ELLIOTT, B.C.

WILSON, G.J.

English

ISBS (International Society of Biomechanics in Sports): Conference Proceedings Archive

THE USE OF ELASTIC ENERGY: A MATTER OF TIMING ELLIOTT, B.C.; WILSON, G . J .  Department of Human lloverment and Recreation Studies The University of Western Australia * Australia Twelve experienced aale weight lifters completed a series of bench press l i f ts  a t  958 of tbeir raxim. m e  l i f t s  included a rebound bencb preu, a purely concentric bench press, and benth presses vitb a long and short pause khreen d e s m t  and ascent phases of th l i f t .  Force, electro~yograpbic and cineaatcqraphic data were collected froa these lifts.  Tbe elastic ewgy stored during tbr eccentric phase of the l i f ts  was observed to dissipate as a function of the puse period. This relationship vas significantly (p<0,01) representPd by a negative q n e n t i a l  equation vith a 0.85 s balf-life of decay. Curing the concentric pbase of the RBP l i f t  the stored elastic energy n s  quickly recovered and only assisted in the prodtictian of force for th initial 0.23s of concentric activity. The nature of this decay of elastic energy vith increasing pause periods and the rapid dissipation of recovered elastic energy durhq the concentric phase are discussed with reference to tbeir hplication for performance of stretch-sborten cycle activities. 'Repare early' i s  a comn phrase used by coacks i n  hitting sports. The logic behind such a statement i s  that for the ball to be hit at the appropriate time and not .late8 requires this early preparation. The question then arises as to vhether performance i s  in fact hindered by this early preparation, as elastic energy stored during the stretch cycle of the mvelaent (bachving) my mt be of benefit durinq the shorten cycle of the activity ([ormrd sving). This st:etch shorten cycle is generally observed in sport as a counter-mvewnt dcring the backing or preparation phase of the activity (the stretchiy phase) that precedes the actual fsriard or upward movemt (the shortening phase). Biomechmial research has shobn that the use of elastic enerqy has resulted in augnentation  ti^ performce derived from prior stretch. hmussen and Bonde-Petersen (1971) observed a rean increase of 10.39 in total heiqht juped and Komi ad Bosco !1973) observed a mean irrrease of 16.71 in total height j q d  by skilled prforsers, both due to prior stretch. Bober et al.  (1980) reported a wan auqwntation to concenric velocity i n  a seated pushinq movenent of 11.71 due to prior stretch of the upper body musculature. The purpse of this s t~dy  uas to examine changes in the augmentation to performance derived froa prior stretch with increasing Fause time in a bench press. Pupher, the timing of this augmentation vith reference to the ascent of the bar vil! be determined. S~ jects Tdelve experienced nale bench pressers of varyi?q akility, vho vere in current training, served as sbject j  in t . i s  s t~dy.  Each subject performed four bench press l i f ts .  'Itme lncluded a rebound bencb press (REP), a bench press with a short pause betdeen the eccentric and concentric phases (SPBP), a press vith a lorq pause beteen the eccentric and concentric phases (LTBP), and a purely concentric bencb press (PCBP), vhere the movement vas initiated from the chest, vithout prior stretch. All bench press lifts were then performed at 95\ of a subject's perceived marmum for each condition. 1 The REP vas performed flrst by all subjects. Such a procedure was deemed necessary as the REP is the l i f t  mst frequently undertaken by the subjects and therefore provided the most reliable examination of the accuracy of the subject's perception of their maximu. This alloved for slight mdification of the lcads to occur, based on the sub]ect4s s t reqth level on the testing day. Tbe order of perfomnce of the remaining three l i f t s  vere randomized. Rior to lifting, each subject perfomd their standard varm-up, vhich included several sets of prcqressively increasing loads. Subjects i-ere instructed to perform at as high force output as possible. A11 VIII Symposium ISBS - 61 - Prague 1990 subjects vere given as uch recovery time betveen l i f ts  as desired, and typically required a five to eight unute rest pricd. The duration of pause for the SPBP and IgBP mveaents mere subjectively de te ra id  by an experiuwd pmerlifting official. short pawe was defined as a standard conpetition pause, while the long pause uas 3 defined as tvice this duration. The pause duratim w e  subsqmtly ad accurately quantified by w of fib data, as the