A Round Off is a unique skill in Gymnastics. It is the most common skill for gymnasts to use when they begin their tumbling lines. A poorly performed round off means a poor tumbling performance as gymnasts will not have the power or momentum to perform their following skills in their tumbling line (lack of power and height resulting in poor quality skills and less difficulty/number of skills.) A round off allows a gymnast to use their forward momentum (from running) and transfer that into backward momentum and perform backward moving skills. A common example is a round off back tuck saulto or a round off back handspring. A round off also transfers horizontal velocity into vertical (backward) velocity. The different biomechanical principles involved in performing a round off will be examined and optimal techniques and focus points will be discussed to provide a description of a successful round off.
|A Round Off Back Layout (saulto) [http://www.christinad.net/tidbits.html]|
The general purpose of the round off has been outlined above. The various biomechanical principles utilised in the performance of a round off will now be discussed. The round off can be divided into phases. These are the Run Up, the Lunge, the Cartwheel, the Side Handstand Snap, the Handspring and the Landing (rebound.)
The Run Up creates the linear momentum.It is horizontal momentum and is formed in the approach to the skill. The momentum itself carries on throughout the skill and into the landing. Good momentum helps ensure a good landing.
This momentum is created through running. Running itself will not be delved into however the impulse-momentum relationship will be briefly explained. Each step the gymnast takes has a reaction with the ground. The more force that is applied, the greater the reaction and therefore the acceleration.Momentum occurs from this. The larger the mass and velocity (or speed) the greater the momentum. Gymnasts only have a short run up in which to perform their round off. This run up is primarily to increase momentum. On top of having a high velocity and speed, momentum can also be increased by increasing the impulse. This is called the impulse-momentum relationship.
Impulse (J) = Force (F) x Time (t)
Each time a gymnast contacts the ground in their run an impulse is created. To optimise a gymnasts run up, they should try and minimise their braking impulses and increase their propulsive impulses. This can occur by having each foot land only slightly infront of the body with each step. The lunge will go against this as a gymnast steps very far forward of their body bends their knees, dramatically increasing the braking impulse however the force generated from this impulse will be greater (because their foot is infront of their centre of mass.) This is the needed outcome from the lunge and so is the only exception in the run up phase where the optimal technique just mentioned does not apply.
The Lunge is all about angles and force.Angular position and angular momentum ensure the skill can be completed efficiently. The 'tripping' effect assists with the transfer of momentum as the lunge is executed and the resulting force assists the gymnast throughout the stages of the skill. Newton's laws are at play here and projectile motion influences how the lunge should be executed.
The tripping effect occurs when the gymnast, during their run up, hurdles and steps forward into their lunge. This hurdle and brief stop of the feet creates a force halting their momentum. As the gymnast is travelling with great horizontal momentum the force causes a 'trip' where the gymnast continues to move forward yet their feet are slightly left behind. At this point the gymnasts centre of mass has moved forward over their feet and the lunge and initial cartwheel stage take place before the gymnast does fall from their sudden pause.
Newton's First Law states:
An object will remain at rest or continue to move with constant velocity as long as the net force equals zero.Newton's Second Law states:
The acceleration of an object is proportional to the net force acting on it and inversely proportional to the mass of the object: F=maNewton's Third Law states:
For every action, there is an equal and opposite reaction.Relating this back to the lunge phase in the round off assist with explaining how the transfer in momentum occurs. The velocity from the run up remains constant until a force (the lunge) is applied. In order to accelerate the body fast and smoothly through the skills' phases a greater application of force is required. As the gymnast applies a downward force on the ground, the ground itself reacts by applying an upward force. In order to change the horizontal momentum and begin rotation to vertical momentum, the lunging foot must apply both a vertical and horizontal force. This force, if strong enough, will accelerate the gymnast forwards, overcoming their inertia.
Projectile motion relates to the motion of a projectile and its angle. In this case the projectile is the gymnast and the angle is the one created as the gymnast lunges to the ground. Understanding projectile motion and projectile angle allows optimal projectile range to be discovered. Finding this can assist the gymnast with knowing how they should approach their lunge and at what angle their arms, body and legs should be at. The general optimum projection angle is 45 degrees as range is maximised because vertical and horizontal velocity are equal. In a round off however this angle should be less than 45 degrees. Gymnasts should take care to not 'jump' or move upwards in any part of their lunge. This will cause them to lose both momentum and velocity.
The Cartwheel continues the transfer of horizontal momentum to vertical momentum. Angular velocity again plays a part in this. Of particular importance is the manipulation of the gymnasts centre of mass.
The gymnasts centre of mass begins at the mid-line of their body. The slightly forward lean in their run up and then deeper lean in their lunge allows the trip effect to occur. As the gymnast kicks up to the cartwheel their centre of mass is manipulated to be above their hands. The gymnast should kick to be upside down in a vertical position and manipulating their centre of mass allows them to balance and control their body in this position. Their hands form a T-shape with the left hand being 90 degrees to the side and their right hand turned 180 degrees so it faces back the direction the gymnast started. This hand position allows for better body control throughout the positions required in the round off skill and also open up the shoulders again allowing for the manipulation of the gymnasts' centre of mass.
|Hand Placement [http://g-wizbusinessacademy.com/how-to-teach-round-offs/]|
The Side Handstand Snap relates to rotation of the body and how this can influence momentum (by stabilising or decreasing it.)
