relaçoes de forca e estrutura MMII e MMSS

RELATIONSHIPS BETWEEN LOWER-BODY MUSCLE

STRUCTURE AND LOWER-BODY STRENGTH, POWER,AND MUSCLE-TENDON COMPLEX STIFFNESS

JOSH L. SECOMB,1,2 LINA E. LUNDGREN,1,2 OLIVER R.L. FARLEY,1,2 TAI T. TRAN,1,2

SOPHIA NIMPHIUS,2 AND JEREMY M. SHEPPARD1,2

1Strength and Conditioning and Sport Science Department, Hurley Surfing Australia High Performance Center, CasuarinaBeach, Western Australia, Australia; and 2Center for Exercise and Sport Science Research, Edith Cowan University,Joondalup, Perth, Australia

ABSTRACT

Secomb, JL, Lundgren, LE, Farley, ORL, Tran, TT, Nimphius, S,

and Sheppard, JM. Relationships between lower-body muscle

structure and lower-body strength, power, and muscle-tendon

complex stiffness. J Strength Cond Res 29(8): 2221–2228,

2015—The purpose of this study was to determine whether any

relationships were present between lower-body muscle structure

and strength and power qualities. Fifteen elite male surfing ath-

letes performed a battery of lower-body strength and power

tests, including countermovement jump (CMJ), squat jump (SJ),

isometric midthigh pull (IMTP), and had their lower-body muscle

structure assessed with ultrasonography. In addition, lower-body

muscle-tendon complex (MTC) stiffness and dynamic strength

deficit (DSD) ratio were calculated from the CMJ and IMTP.

Significant relationships of large to very large strength were

observed between the vastus lateralis (VL) thickness of the left

(LVL) and right (RVL) leg and peak force (PF) (r = 0.54–0.77,

p , 0.01–0.04), peak velocity (PV) (r = 0.66–0.83, p , 0.01),

and peak jump height (r = 0.62–0.80, p , 0.01) in the CMJ and

SJ, as well as IMTP PF (r = 0.53–0.60, p = 0.02–0.04). Further-

more, large relationships were found between left lateral gastroc-

nemius (LG) pennation angle and SJ and IMTP PF (r = 0.53, p =

0.04, and r = 0.70, p, 0.01, respectively) and between LG and

IMTP relative PF (r = 0.63, p = 0.01). Additionally, large relation-

ships were identified between lower-body MTC stiffness and

DSD ratio (r = 0.68, p , 0.01), right (LG) pennation angle (r =

0.51, p = 0.05), CMJ PF (r = 0.60, p = 0.02), and jump height

(r = 0.53, p = 0.04). These results indicate that greater VL thick-

ness and increased LG pennation angle are related to improved

performance in the CMJ, SJ, and IMTP. Furthermore, these re-

sults suggest that lower-body MTC stiffness explains a large

amount of variance in determining an athlete’s ability to rapidly

apply force during a dynamic movement.

KEY WORDS jumping, muscle thickness, pennation angle,

correlations

INTRODUCTION

Lower-body strength and power are key physicalqualities that underpin performance in a multitudeof sports (26,28). It has been well established thatthe force-generating capacity of muscle is highly

influenced by the structural arrangement of the fascicles(1,19). Furthermore, it has been noted that the interactionbetween lower-body muscle and tendon structures largelydetermines an athlete’s ability to express power duringa stretch-shortening cycle (SSC) activity (11,21). As musclestructure demonstrates large plasticity, and therefore is highlyadaptive to training, understanding the relationships betweenmuscle and tendon structures and physical performance is ofgreat importance to strength and conditioning practitioners(1,8,23). Because of this, it is necessary to first identify thespecific lower-body muscle structures, which may be relatedto strength and power variables of interest, through cross-sectional analyses. Identification of the muscle structures thatmay be related to more highly developed lower-bodystrength and power qualities may greatly assist practitionerswith talent identification and to provide a basis of rationalefor longitudinal studies. Such studies would allow for deter-mination of whether changes in the muscle structure lead toenhancements in strength and power (7).

Previous cross-sectional studies have identified that thereare significant relationships between specific lower-bodymuscle structures and performance in the back squat,countermovement jump (CMJ), and squat jump (SJ).Research by both Brechue and Abe (3) and Nimphiuset al. (23) have reported significant relationships betweenvastus lateralis (VL) thickness and 1 repetition maximum(1RM) back squat (r = 0.82, p , 0.01) or relative 1RM backsquat (r = 0.57). Although these studies suggest that greater

Address correspondence to Josh L. Secomb, [email protected].

