INFLUENCE OF JUMPING STRATEGY ON KINETIC AND KINEMATIC VARIABLES

Transcription

1 INFLUENCE OF JUMPING STRATEGY ON KINETIC AND KINEMATIC VARIABLES BORIS JIDOVTSEFF 1,2. JACQUES QUIEVRE 3. NIGEL K. HARRIS 2. JOHN B. CRONIN 2,4 1 Service of Trining nd Assessment of Physicl Fitness, Deprtment of Sports Sciences, University of Liege, Liege, Belgium 2 Sport Performnce Reserch Institute New Zelnd, Aucklnd University of Technology, Aucklnd, New Zelnd 3 Lortory of Biomechnics nd Physiology, Ntionl Institute of Sport nd Physicl Eduction, Pris, Frnce. School of Exercise, Biomedicl nd Helth Science, Edith Cown University, Perth, Austrli. Note : This reserch hs een presented in the 35th Congress of the Société de Biomécnique, Lemns, August Corresponding Author : Boris Jidovtseff Service of Trining nd Assessment of Physicl Fitness, Deprtment of Sport nd Rehilittion Sciences, University of Liege, Allée des sports, Liege, Belgium

2 INFLUENCE OF JUMPING STRATEGY ON KINETIC PARAMETERS Astrct Aim: Different jumping strtegies cn e used during plyometric trining. Understnding how mnipulting vriles such s the counter-movement, flexion mplitude, the drop nd the lod could influence neuromusculr dpttion would e eneficil for coches nd thletes. The purpose of this study ws to nlyze how these vritions in the verticl jump influenced kinemtic nd kinetic prmeters s mesured y force pltform. Methods: Ten mle sujects performed, eight kinds of verticl jumps on force pltform : (1) squt jump (SJ); (2) shllow counter-movement jump (S-CMJ); (3) nturl counter-movement jump (N-CMJ); (4) deep countermovement jump (D-CMJ); (5) loded (20kg) counter-movement jump (20-CMJ); (6) shllow drop jump (S-DJ); (7) deep drop jump (D-DJ); (8) six consecutive jump test (6CJ). Customised Lview softwre ws used to clculte time, displcement, velocity, ccelertion, force, power, impulse nd stiffness. After sttisticl nlysis, jumping vriles were grouped to chieve spécific trining ojectives. Results: The mechnicl prmeters were lrgely influenced y the jump strtegy, ll the deep jumps produced superior jump heights nd concentric velocities s compred to the shllow jumps. The exercises ssocited with greter power outputs were the S-DJ (5386±1095w) nd 6CJ (5795±1365w) tht involved short impulse durtions nd very high ccelertions. The gretest vlues of muscle stiffness were not recorded during the highest verticl jumps, mening tht stiffness is not criticl for jumping high. Conclusion: This study gives n overview of wht is chnging when we mnipulte jumping vriles nd instructions given to the thletes. Plyometric exercises should e crefully selected ccording to the sport nd specific individul needs. Key words : verticl jump, iomechnics, plyometrics, instruction, trining

