Biomechanics Laboratories Utilizing the Kinematic Measurement System

Laboratory 1

Stretch Shorten Cycle Capacity

Equipment

  • 4 x contact mats and KMS interfaces
  • 4 x computers with Kinetic Measurement System (KMS) Software
  • 16 x 25cm step benches

Note: lab can be completed with just a single KMS if required.

Introduction

Most human movement activities involve a counter movement during which the muscles involved are first stretched and then shortened to accelerate the limb. This action of the muscle is called a stretch shortening cycle (SSC) (Komi, 1986) and involves some interesting neural and mechanical processes. A great deal of research has been directed toward the study of the stretch shortening cycle (Bosco & Komi, 1979; Bosco, et al., 1982; Ettema, et al., 1990; Gollhofer & Kyroelaeinen, 1991; Hakkinen, 1989; Komi, et al., 1982; Schmidtbleicher, 1988) because it has been observed that jump performance is potentiated by the prestretch experienced during a counter movement jump (Bosco & Komi, 1979). One study by Bosco et al. (1982) found differences between squat jump (SJ) and counter movement jump (CMJ) heights of 18%-20%. The CMJ jump is higher because as the jumper approaches the end of the decent, the muscle begins to act eccentrically to slow the body and initiate the upwards movement. As the muscle is activated, force is increasedin the tendomuscular complex increasing its stiffness or resistance to stretching. The result is a storage of elastic energy in the muscle and tendon which is recovered during the subseqent concentric phase making it more powerful (Bosco & Komi, 1979). Also contributing to the potentiation of the concentric muscle action is a reflex increase in neural stimulation to the muscle, brought about by the sudden stretch stimulus (Gollhofer & Kyrolainen, 1991; Schmidtbleicher, et al., 1988). Studies by Bosco and Komi (1979) demonstrate that jump performance increases with increasing stretch loads applied. For example, during drop jumping, the height of the subsequent jump increases with increases in drop height. This occurs only up to a point.

There is a threshold at which the stretch load is too great and the golgi tendon organ reflex causes an inhibition of muscle contraction reducing the jump height attained (Gollhofer & Kyrolainen,1991; Schmidtbleicher et al., 1988). It should be noted that athletes unaccustomed to intense SSC loads may produce his/her best performance during a CMJ and the drop jump heights will be even lower than the SJ (Schmidtbleicher, 1992).

Practical Exercise A

1. Divide into 4 groups of 4 students

2. Select one student to be the subject and instruct them in a 5 minute warmup for the lower body.

3. Instruct the subject to perform three SJ's and three CMJ's and record the height jumped for each. Alternate between SJ and CMJ to reduce order effects. Ensure that the subject does not perform any dip movement prior to the SJ. All jumps should be completed with the hands on the hips and maximal height should be attempted.

Squat Jump Height (m) / Counter Movement Jump Height (m)
Trial 1
Trial 1
Trial 1
Average

Study Questions:

What was the percentage difference between SJ and CMJ heights?

How does this result compare with that of the literature?

What type of training methods could be used to increase the subject’s abilityto utilize the stretch shortening cycle?

Practical Exercise B

Drop jump training is a common form of plyometric drill. By increasing the height of drop, one can increase the stretch load imposed on the muscle. Depending on the training history of the athlete greater drop height should result in higher subsequent jump as they are able to utilize the increased stretch load to facilitate impulse applied to the ground and so increased takeoff velocity and therefore jump height. However, they will reach a drop height at which the tension in the musculotendinous units of the lower limbs will exceed the threshold of the Golgi tendon organs initiating a reflex to limit force production so as to protect the muscle and tendon. This reflex causes inhibition of the primary neural drive, reducing muscle force production and therefore jump height. Athletes with limited training history in high tension stretch shortening cycle movements may produce the greatest jump height from the ground, a zero drop height which is a counter movement jump or a relatively low drop height of 10 to 20 cm (Figure 1).

Reactive Strength Index

The ability to tolerate very high stretch loads without reflex inhibition is termed the reactive strength index (RSI) and is an excellent measure of an athlete’s ability in the stretch shorten cycle. The reactive strength index is generally calculated in two ways:

RSI = flight time / contact time

RSI = jump height / contact time

Regardless of the method chosen the two are essentially identical as there is a perfect relationship between the two calculations.