The body rotates over three different axes throughout the execution of the round off. The initial lunge rotates the body forward on the lateral axis (the axis going through the 'tummy' to your 'back'.) As the gymnasts' hands make contact with the ground the body is rotating 90 degrees (to the direction of the first hand) on the longitudinal axis as well as continuing to rotate on the lateral axis. As the cartwheel reaches this point (180 degrees rotation of gymnast) the gymnast continues to rotate using both axes. Just after 180 degrees the gymnasts legs are 'snapped' together from the open cartwheel position. This keeps the gymnasts' body mass close to the centre of rotation and makes overcoming inertia and conserving momentum easier. At this point the handspring phase of the skill occurs. During this time the body continues to rotate on both axes until the gymnast ends the skill, facing back towards the direction they started in an upright position.
|George, G. [http://archive.constantcontact.com/fs047/1103255370515/archive/1108424690046.html]|
Throughout the rotations occurring the gymnast should ensure they stay in a straight line. This allows for the linear horizontal momentum to better and more smoothly transfer and continue throughout the duration of the skill (into vertical momentum.)
The Handspring is an important phase where the transfer of momentum occurs. Moment of inertia and torque are both involved as the gymnast redirects their movement from having both hands on the ground and being upside down with their feet in the air to landing upright on their feet (with hands in the air.) The "push" movement here is the gymnast producing force to counter the moment of inertia and conserve their momentum.
Angular Acceleration (a) = Torque (t) / Inertia (I)
The moment of inertia becomes an issue when the gymnast goes from their handstand position back to standing up. The whole execution of the skill is poor if this phase cannot be completed efficiently. Torque is the force that counteracts this inertia. Increasing torque and decreasing the moment of inertia will result in greater acceleration. So the greater the 'push' is by the gymnasts the greater the torque will be and therefore the quicker their movement to the landing of the round off. Torque is the forced produced by the muscles, in this case it will be the deltoids (muscles in your shoulder.) The moment arm will be between these muscles and the wrist/arm joints (that is where the force is applied directly to the ground.) A gymnast can optimise their handspring phase of the round off by improving the muscle forces involved and improving the torque. Doing this will increase the angular velocity of the arms to the body which in turn presents angular momentum. All of this assists with providing the transfer of momentum required to perform this skill at a high quality.
|The Handspring to Landing [http://cheerleading.isport.com/cheerleading-guides/how-to-do-a-round-off-back-tuck-in-cheerleading]|
The Landing of this skill should produce a rebound by the gymnast whether they jump after wards or continue to move (run/walk/fall) in a backwards fashion. The intent of the round off is to lead into other skills and so a successful round off will have the gymnast producing excess vertical momentum at the end of the skill (to spare or use on the next skill rather than landing still and absorbing their momentum.)
The optimal landing position will have gymnasts landing with their legs only very slightly bent. As soon as the gymnast does land and make contact with the ground they should be rebounding straight away. This means that the legs will not bend to absorb the momentum and land but rather the lower leg muscles will recoil and push the body back upwards straight away. The gymnast's landing position should be leaning backwards with a slight curve in their back and ever so slightly rounded shoulders (optimal for continuation with back handspring.) Their feet will be infront of them and the centre of mass will be in the core body. This will result in the gymnast jumping or falling back when they land which is what a successful round off should look like.
There are many biomechanical principles involved in performing a successful round off. These include Angular Velocity, Projectile Motion, Newton's Laws, Impulse-Momentum Relationship, Centre of Mass and Moment of Inertia/Moment of Force. The most important principles in the take off stage of the round off is Angular Velocity, the Impulse-Momentum Relationship and Newton's Laws. The gymnast needs to have a good impulse-momentum relationship to ensure they achieve maximum momentum from their run up. Angular velocity is important as the gymnast lunges into the round off and creates the force required to accelerate the body into the different phases of the round off. Newton's Laws are the base for understanding this knowledge and what it means for the gymnast. The cartwheel and side handstand snap optimise the maniuplation of centre of mass and projectile motion. A gymnast needs to understand their centre of mass and how to manipulate it in order to balance better and transition through the motions in the round off successfully. The handspring and landing phases of the round off use primarily the moment of inertia and moment of force biomechanical principles. The gymnast will need to understand the correlation between these two principles and find ways to improve their torque to assist with achieving this.A successful round off will combine all these principles and transfer the various momentum throughout the skill smoothly to achieve a landing which has the gymnast rebounding or moving in a backwards fashion (as would be needed for their tumbling line.)
HOW ELSE CAN WE USE THIS INFORMATION?
This information can be of great benefit to coaches and participants involved in gymnastics. However that is not all it can be used for. All of the biomechanical principles that have been discussed apply to skills that can be used in all kinds of sports, games or physical activities. Understanding the various principles and how they coexist with each other and within the same skill allow for greater individual knowledge on these principles. These biomechanical principles help explain how 'things' happen, for example how it is possible for a gymnast to perform a tumbling line. On top of all the different skills and abilities that the gymnast must have, the biomechanical principles explain exactly what happens during each motion/movement. This allows the execution of the skill to be analysed and improvements, errors or strengths/weaknesses to be discovered and worked on for the benefit of the athlete and their performance outcomes. It is this information that can be used in many ways and for many different activities, the round off explanation simply provides another example of how knowing about these principles can improve and optimise one's performance.
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