BIOMECHANICS MEASUREMENT REPORT 3

CALUM NICHOLL (201033796)

GROUND REACTION FORCE (GRF)

CONTERMOVEMENT AND SQUAT JUMP KINETICS

BIOMECHANICS LAB 3- CALUM NICHOLL

Introduction

Using a motion analysis system (Vicon Nexus System) the class were able to analyse the forces generated whilst the subject carried out counter movement and squat jumps. The T-Series cameras (6xT160, 6xT40) and the force plate that is dived into 4 sections were used to record the forces. The subject uses the stretch shortening cycle to gain height when jumping.

There are threes phases that make up the stretch shortening cycle (SSC), the concentric, amortization and eccentric contractions. These three phases refer to muscular contractions, which form the stretch shortening cycle. This lengthening and shortening cycle of muscle actions is fundamental to athletic movement. The initial eccentric phase of contraction involves the muscle lengthening or stretching under tension. This type of contraction is used to decelerate the body. A basic example of this is the running motion, when the athlete makes contact with the ground their centre of gravity drops quickly. The muscles contract and control this sinking motion. The concentric contraction quickly follows the eccentric contraction and involves a rapid shortening of the muscles. The period of time between the beginning of the eccentric contraction and the start of the concentric contraction is known as the amortization phase and is the most important part of a plyometrics exercise. This phase is marked by a very brief period where the muscle is completely stationary which is referred to as an isometric contraction.

Sports Motion Injuries (jumping)
Jumping is a form of plyometric exercise used to increase power and strength into the muscles. Power and strength are used in the vast majority of sports, so plyometrics can be used to improve these attributes that many athletes desire. Plyometric training does carry a high injury risk due to its explosive nature. For this reason athletes should firstly develop a base level of strength through a standard resistance programme. A thorough warm-up and cool down will also help to prevent injuries, as will a gradual development of the intensity of the exercises performed. Delayed onset muscle soreness (DOMS) is a common complaint after plyometric training. The eccentric muscle contractions that consist of jumping or any other plyometric exercise can result in DOMS. DOMS usually occurs 12-48 hours post-exercise. Muscle aching and tightness are the most common symptoms, often resulting in a decreased range of motion. Often the discomfort eases within 3 days post exercise and a full range of movement is restored.

Objectives

Using the force plate in the bio engineering lab, a comparison will be made between the a squat jump and a counter movement jump. Once the results have been generated and the data produced, calculating the maximum power generation, max height and optimum velocity differences between the jumps will be carried out. Also the acceleration, force, velocity and power will be compared graphically. A discussion with regards to the results will then be made, disusing how the stretch shortening cycle can help to increase vertical.

Methods

1. Shown how to set up cameras in the lab for our capture volume space. 2. The Vicon Nexus System was then calibrated, firstly dynamically then statically. The wand was waved around the capture volume space and was picked up by all the required cameras so the system could be calibrated dynamically. The wand was the place in the centre of the space to statically calibrate the system. (All the image errors were to be as close 0.5 as possible) 3. The subject was then asked to stand within one of the 4 sections of the force plate. 4. The subject then completed a trail run to assure that the system had been calibrated correctly. 5. 3- squat jumps were then performed, and then 3-counter movement jumps. 6. Results were stored and the data published.

Results

The results shown below are the graphs of the averages of the taken for the three trials of squat jumps with regards to force, velocity, acceleration and power against time.

[pic]

[pic]

[pic]
[pic]

The results shown below are the graphs of the averages of the taken for the three trials of countermovement jumps with regards to force, velocity, acceleration and power against time.

[pic]
[pic]
[pic]
[pic]

Discussion
The results show the difference with regards to countermovement jumps and the squat jumps. Averages were taken for each of the three attempts of the different jumps and then compared to each other under the specific criteria’s. This includes force, acceleration, velocity and power. The averages were taken and compared for also the max power, max height and optimum velocity. These are values that are of the most interest as they give solid evidence of the maximum achieved by the different jumps.

SQUAT JUMP COUNTERMOVEMENT JUMP
Max Power 4471.505198 W 4977.630474 W
Max Height 0.423297922 m 0.484134626 m
Opt. Velocity 2.718753339 m/s 2.784682907 m/s

The graphical results show little difference between squat and countermovement jumps. The quantities generated through the testing are relatively high and when graphically show on a small-scaled graph the difference are not made clear. Therefore the average max height reached gives the clearest evidenced of the difference in jumping techniques. It is clear to see from the results above that countermovement jumps produce a slightly larger result under all three criteria. The results are not too different, which is why the graphs did not show clearly a difference, but the higher values produced through the countermovement were to be expected. There are many explanations as to why countermovement is greater than squat. The first comes down to the stretch shortening cycle (SSC). During the CMJ the active muscles are pre stretched and absorb energy, part of the energy is stored in elastic elements and later reutilised in the concentric phase. Basically it is the storage and reutilisation of elastic energy from the eccentric phase to the concentric of the jump. The squat jump only has a concentric phase therefore no elastic energy can be built. This is the most likely reason for a difference in jumps.
The second possible reason why the there was a difference is down to the subject not knowing how to properly control a squat jump. If the control is not optimal then the jump height will be less than maximum height achievable. The initial conditions for the jumps vary and the subject my not have the control to hold the eccentric state required before a SQJ, which will reduce the effective energy and give a sub-maximal performance.
The third reason as to why there is a variation in jumps is that in SQJ’s is the muscles are unable to gain a high level of force before the jump. This relates to the first suggestion in that speed can be generated through a CMJ to increase the muscle stimulation.
The final possible explanation is that the muscle stretch that occurs during the CMJ triggers spinal reflexes that help to increase muscle stimulation during the concentric phase to a level above that achieved in SQJ’s, where no pre stretch occur. The higher stimulation level the muscles produce a larger force and create more work during the concentric phase.

Conclusion

The lab was completed successfully although unfortunately the graph did not show clear evidence between the CMJ and the SQJ. The graphs gave us a better understand of the power, force, velocity and acceleration involved in the jumps however due to the finite difference between the two jumps they were to difficult to compare on the basic graphs. Therefore a well digested comparison between the two jumps was made, comparing the averages of the max jump heights, power and velocities, made through excel calculations. The CMJ rated higher under all three categories therefore there must have been a specific reason. Looking at through the literature there were a few reasons as to why there may have been a difference and when comparing the different suggestions there were a few possibilities that would have made the results vary. Although compariong the different suggestions it seems like the most likely reason why CMJ was higher than SQJ is due to the elastic energy built up during the rapid change from eccentric to concentric contractions.

References

1. Why is a countermovement jump height greater than a squat jump height? Maarten F. Bobbert, Karin G. M. Gerritsen, Maria C. A. Litjens and Arthur J. Van Soest 2. Biomechanics lecture notes.