Figure 1. Relationship between drop height and jump height for different levels of athlete and training history.

In this activity, use the contact mat to measure jump heights and contact times from 0.25m, 0.50m, 0.75m and 1.0m drop heights. Complete three trials at each height and record the trial with the highest jump height.

Drop Height / Contact time / Flight time / Jump height / RSI
0
0.25
0.50
0.75
1.0

Plot a graph of contact time and flight time versus drop height.

Plot a graph of RSI versus drop height.

What was the effect of increasing drop height on contact time and jump height?

What would be the significance of the results if one athlete produced the greatest jump height from the 0.5 m drop height and a second athlete who produced the greatest jump height during the CMJ with no drop?

Laboratory 2

Load Velocity Power Relationship

Equipment

  • 4 contact mats and KMS interfaces
  • 4 computers with Kinetic Measurement System Software
  • 4 pairs of dumbells: 5kg, 10kg, 15kg and 20kg each (weights may be varied depending on strength of subject)

Note: lab can be completed with just a single KMS if required.

Introduction

The predominant requirement in a large number of sports is explosive power. For the lower body this is perhaps best exemplified in the vertical jump.Here the muscles about the hip, knee and ankle act rapidly and with high force to produce the greatest possible velocity of the body as it leaves the ground.The jump height produced is determined purely by this takeoff velocity. This laboratory session addresses the interaction of load, velocity and power during vertical jump performance and illustrates their significanceto the testing and training of human performance in general.

Strength Versus Power

Strength is the ability of the muscle to exert a high force or torque at a specified velocity (Knuttgen & Kraemer, 1987) and varies for different muscle actions such as eccentric, concentric and isometric (Kraemer, 1992). Dynamic strength is often assessed using a 1 repetition maximum (1RM) test in which strength is assessed as the maximum weight the athlete can lift once through the complete movement. The development and assessment of strength has received a great deal of research attention (Atha, 1981; Berger, 1962; Hakkinen, et al., 1987; Schmidtbleicher, 1988) and the interested reader may refer to the relevant literature. Pure 1RM strength however, is a requirement of a limited number of athletic endeavours (e.g., Power Lifting). Most sports require the explosive application of force to accelerate the body, limb or implement resulting in a high velocity at the point of impact or release. This aspect of performance has been termed explosive power or speed strength (Young, 1993).

The key difference between strength and power in concentric movements is the speed of muscle action. Strength is the force that the muscle can exert and is maximized during very slow concentric muscle actions. This is due to the force velocity relationship for muscle (Figure 2.) The faster the velocity of concentric muscle action, the lower the force that can be produced (Hill, 1938). Pure 1RM strength is required in the sport of Power Lifting because there is no requirement for the weight to be lifted quickly as the athlete is attempting to lift the maximum amount of weight. This requires movement velocities which are just higher than zero. However, most human sporting activities occur at faster velocities of movement.

In terms of testing and training, velocity specific effects are apparent (Kaneko, et al., 1983; Moritani, et al., 1987; Newton & Wilson, 1993). Therefore, strength testing using heavy loads and low velocity of movement may have limited predictive ability to high speed performance. Thus, it may be much more useful to assess force and power output at or near the velocity of movement used in the event. In terms of training, a number of studies have shown limited performance improvements in explosive activities resulting from heavy strength training (Hakkinen, et al., 1985a; Wilson, et al., 1993) and it may be much more effective to train with lighter loads using explosive ballistic movements.

A number of studies (Faulkner, et al., 1986; Hill, 1938, Newton & Wilson , 1993) have shown that mechanical power output is maximized at approximately30% of maximum shortening velocity and a load of 30% of maximum isometric strength (MVC). Because of this relationship, the 30% MVC load has been proposed as the optimal load for the development of mechanical power (Kaneko et al., 1983; Wilson et al., 1993) and have suggested that ballistic weight training should be performed using this load. The optimal load can be determined for squat jumps using the load height test proposed by Bosco (1992).

