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ArticleTitle Neuromuscular differences between prepubescents boys and adult men during drop jumpArticle Sub-Title

Article CopyRight Springer-Verlag(This will be the copyright line in the final PDF)

Journal Name European Journal of Applied Physiology

Corresponding Author Family Name KotzamanidisParticle

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Division Laboratory of Coaching and Sport Performance, Department of PhysicalEducation and Sport Sciences

Organization Aristotle University of Thessaloniki

Address Thessaloníki, 54006, Greece

Email kotzaman@phed.auth.gr

Author Family Name LazaridisParticle

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Division Laboratory of Coaching and Sport Performance, Department of PhysicalEducation and Sport Sciences

Organization Aristotle University of Thessaloniki

Address Thessaloníki, 54006, Greece

Email

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Division Laboratory of Coaching and Sport Performance, Department of PhysicalEducation and Sport Sciences

Organization Aristotle University of Thessaloniki

Address Thessaloníki, 54006, Greece

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Division Laboratory of Coaching and Sport Performance, Department of PhysicalEducation and Sport Sciences

Organization Aristotle University of Thessaloniki

Address Thessaloníki, 54006, Greece

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Division Department of Physical Education and Sport Science, Institute of HumanPerformance and Rehabilitation, CERETETH

Organization University of Thessaly

Address Thessaly, Greece

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Division Department of Sport and Sport Science

Organization University of Freiburg

Address Freiburg, Germany

Email

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Received

Revised

Accepted 18 March 2010

Abstract The purpose of the present study was to determine the lower extremities biomechanical differences betweenprepubescent and adult males during drop jumps (DJs). Twenty-four untrained males (12 prepubescents, 12adults) performed DJs from 20 cm height. Kinematics of the lower extremities were captured, in additionwith vertical ground reaction forces (vGRFs) and EMG activity of the gastrocnemius medialis (GM), soleus(SOL) and tibialis anterior (TA) muscles. The results showed that men jumped higher, as expected, but theirknees were more flexed prior to landing, and their preactivation level was higher and longer in durationcompared to prepubescent boys. During landing, men had shorter contact times, lower vGRF normalized tobody mass, and less maximal knee joint flexion. Regarding EMG activity men presented higher stretch reflexand higher EMG activity during the braking phase but the level of coactivation (TA to GM + SOL ratio) waslower. It is seems that pre-landing and landing patterns during a complex task such as DJ are affected byphysical development. There are indications that men had higher performance in a DJ than prepubescent boysbecause they activated more effectively their muscles during the preactivation and braking phase. The above-mentioned data support the hypothesis that prepubescent boys might be inferior in optimal regulation of theirmuscle–tendon unit stiffness.

Keywords (separated by '-') Prepubescent - Drop jump - Preactivation - Kinematic - Electromyography

Footnote Information Communicated by Fausto Baldissera.

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ORIGINAL ARTICLE

Neuromuscular diVerences between prepubescents boys and adult

men during drop jump

S. Lazaridis · E. Bassa · D. Patikas · G. Giakas ·

A. Gollhofer · C. Kotzamanidis

Accepted: 18 March 2010 Springer-Verlag 2010

Abstract The purpose of the present study was to deter-mine the lower extremities biomechanical diVerencesbetween prepubescent and adult males during drop jumps(DJs). Twenty-four untrained males (12 prepubescents, 12adults) performed DJs from 20 cm height. Kinematics ofthe lower extremities were captured, in addition with verti-cal ground reaction forces (vGRFs) and EMG activity ofthe gastrocnemius medialis (GM), soleus (SOL) and tibialisanterior (TA) muscles. The results showed that men jumpedhigher, as expected, but their knees were more Xexed priorto landing, and their preactivation level was higher andlonger in duration compared to prepubescent boys. Duringlanding, men had shorter contact times, lower vGRF nor-malized to body mass, and less maximal knee joint Xexion.Regarding EMG activity men presented higher stretchreXex and higher EMG activity during the braking phasebut the level of coactivation (TA to GM + SOL ratio) waslower. It is seems that pre-landing and landing patterns dur-ing a complex task such as DJ are aVected by physicaldevelopment. There are indications that men had higher

performance in a DJ than prepubescent boys because theyactivated more eVectively their muscles during the preacti-vation and braking phase. The above-mentioned data sup-port the hypothesis that prepubescent boys might beinferior in optimal regulation of their muscle–tendon unitstiVness.