3 Introduction It is widely ccepted tht plyometric type trining is eneficil for developing explosive power. As consequence, over the lst few decdes, plyometrics hs ecome prticulrly common nd ccepted form of trining utilized y thletes seeking to improve their musculr power nd jumping ility (1-3). While verticl jump exercises re the most widely used, there is lrge vriety of exercises ville to the strength nd conditioning prctitioner. Given tht the choice of exercise nd the strtegy used during the verticl jump cn result in very different neuromusculr ptterns nd outputs, it would seem prudent to understnd how different vriles influence the kinemtic nd kinetic outputs of respective exercises. Such informtion would ssist in the stremlining of ssessment nd progrmming in reltion to the individul needs of n thlete, ctivity nd/or sporting event. Reserchers hve compred concentric squt jumps (SJ) to counter-movement jumps (CMJ) nd oserved tht the use of musculr pre-stretch improved susequent concentric performnce nd consequently jumping height y 10-20% (4-8). This eccentric-concentric coupling is known s the "stretch-shorten cycle, is implicted in plyometric trining nd the stretch ugmenttion cn e explined y the product of different physiologicl mechnisms such s recoil of elstic energy, spinl reflex ctivity, muscle pre-ctivtion nd fvorle muscle- tendon configurtion (9-12). Drop jumps (DJ) re mong the more widely used exercises selected for plyometric trining. Erly Soviet reserch (Verhoshnski, cited y (3)) concluded tht drop jumps, y emphsizing the stretch-shorten cycle nd eccentric loding which could hve positive influence on concentric work, were n effective method for improving strength power cpilities. During the seventies, severl reserches hve clerly highlighted tht different heights for drop jump trining resulted in different performnce enhncement (4, 7, 8). Since these seminl studies, severl studies hve investigted the iomechnics ssocited with drop

4 jumps from different drop heights (13-15) nd from different jumping strtegies (16, 17). For exmple, Morn nd Wllce (17) hve demonstrted tht for given drop jump height, chnge in knee flexion hd significnt consequences on oth kinetic nd kinemtic vriles. In fct, knee flexion mplitude ws criticl vrile tht influenced jump height in ll verticl jumps (SJ, CMJ nd DJ) (18). Boert et l. (18) hve simulted iomechnicl models for squt jumps nd confirmed tht knee flexion mplitude influences suject ccelertion nd tke-off velocity. The level of knee flexion during plyometric exercise lso ppers to influence the rte of force development (19). As reported y vrious reserches (13, 16, 20), the instruction during plyometric trining is lso criticl. For exmple, Young et l. (20) hve shown tht when instruction ws to chieve solute height regrdless of ground contct time, DJ nd CMJ performnce were similr. By contrst, when contct time hd to e reduced s much s possile, DJ performnce ws different when compred with the CMJ. Using dditionl weight during plyometric trining is wide utilised method tht ims to improve the work performed y the muscle. Reserchers hve profiled the lod-power reltionship for squt jumping nd contrdictory to most coch s thoughts, mximl power output ws oserve t very low lods (21-23). Whilst gret del of reserch hs investigted the power-lod reltionship (21-23) the mechnicl profile nd hence mechnicl dvntge of loded jumps hs not een compred to other plyometric ctivities. From this rief tretise of the literture, it is ovious tht there re mny jump types nd vriles tht cn e used for the trining of thletes. Tht is, jump trining cn occur with or without counter movement, dropping from height, with dditionl lods, with short or mple knee ngle flexion, nd so on. Ech kind of jump will offer unique nd different mechnicl stimuli, which with repeted ppliction will led to specific neuromusculr dpttion. With this in mind creful selection of exercises nd instruction is fundmentl to optimize sport specific nd individul needs. For exmple, drop jump exercises my optimize

5 performnce in sports tht require reounds nd high eccentric muscle contrctions s in thletic jumps, gymnstics nd sketll. CMJ exercises on the other hnd my e est suited for sports involving high verticl jumps nd chnge of direction s in soccer, volleyll, sket ll, ski jumping or diving. It is therefore importnt to understnd how ech of these jumps differ in terms of the mechnicl output they offer nd how jump trining cn e conducted in order to ccentute certin trining ojectives e.g. eccentric, rking phse, stiffness, high concentric power outputs, nd so on. A review of the literture hs shown tht most investigtions hve focused on one or two jumping vriles within their reserch prdigm. Consequently, comprisons etween descriptive dt of the jump vriles etween studies is prolemtic given the differences in protocols, suject gender nd trining sttus, technology nd dt nlysis procedures, etc. Compring the mechnicl chrcteristics of numer of jump types in one study would ddress these limittions nd provide vlule informtion to the strength nd conditioning coch. Consequently, the im of this study ws to compre the mechnicl chrcteristics of different verticl jump vriles (e.g. influence of the counter-movement, the influence of flexion mplitude, the influence of the drop nd the influence of lod). This study should led to etter understnding of differentil dpttion when certin vriles re mnipulted vi jump type nd instructions given to the thletes. With such understnding, coches should e etter le to prescrie exercise in ccordnce with the specific trining ojectives/needs of the individul nd/or sport. Mteril nd methods Sujects

