Biomechanics and Kinematics of a Leap Dance Term Paper

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Jete'

Analysis of the Muscles Involved

The Center of Gravity

Analysis of the Injuries Prone to the Movement

It is not known when leaping first appeared in dance. Many ancient forms of dance involve leaping. The most common connotation of leaping is found in ballet. Ballet reached the height of its popularity during the reign of Louis XIV at the end of the late 17th and early 18th centuries. It was because of leaps that the ballet dancers shortened their skirts. The French Dancer, Marie Camargo broke convention and shortened her skirts to emphasize her leaps and jumps (Thinkquest, 2002).

Leaping refers to a movement taking off from one foot and landing on the other foot. There are five types of jumps in ballet. This particular type of leap is called a jete' A leap in dance is often proceeded by a preparatory move such a one or more approach steps or a plie. A plie means bending at the knees. Preparatory steps are designed to give the dancer momentum so that they can achieve greater height and distance. Emphasis in a ballet leap is on proper form in all stages of the leap from preparation to take off to flight, and landing. A proper leap will include a lifted upper body, proper arm placement, extension, tight legs, and pointed toes. A proper leap has both height and distance. The leap should give the illusion of flight.

Kinesthesiology is a branch of physics dedicated to the physics involved in movement. Several scientists of note contributed to this field including Aristotle, who applied geometry to the acts of walking, running and jumping. Barelli, a student of Galileo, noted that the bones act as levers. The forces that act on the body are both internal from muscles and bones, and external forces, such as gravity and inertia. As a dancer moves across the stage force is transferred from one element to another. This paper will explore the body mechanics involved in all phases of the ballet leap, called the jete'.

Phases of the Jete'

The leap can be divided into three major portions, the take off, flight, and landing (Becker, 1964). In addition there is a preparation stage and a recovery stage after landing. The preparation and recovery stages can be have many forms and variations.

The purpose of the preparatory stage is to provide momentum. In order for maximum upward and forward momentum to be exerted, several actions must take place, a change in direction and a burst of energy from the feet pushing against a solid object. These two forces interact to launch the dancer into the air. Ballet dancers use many preparations in their leaps. They may give a plie, bending of the knees to varying degrees. They may also use a series of steps prior to the leap.

The take off is the most important part of the leap. In order to change direction the feet must impact a solid surface (floor). At this point the legs move from a bent positions to a straight position and then one leg extends forward and one extends backward into flight position. At the same time the arms and hands rise from the preparatory positions, usually either hanging to the sides or at 90 degree angles to the floor and torso. They rotate into final flight position.

Flight involves forward motion. At this point legs and arms are positioned for maximum aerodynamics and extension. Many arm positions are possible. Legs are extended rigidly in a split position. Toes are pointed. The body travels in an arch.

The forward foot is the first to impact the floor. Specifically the big toe, ball of foot, then rolling to the heel. The second leg joins the first, which is already firmly planted. Knees are slightly bent to absorb impact. The recovery involves a bend in the knees to absorb shock. Arms return to natural position. The knees straighten to a standing position ready for the next move.

Analysis of the Muscles Involved

The forces exerted during the leap are both internal and external. Internal forces come from the muscles, bones, and tendons. When a muscle contracts it exerts active force. A passive force absorbs energy that is transferred to it by the active force, such as when the muscle tries to move the joint. Gravity and inertia are examples of external forces that are exerted on the dancer. An load is applied when a dancer lifts another dancer Dancing involves many types of movements.

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However, physics primarily recognizes two types of movements, linear and rotational (Kent School District, 1998).

For the leap, the body can be divided into two portions, the upper body and lower body. The upper body including the head, shoulders, arms, and torso are used primarily in the leap for stability, balance and aerodynamics. All major muscle groups in the upper body are involved in this activity. However, none of them has excessive force exerted on them, except for friction from the air and gravity.

The legs do the major portion of the work in the leap. At the take off, primary stress is placed on the calves, ankles, hamstrings, and gluteus maximus. They work in combination to provide the change in direction and propulsion. During the flight phase, these same muscles must transition to maximum extension to provide balance and stability while in the air. During the landing, these same muscles must absorb the impact of the landing and work to return the body to a stable position.

A leap is complex and involves every muscle in the body at one point. The arms and upper body do some work. However, it is the lower body that must bear the brunt of the burden. A jete' is one of the most complex movements in dance.

The Center of Gravity

The object of the leap is to keep the center of gravity just above the contact points with the floor. Any variance from this will cause the dancer to become unstable and possibly lose balance and fall. Failure to maintain proper center of gravity will severely reduce the dancer's distance and height in the leap. This is perhaps the most critical element of the leap and the most difficult as well.

Proper positioning of the body is the key to maintaining a proper center of gravity. A dancer practices each phase of the leap separately until proper body position is easily maintained throughout all phases. A center of gravity, which is too low will cause a reduction in height and reduce the parabolic flight path. A center of gravity, which is off center will cause the dancer to have to compensate, which could ultimately cause a fall.

The dancer seeks to raise the center of gravity just above the area of contact with the floor. Otherwise they will tip over. A smaller area of contact makes balance more difficult. In a leap there is a time during the flight phase where nothing is in contact with the floor and the center of gravity is quite high. This is the most difficult phase in which to maintain balance. In addition if a dancer gets off balance, they will have to compensate in the landing phase to prevent a fall.

In physics terms, the effect of gravity on center of gravity is the same as if gravity were acting on the entire body. At the center of gravity force = 0 (Laws, 1994). If the center of gravity drifts off center the object (dancer) will develop torque (spin) which increases the risk of a fall. Gravity and friction are the main forces acting on a dancer in the air. Maintaining a correct level and balanced center of gravity is the key to flight.

Analysis of the Injuries Prone to the Movement

The body is subject to various injuries throughout all phases of the leap. The upper body, arms and shoulder are at risk of hyperextension and injuries involving muscles, tendons, and joints. Maintaining proper positioning and center of gravity are the keys to preventing these types of injuries. The less these body parts need to compensate, the less likely they will sustain injury.

The legs and lower body bear the brunt of the force and have the greatest risk of injury. During the take off stage, there is a risk of injury to the major muscle groups of the legs and buttock areas. All muscles are at equal danger of risk. The primary risk is that of excessive force beyond the muscle capacity to bear the force. Primary injuries would be to muscles, tendons and joints. The risk of injury to the lower body is primarily the same during the flight phase. Muscles, tendons, and joints run the risk of hyperextension.

The landing phase of the leap is by far the most dangerous. If the body is not positioned properly to absorb the impact, injuries could be extensive, particularly if they cause a fall. Every part of the body.....

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