The Relationship Between Force Velocity And Power Physical Education Essay

In many sporting activities it is particularly important for success and when it happens the body is considered to have the ability to quickly accelerate from a resting position. As most daily and sport activities involve the generation of force through ranges of joint movements it is important to understand the relationship between the force, velocity and power of the movement.

The concept of a force- velocity relationship was based on the research performed by A.V Hill on the muscle mechanical model (Hill, 1938). Based on his research the force – velocity relationship is described as the amount of force generated by a muscle as a function of the speed at which a muscle length is shortening by stimulating the muscle under isotonic conditions. At a fastest speed of shortening the muscle generates the lowest force but when the speed of shortening decreases the force output of the muscle in concentric contractions increases that maximum force occurs. However as the load on the muscle continues to increase the speed of shortening will decrease due to the load becoming greater than the ability of the muscle to generate enough force to overcome the imposed load. Small loads enable the muscle to react with high shortening velocities and heavy loads require most of the cross-bridges for lifting the load rather than accelerating it. 2

The other relationship to consider is the power – velocity relationship. Power is the rate at which a muscle can produce force and is defined as a product of force multiplied by velocity. The peak power that a muscle can generate increases rapidly as velocity increases until it reaches speeds up to 200 – 300 degrees-s-1, when velocity increases beyond these values the power output of the muscle declines due to the reduced overlap between actin and myosin filaments. Peak power is also related to the fiber type composition of the muscle. Muscles that have a greater percentage of fast-twitch fibers generate a greater peak power than muscles that are mostly composed of slow-twitch fibers because fast-twitch fibers contain higher ATPase activity than slow-twitch therefore ATP is broken down more rapidly. For athletes who engage in sports involving brief spurts of high power the explanation of the force, velocity and power relationship shows that these athletes generally contain a higher percentage of fast-twitch muscle fibers than slow-twitch.

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Basketball is among the most popular sports internationally. It is a dynamic game with intermittent high intensity exercise that incorporates frequent sprinting, rapid and repeated sudden changes in direction and jumping. These actions require adequate leg muscle strength and anaerobic power that are frequently required during critical episodes of the game. Jumping is the most dominant act performed by a basketball player as it plays a part of various defensive (blocking, rebounding and stealing)and playing offensive ( passing, shooting, lay – ups and rebounding) manoeuvres during practices and games. When under real game situations are not only required to perform numerous high jumps but they have to perform them in a competitive and demanding environment against the opposing team. Not only are they required jump higher they also have to jump higher than their opponents in a given defensive or offensive situation. As a result it is one of the desired goals of basketball players, regardless of their playing position whether it is a guard, forward, or centres, to improve their jumping ability in addition to them achieving a high level of competence in the other defensive and offensive plays they have to perform during the game.

A number of studies have assessed vertical jump heights in basketball players and come across mean values of jump heights ranging from 24.8cm3 in one study up to 48.2cm4 in another. However most studies have found values that were above 40cm. For female players they can have values of 22 – 48cm and male players 40-75cm1. Also from one of the studies they found that vertical jump heights improved after part taking in plyometric training and when just taking part in resistance training alone the results showed that there was a lack of improvement in jump height but was a significant improvement in strength.

The purpose of this investigation is to look at the relationship between force, velocity and power by taking part in commonly used resistance exercises the flat bench press and dumbbell squat jumps. Also to design a training session for a basketball player to improve their fitness in the aspect of power.

Method

The participant that volunteered in this study was a 21 year old male student studying level 2 sport at Sunderland University and plays for the university’s basketball team. The student is 5ft 10 inches and a weight of 71kg. The subject was informed about the procedures and demands of the study and signed a written consent form which was approved by the ethics committee at the university. As the subject is a basketball player they are regularly involved in team practices and matches and have had some experience in weight training as trains for strength 3 times a week.

