Effects of weight lifting training combined withplyometric exercises on physical fitness, bodycomposition, and knee extension velocity duringkicking in football

Jorge Perez-Gomez, Hugo Olmedillas, Safira Delgado-Guerra, Ignacio Ara Royo,German Vicente-Rodriguez, Rafael Arteaga Ortiz, Javier Chavarren, andJose A.L. Calbet

Abstract: The effects of a training program consisting of weight lifting combined with plyometric exercises on kickingperformance, myosin heavy-chain composition (vastus lateralis), physical fitness, and body composition (using dual-energyX-ray absorptiometry (DXA)) was examined in 37 male physical education students divided randomly into a traininggroup (TG: 16 subjects) and a control group (CG: 21 subjects). The TG followed 6 weeks of combined weight lifting andplyometric exercises. In all subjects, tests were performed to measure their maximal angular speed of the knee during in-step kicks on a stationary ball. Additional tests for muscle power (vertical jump), running speed (30 m running test), anae-robic capacity (Wingate and 300 m running tests), and aerobic power (20 m shuttle run tests) were also performed.Training resulted in muscle hypertrophy (+4.3%), increased peak angular velocity of the knee during kicking (+13.6%), in-creased percentage of myosin heavy-chain (MHC) type IIa (+8.4%), increased 1 repetition maximum (1 RM) of inclinedleg press (ILP) (+61.4%), leg extension (LE) (+20.2%), leg curl (+15.9%), and half squat (HQ) (+45.1%), and enhancedperformance in vertical jump (all p £ 0.05). In contrast, MHC type I was reduced (–5.2%, p £ 0.05) after training. In thecontrol group, these variables remained unchanged. In conclusion, 6 weeks of strength training combining weight liftingand plyometric exercises results in significant improvement of kicking performance, as well as other physical capacities re-lated to success in football (soccer).

Key words: plyometric training, weight training, vertical jump, 1 RM, MHC, soccer, football.

Resume : Dans cette etude, on analyse l’effet d’un programme d’entraınement constitue d’exercices de musculation etd’exercices pliometriques sur un botte, la composition de la myosine a chaıne lourde (vaste externe), la condition physiqueet la composition corporelle (DXA) chez 37 etudiants en education physique divises aleatoirement en deux groupes dontun s’entraınant (TG, 16 sujets) et l’autre servant de controle (CG, 21 sujets). Le TG s’entraına durant 6 semaines aumoyen d’exercices de musculation et d’exercices de pliometrie. Les tests passes par tous les sujets consistent en des mesu-res de la vitesse angulaire maximale du genou au cours d’un botte du cou-de-pied sur un ballon stationnaire. Les sujetspasserent aussi des tests de puissance musculaire (saut vertical), de vitesse de course (sprint sur 30 m), de capacite anaero-bie (test de Wingate et course sur 300 m) et de puissance aerobie maximale (test de navette de 20 m). Le programmed’entraınement suscite l’hypertrophie musculaire (+4,3 %) et une augmentation de la velocite angulaire de pointe du genouau cours d’un botte (+13,6 %), du pourcentage de myosine a chaıne lourde (MHC) des fibres du type IIa (+8,4 %), de lacharge maximale levee (1 RM) au cours d’un developpe incline des jambes (ILP, +61,4 %), de l’extension des genoux(LE, +20,2 %), de la flexion des genoux (+15,9 %), des demi-redressements assis (HQ), +45,1 %) et de la hauteur du sautvertical (toutes les differences significatives a p £ 0,05). Par contre, la quantite de MHC des fibres de type I est diminuee(–5,2 %, p £ 0,05) a la suite du programme d’entraınement. On n’observe aucun changement dans le groupe de controle.En conclusion, un programme d’entraınement d’une duree de 6 semaines et comprenant des exercices de musculation etdes exercices pliometriques suscitent une augmentation de la performance au botte et des autres capacites physiques perti-nentes au football (soccer).

Received 13 July 2007. Accepted 8 January 2008. Published on the NRC Research Press Web site at apnm.nrc.ca on 25 April 2008.

J. Perez-Gomez. Department of Physical Education, University of Las Palmas de Gran Canaria, Las Palmas de Gran Canaria, CanaryIsland, Spain; Department of Physical Education, Catholic University of San Antonio, Murcia, Spain.H. Olmedillas, S. Delgado-Guerra, J. Chavarren, and J.A.L. Calbet.1 Department of Physical Education, University of Las Palmas deGran Canaria, Las Palmas de Gran Canaria, Canary Island, Spain.I.A. Royo and G. Vicente-Rodriguez. Department of Physical Education, University of Las Palmas de Gran Canaria, Las Palmas deGran Canaria, Canary Island, Spain; Department of Physiatrist and Nursing, Faculty of Health and Sport Sciences, University ofZaragoza, Spain.R.A. Ortiz. Department of Physics, University of Las Palmas de Gran Canaria, Las Palmas de Gran Canaria, Canary Island, Spain.