6 Ten mle sujects, prticipted in this study (ge: 26±4 yers; height: 1.80±0.05 m; mss: 77±9 kg). All hd recretionl sports ckground, nd were free from injury. The sujects were informed out the potentil risks involved with prticipting in the study nd gve their written consent. The experimentl procedures were pproved y the Ethicl Committee of the University of Liege. Equipment A force pltform (Kistler, type 928A11, Switzerlnd) ws used to mesure the verticl component of the ground rection force during ech jump. The signl ws collected t 500Hz vi n cquisition crd (type ATMIO16, Ntionl Instrument) driven y specific softwre (Dqwre, Ntionl Instrument). Procedures Sujects hd to refrin from strenuous physicl ctivity for 24 h prior to the testing session. They were instructed to wer their usul trining shoes. After stndrdized wrm-up, ll sujects performed, in rndomized order, eight kinds of verticl jumps on the force pltform: (1) squt jump (SJ); (2) short counter-movement jump (S-CMJ); (3) nturl countermovement jump (N-CMJ); (4) deep counter-movement jump (D-CMJ); (5) loded (20kg) counter-movement jump (20-CMJ); (6) short drop jump (S-DJ); (7) deep drop jump (D-DJ); nd, (8) six consecutive jumps (6CJ). Jump order ws rndomized to prevent ny order nd ftigue effects with the exception of the six consecutive jump test (6CJ), which ws the lst test to e performed. All jumps were repeted for three trils with one-minute inter-tril rest periods, except for the 6CJ where two minutes rest ws tken. Three minutes rest ws

7 llocted etween the different jumps. Before ech jump, the sujects were issued stndrdized nd specific instructions ccording to the detils listed in Tle 1. Tle 1. Stndrdized nd specific instructions ddressed to the sujects for ech jumping modlity. Jump Squt Jump (SJ) Short counter-movement jump (S- CMJ) Instruction "Strt with 90 knee flexion nd jump s high s possile without ny counter-movement" "Jump s high s possile with shllow nd quick counter-movement" Nturl counter-movement jump (N-CMJ) Deep counter-movement jump (D- CMJ) Loded (20kg) counter-movement jump (20-CMJ) Short drop jump (S-DJ) Deep drop jump (D-DJ) Six consecutive jump test (6CJ) "Jump s high s possile with nturl (self-selected) countermovement" "Jump s high s possile with deep nd fst counter-movement" "Jump s high s possile with nturl (self-selected) countermovement" "Strt on the ox, step off, when you touch the ground jump s high s possile with miniml ground contct time, nd very little knee flexion" "Strt on the ox, step off, when you touch the ground jump s high s possile with long ground contct time deep knee flexion" "Execute 6 consecutive mximl jumps with miniml ground contct time" Dt Anlysis The verticl component of the force signl ws therefter nlyzed using customized softwres (Lview 8.5, Ntionl Instrument, USA) specificlly developed for the jump nlysis, the softwre clculting the vriles of interest. Center of mss verticl ccelertion ws directly mesured from the force signl y using the following formul: Accelertion = (force/mss)-9.81 A single integrl of the ccelertion signl ws used to otin verticl velocity (V) nd doule integrl ws used to determine verticl displcement (D). The verticl power output ws determined from the product of the force nd velocity signls. As recommended y other uthors (24), sujects were instructed not to move just efore nd just fter ech jump for one second in order to record with the force plte flt signl t the eginning nd t the end of

