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North American Congress on Biomechanics Canadian Society for Biomechanics - American Society of Biomechanics University of Waterloo Waterloo, Ontario, Canada August 14-18, 1998 |
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The work done by a diver during springboard depression has an important bearing upon the maximum height achieved in the dive. This contention is supported by the significant increase in the diver's absolute vertical velocity between the start and end of the takeoff (Miller, 1984). In addition, only about half the variance in dive height can be accounted for by vertical velocity at the start of the takeoff (Sanders et al., 1988). Strategies employed during board depression should be able to increase maximum deflection resulting in more elastic strain energy being stored in the board and available to project the diver upward during recoil.
The key role of the lower extremities in springboard depression has been identified by Miller et al. (1984). Subsequently, Saunders et al. (1992, 1993) investigated how 6 subjects increased their height from a .4 m drop jump onto a sprung surface (k~23 kN/m compared with ~6 kN/m for a competition diving board). They found that with practice subjects initiated hip extension prior to contact and reduced knee flexion after contact with the surface resulting in a faster loading rate. No significant correlation was found between peak ankle, knee or hip joint torque and jump height.
The purpose of the present investigation was to employ the reverse dive pike (301B) of Olympic competitors as a performance model to identify knee motion strategies used by elite divers to aid in depressing the springboard (Fig. 1). The 301B was chosen because it does not involve a change in trunk rotational direction during the takeoff, requires a minimum of angular momentum for the flight and is executed by almost all divers in competition. It was hypothesized that a common knee extension pattern during board depression would characterize the best divers.
Figure 1: Initial contact (IC), maximum knee flexion, maximum board depression (MD) and last contact (LC) of the 1996 Olympic silver medallist performing the takeoff for a reverse dive pike (score 8.6).
Two Panasonic camcorders positioned as far back from the springboards as possible within the constraints of the diving facility were used to videotape the hurdle, takeoff and portion of the flight above the board of the 3-m semifinal dives at the 1996 Olympics. Each camcorder was level with its optical axis perpendicular to the length of the diving boards and the center of the video field approximately at hip height. Black tape on the sides of each of the three Maxiflex boards provided a linear scale. An observer seated above but in line with the fulcrum assemblies recorded the setting of each dive.
A Peak5 system was used to digitize the reverse dives pike performed by 16 male and 18 female semifinalists. The time interval between fields analyzed was 1/60 s. Bilateral symmetry was assumed as the arms were primarily in the principal plane of motion during the takeoff. Position data were digitally filtered prior to differentiation.
Consistent with previous reports, there was an increase in absolute vertical velocity between the start and end of the takeoff with greater gains being achieved by the male divers (Table 1).
| Gender | Female | Male |
|---|---|---|
| IC | -3.7±0.2 | -4.1±0.2 |
| MD | 0.8±0.2 | 0.8±0.5 |
| LC | 4.8±0.2 | 5.8±0.2 |
Table 1: Means and standard deviations for the vertical velocity (m/s) at initial contact, maximum depression and last contact in the 301B takeoff .
The takeoff began with a brief period of knee flexion as the divers contacted the moving, compliant surface (Table 2). Knee extension followed.
| F | M | |
|---|---|---|
| IC | 127±7 | 122±7 |
| Max Flexion | 115±4 | 107±5 |
| MD | 145±9 | 132±8 |
| DepTime | 236±15 | 265±28 |
Note: p<.001 for all M/F differences except IC.
Table 2: Knee angles (deg) at IC, greatest flexion and MD as well as depression time (ms).
The configuration of the knee extension angular velocity time-curve during springboard depression was characterized by either 2 or 3 slopes (Fig. 2). The latter pattern was evident in the takeoffs of 39% of the women and all except 1 of the men. The 3-slope pattern included a positive, zero or negative middle slope.
Figure 2: Knee angular velocity patterns during takeoff (IC to LC). The vertical line (MD) divides board depression from recoil. Positive signifies knee extension. Data from 2 divers with the 2-slope and 2 with the 3-slope pattern are shown.
The change in slope signifying a slowing in the rate of knee extension coincided with the start of positive acceleration of the board tip. Thus when divers initially encountered increased resistance to depressing the board, their knee extension angular velocity decreased. Upward acceleration of the arms with respect to the shoulders also overlapped this region. In general, divers with the 3-slope pattern had a relative maximum angular acceleration in the region of board maximum depression (Fig. 3).
Figure 3: Knee angular acceleration related to the 2- and 3-slope angular velocity patterns.
To reduce the effect of gender, takeoffs of the women who clearly had either a 3-slope (N=7) or 2-slope (N=9) pattern were compared. Although differences between the groups were not statistically significant, the power of these tests was low. The 3-slope group however had an over-representation of divers ranking highest in their competition (i.e., including places 1,2,4,5,7 and 8 out of 18).
The results of the present study suggested that an effective pattern for board deflection incorporates a slowing followed by an increase in knee extension velocity allowing divers to push through maximum board depression. Seven of the 9 divers who scored 8.0 and above on the reverse dive pike employed this strategy.
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Sanders R.H. et al. Int. J. Sport Biomech. 4, 231-259, 1988.
Sanders R.H. et al. Hum. Move. Sci. 11, 593-614, 1992.
Support for this study provided by the United States Olympic Committee, United States Diving and the Medical Commission of the International Olympic Committee.