1Corresponding author (e-mail: lopezcalbet@gmail.com).

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Appl. Physiol. Nutr. Metab. 33: 501–510 (2008) doi:10.1139/H08-026 # 2008 NRC Canada

Mots-cles : entraınement pliometrique, programme de musculation, saut vertical, 1 RM, MHC, soccer, football.

[Traduit par la Redaction]

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IntroductionSuccess in football (or soccer, as it is known in North

America) depends on kicking performance (Lees and Nolan1998), among other factors (Arnason et al. 2004). Most stud-ies have concluded that strength training (6–15 weeksduration) improves kicking performance (De Proft et al.1988; Dutta and Subramanium 2002; Jelusic et al. 1992;Manolopoulos et al. 2006; Manolopoulos et al. 2004),though some investigations have failed to find an improve-ment (Aagaard et al. 1996; Trolle et al. 1993). The discrep-ancies between studies may be explained by markeddifferences in the strength-training program: some investiga-tors have used strength training alone (Aagaard et al. 1996;Trolle et al. 1993) or combined with football-specifictraining (De Proft et al. 1988; Manolopoulos et al. 2006;Manolopoulos et al. 2004) and loaded kicking movements(Aagaard et al. 1996; Jelusic et al. 1992; Trolle et al. 1993),as well as isokinetic strength training combined with spe-cific training for football kicking (Dutta and Subramanium2002).

Weight training improves maximal dynamic force(Campos et al. 2002) and plyometric training has positiveeffects on speed and force of muscle contraction (Malisouxet al. 2006a; Malisoux et al. 2006b; Saunders et al. 2006).Some studies have shown that sprinting and jumping abilitymay be improved with plyometric training (Diallo et al.2001; Moore et al. 2005; Rimmer and Sleivert 2000; Siegleret al. 2003), even with only 4 weeks of training (3 sessions/week) (Impellizzeri et al. 2008). Nevertheless, the effect ofthis kind of training on other components of physical fitness,particularly peak knee-extension velocity during kicking infootball, have not been examined.

Although the efficacy of plyometric training is well estab-lished (Bobbert 1990), less is known about the mechanismby which plyometric training may enhance muscle powerand sprinting abilities. Muscle power depends on myosinheavy-chain (MHC) composition (Schiaffino and Reggiani1996). Some studies have shown a decrease in type I and in-crease of type IIa MHC isoforms with 3 months of trainingin sprinters using diverse plyometric and strength-trainingexercises (Andersen et al. 1994b). Plyometric training alonehas been reported to elicit an increase of MHC IIa(Malisoux et al. 2006a). In contrast, Andersen et al. (1994a)reported a reduction in type IIa muscle fibers (using tradi-tional myofibrillar ATPase histochemistry) without signifi-cant changes in the proportion of MHC isoforms(determined by electrophoresis) with 3 months of strengthtraining in soccer players. In general, most studies report ashift in MHC composition from IIx to IIa combined with nochange (or reduction) of MCH I with strength training(Campos et al. 2002; Putman et al. 2004; Raue et al. 2005).It remains unknown if vastus lateralis MHC compositionmay be affected by 6 weeks of strength training combinedwith plyometric exercises.

Thus, we hypothesized that the combination of weight

lifting with plyometric training would promote an improve-ment in knee-extension velocity during kicking in football,as well as an improvement in vertical jump and runningspeed, while eliciting an enhancement of type IIa and a re-duction of type I MHC composition.

Therefore, the aim of this study was to determine whethera 6-week strength-training program combining weight liftingand plyometric exercises elicits the appropriate adaptationsto improve kicking velocity and performance in other skillsrelevant to football success, such as sprinting capacity,jumping, and endurance. Another aim of this study was todetermine if this training program has other potential benefi-cial effects on physical performance and body compositionthat could be of benefit for football players.

Materials and methods

SubjectsForty-two physical education students were randomly as-

signed to a strength-training group (TG) (n = 16; age,23.4 ± 0.5 years; height, 174.9 ± 1.7 cm; body mass, 71.2 ±1.9 kg (all mean ± SEM)) and control group (CG) (n = 21;age, 24.3 ± 0.5 years; height, 177.0 ± 1.5 cm; body mass,75.7 ± 2.5 kg (all mean ± SEM)). Five subjects were ex-cluded from the training group due to failure in accomplish-ing the training program and (or) performance tests. Thedata reported correspond to the 16 subjects that finished thestudy.