8 ech test. Such instructions were very importnt in order to djust for possile signl drift tht cn e oserved fter single nd doule integrtion. Signl drift ws utomticlly corrected y the Lview softwre. An exmple of record otined from the pltform during CMJ cn e oserved in Figure 1. Four different phses cn e identified (eccentric-flexion, concentricextension, flight nd lnding). The eccentric flexion phse (ll jumps except SJ) included lightening su-phse where gonist muscles relx during initil flexion (except for D-DJ, S-DJ nd 6CJ) nd rking su-phse where gonist muscles strt the rking contrction. The ground contct time (GCtime) cn e split into three prts: lightening time (Ltime); rking time (Btime) nd concentric time (Ctime). Ltime corresponds to the initil prt of knee flexion during which the ground rection force (GRF) is elow the Figure 1. Force-time (), velocity-time (), displcement-time (c) nd power-time (d) curves for counter-movement jump with phse identifictions nd selected prmeters. ody mss force. Btime corresponds to the

9 prt of eccentric flexion during which GRF is over the ody mss force (24). Becuse of movement chrcteristics, Btime is not present in the SJ nd Ltime is not present in SJ, S-DJ, D-DJ nd 6-CJ. Center of mss t lowest position (Dmin), eccentric pek force (EF), eccentric pek velocity (EV), nd eccentric pek power (EP) were determined during the flexion phse of the jump. In this phse, eccentric impulse (Eimp) ws estlished s the re under force curve during the rking su-phse. Concentric pek force (CF), concentric pek velocity (CV), nd concentric pek power (CP) were determined during the extension phse of the jump. The concentric impulse (Cimp) corresponded to the re under the force curve during the extension phse. Totl impulse (Timp) is the sum of Cimp nd Eimp. Jump height (Dmx) corresponds to the center of mss pek position during the flight phse, nd ws clculted from the flight time. The leg stiffness (Stif) of the jump ws lso mesured. Leg stiffness distinguishes the rtio etween pek ground rection force nd pek center of mss displcement (25). In the present study stiffness ws mesured t mximl center of mss lowering using the following eqution: Stif = F/Dmin, where F represents the force t Dmin. Sttisticl Anlysis Mens nd stndrd devitions were employed throughout s mesures of centrlity nd spred of dt. Shpiro-Wilk test ws used to test for normlity. Ten out of 136 comprisons (17 prmeters x 8 modlities), concerning 7 different prmeters, were not normlly distriuted.. For these comprisons, Friedmn repeted mesures were used to determine significnt differences. Wilcoxon test ws then used to determine significnt differences etween the jumps. For the other comprisons, repeted mesures nlysis of vrince

10 (ANOVA) nd Tukey post hoc comprisons were used to determine differences etween jumping strtegies. The sttisticl significnce ws set t n lph level of p Results The influence of jumping strtegy ws found to significntly (p 0.05) influence mny of the vriles of interest in this study. As similrities were less frequent thn differences nd in order to void dt/nlysis overlod, the sme letter hs een used to denote when different jumping modlities re identicl for given prmeter. As consequence ll vriles with the sme letter were not sttisticlly different nd due to the lrge numer of comprisons nd differences only the more importnt findings re discussed herewith Displcement (m) cd c d SJ S-CMJ N-CMJ D-CMJ 20-CMJ D-DJ S-DJ 6-CJ c d cd Figure 2. Men (±SD) mximl displcement (Dmx) nd miniml displcement (Dmin) of the centered mss(cm) in different jump conditions. Men±SD. It cn e oserved from Figure 2 tht jump height ws sttisticlly greter in three jump conditions: N-CMJ (0.42±0.06 m), D-CMJ (0.43±0.05 m) nd D-DJ (0.42±0.06 m), lthough these three jumps were not significntly different from ech other. The lowest jump heights were oserved in 20-CMJ (0.32±0.05 m), S-DJ (0.32±0.06m) nd 6CJ (0.31±0.06 m). Dmin ws influenced y the jumping strtegy with deepest flexion oserved in D-CMJ nd D-DJ nd the shllowest flexion oserved for S-DJ nd 6CJ.

