Free Fall Acrobatics to Reduce Neck Loads During Parachute Opening Shock: Evaluation of an Intervention.
Condition: Pain; Athletic Injuries; Whiplash Injuries; Biomechanical Lesion, Unspecified
MeSH terms: Wounds and Injuries; Whiplash Injuries; Athletic Injuries
Study type: Interventional

Summary: 
      This study aims to evaluate the use of an aerial human body manoeuvre to reduce the
      biomechanical load on the neck of a parachutist during the parachute opening, in order to
      create a basis for future prevention of skydiver neck pain in the parachutist population.
    

Detailed description: 
      Elevated neck pain prevalence among skydivers is associated with exposure to repeated
      parachute openings. The parachute opening shock (POS) is a sudden and brutal deceleration of
      a human being. In skydiving (sport parachuting from aircraft), it slows a free falling
      skydiver from a velocity >200 km/h to <30 km/h within a few seconds. POS deceleration
      magnitudes 9-12 times Earth's gravitational acceleration (a dimensionless ratio denoted G)
      have been measured. These hard openings were painful, and a number of very hard openings have
      generated injuries visible to health care systems. During subjectively normal openings,
      decelerations measured on the human neck exceed 4 G with initial onset rates (jerks)
      exceeding 20 G/s. Considering that active skydivers may do ten jumps per day and may
      accumulate well over a thousand jumps during a parachuting career, these are problematic
      values. Fighter pilots have suffered neck pain after less accelerative exposure. In the
      Swedish skydiving population, the neck pain prevalence is 45%, to be compared with a general
      population estimate of 37%. Recently published data show that skydiver neck muscles are under
      excessive strain during POS, and data from our group suggest POS as composed of
      biomechanically discrete phases. A first phase contains an initial jerk in ventral to dorsal
      direction, i.e. "pulled backwards", denoted negative Gx, when the skydiver is rapidly rotated
      from a prone belly-to-earth body position to an upright position. During this phase, the
      moment arm from the center of mass of the head to the parachute connection point at the
      shoulders is long and likely to yield a high torque in the neck. The second phase, denoted
      positive Gz, contains the bulk of POS-deceleration directed caudally to cranially. Entering
      the second phase with the neck flexed forward from the jerk would put the neck muscles in a
      clear disadvantage.

      In a sport, it would seem desirable to prevent injuries by the way the sport is practiced. A
      number of techniques to reduce POS neck loads have been suggested among athletes, two of
      which are biomechanically appealing: Reducing parachute deployment airspeed and positioning
      the human body head high prior to main parachute extraction. Whether these techniques
      actually reduce neck loads during POS has not been systematically evaluated. From an
      empirically determined relation between maximum POS deceleration and free fall velocity, it
      can be calculated that a decrease in velocity from 220 km/h to 190 km/h may reduce the
      maximum deceleration and thereby (constant mass) force 25%. Such a velocity reduction is
      possible using the human body only. Our static anthropometrical assessments suggest that,
      unless a flexion forward of the head occurs, pitching up the body head high to an angle of 45
      degrees from the flat-belly-to-relative-wind plane may reduce the head-neck lever arm 30%.
      Thus, a successful combination of velocity reduction and head-neck lever arm reduction holds
      the promise of an approximately halved torque in the neck during POS. Such a substantial
      mechanical change can be hypothesized to have biological effects.

      The intervention protocol is based on observational data and static anthropometrical
      calculations and inspired by anecdotal information. It has been systematically validated by
      subject matter experts, and the results of this validation has been published. The
      intervention is conceived as a combination of velocity reduction and head-neck lever arm
      reduction, main outcome variables being magnitudes of decelerations and jerks, expressed in G
      and G/s, respectively, and research subjects being experienced skydivers completing two
      consecutive skydives on the same day with random ordering (one being the control jump and one
      being the intervention jump).