Their usual physical activity was limited to some partici-pation in sports and exercises related to their studies, butnone had been doing strength training during at least thelast 6 months. Subjects did not train or perform any kind ofintense or unusual physical activity during the 24 h beforethe test and subjects were always tested in the morning.Subjects were informed about the aims, benefits, and risksof the study, which was approved by the Ethical Committeeof the University of Las Palmas de Gran Canaria and per-formed in accordance with the Helsinki Declaration of 1975in regards to the conduct of clinical research. All volunteersprovided their written consent before participating in thestudy.

Training programThe TG followed a periodised 6-week training program

consisting of 3 sessions/week, scheduled on Monday, Wed-nesday, and Friday. During the training, they executed bilat-eral plyometric exercises combining unloaded drop jumpsfrom a height of 40–60 cm and explosive hurdle jumps, us-ing 5 hurdles spaced 1 m apart at a height of 50 cm. Afterthe completion of the plyometric exercises they performedthe weight lifting part of training, consisting of bilateral in-clined leg press (ILP), leg extension (LE), half squat (HQ),and leg curl (LC). The intensity, repetitions, and sets persessions are described in the Table 1. The subjects were in-structed to jump as high as possible over the hurdles during

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plyometric training and to minimize contact time with theground as much as possible and to lift the load as quicklyas possible during weight lifting. The weight lifting trainingwas not technically explosive, i.e., no release of the weightoccurred, but the subjects were encouraged to do the exer-cises quickly. After 3 weeks of training 1 RM tests wereperformed and the absolute training load was adjusted ac-cordingly. The CG did not perform any kind of training.

TestsTests were carried out over 5 days. The first testing day

started with the determination of body composition andkicking performance, followed by the assessment of jumpingperformance and maximal dynamic strength. On a differentday, sprint performance and anaerobic capacity were as-sessed. The anaerobic capacity tests were started at least30 min after the sprint tests. The next day, the Wingate testwas carried out. The 4th and 5th testing days were used todetermine the maximal aerobic power and to obtain amuscle biopsy, respectively.

Lower-limb lean massLean mass of the lower limbs (lower limb mass – [lower

limb fat mass + lower limb bone mass]) was assessed bydual-energy X-ray absorptiometry (DXA) (QDR-1500, Holo-gic Corp., Software version 7.10, Waltham, Mass.) as re-ported in Calbet et al. (2001) and Ara et al. (2006). DXAequipment was calibrated using a lumbar spine phantomand following the manufacturer’s guidelines. Subjects werescanned in the supine position and the scans were performedin high resolution. Lower-limb lean mass (kg) was calcu-lated from the regional analysis of the whole-body scan andwas considered equivalent to the lower-limb muscle mass(Perez-Gomez et al. 2008). The coefficient of variation forthe assessment of lower limb lean mass in young volunteers(n = 9) with repositioning was 1.5% (Calbet et al. 1998).

Kicking performanceA telemetric electrogoniometer (Gait Analysis System,

Mie Medical Ma 695110, Leeds, UK) was firmly attachedto the lateral aspect of the right knee centred on thecondyle–tibial joint line to measure angular velocity of theknee joint during a maximal instep kick. In addition, anaccelerometer (Kistler 8632C50, Winterthur, Switzerland)was also firmly attached to the medial aspect of the tibiawith tape, just below the tibial tuberosity and used to iden-tify the time at which the leg impacted the football (Mikasa,Official size 5, Hiroshima, Japan) during kicking. The kneeangular velocity reached just 10 ms before impact was takenas the maximal knee extension velocity. All data weresampled at 1000 Hz and recorded on a PC using a data ac-quisition system (MacLab/8e, ADInstruments Pty. Ltd., Cas-tle Hill, Australia). Subjects performed 3 maximal instepkicks on a stationary ball, as quickly as possible, withoutany special attention to the accuracy of the kick. The sup-porting leg was situated 10 cm to the side and 10 cm behindthe ball. The best of the three trials was selected as the rep-resentative value of the kicking performance. The intraclasscorrelation coefficient a (Cronbach) for the peak knee angu-lar velocity in 14 subjects was 0.98.

