This study describes the characteristics of an aerial 'mechanic' developed to utilise the rebound characteristics of bungy cord and used in the teaching of intermediate and advanced trampoline skills. An elite trampolinist was filmed while in the harness and take-off and landing velocities were derived at maximum tension and in free bouncing. Time in the air, and support during flight, were identified by the coach as positive characteristics of the system. Five athletes exposed to nine coaching sessions using the system commented positively.

Keywords: trampoline, coaching, harness, bungy.

Introduction

The basic requirements of a competent trampoline performance are determined by the characteristics of all projectile motion as applied to humans in flight. (Brancazio 1984, Hay 1993, Yeadon 1997) The trampolinist is required to perform a number of somersaulting and twisting movements while in the air, and as the routine is comprised of ten consecutive bounces it is imperative that at each take-off there is a minimum horizontal component of the take-off velocity. The basic mechanical requirements then are to gain maximum time in the air, and to confine the take-off and landings to a relatively small area of the trampoline bed.

The trampoline itself is a nylon weave mat attached to a rigid steel frame standing 1.2m above the floor. With rules of competition providing for a minimum ceiling height of 8m it can be seen that a competitor 1.8m¹ in height who will perform at least the preliminary bounces in an upright stance may have available a displacement maximum of 5m. A competitor will rarely bounce too close to the ceiling, but then too will rarely be in an upright position at peak flight. The parameters of this study are based on a maximum displacement of the athlete of 5m, providing a maximum time in the air of approx 2secs and a take-off velocity in the region of 9.9m/s.

The trampolinist in performing a routine is obviously restricted to the area of the bed from which to bounce but prefers to stay within a more limited area of approximately 1 x 2m in the centre of the bed. Therefore while the performer must adopt a stance while in contact with the mat to initiate rotation at take-off - in order to complete the required rotations - there must also be minimal horizontal component of take-off velocity to keep the performer within the confines of the trampoline. This requires considerable control and the techniques employed in maintaining that control form a large portion of practice and learning time. Time in the air is also essential, to complete complex moves, hence the trampoline coach places considerable emphasis on extending that time in the air in the first learning phase.

The adjustable bungy guidance system described in this paper provides the required increased time in the air for the completion of more advanced skills, but also removes the need of the learner to attend completely to the directional control of take-off. The system provides a safe learning environment by slowing the athlete during descent, and maintains the specificity of trampoline activity by always initiating another take-off after each landing. The major benefit perceived by the authors however is the ability to adjust the tension within the system, providing a graduated guidance not available with traditional harness systems.

There is a considerable difference of opinion on the usefulness of harnesses in the coaching of trampoline, and while there is considerable literature on the detail of twisting/somersaulting movement (Yeadon 1993, Sanders 1995) there is certainly a paucity of information (Hans 1999) on the subject of harness use. For the purposes of this study four NZ coaches were asked for an opinion on the use of harnesses.

¹ = cog 55% of standing ht
² = Personal communication

Trampoline Competition

International trampoline competition until 1999 was conducted and controlled by the International Trampoline Federation, but is now the responsibility of the International Gymnastics Federation. While the grade system begins at level 7 and moves through to 'elite' level, competition in New Zealand begins at level 3. A competition performance is comprised of a compulsory routine performed by all competitors in the grade, and either one or two (elite only) optional routines developed from the 'moves³' available for that grade. A routine is comprised of ten 'moves' performed in sequence. Normally there would be no 'straight' (non-rotating or twisting) bounces between moves although there is no regulation that forbids the use of a single 'straight' bounce within a routine⁴. All routines are graded for difficulty using the accepted FIT rating scale and there are ceiling limits on difficulty at each grade. A competitor competing in grade 3 for instance may perform no more than two of three specific moves that signal a competitor's ability to compete in grade two. At grade one there are five moves which if used, take that routine into the elite grade. All open international competition is competed for amongst elite grade performers.

All routines are judged for form, with the difficulty component included in the final score. As in similar activities such as diving and gymnastics a performer may select a performance of lesser difficulty that may enhance the ability to control form, or increase the difficulty rating that may compromise form. Form is judged on a 10-point scale with five judges scoring the routine. Scores at both ends of the range are omitted, the remaining three totalled to find a final score for form, to which the difficulty component is added⁵.

Difficulty is calculated from the written description of the routine delivered to the judging panel prior to competition. During the performance a 'difficulty' judge checks that the routine is delivered as described, and makes adjustments as required. The final score is derived from both scores by adding the total (3 judges) form mark to the difficulty mark. At the very elite end of competition difficulty may exceed 14, while at level three difficulty ratings may begin as low as 3.

³ = What happens in the air between take-off and landing
⁴ = A straight bounce must not be used to begin or end a routine
⁵ = In diving the form score is multiplied by the degree of difficulty (DD)

The Bungy Guidance System

Bungy cord is constructed from round section, extruded latex rubber thread, bound into an elastic rope. At commercial bungy jumping facilities there will usually be two ropes, one for those weighing less than 75kg, and another for those over 75kg. The rope is constructed to allow for the height of the jump and the required closeness to the ground or water at full stretch. The important factor in each case is the fully extended length of the rope with a given load, as is the case in this system.