Keywords Prepubescent · Drop jump · Preactivation · Kinematic · Electromyography

Introduction

Performing plyometric exercises, such as drop jumps (DJs),involves the so-called stretch-shortening cycle (SSC),during which a muscle is stretched before its explosive con-traction. The SSC enhances the performance as comparedto pure concentric contractions of isolated muscles or intacthuman muscles (Cavagna et al. 1968) due to the storageand the recoil of the elastic energy (Gollhofer et al. 1984).

It has been reported that the performance eYciency dur-ing a DJ is depended mainly on the stiVness increment ofthe muscle–tendon unit (MTU) during the braking phase(HoVrén et al. 2007; Horita et al. 2002). The MTU stiVnessis regulated by the stretch reXex amplitude and the muscleactivation before the foot contact (preactivation) and duringthe braking phase (HoVrén et al. 2007; Horita et al. 2002).

Previous studies (Horita et al. 2002) reported two typesof DJ in adults: the bouncing type (good jump) and theabsorbing type (poor jump). The bouncing type involves,during the preactivation phase, higher knee Xexion, earlierEMG onset and higher EMG activity. Furthermore, duringthe braking phase good performers presented shorter brak-ing time, decreased knee Xexion at the end of the brakingphase and higher stretch reXex and EMG activity. Viitasalo

Communicated by Fausto Baldissera.

S. Lazaridis · E. Bassa · D. Patikas · C. Kotzamanidis (&)Laboratory of Coaching and Sport Performance, Department of Physical Education and Sport Sciences, Aristotle University of Thessaloniki, 54006 Thessaloníki, Greecee-mail: kotzaman@phed.auth.gr

G. GiakasDepartment of Physical Education and Sport Science, Institute of Human Performance and Rehabilitation, CERETETH, University of Thessaly, Thessaly, Greece

A. GollhoferDepartment of Sport and Sport Science, University of Freiburg, Freiburg, Germany

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et al. (1998) added that the experienced jumpers activatedearlier and to a greater extend their muscles during the pre-activation phase compared to the untrained subjects.

There are several studies examining the task of landingfrom a height in children with and without subsequentjump. However, these two tasks are quite distinct, withdiVerences in muscle activation and stiVness regulation(Leukel et al. 2008). More speciWcally for the DJ in chil-dren, according to the current literature there are studiesthat analyzed either only kinetics (McKay et al. 2005) orkinematics of the knee and hip joint (Noyes et al. 2005;Barber-Westin et al. 2005, 2006; Hewett et al. 2006) duringthe time of initial contact, maximal knee Xexion and takeoV. These studies were mainly focused on the loading ofthe anterior cruciate ligament. Nonetheless, there are nodata available related to EMG activity and kinematics dur-ing preactivation, or agonist/antagonist and reXex activityduring braking and propulsion phase. From this point ofview there is no adequate information about stiVness regu-lation in children and consequently which strategy (absorbor bounce) prepubescent children might use during landingfor a DJ. Furthermore, prepubescents have a more compli-ant MTU than adults (Kubo et al. 2001; Lambertz et al.2003) and therefore it is not clear yet how and if the neuro-muscular system compensates for this diVerence during DJ.

Keeping in mind that the majority of the literature refersto physically active subjects (athletes) and the jumping taskwas performed with the hands hanging freely, we tried toclarify the above mentioned issues from a biomechanicalpoint of view. For this reason, our aim was to identify thebiomechanical diVerences in DJ between prepubescentsand adults. This will possibly provide evidence for thecauses of the performance deWcit in prepubescents and willhelp to develop methods in order to improve their perfor-mance.

Methods

Participants

Volunteers were selected according to guidelines estab-lished by Tanner (1986). Twenty-four males (12 prepube-scents, 12 adults) participated in this study. Prepubescentboys were 9–11 years old (age 9.8 § 0.6 years, body mass37.0 § 4.7 kg, body height 1.41 § 0.06 m) and men were19–27 years old (age 25 § 2.7 years, body mass 73.6 §

8.9 kg, body height 1.77 § 0.05 m). All participants wereuntrained and did not participate systematically in anysports training program during the past 2 years. They werefree of any neurological deWcit that could inXuence thelower-extremity performance and they did not have anyback or lower extremity injury history. Before testing, the

adult participants and the prepubescents’ parents read andsigned a written informed consent statement.