      Based on an estimated real-world 30% effect size and a desired 0.9 power level, sample size
      calculation suggest 16 subjects as sufficient for parametric analyses. To allow for dropout,
      the sample size will be increased to 20 subjects. Thus, twenty highly experienced skydivers
      will be recruited to the study, to perform the two consecutive terminal velocity skydives on
      the same day. Their demographics and background data will be obtained with use of a validated
      web-based questionnaire for skydivers. The test subjects will be recruited through electronic
      fora, including e-mail lists, for highly experienced skydivers. They will use their own sport
      parachute systems packed and maintained by themselves. The rationale for this is to maintain
      typical and comfortable conditions for each subject, whereas an unfamiliar system may
      introduce undesired psychomotor confounders, and also to maintain a high degree of external
      validity (as compared to using only one type of standardized parachute).

      A detailed description of the planned measuring instrumentation has been published
      separately. The equipment setup in its entirety is approved for aerial use by the National
      Safety Officer of the Swedish Parachute Association. Multiple triaxial accelerometers are
      used to measure decelerations and jerks, and videography to record complex movements,
      including the parachute opening and head motion. Altitude and falling speed data will be
      collected with barometric and Global Positioning System (GPS) altimetry, using
      state-of-the-art skydiving devices.

      Two consecutive skydives from 4000 m altitude will be completed by each subject on the same
      day; one being the control jump and one being the intervention jump. For safety reasons, main
      parachute terminal velocity deployment altitude will be elevated above normal and high-speed
      landings forbidden. In one of the two skydives, the test subject will perform an intervention
      consisting of a sequence of free fall manoeuvres performed using the human body: A free fall
      velocity reduction prior to main parachute deployment followed by a head high body attitude
      prior to main parachute extraction. Details of the manoeuvres will be given to the test
      subject in a written instruction.

      This study will use a cross-over, within-subject, repeated measures design with random
      ordering of performing intervention jump or "ordinary" jump in the first jump of two.
      Computerized randomization will be performed in blocks, i.e. to have equal numbers of
      subjects starting with intervention jump vs. control jump in a block of every four subjects.
      A detailed description of the planned methodology for treatment and analysis of data has been
      published separately. Repeated within group measures ANOVA is planned to be used to measure
      effects, and regression used to test for relationships.

      The results of this study are expected to contribute to a basis for future prevention of neck
      pain among skydivers, which is known to have a relation to repeated parachute opening
      exposure. In work addressing this health problem, our study may create a logical fork with
      important future implications. From static biomechanics and theoretical calculations, the
      planned intervention holds the promise of a halved torque in the neck during parachute
      opening. Though real-world results may not show such an impressive effect, it is important to
      examine this assumption; if it can be demonstrated to have some merit, further large-scale
      population studies and implementation would seem warranted, possibly offering an elegant
      solution to a widespread health problem in this population. If, on the other hand, the
      planned intervention can be demonstrated to have little or no effect on its biomechanical
      outcome variables, doubt will be cast upon the conventional wisdom skydiver good advice upon
      which it is based, shifting future attention more pointedly towards manufacturers of
      parachute systems.

      This study will be conducted in Swedish airspace in cooperation with and under the
      supervision of the National Safety Officer of the Swedish Parachute Association. All safety
      resources for sport parachuting available within the Swedish Parachute Association will be
      employed. This study will be conducted in compliance with the Declaration of Helsinki. All
      subjects will receive oral and written information about the study, including safety aspects
      (e.g., agreeing not to perform high-speed landings during the study), and sign a written
      consent to participate. All subjects will be informed that they can end the study
      participation at any time.
    

Intervention name(s): Intervention
Intervention type(s): Behavioral
Intervention description(s): Standard skydive from 4 000 m above mean sea level (AMSL) following standard safety recommendations and procedures including, if necessary, standard reserve parachute activation procedures. If necessary for safety, participants are asked to immediately leave the study at will. At 1 500 m AMSL, the participant is asked to begin to slow down the fall rate by increasing the body surface area to the relative wind. At no lower than 1 200 m AMSL, the participant is asked to deploy the main parachute. At main parachute deployment, while maintaining a stable body position with shoulders level to the horizon and unaltered heading, the participant is asked to increase the pitch angle of the long body axis attitude, raising the head, shoulders, and upper body up from the flat belly-to-relative-wind plane to a head-high body position, using any free fall technique the participant is comfortable with - as long as there is NO RISK for an unintentional backflip.
Allocation: Randomized
Intervention model: Crossover Assignment
Masking: None (Open Label)
Primary purpose: Prevention
Keywords: Aviation; Parachuting