11 The gretest concentric velocity (see Figure 3) ws oserved in the N-CMJ (2.79±0.19 m.s -1 ), D-CMJ (2.80±0.17 m.s -1 ) nd D-DJ (2.78±0.17 m.s -1 ) while the lowest velocity (<2.5m.s -1 ) ws noted for the 20-CMJ, S-DJ nd 6CJ. EV ws independent from CV. The hierrchy ws s follow: 6CJ (-2.39±0.19 m.s -1 ). < D-DJ (-1.90±0.35m.s -1 ) nd S-DJ (-1.85±0.23m.s -1 ) < N- CMJ nd D-CMJ < S-CMJ nd 20-CMJ (p<0.05) Velocity (m.s -1 ) 3.00 c de cd e SJ S-CMJ N-CMJ D-CMJ 20-CMJ D-DJ S-DJ 6-CJ c c Figure 3. Men (±SD) pek concentric velocity (CV) nd pek eccentric velocity (EV) for the different jump conditions c GCTime c CTime BTime LTime SJ S-CMJ N-CMJ D-CMJ 20-CMJ D-DJ S-DJ 6-CJ Figure 4. Men (±SD) lightening (Ltime), rking (Btime) nd concentric (Ctime) phse time in different jump conditions. Ground contct time (Ltime+Btime+Ctime) for the D-CMJ (0.81±0.12 s) nd 20-CMJ (0.84±0.12 s) ws more thn four times greter thn the S-DJ (0.19±0.05 s) nd 6CJ (0.15±0.02 s) see Figure 4. Btime ws shorter in 6CJ (0.07±0.01 s), S-DJ (0.10±0.03 s) nd

12 S-CMJ (0.09±0.01 s) nd longer for the D-DJ (0.25±0.06 s) nd 20-CMJ (0.27±0.05 s). Ctime ws the shortest in the 6CJ (0.08±0.01 s), followed y the S-DJ (0.1±0.02 s) nd the S-CMJ (0.15±0.02 s). The longest Ctime (p<0.05) were recorded in SJ (0.30±0.10 s), D-CMJ (0.26±0.02 s) nd 20-CMJ (0.27±0.05 s) EF CF Pek Force (N) < < = = < = > = SJ S-CMJ N-CMJ D-CMJ 20-CMJ D-DJ S-DJ 6-CJ Figure 5. Men (±SD) pek eccentric (EF) nd concentric force (CF) for the different jump conditions. (< is indicted when CF is greter (p<0.05) thn EF ; > is indicted when EF is greter (p<0.05) thn CF ; nd = is indicted when there is no significnt difference etween EF nd CF). Highest force development (>4000 N) cn e oserved in the 6CJ nd S-DJ conditions (see Figure 5). By contrst, SJ, N-CMJ, D-CMJ, 20 CMJ nd D-DJ were typified y sustntilly lower pek forces (<2100 N)(p<0.001). S-CMJ (2579±349 N) resulted in the est PF of ll the CMJs. Comprison etween EF nd CF reveled tht CF ws greter thn EF in S-CMJ (+5%, p<0.05) nd 20-CMJ (+4%, p<0.05), wheres in the S-DJ, the converse pplied (-4%, p<0.05). For ll other modlities, there were no significnt differences etween EF nd CF. Totl impulse, which included Eimp nd Cimp were greter (see Figure 6) in the loded (20- CMJ; 782±130 N.s) nd deep flexion exercises such s D-DJ (716± 99 N.s), D-CMJ (645±77 N.s) nd N-CMJ (589±96 N.s). Eimp ws the gretest for the D-DJ (329±49 N.s) while Cimp ws the highest in the 20-CMJ (490±77N.s).

