Parachute opening shock is the result of a rapid change in the velocity of the parachute-parachutist system relatively to the earth (basically 50 m.s-1 during free-fall, 5 m.s-1 during descent under open canopy). This decceleration is caused by an increase of air drag resulting from parachute canopy deployment. Physically, it means that the mechanical energy of the parachutist after the opening is considerably reduced and can be safely dissipated through the work of lower limbs during landing. Analysis of the mechanical aspects of parachute opening shows that shock intensity, considered as mean acceleration, is influenced by several main factors : mass of the parachutist, density of the air (i.e. altitude), initial velocity, duration of the opening, initial and final surface and shape of the system. From a biomechanical standpoint, the force produced by the parachute canopy deployment (riser force) is transmitted to the parachutist's body near the junction of the cervical and thoracic spine. To one side of this point, a forward flexion of the head and the neck occurs; on the other, the parachute harness induces a compression of the lumbar and thoracic spine. The time history of the riser force vector has considerable biomechanical significance. Injuries observed during parachute opening usually affects the cervical and thoracic spine. Over the last ten years, there has been a steady rise in opening shock of parachutes. Only the acute consequences of the shock are considered here. Egress at high speed and low altitude from high performance combat aircraft is one main concern. High initial velocity and short duration of the opening are the acting factors related to the intensity of shocks. Additional weight of equipment on the head of the pilot worsens the risk of head-neck injuries. Paratroopers are also exposed to severe opening shocks. The weight of military burden and sometimes openings at very high altitude with sophisticated canopies are the main causes in this case. Technical improvement of sport parachutes has considerably reduced the occurence of severe shocks. Acceptance tests of parachutes include drops with dummies and test parachutist jumps. In France, the current standard for opening shock limitation states that the peak riser force must not exceed 1200 daN. Since it does not take into account the time history of acceleration and the biomechanical response of the human body, this standard is now recognized as obsolete and unadapted to modern use of parachutes. There is a need to replace the current standard by a more comprehensive one. An overview of the literature shows that many studies have been devoted to the effects of shocks on the human body, mostly related to road safety problems. Very few have dealt directly with the problem of opening shocks and, from a practical standpoint, are difficult to use for standardization purposes. A diagram of cervical and dorsal spine tolerance, function of acceleration characteristics, has been generated from a triangular waveform stimulation. Real opening shocks are usually more complex. Data recorded during drops using antropomorphic dummies could be used in combination with mathematical modeling of the human body response to opening shocks. A simplified, easy to use model has been developed in the laboratory for this purpose. Additional validation is still needed before it can become an operational standardization tool. Reliable tolerance criteria also need to be established for head-neck and dorsal spine.