Instrumentation

Kinematic data were captured using a six-camera 3Dmotion analysis system (VICON 612, Oxford Metrics Ltd,Oxford, Oxfordshire, UK), sampling at 100 Hz. Groundreaction forces were recorded with a ground mounted40 £ 60 cm force plate (Bertec Type 4060, Bertec Corpora-tion, Columbus, OH, USA). The EMG activity wasrecorded with a BTS Telemg EMG device (Milano, Italy),using bipolar surface Ag/AgCl electrodes (contact surface0.8 cm, inter-electrode spacing 2 cm). The signal was pre-ampliWed (1,000£).

All devices were triggered by the motion analysis sys-tem. The sampling frequency for the EMG and groundreaction force signals was set at 1 kHz and 100 Hz, respec-tively.

Procedures

Initially, the investigator who assessed all evaluations mea-sured the anthropometric dimensions that are required forthe kinematics and ground reaction force processing. Thenparticipants were warmed up on a cycle ergometer and per-formed submaximal and maximal self initiated jumps, andthree DJ from 20 cm. We avoided doing more familiariza-tion because that would alter the original jumping strategy,as reported by Oñate et al. (2005). After warming up, theEMG electrodes and the reXective markers for the kine-matic capture were placed.

EMG was measured for the dominant limb only, asdetermined by the preference limp to kick a ball intuitively(Wikstrom et al. 2006). The EMG electrodes were placedfollowing the SENIAM guidelines (Hermens et al. 2000).The gastrocnemius medialis (GM) and soleus (SOL) EMGactivity was captured as agonist muscles and the tibialisanterior (TA) as antagonist one. The ground electrode wasplaced over the bony surface of the contra lateral wrist.Skin was shaved and cleaned with an alcohol solution andskin impedance was maintained below 2 k�. EMG signalswere Wltered with a zero lag, second order, band-pass(10–500 Hz), Butterworth Wlter, followed by full-waverectiWcation and then low-pass Wltering at 50 Hz.

For the kinematics, 16 reXective markers (14 mm diame-ter spheres) were placed at anatomical bony landmarks oflower extremities according to Davis et al. (1991). Thestatic and dynamic calibration for the motion analysis sys-tem was assessed by the same investigator according to themanufacturer recommendations before each data collectionsession. Trajectories were Wltered with the generalizedcross validated splines (Woltring and Fortran 1986).

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The DJ test included three DJs from 20 cm height, with2-min rest interval in between. Subjects landed on both feetover the force platform, which was placed approximately8 cm in front of the jumping platform edge. Hands wereplaced on the hips and subjects were instructed to jump ashigh as they can. During the test no verbal motivation orany kind of feedback about their performance was pro-vided. Three trials with maximal eVort were captured withforefoot landing, as judged from kinematic data and withthe typical shape of force–time curve, as described byKovács et al. (1999).

DJ performance is aVected by drop height (Walshe andWilson 1997; Arampatzis et al. 2001; HoVrén et al. 2007).However, based on previous studies (Noyes et al. 2005;Barber-Westin et al. 2005, 2006; Hewett et al. 2006) weused a Wxed DJ height (20 cm) for both groups becausesuch a height is common practice in training for childrenand adults (Kotzamanidis 2006).

Data analysis

Data were further processed oZine using scripts of Matlab6.1 (The MathWorks Inc.). All three maximal trials wereaveraged. Jump height was estimated taking into accountthe impulse which was recorded from the vertical groundreaction force (vGRF)–time curve. The peak vGRF wasnormalized to the body weight of each subject.

Each DJ was divided into three phases: preactivation,braking and propulsion phase. These phases were deter-mined taking into account the preactivation onset, the timeof contact with the ground, the maximal knee Xexion angle,and the toe oV (Viitasalo et al. 1998). The time of contactwith the ground was derived from the force plate after thevGRF exceeded 20 N.

Knee Xexion angles were calculated 100, 80, 50, and30 ms before touchdown, during touchdown and when knee

angle reached its maximum value. Furthermore, the hipangles at 50 ms before touchdown, during touchdown andthe maximum value were estimated. Finally, the knee rangeof motion and peak angular velocity during the braking andpropulsion phase was calculated.

The onset for the preactivation of the GM and SOL mus-cles was set when the processed EMG amplitude exceededtwo standard deviations of the processed EMG signal dur-ing standing prior leaving the jumping platform (Arampatziset al. 2001). The onset of the short latency reXex (SLR) ofthe GM and SOL muscles was expected between 40 and60 ms after ground contact (Marsden 1973; Komi 2000)and was set visually by two independent observers.

The average SLR and the EMG amplitude calculatedover each DJ phase was normalized to the maximum EMGvalue over the DJ (Yang and Winter 1984). The coactiva-tion of the TA was expressed as the ratio of TA/(GM + SOL) during each subphase (Ford et al. 2008).