13 TImp Cimp Eimp SJ S-CMJ N-CMJ D-CMJ 20-CMJ D-DJ S-DJ 6-CJ Figure 6. Men (±SD) totl impulse (Timp), represented s eccentric impulse (Eimp) nd concentric impulse (Cimp) for the different jump conditions. Eccentric power ws similr (p > 0.05) for ll CMJ modlities. Drop jump exercises nd repeted jumps resulted in the gretest eccentric power: -3073±631 W for D-DJ; -4954±1416 W for S-DJ nd -6354± 1126 W for 6CJ (Figure 7). Pek concentric power ws the gretest in the S-DJ (5386±1095 W) nd in 6CJ (5795±1365 W) conditions. Amongst the CMJ conditions, S-CMJ (4291±876 W) nd N-CMJ (4121±640 W) resulted in superior concentric Power (w) c c d e e de SJ S-CMJ N-CMJ D-CMJ 20-CMJ D-DJ S-DJ 6-CJ CP EP Figure 7. Men (±SD) pek concentric power (CP) nd pek eccentric power (EP) for the different jump conditions. Stiffness (Figure 8) t Dmin ws gretest in S-DJ (29343±12200 N.m -1 ) nd the 6CJ (38712±1378 N.m -1 ). S-CMJ (13794±3624 N.m -1 ) produced the gretest stiffness mong ll

14 CMJ conditions (<7000 N.m -1 ). The comprison of the two DJ modlities reveled tht stiffness ws more thn four time superior in S-DJ in comprison with D-DJ Stiffness (N.m -1 ) SJ S-CMJ N-CMJ D-CMJ 20-CMJ D-DJ S-DJ 6-CJ Figure 8. Men (±SD) stiffness (Stif) for the different jump conditions. Discussion Mny studies hve investigted the vrious forms of verticl jumps (1, 15, 16, 18-20, 24, 26-30) highlighting how one or nother vrile my ffect iomechnics nd jumping performnce. However, to the knowledge of the uthors, this study is the first tht hs reported the kinetics nd kinemtics of rod rnge of verticl jumps. It is well ccepted tht counter-movement induces musculr stretch improving susequent concentric performnce nd consequently incresing jumping height, velocity, power nd force (4-8). In the present study, the differences of ~16% etween SJ nd N-CMJ ppered quite high in comprison to previous reserch (8, 29-31) lthough some reserchers hve reported similr results (4, 17). While the use of counter-movement improved Dmx, CV nd CP surprisingly it ws not the cse for either CF or Cimp. Such findings re in disgreement with Boert nd Cssius who simulted the force-time curve for oth CMJ nd SJ, nd reported tht pek force ws greter during the CMJ (32). However, recent literture hs provided no evidence tht pek concentric force is superior in the CMJ in comprison with the SJ (33-35). The greter jump height is ttriuted to the fct tht the counter-movement llows the suject to ttin greter force levels t the initition of the concentric contrction (26), which does not

15 necessrily coincide with the occurrence of pek force. It is interesting to note tht Cimp ws not significntly different etween SJ nd CMJ-N. These results my e surprising s impulse hs een considered s n importnt determinnt of tke-off velocity given the impulse momentum reltionship. However, Linthorne (36) nd Reiser et l (37) reported tht tke-off velocity ws not directly relted to the Cimp ut rther to the impulse due to suject center of mss ccelertion (Cimp cm = m..ctime) which corresponds to the difference etween Cimp nd the impulse due to the jumper's ody weight (Cimp w =m.g.ctime) : Cimp cm =Cimp - Cimp w. During squt jump, Cimp w is improved y the longer Ctime while Cimp cm is reduced y the lower ccelertion level. All the deep jumps (N-CMJ, D-CMJ nd D-DJ) produced superior Dmx nd CV s compred to the shllow jumps (S-CMJ, S-DJ nd 6CJ). Our results support previous reserch findings tht report n insufficient center of mss lowering ( comintion of nkle, knee nd hip flexion) leds to decresed jumping performnce (17, 18). In fct, the jumps with shorter rnge of movement produced very short Ctime reducing concentric impulse nd consequently velocity development which is necessry to jump high (37). Interestingly, n unnturl jumping strtegy (D-CMJ) produced equl jump height to the N-CMJ. These findings re in greement with recent studies (18, 19) demonstrting tht the use of deeper knee flexion thn nturlly selected did not reduce jump height in comprison with self-selected depth jumps. Consequently, it ws not surprising to oserve in our study tht 5 out of 10 sujects jumped higher during the D-CMJ condition. Indeed, it hs een demonstrted tht when n pproprite coordintion is dopted, using deeper position my result in incresed jump height thn from the preferred position (18, 19). Coches should e wre tht in some thletes, using dditionl squt depth could led to etter jumping performnce. While using deep squts is not recommended in some ctivities, such strtegy my e pertinent for some