Maximal dynamic force (1 RM)Maximal strength was assessed using the 1 RM of ILP,

LE, HQ, and LC exercises. For the ILP and HQ, subjectswere required to lower the load so that 908 of knee flexionwas achieved. For the LE, each participant lifted the weightto the full extension of the knee. For the LC, each subjectlifted the device until contact with the thigh. Before the first1 RM attempt subjects warmed up by doing 10 min of sta-tionary cycling followed by 10 repetitions with approxi-mately 50% of perceived maximum load. Then, subjectsperformed 4–5 lifting attempts with progressively heavier

Table 1. The combined strength and plyometric training program.

Weight lifting Plyometric exercises

Weeks Sessions (% 1 RM) Sets Repetition Drop jumps Hurdles1 1 50–70–90 1–1-1 12–6-2 4�5 (40 cm) 4�5

2 50–70–90 1–2-1 12–6-2 5�5 (40 cm) 5�53 50–70–90 1–3-1 12–6-2 6�5 (40 cm) 6�5

2 1 50–70–90 1–3-1 12–6-2 5�5 (40 cm) 5�52 50–70–90 1–3-2 12–8-3 6�5 (40 cm) 6�53 50–70–90 1–3-1 12–8-2 7�5 (40 cm) 7�5

3 1 50–80–90 1–3-2 12–8-3 5�5 (50 cm) 5�52 50–80–90 1–3-2 12–8-3 6�5 (50 cm) 6�53 50–80–90 1–3-2 12–8-3 7�5 (50 cm) 7�5

4 1 (ST) 50–70–90 1–3-1 12–6-2 6�5 (50 cm) 6�52 50–70–90 1–2-1 12–8-2 7�5 (50 cm) 7�53 50–70–90 1–3-1 12–10–2 8�5 (50 cm) 8�5

5 1 50–70–90 1–3-1 12–10–2 6�5 (60 cm) 6�52 50–70–90 1–3-1 12–10–3 7�5 (60 cm) 7�53 50–70–90 1–3-2 12–10–2 8�5 (60 cm) 8�5

6 1 50–80–90 1–3-1 12–8-3 7�5 (60 cm) 7�52 50–80–90 1–3-2 12–8-3 8�5 (60 cm) 8�53 50–80–90 1–3-2 12–8-3 9�5 (60 cm) 9�5

Note: After 3 weeks of training 1 RM performance was measured and the absolute training load ad-justed accordingly. ST, strength test.

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loads until the 1 RM was determined. To minimize fatigue,3–5 min resting periods were allowed between attempts.

Muscle biopsiesNeedle muscle biopsies were obtained from the middle

section of the vastus lateralis muscle under local anaesthesiawithout suction, but with mild pressure on the lateral aspectof the thigh. Biopsies before and after the 6-week periodwere obtained from 25 of the subjects (15 from the TG and10 from the CG). The muscle samples were immediatelymounted with Tissue-Tek and frozen in isopentane cooledwith liquid nitrogen, and stored at –80 8C. MHC analyseswere performed on the muscle biopsies using sodiumdodecylsulfate polyacrylamide gel electrophoresis (SDS–PAGE), as reported by Larsson et al. (2002). From eachbiopsy 20–40 serial cross sections (10 mm) were cut andplaced in 200–500 mL of lysing buffer and heated for 3 minat 90 8C. Between 2 and 12 mL of the myosin-containingsamples were loaded on a SDS–PAGE. Gels were run at70 V for 43 h at 4 8C. Subsequently, the gels were Coomas-sie stained and MHC isoform bands (I, IIa, IIx) were deter-mined based on known migration patterns and quantifiedwith Un-scan-it gel software (Orem, Utah). A representativeexample is depicted in Fig. 1.

Vertical jump performanceThe forces generated during vertical jumps were meas-

ured with a force plate (Kistler, Winterthur, Switzerland), asreported in Ara et al. (2006). During the jumps, the subjectswere asked to keep their hands on their hips and to mini-mize horizontal and lateral displacement. They were awarethat the jumps had to be executed explosively to achievemaximum height. Two kinds of jumps were performed:squat jumps (SJs), in which countermovement was not per-mitted, and countermovement jumps (CMJs), in which sub-jects were asked to perform a countermovement fromstanding, intending to reach knee-bending angles of around908 just before impulsion. A digital goniometer (LafayetteInstrument Company, Lafayette, Ind.) was used to verifythat knees were bent at 908 before jumping for the SJ. Thevertical velocity at takeoff (VT), height jumped (VJH), themean rate of force development (RFD), positive impulse(PI), mean power (MP), maximal instantaneous power(MIP), and maximal instantaneous vertical velocity (MIV)generated were determined in the best of the 3 trials for SJsand CMJs. The RFD was obtained by linear regression ofthe force–time relationship during the impulse phase of theSJ and CMJ between 25% and 75% of the peak force. Theintraclass correlation coefficients a (Cronbach) for VT, VJH,RFD, PI, MP, MIP, and MIV during the SJs were 0.98, 0.98,0.88, 0.99, 0.86, 0.99, and 0.99, respectively, in 10 subjectswho repeated the jumps 3 times. The corresponding intra-class correlation coefficients for the same variables duringthe CMJs were 0.99, 0.99, 0.88, 0.99, 0.97, 0.99, and 0.99,respectively.