Statistics

Statistics were performed with the SPSS/PC 16.0 (SPSSInc.) statistical package. Mean, standard deviation of themean was assessed for all dependent variables. Mann–Whitney non-parametric test has been used to identifysigniWcant diVerences between the groups. The signiWcancelevel was set to 0.05 with Bonferroni correction.

Results

Boys’ performance in DJ height was 15.1 § 1.7 cm andthis was signiWcantly lower (P < 0.001) than the valuesachieved by men (32.7 § 4.0 cm, Table 1).

As shown in Fig. 1 the knee joint of the men’s groupprior landing was more Xexed compared to boys’ and this

Table 1 Kinematic and kinetic data during the landing phase of DJ from 20 cm in men and boys

Men Boys z score P value

Braking phase

Knee velocity (°/s) 427.7 § 68.9 560.2 § 96.2 2.94 0.002*

Duration (ms) 142.9 § 18.1 177.0 § 35.9 2.31 0.020*

Propulsion phase

Knee velocity (°/s) 778.1 § 31.3 688.0 § 46.7 4.16 <0.001*

Duration (ms) 158.3 § 24.2 226.2 § 60.1 2.83 0.004*

Jump height (cm) 32.7 § 4.0 15.1 § 1.7 4.16 <0.001*

Peak vertical ground reaction force (times body weight)

3.4 § 0.5 4.3 § 1.0 2.23 0.024*

Hip angle (°)

50 ms before touchdown 36.9 § 8.3 39.9 § 8.7 0.66 0.514

Instant of touchdown 41.8 § 9.9 44.9 § 8.9 0.75 0.478

Maximal Xexion 66.2 § 8.0 79.2 § 9.7 3.00 0.002*

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diVerence was signiWcant 30 ms before touchdown and dur-ing touchdown (P < 0.008).

Men had longer (earlier) preactivation than boys for theGM (P = 0.005) but not for the SOL muscle (P = 0.018,Fig. 2a). Concerning the EMG amplitude during preactiva-tion, men had signiWcantly higher values for the GM mus-cle (P = 0.005). The SOL and TA EMG amplitude showedno statistically signiWcant diVerence (P > 0.008, Fig. 2b).

The kinematics and vGRF data during the two phases oflanding (braking, propulsion) are shown in Table 1. SpeciW-cally, the duration of the braking and propulsion phase wassigniWcantly shorter in men compared to their counterparts(P = 0.020 and P = 0.004, respectively). Furthermore, thenormalized to body weight peak vGRF was signiWcantlygreater in boys than in men (4.3 § 1.0 and 3.4 § 0.5,respectively, P < 0.001).

Prepubescent boys presented a greater maximum kneejoint Xexion (P = 0.002) and regarding the knee angularvelocities they demonstrated higher values during brakingand propulsion phase (P = 0.002 and P < 0.001, respec-tively, Table 1). Regarding the hip joint, prepubescent boyspresented similar angles of hip Xexion with men 50 msbefore touchdown (39.9° § 8.7° vs. 36.9° § 8.3°, P > 0.05)

and at the instant of touchdown (44.9° § 8.9° vs.41.8° § 9.9°, P > 0.05). Nonetheless, boys exhibited statis-tical signiWcantly greater levels of maximum hip joint Xex-ion compared to men (79.2° § 9.7° vs. 66.2° § 8°,P < 0.001).

A typical example of the EMG signals of one boy andone man including the SLR is shown in Fig. 3. On averagemen had statistically signiWcant greater SLR responsescompared to boys in GM (P = 0.002) and in SOL(P = 0.002) muscle (Table 2). The onset of the SLR wasearlier for children and this diVerence was signiWcant forthe GM and the SOL muscle (P = 0.012 and P = 0.008,respectively).

During the braking and propulsion phase men had statis-tically greater GM EMG (P = 0.012 and P = 0.001, respec-tively). For the SOL the diVerences did not exceed thethreshold of statistical signiWcance (braking phase P = 0.089,propulsion phase P = 0.143, Fig. 4). The level of coactiva-tion in boys was higher compared to men, reaching thesigniWcance level during both braking and propulsion phase(P = 0.006 and P = 0.004, respectively, Table 3).

Discussion

The data of the present study showed that during DJ menhad earlier and higher preactivation, compared to boys.Men achieved higher performance than boys did, and dur-ing the braking and propulsion phase they had higher GMactivity, as well as increased stretch response and lowercoactivation.