16 tsks nd led to improved performnces. However, n excessive knee flexion in the squt my reduce performnce ecuse muscles my e tken eyond their optiml length (37). Loding counter-movement jump with 20 kg (20-CMJ) s expected reduced Dmx nd CV, nd incresed ground contct time ut interestingly, didn't produce greter pek forces thn the unloded jumps. Force is dependent on mss nd ccelertion nd in the present countermovement context, the 20 kg increse in mss ws countered y corresponding decrese in ccelertion. It ws interesting to oserve tht in the 20-CMJ, CF ws greter thn EF (p<0.05). This phenomenon my e ttriuted to the lower velocity nd ccelertion oserved during the eccentric phse in the 20-CMJ. It is possile tht in order to preserve their muscles from ny extreme eccentric loding nd potentil risk for injuries, sujects nturlly dopted jump strtegy incorporting less velocity nd ccelertion during flexion. Loding CMJ with 20 kg induced longer movement (GCtime) which influenced the forcetime curve nd resulted in the highest Timp. Despite such high Timp, mximl velocity nd jump height ws reduced in the 20-CMJ in comprison with N-CMJ. As discussed previously, jumping performnce does not depend on Timp ut the difference etween Timp nd the impulse due to the suject s weight, tht is 20 kg greter in the 20-CMJ jump condition. Despite the decresed eccentric nd concentric velocities, loded jumps pper to e n excellent exercise to solicit high force level in specific durtions, nd such the longer impulses ssocited with this jump my e importnt to improve ctivities such s the initil ccelertion phse in the sprint nd inititing throw such s shot put. In the present study, CP ws higher in the unloded jump (N-CMJ) thn the loded jump (20- CMJ). These findings re in greement with severl reports on the lod tht mximizes power

17 output (Pmx) in squt jumps (17-19, 21-23, 38, 39). For exmple, Cormie et l. (23) oserved tht Pmx ws significntly superior t 0% thn 12% of 1RM. We included odymss in the eqution for power (so-clled 'system-mss'). Not doing so cuses sustntil shift in Pmx towrd the hevier end of the lod spectrum nd cuses proportionlly lrger error in clcultion of power t lighter lods (38). It ws lso demonstrted tht the more powerful exercises were the S-DJ nd 6CJ tht involved short nd very high ccelertion levels. In contrst, lrge rnge of motion seemed to decrese power development, s demonstrted y the lower force nd power production during D-DJ nd D-CMJ in comprison with S-DJ nd D-CMJ respectively. The results re in greement with the reserch of Boert t l.(1) tht hs found tht the sujects mking drop jump of smll mplitude presented higher force nd power output in comprison with those who were mking drop jump with lrge mplitude. Such finding indictes tht verticl jump performnce nd pek power output re not necessrily linked. According to previous reserch, power output is lrgely influenced y the jumping strtegy nd could not e ccurtely predicted from single ssessment of verticl jump height (40). Oviously, in given jumping modlity, the power output is relted with jump height nd improvement in power should led to n improvement in jumping performnce (28). With regrds to the eccentric phse it ppers tht eccentric loding (EF) is emphsized y short impulse time jumps (S-CMJ, S-DJ nd 6CJ). In these modlities, the high lnding negtive velocity (corresponding to EV) nd the short flexion level involved n enhnced rking ction leding to very high rte of eccentric force development. These findings re in greement with other studies (14, 17, 31, 41) nd underlines the importnce of such exercises for loding the eccentric phse nd improving eccentric rking ction. However, s demonstrted y the works of Morn nd Wllce (17), Lin et l (41) nd Wlsh et l. (16),