All-out 30 s sprint test (Wingate test)Wingate tests were performed on a modified mechanically

braked ergometer (Monark 818E, Monark AB, Vargerg,Sweden) equipped with an SRM power meter (Schoberer,Germany) with a braking load equivalent to 10% of body

mass (Calbet et al. 2003). Peak power output (PPO) was re-corded as the highest work output performed during a 1 sinterval of the test, and mean power output (MPO) was re-corded as the average power developed during the 30 s per-iod. The intraclass correlation coefficient a (Cronbach) forMPO and PPO was 0.98, in 19 subjects assessed in our lab-oratory twice in a single day (Calbet et al. 1997).

Running sprint testsFollowing an individual warm-up, subjects performed 3

maximal indoor short sprint trials, each separated by at least5 min. The time required to cover 30 m was recorded withphotoelectric cells (General ASDE, Valencia, Spain). Thetimer is automatically activated when the subject crossesthe first cell, and every 5 m thereafter. The subjects wereencouraged to run as fast as they could. A standing startwas used and the best of the 3 trials was selected as the rep-resentative value of this test (Vicente-Rodriguez et al. 2004).The intraclass correlation coefficient a (Cronbach) for therunning times at 5, 10, 15, 20, 25, and 30 m were 0.91,0.97, 0.98, 0.99, 0.99, and 0.99, respectively, in 14 physicaleducation students who repeated the test 3 times.

Anaerobic capacityAn all-out 300 m running test was used to estimate the

anaerobic capacity, since the anaerobic metabolic pathwayscontribute more than 50% to the overall energy expenditureduring all-out exercise tests with a duration between 30 and60 s (Calbet et al. 1997; Calbet et al. 2003; Medbo andTabata 1993). The test was performed on a 400 m track,and the time was recorded manually with a digital stop-watch.

Aerobic maximal powerThe maximal oxygen uptake (VO2 max) was estimated us-

ing the maximal multistage 20 m shuttle run (Leger et al.1988). Subjects were required to run back and forth on a20 m course and be on the 20 m line at the same time thata beep was emitted from an audiotape. The frequency of thesound signals increased in such a way that running speedstarted at 8.5 km�h–1 and was increased by 0.5 km�h–1 eachminute. The time during which the subjects were able to runwas recorded to calculate VO2 max. This test has a test–retestreliability coefficient of 0.95 for adults (Leger et al. 1988).

Statistical analysisMean and standard error of the mean (SEM) are given as

descriptive statistics. To identify potentially significantgroup by time interactions, separate 2 � 2 (group � time)

Fig. 1. Identification of the 3 bands corresponding to MHC iso-forms I, IIa, and IIx in muscle biopsies using sodium dodecylsulfatepolyacrylamide gel electrophoresis.

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analyses of variance (ANOVA) with repeated measures wereused. When the group by time interaction was statisticallysignificant, post hoc pairwise comparisons were performedusing the Tukey’s post hoc test. The relationship betweenvariables was assessed by linear regression analysis andPearson’s correlation coefficient was calculated. SPSS soft-ware (SPSS Inc., Chicago, Ill.) was used for the statisticalanalysis. Statistical significance was set at p £ 0.05.

Results

Body compositionThe subject’s physical characteristics are summarized in

Table 2. Training resulted in an enlargement in lower limblean mass, which was significantly greater than the small in-crease observed in the control group (ANOVA group � timeinteraction: p = 0.05).

Kicking performance, vertical jump, and maximaldynamic force (1 RM)

Kicking performance, vertical jump, and maximal dy-namic force values pre- and post-training are presented inTable 3. Significant improvements were obtained in themaximal angular velocity of the knee in the experimentalgroup (from 21.9 ± 1.3 before training to 24.5 ± 1.2 rad�s–1

after training, p £ 0.01), whereas no significant changeswere observed in the control group (ANOVA group � timeinteraction: p £ 0.001). However, there was no correlationbetween the increments in angular velocity of the knee andthe changes in performance in the other variables assessed inthis study.