Previous studies have shown that during the brakingphase of a DJ the level of joint stiVness of MTU is a regu-lating factor for the power output (HoVrén et al. 2007;Ishikawa et al. 2005; Liu et al. 2006). The MTU stiVeningduring the DJ is aVected by various factors such as theincreased level of reXex activity (Horita et al. 2002; Sinkjaeret al. 1988) and the muscle activation prior touchdown andduring the braking phase (Horita et al. 2002; HoVrén et al.

Fig. 1 Knee Xexion angle 30–100 ms before touchdown, duringtouchdown, and maximal value (MV) during DJ in men and boys.Asterisks indicate statistical signiWcance between the groups(P < 0.008)

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2007; Liu et al. 2006; Yoon et al. 2007). In the presentstudy boys had lower DJ performance than men and thiscould be attributed to their inability regulate optimally theirstiVness, as men do. This argument is based on the Wndingthat boys’ EMG amplitude during preactivation and brak-ing phase was lower compared with that of men. To ourknowledge there are no relevant comparable EMG data for

Fig. 3 Vertical ground reaction force and gastrocnemius media-lis rectiWed EMG of a represen-tative man (a) and a boy (b) during drop jump from 20 cm height

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Preactivation Preactivation

Vert

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Table 2 Short latency reXex onset (in respect to contact time) and amplitude (normalized to the maximal DJ EMG value) during the braking phase of DJ from 20 cm in men and boys

Men Boys z score P value

Stretch reXex latency (ms)

Gastrocnemius medialis 45.2 § 6.5 39.6 § 6.0 2.48 0.012*

Soleus 46.1 § 3.8 41.0 § 3.9 2.60 0.008*

Stretch reXex amplitude

Gastrocnemius medialis 0.15 § 0.05 0.07 § 0.04 2.10 0.002*

Soleus 0.17 § 0.05 0.11 § 0.03 3.00 0.002** P < 0.013

Fig. 4 GM and SOL EMG amplitude during DJ in men and boys. Asterisks indicate statisti-cal signiWcance between the groups (P < 0.013)

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Table 3 TA coactivation level [TA/(GM + SOL)] during landingphases of DJ from 20 cm in men and boys

* P < 0.025

Men Boys z score P value

Braking phase 0.46 § 0.2 0.82 § 0.36 2.71 0.006*

Propulsion phase 0.20 § 0.07 0.33 § 0.13 2.83 0.004*

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children and adults after a DJ during the braking phase. Forthis reason, close to our study, we considered studies thatcompared diVerent age groups such as young adults andelderly people (HoVrén et al. 2007; Liu et al. 2006). Elderlypeople exhibited less EMG activity during braking phaseand less MTU stiVness compared to adults. This was attrib-uted either to a neuromuscular deWcit in elderly (Liu et al.2006) or to diVerences on the level of muscle Wber shorten-ing during braking phase (HoVrén et al. 2007). Concerningneuromuscular diVerences between children and adults,there are only a few studies which reported that both groupscan recruit their motor units in similar extent when examin-ing the plantar Xexors and knee extensors (Grosset et al.2008; Streckis et al. 2007). However, Grosset et al. (2008)reported that children exerted lower EMG activity duringMVC and relatively higher EMG activity during submaxi-mal intensities compared to adults. This indicates that chil-dren do not control optimally their muscle activation duringvariable contraction intensities and possibly during com-plex tasks such as DJs. This might explain why boysexerted lower EMG during braking phase compared tomen.

There is not much information in the literature regardingthe mechanical properties of children’s MTU. SpeciWcally,it is known that children have lower musculo-tendinousstiVness (Lambertz et al. 2003) and a more compliant ten-don (Kubo et al. 2001) than adults. This condition could bebeneWcial for storing more elastic energy during a SSCexercise (Kubo et al. 1999). However, the obtained dataindicate that although boys may possibly store more elasticenergy, they are less eYcient compared with adults, whouse their stored energy more optimally, if we consider theiradvanced performance. Similar behavior has been previ-ously observed in the elderly people (HoVrén et al. 2007;Liu et al. 2006), who have also more elastic tendon, com-pared with adults. Additionally, it is possible that boys failto regulate their active stiVness which is time and taskdependent (Zatsiorsky 2002). At this point we must com-ment that the lower stiVness during the braking phase inboys is derived by the higher knee angular velocity duringthis phase and the increased maximal knee Xexion (deepestposition) compared with men.