18 the knee flexion mplitude ppers to e the criticl determinnt for eccentric loding intensity. In comprison with lrge mplitude stretch drop jumps, short-rnge stretch drop jumps led to greter pek force nd ccelertion during oth concentric nd eccentric phses. By contrst, s discussed ove, the more intense S-DJ led to lower jumping performnce, reinforcing the theory tht solute force nd power development re not directly linked to jump height. As reported y other reserchers, drop jumps enhnce eccentric loding nd pek force levels, ut do not necessrily produce greter jump heights thn CMJ when rnge of knee flexion is comprle (1, 8, 17, 31, 33). Stiffness hs een reported to e key determinnt of sport performnce, especilly in high power tsks like jumping nd sprinting (42-44). Our results showed tht this prmeter is highly dependent on the jumping strtegy. Interestingly, gretest stiffness vlues were not recorded during the highest verticl jumps.. These findings re not surprising s the highest jumps re the deepest ones. A study of Armptzis et l. (13) hs demonstrted tht the sme jumping performnce cn e chieved with different level of leg stiffness. A decrese in stiffness is counterlnced y proportionl increse in the GCT. These results re in ccordnce with those of Hor et l. (45) who hve demonstrted tht leg stiffness incresed with hopping frequency. Stiffness ppers to e criticl to the rte of eccentric force development (46) nd in mintining positive energy lnce (33), which re key points for short durtion nd high impulse ctivities encountered in severl sporting contexts like sprinting, thletics jumping, ounding or chnging direction. This study demonstrted tht kinemtic nd kinetic outputs re lrgely influenced y the style of jump. However, in most cses, coches use wide rnge of plyometric jumping exercises in order to improve lower lim function without mking ny distinction etween the

19 distinctive neuromusculr stresses nd susequent enefits tht prticulr exercises provide. Different trining ojectives cn e defined: (1) jumping high is importnt for ll sport tht required to jump s high s possile like in sket ll, volley ll or thletics jumps; (2) muscle stiffness hs to e emphsized in ll the sports where lim deformtion t ground impct hs to e reduced like in sprinting or thletic jumps; (3) impulse is trining ojective for ll sport ctivities tht require the development of high level of force during long lsting impulse like in weightlifting or in rowing; (4) the development of mximum power output remins n importnt trining ojective for mny sports nd it is importnt to know the jumping exercises tht mximize power output; (5) eccentric loding ppers to e importnt in ll sports tht required high level of eccentric force, either to void lim deformtion t impct (reounding, thletic jumps), or to ensure sfe rking ction (sket ll, ski jumping). A clssifiction of the verticl jump exercises in ccordnce with these five specific trining ojectives (jumping high, muscle stiffness, impulse, eccentric loding, nd mximl power) is represented in the Tle 2. This tle cn e used to inform the prctitioner how jumping vriles my e mnipulted in order to chieve trining ojective. Tle 2. Clssifiction of jumping exercise ccording to specific trining ojective. Jumping performnce Stiffness Impulse Eccentric loding Power Ctegories Depth jumps Short impulse jumps Depth nd loded jumps SJ V V Drop nd continuous jumps Short impulse nd nturl jumps S-CMJ V V N-CMJ V V V D-CMJ V V 20-CMJ V D-DJ V V V S-DJ V V V 6CJ V V V

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