Strength training also resulted in improvements incountermovement jump vertical velocity at takeoff, heightjumped, maximal instantaneous vertical velocity, and maxi-mal instantaneous power.

The experimental group improved 1 RM performance ininclined leg press, leg extension, and half squat.

Wingate and running testsThere were no significant effects of strength training in

any of the variables analysed during the Wingate tests andrunning tests. The data are presented in Table 4.

Myosin heavy-chain isoform distributionStrength training resulted in an increased amount of MHC

type IIa (+8.4%, p £ 0.05; ANOVA group � time inter-action: p £ 0.05) and a reduction in the amount of MHC

type I (–5.2%, p £ 0.05; ANOVA group � time interaction:p = 0.05) (Table 5). No significant correlations were ob-served in the training group between the change in MHCtype IIa composition and the improvement of kicking per-formance, jumping performance, and maximum dynamicstrength.

DiscussionThe main finding of this study was that 6 weeks of

strength training consisting of weight lifting combined withplyometric exercises in the same training session signifi-cantly improved kicking performance, vertical jump, andmaximal dynamic force (1 RM) in physical education stu-dents; however, the training failed to increase running veloc-ity or predicted maximal aerobic power. These effects wereassociated with greater lean mass in the lower extremities,higher proportion of MHC type IIa, and reduction of MHCtype I.

The maximal knee angular velocities observed in thisstudy ranged between 21.3 and 24.5 rad�s–1, and are similarto those reported by Lees et al. (2005) but a little lower thanthose recorded by Manolopoulos et al. (2006) in footballplayers. In general, kick performance has been determinedby measuring the distance reached by the ball after kicking(De Proft et al. 1988), or by the velocity of the ball after ithas been hit (Aagaard et al. 1996; Trolle et al. 1993). Intheory, the velocity of the ball may vary depending on thecharacteristics of the ball and the technique of kicking. Inaddition to these factors, the kicking distance depends alsoon the take-off angle of the ball, wind direction, air density(altitude), and the amount of imparted spin. Thus, to betterisolate the effect of the strength-training program on the ca-pacity to kick the ball harder, we measured the angularspeed of the knee, which is the main factor determining thevelocity of the ball (Dorge et al. 1999; Lees and Nolan2002). We did not assess, however, the process of transferof energy from proximal (upper leg) to distal (lower leg)segments, which is also crucial in imparting a high velocityto the ball (Dorge et al. 1999; Wickstrom 1975).

Kicking performance has been related to leg musclestrength (De Proft et al. 1988; Dutta and Subramanium2002; Jelusic et al. 1992; Manolopoulos et al. 2004). DeProft et al. (1988) noted that after a specific leg strength-training program during a full football season the concentricstrength of the knee increased and kick performance (meas-ured as kicking distance) improved (De Proft et al. 1988).

Table 2. Subjects’ anthropometrics results, body mass, and leg muscle mass.

Group Age (y) Height (cm) BM (kg) LM (kg)* Body fat (%)

Pre-testTG (n = 16) 23.4±0.5 174.9±1.7 71.2±1.9 9.3±0.3 15.5±1.1CG (n = 21) 24.3±0.5 177.0±1.5 75.7±2.5 10.0±0.3 15.8±1.4

Post-testTG (n = 16) 23.8±0.5 174.9±1.7 71.9±1.6 9.7±0.3{ 15.4±0.9CG (n = 21) 24.7±0.5 177.0±1.5 76.5±2.4 10.2±0.3{ 15.8±1.3

Note: CG, control group; TG, training group; BM, body mass; LM, leg muscle mass. Data are expressed asmean ± SEM.*Statistically significant group � time interaction in the ANOVA test for repeated measures.{p £ 0.05 between pre-test and post-test.

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Table 3. Changes in maximal angular velocity of the knee, 1 RM of inclined leg press, leg extension, half squat, leg curl, squat jump, vertical velocity at takeoff, height jumped, rateof force development, positive mechanical impulse), mean power, maximal instantaneous power, maximal instantaneous vertical velocity, and countermovement jump.