Figure 2.Force velocity power relationship for skeletal muscle. Vm, Pm and Fm are maximum movement velocity, maximum power output and maximum isometric force output respectively (adapted from Faulkner, et al., 1986).

Here the athlete performs CMJ's with increasing additional loads. The height of jump and power output is recorded and the load which produced the greatest power output is determined.

Practical Exercise A

1. Divide into 4 groups of 4 students

2. Select one student to be the subject and instruct them in a 5 minute warmup for the lower body.

3. Instruct the subject to perform three CMJ's with no additional load as well as each of the four loads. Exchange dumbells with the other groups and randomise the order. All jumps should be completed with the hands on the hips and maximal height should be attempted. For the dumbell trials, the weights should be held in the hands, firmly against the hips. Make sure that you add the weight to the subject's body weight and enter it into the KMS so that power can be correctly calculated.

4. Record the height jumped and power output for each trial.

Load (kg) / Jump Height (m) / Jump Power (W)
0
5
10
15
20

Plot a graph of jump height and power against load.

Discuss why the graph above is similar to the force, velocity, power graph typical of skeletal muscle.

What was the optimal load for power output? Why is this significant?

Discuss the theoretical effects on the graph above if the subject completed a program of heavy weight training versus explosive jump training.

Laboratory 3

Biomechanics of Plyometric Training

Equipment

  • 4 x contact mats and KMS interfaces
  • 4 x computers with Kinetic Measurement System (KMS) Software
  • 16 x 25cm step benches

Introduction

Vertical jump performance has been shown to respond to training which involves the athlete performing SSC movements with a stretch load greater and more rapid than to which they are accustomed. These activities have been termed plyometrics and have been found, in a number of studies, to be effective for increasing jumping ability (Adams, et al., 1992; Clutch, et al., 1983; Schmidtbleicher, et al., 1988; Wilson, et al., 1993). Plyometric training results in an increase in the overall neural stimulation of the muscle and thus force output, however, qualitative changes are also apparent.

In subjects unaccustomed to intense SSC loads, there is a reduction in EMG activity starting 50-100 ms before ground contact and lasting for 100-200 ms (Schmidtbleicher, et al., 1988). Gollhofer (1987) has attributed this to a protective mechanism by the golgi tendon organ reflex acting during sudden, intense stretch loads to reduce the tension in the tendomuscular unit duringthe force peak of the SSC. After a period of plyometric training the inhibitory effects are reduced, termed disinhibition, and increased SSC performance results (Schmidtbleicher, et al., 1988).

Practical Exercise A

1. Divide into 4 groups of 4 students

2. Select one student to be the subject and instruct them in a 5 minute warmup for the lower body.

3. Instruct the subject to perform drop jumps from the bench. Three conditions are to be completed:

  1. Attempt to minimise the time spent on the ground (contact time);
  2. Attempt to maximise the jump height;
  3. Attempt to maximise the jump height while minimising the contact time.

Complete three trials of each condition and record the best performance for each condition. All jumps should be completed with the hands on the hips

4. Record the height jumped, contact time and power output.

Condition / Contact Time (ms) / Flight time (ms) / Jump Height (m) / Jump Power (W)
1
2
3

Power output is calculated as: Power = mgh / time

Where: m = mass of subject

g = acceleration dur to gravity (9.81 m.s-2)

h = height of jump calculated from flight time

t = contact time prior to jump

Which jump condition resulted in the greatest power output? Explain why.

Discuss why it is important to minimise the contact time during plyometric training.

Practical Exercise B

Use the contact time test on the Kinematic Measurement System to determine the averagecontact time for various activities. Record the contact time for each person

in the group completing:

  1. Double leg broad jump off the mat with a run-up;
  2. Single leg broad jump off the mat with a run-up;
  3. Sprint running over mat (place carpet or rubber over mat to prevent sliding).

Record the average contact time for your group for each activity in the table below.

Condition / Contact Time (ms)
double leg takeoff broad jump
single leg takeoff broad jump
sprint running

Discuss the differences in plyometric training for sprinting versus vertical jump in terms of contact time.

How do the contact times measured in Exercise A compare to the contact timesduring actual sport activities?

Reference List

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