to avoid confusion) by squatting in a slow and controlled manner until they could feel the bar on their hips as shown in Figure 3.15 (a) and perform the jump as shown in their effort (figure 3) with maximum effort. Then the subjects rested for 180 seconds for strength recovery, and in the meantime the horizontal bar on the jump stand was adjusted to the desired height.
Data Analysis
Introduction
Objective I: To Determine the Warm-Up Routine .1 Intensity
Duration of Rest Between Warm-Up and Squat Jump Test
The second purpose of the pre-test was to identify the appropriate rest period for full energy recovery after warm-up. The volunteers were asked to rest for three minutes after the warm-up and were asked to report the progress of their strength recovery every minute based on their perceptions. Therefore, three minutes/180 seconds was the appropriate duration for the rest period between the warm-up session and the SJ test session.
Objective II: To Determine the Method of Manipulating the Initial Knee Flexion Angle
- Measurement of Knee Flexion Angle with Goniometer and Self- Assembled Jump Stand Version I
- Measurement of Knee Flexion Angle with Goniometer and Self- Assembled Jump Stand Version II
- Pilot Study Results
- Measurement of Squat Depths with Metre Ruler and Self- Assembled Jump Stand Version III
- Biomechanical Parameters at Different Knee Flexion Angle Table 5.2 has recorded the biomechanical parameters (i.e., flight time, peak
- Correlation between the Biomechanical Parameters and Knee Flexion Angle
- Correlation between the Biomechanical Parameters
When the desired knee flexion angle was reached, the screws were tightened so that the position of the horizontal bar was fixed. A pilot study was conducted on Volunteers C and D to test the acceptability and feasibility of measuring knee flexion angle with a goniometer and self-assembled jumping stance version II. The biomechanical parameters (i.e. flight time, peak speed, propulsive peak force, maximum concentric force and flight height) measured by BTS G-Walk® sensor at different knee flexion angles are plotted in Figure 4.6.
To address the issues identified in section 4.3.2.1, squat depth was indicated by the height of the horizontal bar instead of the knee flexion angle. The results showed that as the knee flexion angle increased, the flight time, peak speed and normalized flight height of the SJ decreased, but the normalized propulsive force increased. A quadratic regression analysis was applied to find the correlation between knee flexion angle and biomechanical parameters.
The results in Table 5.3 have shown that there were significant correlations between the knee flexion angle and all biomechanical parameters (p < 0.05). The r2 value summarizes the proportion of variance in the biomechanical parameters associated with the knee flexion angle.
Discussions
In contrast to flight time and peak speed, maximum propulsive peak power was observed at the greatest knee flexion angle, confirming findings from previous studies (Torre et al., 2010; McBride et al., 2010; Ghellar et al., 2015; Mitchell et al., 2017). In the current study, a smaller knee flexion angle indicated slower eccentric squat and lower RFD while less force was generated, and more force was expended as more muscle contraction was involved just before the jump was initiated. In the present study, the highest maximum concentric power was obtained at 75º. The results do not agree with the findings reported by Ghellar et al. 2015), where peak force increased with decreasing knee flexion angle, but is consistent with the findings reported by Torre et al.
The reason may be due to the smaller range of knee flexion angle (70º – 110º) in the study conducted by Ghellar et al. The kinetic parameters, (i.e. force and power) appeared to be good predictors of vertical jump performance in other investigations (McBride et al., 2010). This may be because there was no significant difference present in the lower limb muscle strength and power of the lightly active and inactive subjects.
Conclusion
Recommendations for Future Work
Dependence of human squat jump performance on the elastic compliance of the triceps surae series: A simulation study. Effects of additional shock absorbing insoles and foam on the protective properties of sports shoes. Effects of external loading on power production in a squat jump on a force platform: A comparison between strength and power athletes and sedentary individuals.
Acute effects of heavy load exercise, stretching exercise, and heavy load plus stretching exercise on squat jump and counter-movement jump performance. Ankle joint range of motion and its effect on squat jump performance with and without arm swing in adolescent female volleyball players. Acute effects of two different warm-up protocols on lower limb flexibility and explosive performance in high-level male and female athletes.