(a) 1 RM

Group Kav (rad�s–1)* ILP (kg)* LE (kg)* HQ (kg)* LC (kg)*TG (n = 16) 21.9±1.3 203.5±10.9 67.2±3.3 145.3±6.5 52.2±1.9CG (n = 21) 22.5±1.0 255.7±16.8 69.8±2.9 159.3±8.2 58.4±2.2TG (n = 16) 24.5±1.2{ 325.0±13.8{ 84.3±3.5{ 208.2±7.0{ 60.5±1.8{

CG (n = 21) 21.3±0.8 267.4±18.6{ 70.9±3.2 155.8±7.2 58.7±2.4

(b) Squat jump

Group VT (m�s–1) VHJ (cm) RFD (kgf�s–1) PI (kgf�s–1) MP (W) MIV (m�s–1) MIP (W)

Pre-testTG (n = 16) 2.46±0.04 0.31±0.01 675.5±80.9 18.0±0.5 633.5±37.5 2.52±0.04 3341.6±119.5CG (n = 21) 2.40±0.04 0.30±0.01 808.9±83.4 18.6±0.6 655.6±27.4 2.48±0.04 3471.0±116.0

Post-testTG (n = 16) 2.52±0.04 0.33±0.01 618.0±107.2 18.5±0.5 631.2±44.3 2.59±0.04 3545.3±123.6CG (n = 21) 2.41±0.03 0.30±0.01 748.0±65.9 18.8±0.5 648.3±28.4 2.49±0.03 3504.4±99.6

(c) Countermovement jump

Group VT (m�s–1)* VHJ (cm)* RFD (kgf�s–1) PI (kgf�s–1) MP (W) MIV (m�s–1)* MIP (W)*

Pre-testTG (n = 16) 2.64±0.05 0.36±0.01 843.3±73.4 19.1±0.5 923.3±48.8 2.71±0.05 3484.0±114.6CG (n = 21) 2.56±0.05 0.34±0.01 741.4±50.1 19.7±0.6 926.2±41.4 2.65±0.04 3574.3±115.4

Post-testTG (n = 16) 2.74±0.05{ 0.39±0.01{ 669.4±72.3 20.1±0.5 993.1±49.7 2.80±0.04{ 3735.7±114.4{

CG (n = 21) 2.58±0.04 0.34±0.01 645.0±54.1 20.1±0.5 932.8±39.5 2.67±0.04 3647.6±101.0

Note: Data are expressed as mean ± SEM. Kav, knee angular velocity; ILP, inclined leg press; LE, leg extension; HQ, half squat; LC, leg curl; VT, vertical velocity at takeoff; VHJ, height jumped, RFD,rate of force development; kgf�s–1, rate of force development in kilograms-force per second; PI, positive mechanical impulse; MP, mean power; MIP, maximal instantaneous power; MIV, maximal instanta-neous vertical velocity.*Statistically significant group � time interaction in the ANOVA test for repeated measures.{p £ 0.05 between pre- and post-test.

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They also reported that the correlations between leg strengthand kick performance improved from the beginning to theend of the season in adolescent football players. In thepresent investigation, both kicking performance and verticaljump were improved with strength training. However, incontrast to De Proft et al. (1988), we did not observe a sig-nificant correlation between vertical jump and peak knee an-gular velocity during kicking in adults.

The results of the present study are consistent with pre-vious research in which vertical jump performance was im-proved following a training program combining weightlifting and plyometric exercises (Adams et al. 1992; Baueret al. 1990; Fatouros et al. 2000; Ingle et al. 2006; Lyttle etal. 1996; Moore et al. 2005). A close analysis of the effectsobserved in both types of vertical jump shows the specificityof this training program (Morrissey et al. 1995; Sale andMacDougall 1981). Training resulted in improvements ofvertical velocity at take off, vertical jumping height, max-imal instantaneous vertical velocity, and maximal instan-taneous power during the countermovement jumps, but notduring the squat jumps. This specificity of training is likelythe reason why no improvements were observed in runningspeed or cycling power (Wingate tests) with this trainingprogram. Indeed, Young et al. (2001) noted that the besttraining to improve performance in a 30 m sprint test wastraining over a similar distance without changes of direction.In soccer, part of training is usually devoted to sprinting ex-ercises; this may be reason why some studies report im-provement in sprinting performance with 4 weeks ofplyometric training (Impellizzeri et al. 2008) and with3 months of weightlifting combined with plyometric exer-cise (Moore et al. 2005). Our results combined with pre-vious studies indicate that strength training alone orcombined with plyometric exercises may fail to enhancesprint performance without adding some sprint-specifictraining.

Myosin heavy chain (MHC) isoforms determine the con-tractile and energetic properties of human muscles fibretypes (Bottinelli and Reggiani 2000). Despite the significantincrement in MHC isoform type IIa in the experimentalgroup there was no correlation between the increment inMHC isoform type IIa and the increment in the angular vel-ocity of the knee, the maximal dynamic force, or heightjumped. Our results are in agreement with those ofAndersen et al. (1994b), who reported an increase of MHCIIa and a reduction of MHC I after 3 months of training inT

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Table 5. Myosin heavy-chain isoforms.