An aerial 'mechanic' was constructed of a pair of 9mm non-elastic nylon ropes passing through two pulleys suspended from the roof approximately 6m from the surface of the trampoline. When used as a normal mechanic these ropes are attached to a rotating harness by two 'D' shackles. When used in the bungy system the bungy cord is attached directly to the harness (Figure 1) providing the required stretch characteristic at the performer end of the system. The pulleys in the roof are 5m apart providing a wide angle at attachment to the harness and minimal interference with the arms of the performer during bouncing. At the coach end of the system the ropes pass through a locking device attached to a stanchion at trampoline level. The locking device provides the means of adjusting tension in the bungy and is an essential element in the guidance system.

For the purposes of this system three pairs of bungy cord were prepared. Each cord is 1.2m in length and constructed of standard 1.2mm, 10 thread tape. The 'stiffest' of the three is made up of 210 threads, the medium cord of 170, and the 'lightest' of the three contains 130 threads. Breaking strain of the cord is 3000kg/mm2 and the elongation of the cord at break is 700%. It can be seen from the description of the facility above that it is not possible in this system for the bungy cord to be stretched more than 400% while in use, and the three cords (Figure 2) easily accommodate a range of masses from 30kg to 70kg.

The locking device in this system provides an ability to adjust tension, or stretch, in the bungy cords. Adjustment of the tension provides a graduated level of support during the learning process. In the first stages the cord may be tensioned to such an extent that when at rest the athlete is only just touching the trampoline bed (Figure 1). This 'maximum' situation provides the athlete with stability and control at take-off, further acceleration after take-off, more time in the air to learn the move, and a slowing descent that brings the learner to a halt just as he/she meets the trampoline bed. Reduction in tension to an 'intermediate stretch reduces the assistance at take-off, continues to provide stability and control at take-off, adjusts time-in-the-air to a more realistic value, and reduces the assistance upon landing. A 'minimal' stretch simulates reality, all vertical assistance is gone, the guidance providing control and stability of the twisting harness.

Method

For the purposes of the development of the bungy system, take-off velocities and time in the air of an elite performer were derived from video record during 'straight' bouncing, both within the bungy harness and without any physical assistance. The video record was gathered using a Panasonic MS4 SVHS movie camera on VHS mode at 25fps and a shutter speed of 1/500th. The video record was captured on Asymetrix 4 video data capture software, take-off velocities and time in the air derived using the 'measurement' mode of Video Expert II.

Results

During unassisted bouncing the performer registered a take-off velocity of 8.2m/s and as would be expected a similar velocity was registered at landing. The accelerations registered during bouncing at various tensions of the bungy harness indicate that when the tension was high the athlete after take-off did not slow as quickly as when bouncing without assistance or restraint. In addition the rate of descent was slowed quite markedly during the latter stages. The following tables demonstrate these characteristics.

A questionnaire was administered to five athletes involved in a skill learning project using the bungy harness. The questions asked were designed specifically for that research project but included an opportunity for the athlete to comment generally on the bungy harness system. All five commented positively on the system, the detail influenced by age and experience.

.

Discussion

When the athlete is strapped into a harness supported by bungy cord on either side, any force in the vertical plane will initiate a bouncing movement, similar to that experienced on a trampoline. With the bungy supported harness, plus the trampoline, the athlete experiences the up and down movement, but with the stretch characteristics of the bungy cord providing a portion of the take-off velocity from the bed. This removes another of the attention requirements of the athlete, 'working' the trampoline bed, and allows the athlete to attend almost completely to the movement in the air. As the athlete rises from the trampoline bed the bungy cord contracts but within a very short distance (Figure 3) the athlete is free in the air as in a trampoline performance and the bungy does not come into effect again until the athlete is approaching landing.

Close examination of the position of the harness on the athlete's body when at maximum tension shows that the harness is in fact slightly higher on the body than the centre of mass of the athlete. This can provide an eccentric force counter to the direction of rotation about the transverse axis and in fact in testing lead to a number of interesting 'non-rotating' somersaults by the principal author. For the competent trampolinists however this slight resistance to forward and backward rotation accentuated the need to develop strong rotational forces at take-off, and is though to be beneficial.

The picture sequence above demonstrates almost all of the perceived advantages of the bungy harness system. As the athlete takes-off the bungy cord provides vertical velocity as in a good trampoline take-off. During ascent, assistance from the bungy is minimal and in effect the flight of the athlete is determined by gravity. As the performer begins to fall there is a normal acceleration due to gravity until the final stages during which the bungy cord slows the rate of descent providing extra time for the athlete to control the landing and prepare for the next bounce.

Conclusion

The authors are convinced of the benefits of the bungy harness guidance system and while there are obvious applications in the teaching of diving and acrobatics, there are many other possible uses of the specific characteristics of bungy in a number of other partially airborne activities.