Another factor which also supports the idea that boyscannot regulate optimally their stiVness as men do, is thelower stretch reXex, so long this reXex is considered as oneof the factors for joint stiVening (Sinkjaer et al. 1988).Lower stretch and tendon tap reXex in children comparedwith adults has been previously reported (Grosset et al.2007) and one possible hypothesis for this diVerence couldbe a less sensitive muscle spindle during the developmentalages. Regarding the latency of SLR the values for the adultsare in accordance to the literature (Grüneberg et al. 2003).Nonetheless, the lower values shown in children compared

with adults could be attributed to diVerences in limp lengthbetween the two groups, as supported by other studies(Potach et al. 2009). This implies that using Wxed time win-dows for the reXex response in children and adults might bemisleading.

Antagonist coactivation decreases the net produced jointmoment and contributes to the joint stiVness increase (Fordet al. 2008; Kellis 1998). The existing data concerning thecoactivation during a complex task in children are conXict-ing. It was shown that during a balance task (Schmitz et al.2002) or walking, running (Frost et al. 1997), coactivationwas higher in children. During prelanding adolescents havehigher coactivation compared to children but during land-ing it is lower (Croce et al. 2004). On the other hand,Russell et al. (2007) reported higher coactivation in adultscompared with children during prelanding, but not duringlanding phase. These authors attempted to link the coactiva-tion Wndings with stiVness regulation (Croce et al. 2004;Russell et al. 2007), although they did not report diVerencesin the agonist activity which is an important factor for thestiVness regulation as well (Horita et al. 2002). However,comparing skilled and non-skilled prepubertal children, theskilled children had lower coactivation with no diVerencein stiVness (Hamstra-Wright et al. 2006). This was attrib-uted to learning factors and this speculation could be sup-ported by Wndings that children consume more energyduring jumping than adults do (Villagra et al. 1993), possi-bly due to lower inter-limb coordination. Despite these newWndings, the antagonist activity in children during landingand jumping needs further investigation as indicated in arecent review (Ford et al. 2008).

The delayed preactivation observed in boys could besupported by the strong relationship between maturationand the ability to predict an event (Assaiante and Amblard1996). Children possibly followed a non-skilled landingstrategy. �his aspect could be supported by Viitasalo et al.(1998), who argued that the less experienced adults, apartfrom lower jumping performance, had more delayed preac-tivation compared to trained ones. Furthermore, adultsXexed their knees more before and during initial contactwith the ground. These are characteristics of “good jump-ers” (bouncing type) as deWned by Horita et al. (2002). Itseems therefore that the behavior of children prior landinghas all the characteristics that the “poor—untrained jump-ers” (absorbing type) adopt.

The developmental process, physical maturation andskill development might better regulate and reduce thevGRFs during the landing impact (McNair et al. 2000; Sigget al. 2001). This could result in more eYcient powerabsorption of the ground reaction forces. Men’s lower nor-malized vGRFs values could be explained by theirincreased knee Xexion during touchdown in contrast to themore extended knee position of boys at this moment. This

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Xexed position is more advantageous for shock absorption(Horita et al. 2002; Louw et al. 2006).

Men and boys exhibited the same hip Xexion during pre-activation and initial contact, but boys Xexed their hip to ahigher extent during the end of braking phase. This couldbe attributed to learning factors (McNitt-Gray 1991). MorespeciWcally, McNitt-Gray (1991) reported that recreationalathletes Xexed their hip joint more during the braking phaseand had longer contact time. This occurred probably due tothe fact that gymnasts must maintain their balance duringlanding. An interesting point in the current study was thatthe greater hip Xexion observed in boys after landing andnot before and during touchdown indicates that hip Xexionmay not be a case of central programming as happened withknee joint but rather a mechanical consequence of thegreater knee Xexion in children or also possibly due to theirinability to optimally regulate their extensor forces in thehip, as indicated by McNitt-Gray (1991).

Conclusions

Pre-landing and landing patterns in a complex task such asDJ from a 20-cm-height is aVected by age during develop-mental period. The superiority of men could be attributed totheir more eVective muscle activation before landing andduring the braking phase, a fact which possibly leads to abetter regulation of MTU stiVness. These diVerencesbetween prepubescent boys and men possibly are aVectedby learning factors since boys followed an immature DJstrategy, which is adopted by poor men jumpers (absorbingtype). Further research is required to Wnd out how this deWcitcan be inXuenced by stimuli such as training, diVerent jumpheights, or other conditions that modulate MTU stiVness.

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