Flat bench press

The protocol for the flat bench press is that the subject was well warmed up and stretched the relevant muscles before beginning the exercise. When ready to perform the exercise the subject would assume a supine position by lying with their back on the bench, once comfortable on the bench they’d then grasp the bar slightly wider than shoulder length with a closed grip. The spotter waits for the subject to give a signal for them to help assist in moving the bar off the supports. All repetitions started from the position of the bar resting lightly while touching the chest with the subject’s elbows flexed. From the start position the subject pushes the bar upwards until the elbows are fully extended and then lowers the bar to touch the chest and not bouncing off it while keeping the wrists rigid and directly above the elbows. The subject will first perform 5 repetitions with the bar unloaded and then increase the load to 10kg each time until they can no longer lift the current load. For this exercise it was important that two spotters were positioned at either end of the barbell (but not too close to distract the subject) and that they kept their hands in a position close to the ends of the barbell. The spotters also paid close attention to the subject’s forearms and the barbell especially when the subject was experiencing difficulties in completing the repetitions. When following the bar they kept their back flat, knees, hips and torso slightly extended when spotting the movements. Velocity, force and power was measured using the Globus machine by having a line attached onto the end of the barbell ensuring that it didn’t interfere with the movement.

Dumbbell Squat jumps

The subject performed a warm up for 5 minutes by cycling at 60 W on a cycle ergometer and then practiced performing squat jumps in a controlled manner with no mass. The athlete squats down until the knees are bent at 90 degrees with weight distributed evenly over both feet which they maintained for couple of seconds and have both arms by their sides with some light bars. The athlete then jumps up vertically as high as possible by applying force as fast as possible and then land back on the floor with both feet landing at the same time. On every squat jump the mass should be increased by 5 or 10kg increments until the subject is unable to perform a controlled squat jump. Every time the squat jump was performed the force, velocity and power was measured using the Globus system by attaching the line onto the end of the dumbbell. The subject also had 2 spotters supervising them while performing the squat jumps making sure there was minimum risk of injury.

The data from both exercises were then presented on the computer and then plotted into force, velocity and power curves.

Fig 1Results

Presented above in figure 1 is a graph of the force, power curve. The power curve is shaped like an inverted u and can see where the maximal amount of power of the muscle has peaked. From the graph power has peaked at 535 and decreased after this point. The subject lifted up to 90kg during the exercise but his maximal power was obtained at 40kg so 45% of the subjects 1RM is at 40kg.

Dumbbell mass (kg)

Total mass (kg)

Velocity

Force

Power

0

75.1

2.64

1819

3270

10

85.1

2.1

1010

1720

28

93.1

1.76

971

1397

34

109.1

1.2

957

970

44

119.1

1.11

940

850

54

129.1

0.8

731

517

Fig 2

Above in figure 2 is a table of the results from the squat jump test. From looking at the first score we can see that there was a significantly high force being generated for the jump and resulted in the high amount of power but when the load was increased the amounts of power within the jumps seemed to decrease.

Discussion

From the results collected in the study the maximal power was found to occur at 40% of the subjects 1RM and compared to other studies it confirmed these results. The reason why the squat jump power scores are low could be down to jump not using any stored elastic energy in the musculotendinous unit due to it being too slow to allow potentiation from the stretch reflex because of there being no eccentric phase.

To help improve jumping ability it may be beneficial to see whether plyometric training would be better instead of regular strength training as Plyometrics training involves lower loads and fast explosive movements using a prestretch or counter – movement that involves the stretch shortening cycle and are related to skills relevant in the game of basketball. The purpose of plyometric exercises is to increase the power of subsequent movements by using both the natural elastic components of muscle and tendon and the stretch reflex. Plyometric training can increase muscle power by the explanation of two proposed models mechanical and neurophysiological. In the mechanical model elastic energy is stored in the musculotendionous unit by the result of a stretch during the eccentric phase of a movement and when followed by a concentric muscle contraction the stored elastic energy is released and increases the total force production. The neurophysiological model involves the stimulation of muscle spindles at the eccentric phase caused by a rapid stretch and making a reflexive movement action meaning when the concentric phase happens after there is increased muscle recruitment. Also the stretch – shortening cycle employs both the storage capabilities of elastic energy and the stimulation of muscle spindles from the stimulation of stretch reflex.

I think the basketball player would benefit greatly from a Plyometric training session as they are required to jump repeatedly throughout the game for rebounds, defending, shooting. By taking part in a mixed upper and lower body session the player would have the ability to produce more force in a shorter time when jumping giving them the chance to jump higher.

Training Session – Plyometrics training

Warm – up