Possible errors in the comparative evaluation of drop ...

E RGO N OM ICS , 1999,

VOL .

42,

NO.

7, 1011 ± 101 4

Technical Note

Possible errors in the comparative evaluation of drop jumps from diŒerent heights A RM IN K IBELE * Institute for Sports and Sport Science, University of Freiburg, Schwarzwaldstr. 175, D-79117, Freiburg, Germany

Drop jumps (DJ) from elevated platforms have become an important training regimen for plyometric training. To ® nd the most bene® cial training stimulus, researchers have tried to identify the optimal dropping elevation which is supposed to lead to a maxim al jump height (e.g. Asmussen and Bonde-Petersen 1974, Komi and Bosco 1978 , Bosco and Komi 1980 , Bosco et al. 1981, Schmidtbleicher 1983 , Bedi et al. 1987, Lees and Fahmi 1994). However, there is little consensus in the literature about what elevation this would be. Am ong several possible reasons for the controversial ® ndings there is one that appears particularly crucial since it has not yet been analysed. So far, no emphasis has been given to the issue of correct dropping technique. The question is whether the distance for the free fall during the drop actually corresponds to the given platform height from which drops are performed. Our experiences with numerous sport science students and athletes from diverse national teams (e.g. ski jumping, track and ® eld, volleyball) supports the assumption that there are considerable diŒerences between the platform levels and the actual dropping

heights. These diŒerences increase with increasing dropping height even though subjects are instructed not to raise or lower their centre of mass (CM ) when stepping forward before the drop. In this respect, Bobbert et al. (1987: 340) wrote `During the execution of DJ40 and D J60, the subjects preferred to assum e a slightly bent posture before jumping down. It was considered irresponsible to prevent them from doing this’ . Using ® lm an alysis, Bobbert et al. found actual dropping heights for a 20, 40 and 60 cm condition to be 20, 31 and 49 cm (n = 6). The statement from Bobbert et al. corresponds well to our own ® ndings. Therefore, we have reason to assum e a tendency for subjects to step down from the platform or to bend forw ard particularly for dropping heights above ~ 20 cm. For those dropping conditions, the actual dropping height is less than the given platform level for most subjects. This paper intends to direct attention to this open problem and to introduce two alternative biom echanical methods that control for the actual dropping height. Assum ing the actual dropping height to be equivalent to the platform

*e-mail: kibele @ ruf.uni-freiburg.de 0014± 0139 / 99 $12.00

Ó

1999 T aylor & Francis Ltd

1012

A. Kibele

level may produce potential errors when using the num erical integration method as suggested by Lees and Fahmi (1994) for the determ ination of the jumping height. Lees and Fahmi have used the platform elevation to calculate the landing impulse by means of the impulsemom entum relationship and consecutively to calculate the take-oŒ impulse. All procedures were based on force-plate data only. Suppose the above-m entioned diŒerences were also present in the Lees and Fahm i study. Consequently, as platform heights increase, subjects would produce smaller landing im pulses than expected by the assumption of a correct dropping technique. For the take-oŒ impulse, calculated as the difference between the total integral minus the landing impulse, jumping heights are estimated too low. However, from the above, we know that for relatively low platform levels, this error would not be large. As a consequence, results are forwarded, which claim to show that dropping from sm all platform levels (12 cm) does produce the largest jumping heights (Lees and Fahmi 1994 ) counter to what was found in the literature so far (e.g. Bedi et al. 1987). Despite the obvious uncertainty involved in the method from Lees and Fahmi, we have no direct evidence to question their results. Nevertheless, these authors and many others did not ® nd the problem of a proper dropping technique worth mentioning and did not com ment on any measure of control to ensure that their subjects did perform a proper dropping technique. It is the same problem that exists for studies that calculated the dropping impulse based on the platform level to distinguish the eccentric and the concentric phase during ground contact (Asmussen and Bonde-Petersen 1974 , K om i and Bosco 1978, Bosco et al. 1982). To prevent this potential error we suggest an analysis method that exam-

ines not only the ground reaction force after the drop but also before and during the dropping movem ent. For that purpose, larger force plates or multiple force plates have to be used. So far, dropping platforms were preferably located outside the force plate surfaces and they could not be used to analyse the dropping dynam ics. In essence, we suggest that subjects drop from platforms or dropping boxes located on the force platforms. Figure 1 shows a force record as well as net displacement (a) and net velocity (b) curves based on force plate data that include the dropping dynam ics. This trial shows what may be the case at the beginning of the dropping movement. CM is slightly raised to step forward before the drop. For this relatively low dropping height, CM is slightly elevated before the free fall as compared with the stance at rest. W ith this information, the actual dropping height can be correctly calculated and fed back to the subjects for them to change the dropping technique. H owever, larger force plates or multiple plates are expensive and may not be available in every laboratory. An alternative method that does not require any additional equipment other than a normal force plate (e.g. Kistler type 9281 ) is based on the assumption that jumping height, JH, may be calculated by m eans 2 of ¯ ight tim e (JH = ô g Ç ( ô t ¯ ig h t ) ). The validity of this m ethod was examined by Frick et al. (1991). They found it acceptable for experienced researchers able to detect postural diŒerences between take-oŒ and landing. Therefore, assuming the ¯ ight time method to be approximately correct and to give adaquate jump heights JH , the take-oŒ impluse m Ç v ta k e -o Œ can be calculated by the impulse-m omentum relationship 2 ( ô m Ç v ta k e -o Œ = m Ç g Ç JH). Consecutively, the diŒerence of the total force integral (corrected for the body weight) and the