epoch during which the bar remined in contact vith the chest. Ground reaction force procedures 9 Subjects perforaed th l i f t s  on a bench vhidi vas designed to  wnfon to Internatiorvl Powrliftiq Federation statdards and uas rigidly mounted to a force p la t fm (Xistler 92818 ) by four screws. The vertical and horizontal forces for each baxb press wvesent v e n  recorded. The analogue signals  fro^ the force platform bere amplified and data collected on an IBH-PC cop t ib le  cooputer. Filning procedures N l  bench press wvuents e r e  filrrd at a nominal rate of loo8r by a 1 6 r  Pbtosonics high speed carera attached to a rigid tripd. The ? a r a  was psitioned prpeodirular t o  ttx sagittal. plane of &ion o t  the subject and !mar. Cinemtograpbic data mere syxhonized vith the farce data by use of a Daw Strobcsi~p. Depression of a ranual trigger to comnce forw d a t a  collection simul~neously activateb. the s t ~ o t e  light; uhich operated at  2108~ and mas positioned i n  the camera's field c i  view. h l y s i s  atd hectment of Data The vertical force, vhich was adjusted for bar weight, uas cobin& with the brizontzll force ,usinq Fythagora's Theorem to derive a resultant force. The aug~ntation dl& t o  prior stretin was quztt:!ied as !ollovs: Bench press impulse -- '123 t FCBP inpuise here impulse mas calculated over the first 0.37s of me concentr:~ phase of ench l i f t .  m i i  perid was determined after examination of fom-time and power-tine m e s  3f the KEP and REP 1i:ts revea!,d !Pa: t:e augmentation to concentric wtion derived frsm prier st;et:h wsp no layer evident 3.77s into ?>e ccnce?!:.: motion for any lifter. A second order Butterfirth recursive low digital filte: was used to swath the position data d C  a c.2: off frequency of 502. Ve1oci:y data were subseqcently derived usiy finite differerre ca!cd17:s. Tie filn 5'. provided positioml inforzdtion which wre used to delimit the bench press aveaent into it; descenr an? Isce:: phases, and allowed quantification of the duratisn of the pause period. The ve!ocity d a b ,  used in csnj,acr::r with force dab,  allowed for the calculation of instantaneous power of the bar [Wils~n eL. a:,, i c  press]. The effect of the imposition of a Fuse betteen the eccentric and concentric i;hases on the 'itillzati3n c: elastic energy was exarined by a one-way repeated measures analysis of variance beteen t!!e variols krch pres: conditions. If a significant (~~3.05) result mas obtained, Scheffe's comparisons vere used to identify his: * conditions mere significantly different. The pallse duratioq versus use of elastic energy data wre also :i:ed to expanential podels. The best rodel mas determined by a minimization of least squares cri?erion. W&TS AND DISCUSSION 4 1. Dissipation of elastic energy due to the imposition of a pause. The wan duration of the pause priod for the SPBP bas 0.6s while the LPBP bad a Dean pause ti= 1.3s. The effect of inposing these pause perids betieen the n w l e  stretch and shorten movecent phases mas t? significantly reduce the ispuhe produced during the bar's ascent as -red to the RBP (Table 1) VIII Symposium ISBS Prague 1990 ?MU 1 Relative Impulse Copared to W l s e  for PCBP During Initial  0.37s of   on centric b t i on  (1) REP 119 5 EBP > SPBP and LPBP SPBP 112 4 SPBP > LPBP LPBP 107 3 The RBP l i f t  produced on average 191 mre hpulse colpared to that produced in the PCBP for the sane time period. The SPBP produced 121 lore impulse and the LPBP 71 aore impulse, as conpared to the PCBP l i f t .  Thus the effect of i q s i n q  a pause was to reduce the use of elastic energy. The augnentation values for the RBP conpared to the PCBP wre sinilar to the rean increase of 16.71 in total height junped due to prior stretch for skilled male jumpers (Komi and Bosco, 1978) and a rean a p n t a t i o n  t o  concentric velocity in  a pushing movement sinilar to the bench press of 13.7t due to prior stretch of the upper body ~usculature (Bokr et  a1.,1983]. I t  i s  evident that the use of elastic energy declined as a f m t i o n  of the pause duration. The dissipation of elastic energy with increased p a w  t ie i s  accurately represented (~(3.01) by a negative exponential equation with a 0.85s half-life of decay (Piqure 1) .  This i s  substantially shorter than the 4s half-life suggested by Shorten (1987) for the decay of stored elastic energy in leg extensor muscles, but corresponds to data presented by Thys e t  a l .  (1975). Therefore after a delay period of approxinately 1 s 5 3  o! the store!! energy i s  lost: after 2 s 80t of the stored energy i s  lost and after a 4 s delay almost a l l  the stcred energy is  last .  ' Pipre  1: Dissipation of the use of elastic energy with increasirq pause time 2. The dissipation of elastic energy throughout the concentric phase of the RBP l i f t .  The wan power-time m e s  of the PCBP and the RBP l i f t s  (Figure 2 )  show that the recovery of elastic enerqy greatly f ac i l i t a td  the production of work over the initial  portion of the correntric phase. Hwever, the VIII Symposium I S B S  - 63 - Prague 1990 augmentation realized was short lived and in fact it w no longer evident approximately 0.23s into the concentric motion. This supported wd by Chapw and Caldwll (1985) who reported that the influence of a prior stretch was m longer evident approxirately 0.25 a into the omcentric action. . 0. 00 0. 10 a m  aX) TIWE 0 Figure 2: Mean instantaneous p e r  curve for the first 3.1 s of bar ascent. Tbis study has not only shovn that elastic energy is important in the bench pess movepenr, but thzt its contribution t o  movement is reduced as the pause between stretch and shorten phases of a movepen: is incread.  Coaches for best effect must aim to reduce this pause to as small a period as is practical vhile s t i l l  mzintaining an efficient technique. That is, in  hitting skills the backwinq an! foria:d wing phases of mvement ws t  flow from one into the other. In wvelents where there i s  a sequencing of motion from the prsxinal to more distal segments then aqain the timifq of the stretch phase of any of these joint actiors must be sucfi that the pause prior to movement is reduced. That is in a tennis serve, the player vho prepares 'tw early' and i s  forced to wait for the ball will qenerally not benefit from the elastic energy stored i n  the muscles duriffi the stretch. * In power liftinq i t  i s  obvious that lifters must practice the bench press with a pause to replica?e competitive conditions. Pipally in events such as tbe clean and jerk it i s  important i n  the 'clean' section of the lift to utilize the elastic energy stored durinq the squattifq action to assist i n  the movepent to an upright position. That i s  the smaller the pause at the bottom of the squat action the more elastic e n e q  can be used to augment the upward mtion. I t  i s  also evident that this elastic enerqy has an a b s t  immediate inflwnce on the concentric ration. Consequently large accelerations can often be achieved in  the early pixie of the mncentric olotioa if elastic energy i s  utilised; which s i p  play a vital role in the early stages of a mvement *ere the inertia of an object mst be overcome or a skill cas be p e r f 0 4  uith the assistance of aqwntation from the elastic energy despite the body being in a mechanically poor position. VIII Symposium ISBS Prague 1990 m- The authors thank the Australian Sports ComPission Applied Sports Research h'ograr fo r  funding t h i s  project  and Craeme A. Yood [PM) f o r  h i s  general assistance. REFERENCES J ASXUSSW,E. and BOND@-PETWSW,?.(1974).Storage of e l a s t i c  energy i n  skeletal  muscles in loan. Acta Physiol. Swd. 91,385-392. BC9W,T., JASROLSX1,E. and WOWACX1,Z. (1983). Study on e c c e n t r i c - c o m t r i c  contraction of the upper body extremity muscles. J. Biomechanics 13,135-138. Q!AJIWI, A.E. and WELL, C,E. (1985). The use of auscle s t r e t ch  in ine r t i a l  loading i n  D.A. Winter, R.V. Nom, R.P. Wells, K.C. b y e s  and A.E. Patla (Eds).Bioaechanics M-A, Human Kinetics, Fubs. Illinois,44-49. KCYI, P.V. wd BCSCC, C. (1973). Uti l izat ion of stored e l a s t i c  energy i n  leq extersor wsc le  by men and m e n .  Hed.Sci. Sports 11(4 ) .  26: - 265. KJtX;ELIN, T . H .  (1984). The biomechanics of puer!iftinq: assistance exercise, developinq the chest and l a t j .  Powr l i f t inq  J.S.A. !!9], 20 - 21. ?dCFTEH, X.R. (1987). k x l e  e l a s t i c i t y  and b w n  p e r l ~ r a n c e .  Xed. S p f  Sci. Karqer. Bass!, Nei York, p~ .1 -13 .  WtS, fi., CMAGNA, G . A .  and NARCARIA, R. (1375). The role  played by e las t i c i ty  in an exprrise involving raovemer.% 0: small a3plit.de. Pfluqers Uch.  154, 281 - 236 .  MLjCH, G.J., ELLIOTT, B.C. and ROD, C.A. (1996). The effect  ot imposinq a delay het.ieen Ue eccentr icand concentric phases of a s t retch - shorten cycle mvement. Accepted Xed. Sci.Spr! and E l . ,  1339. VIJI Symposium TSBS Prague 1990 

THE USE OF ELASTIC ENERGY: A MATTER OF TIMING

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THE USE OF ELASTIC ENERGY: A MATTER OF TIMING

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