Group MHC I* MHC IIa* MHC IIx

Pre-testTG (n = 15) 52.8±2.0 46.0±2.0 1.2±0.7CG (n = 10) 47.7±3.4 51.0±3.4 1.3±0.8

Post-testTG (n = 15) 49.9±2.0{ 49.6±1.9{ 0.5±0.2CG (n = 10) 48.1±3.4 50.9±3.4 0.8±0.5

Note: Data are expressed as mean ± SEM.*Statistically significant group � time interaction in the ANOVA

test for repeated measures.{p £ 0.05 between pre-test and post-test.

Perez-Gomez et al. 507

# 2008 NRC Canada

sprinters who used a combination of strength, plyometric,and sprint-specific exercises. Other researchers have also re-ported an increase in MHC type IIa isoform with plyometrictraining alone (Malisoux et al. 2006a) or combined withweightlifting exercises (Liu et al. 2003).

The study by Liu et al. (2003) is comparable to ours,since they also studied the effect of 6 weeks of strengthtraining on physical education students. In the latter, 12 sub-jects performed combined strength training consisting ofweight lifting and plyometric exercises, while the other 12subjects only trained with weight-lifting exercises (Liu et al.2003). The reduction in MHC type I observed in the presentinvestigation agrees with that reported by Liu et al. (2003)in the triceps brachii when strength training is combinedwith plyometric training.

In contrast to our results, Aagaard et al. (1996) and Trolleet al. (1993) did not find improvements in kicking perform-ance after knee-extension strength training. Several reasonscould explain the differences; the participants of our studywere physical education students, whereas the studies ofAagaard et al. (1996) and Trolle et al. (1993) used elite foot-ball players. It is likely that football players already possessa high kicking performance with less potential for improve-ment. The action of kicking requires a complex series ofsynergistic movements that are difficult to replicate withsimple strength-training movements (Bangsbo 1994). An im-portant difference between the studies of Aagaard et al.(1996) and Trolle et al. (1993) with respect to that of DeProft et al. (1988) is that in the latter study the football play-ers carried out strength training in addition to their ordinaryfootball training, whereas in the former studies only strengthtraining was used. Thus, it appears that combining strengthtraining with technical training involving the actual motortasks is necessary to improve kicking in professional foot-ball players. A difference between our study and those ofAagaard et al. (1996) and Trolle et al. (1993) was that theseinvestigators applied only high resistance, low resistance, orloaded kicking movements in the training process, whereaswe combined weight lifting with explosive actions (plyo-metric leg exercises) in the same session. Thus, the stimulusfor our participants may have been higher and more specificthan for the football players in the studies of Aagaard et al.(1996) and Trolle et al. (1993). A further difference betweenthese studies and ours was that our participants performedmaximal instep kicks as quickly as possible with no regardfor accuracy or direction of the kick, whereas their footballplayers shot towards a handball goal and only shots withinthe goal posts were accepted. This criterion may have lim-ited the players’ ability to use all of their kicking potential,though admittedly this should have affected the kicking per-formance pre- and post-training similarly.

LimitationsThe experimental design used did not allow us to distin-

guish between the effects of the strength and plyometriccomponents of the training program. Another limitation isthat it is uncertain if athletes with high experience instrength training or footballers would respond in the sameway as our physical education students. It remains to be de-termined what is the optimal duration and combination ofweightlifting and plyometric exercise for an optimal im-

provement of kicking performance, power, and sprintingability.

Conclusions

The present results indicate that, in physical educationstudents, 6 weeks of strength training combining weight lift-ing and plyometric exercises is associated with improve-ments in angular velocity of the knee during a kick, onerepetition maximum (1 RM) in leg extension, inclined legpress, leg curl, and half squat, and vertical velocity at take-off, height jumped, maximal instantaneous vertical velocity,and power in a countermovement jump. This training alsoelicited muscle hypertrophy, and increased and decreasedthe amount of MHC type IIa and I, respectively, within skel-etal muscle. Further studies are needed to ascertain if similarresults may be achieved in football players.

AcknowledgementsThe authors wish to thank Jose Navarro de Tuero for his

excellent technical assistance. The specialized advice fromTony Webster in editing the English version of the manu-script is also acknowledged. This study was supported bygrants from the Ministerio de Educacion y Ciencia(BFI2003-09638, BFU2006-13784 and FEDER) and the Go-bierno de Canarias (PI2005/177). Special thanks are given toall subjects who volunteered for these experiments.

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