Drop jumps from different heights

1013

Figure 1. (below) Vertical ground reaction force during a depth jump from 16 cm. The record starts with a subject standing on the dropping platform with hands kept akimbo. (above) Net velocity and net displacement curves for the CM calculated by numerical integration.

take-oŒ im pulse results to the landing impulse m Ç v lan d in g . Further application of the impulse-m omentum relationship gives an estimate for the actual dropping 2 height ADH ( ô m Ç v la n d in g = m Ç g Ç ADH) which m ay be used as feedback measure to subjects’ dropping technique. In the past, we have used this alternative method to estimate the am ount of error involved in the dropping technique. Here we have particularly instructed the subjects not to raise or lower their CM when stepping forward to drop from the given heights. However, we have not told them about their diŒerence between the platform level and the actual dropping height. This instruction was thought to be comparable to instructions given in other studies. As an example, for the Germ an national volleyball team (age

24.6 6 2.8 years, height 195.0 6 5.8 cm, weigh t 86.8 6 9.0 kg) we found average jumping heights from 16, 32, 48 and 64 cm platforms calculated by the ¯ ight time method of 38.3 6 4.3, 39.5 6 6.0, 39.8 6 5.5 and 40.2 6 5.1 cm respectively. The average diŒerence between the actual dropping height and the given platform level was Ð 0.9 6 4.6, Ð 8.2 6 4.7, Ð 11.2 6 6.0 and Ð 17.8 6 6.8 cm with negative values standing for lowering the CM before the free fall. In conclusion, dropping technique is a crucial problem in a comparative analysis of drop jumps from diŒerent platform levels. This is particularly true for those studies that use the given platform level for the calculation of the landing impulse as well as for a division of the ground contact tim e into a

1014

A. Kibele

concentric and an eccentric phase. W e suggest the inclusion of the dropping dynam ics into the biomechanical analysis to control a proper dropping technique. Studies that do not mention any measure of control must be interpreted with care (e.g. Asmussen and BondePetersen 1974, Bosco et al. 1982, Lees and Fahmi 1994). References A SM USSEN , E. and B ON DE -P ETER SEN , F. 1974, Storage of elastic energy in skeletal muscle in man, Acta Physiologica Scandinavica, 92, 385 ± 392. B EDI , J. F., C RESSW ELL , A. G., E NG EL , T. J. and N ICOL , S. M. 1987, Increase in jumping height associated with maximal eŒort vertical jumps, Research Quarterly for Exercise and Sport, 58, 11 ± 15. B OBBERT , M., H U IJING , P. A. and VA N ING EN S CHENAU , G. J. 1987, Drop jumping. II. The in¯ uence of dropping height on the biomechanics of drop jumping, Medicine and Science in Sports Exercise, 19, 339 ± 346. B OSCO , C. and K OMI , P. V. 1980, Potentiation of the mechanical behavior of the human skeletal muscle through prestretching, Acta Physiologica Scandinavica, 106, 467 ± 472.

B OSCO , C., K OM I, P. V. and Ito, A. 1981, Prestretch potentiation of human skeletal muscle during ballistic movement, Acta Physiologica Scandinavica, 111, 135 ± 140. B OSCO , C., V IITASALO , J. T., K OMI , P. V. and L UH TAN EN , P. 1982, Combined eŒect of elastic energy and myoelectric potentiation during stretch-shortening cycle exercise, Acta Physiologica Scandinavica, 114, 557 ± 565. F RICK , U., S CH MID TBLEIC H ER , D. and W OÈ RN , C. 1991, Vergleich biomechanischer Meû verfahren zur Bestimmung der SprunghoÈ he bei VertikalspruÈ ngen (Comparison of biomechanical methods for the evaluation of jump height in vertical jumps), Leistungssport, 2, 48 ± 53. K OM I, P. V. and B OSCO , C. 1978, Utilization of stored elastic energy in leg extensor muscles by man and women, Medicine and Science in Sports Exercise, 10, 261 ± 265. L EES , A. and F AHM I , E. 1994, Optimal drop heights for plyometric training, Ergonomics, 37, 141 ± 148. S CH M ID TBLEICH ER , D., 1983, Welche AbsprunghoÈ hen bei TiefspruÈ ngen? (What dropping heights for depth jumps?), Die Lehre der Leichtathletik, 34, 47 ± 49.