29(8)/2221–2228

Journal of Strength and Conditioning Research� 2015 National Strength and Conditioning Association

VOLUME 29 | NUMBER 8 | AUGUST 2015 | 2221

Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.

lower-body strength, as measured with a 1RM squat, isrelated to increased thickness in the VL, no research to datehas investigated whether similar relationships are present withthe isometric midthigh pull (IMTP). Additionally, it has beenidentified that better performance in the CMJ and SJ wasrelated to decreased fascicle length and greater pennation ofthe lower-body musculature (8). Earp et al. (8) reported thatin 25 resistance-trained male subjects, lateral gastrocnemius(LG) muscle structure could predict jumping ability. Althoughsignificant relationships have been reported between LG pen-nation angle and jumping performance (8), it is yet to bedetermined whether similar relationships exist within eliteathletic populations. Once it is established whether specificlower-body muscle structures are related to performance inthe CMJ, SJ, and IMTP in elite athletic populations, longitu-

dinal analyses can be performed to determine the influence oftraining-specific adaptations in lower-body muscle structureon strength and power qualities.

Fukashiro et al. (11) suggested that the fascicles are theforce generator of the muscle-tendon unit, whereas the ten-don structures act as an energy redistributor and a poweramplifier. It has previously been noted that in better per-formers, the LG will typically perform a powerful concentricor isometric contraction during the eccentric phase of anSSC (11). This allows a larger magnitude of elastic strainenergy to be developed within the tendon, because of thereduced deformation of the muscle, and hence increasedtendon deformation (7,11). Fukashiro et al. (11) indicatedthat the tendons’ ability to effectively store and redistributethis energy will largely determine performance in the CMJ.As a result, the stiffness of the lower-body muscle-tendoncomplex (MTC) is of particular importance to practitionersbecause this may greatly influence the magnitude of elasticstrain energy that can be stored and released during an SSC(10). Understanding the muscle structures that may berelated to lower-body MTC stiffness and the amount of ex-plained variance this stiffness has on strength and powerqualities may provide highly useful information to practi-tioners and coaches who work with athletes requiring highlevels of lower-body power.

Although it has been identified that specific musclestructures are related to performance in the squat, CMJ andSJ, in resistance-trained males, to our knowledge no researchto date has determined whether similar relationships arepresent in elite male athletes and with the IMTP. Further-more, no previous research has investigated the relationshipsbetween lower-body muscle structure and lower-body MTCstiffness. As a result, it is necessary to identify any relationshipsthat may be present between lower-body muscle structureand strength and power qualities, as well as with themechanical properties of the lower-body. This will providestrength and conditioning practitioners and coaches witha sound rationale for talent identification and an understand-ing of the muscle structures related to an enhanced expressionof lower-body strength and power.

METHODS

Experimental Approach to the Problem

The purpose of this study was to determine whether anysignificant relationships were present between lower-bodymuscle structure and lower-body strength and power quali-ties. Furthermore, it was also an aim to identify any relation-ships that were present between the lower-body strength andpower variables measured within this study. This studyinvolved a cross-sectional analysis, whereby subjects wererequired to have their lower-body muscle structure assessedusing ultrasonography, before completing a battery of lower-body strength and power tests that were all completedduring a single session. Strength and power tests includedthe CMJ, SJ, and IMTP.

Figure 1. Ultrasound images of the vastus lateralis. Measurements formuscle thickness were taken from the deep to superficial aponeurosis,and pennation angle (u) was calculated between the deep aponeurosisand the line of the fascicles.

Figure 2. Sagittal ultrasound images of the lateral gastrocnemius.Measurements for muscle thickness were taken from the deep tosuperficial aponeurosis, and pennation angle (u) was calculated betweenthe deep aponeurosis and the line of the fascicles.

Relationships Within Lower-Body Qualities

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Subjects

Fifteen elite competitive male (21.7 6 5.0 years; 176.6 6 5.5cm; 71.0 6 8.5 kg) surfing athletes participated in this study.Inclusion criteria involved the following: (a) actively com-peting at an international level, (b) aged 16–35 years, and (c)currently free of any injury or medical condition, as pera health screening questionnaire. The study and procedureswere approved by the Edith Cowan University HumanEthics Committee (approval number: 10228) and were con-ducted according to the Declaration of Helsinki. All partic-ipants were provided with information detailing the studybefore providing informed consent and were screened formedical contraindications before participation. Age rangeof subjects was 16.4 years to 31.0 years. In the event that

a subject was under 18 years of age a parent or guardiansigned the informed consent form.

Procedures

Ultrasound. Muscle structure was measured with real-timeB-mode ultrasonography (SSD-1000; Aloka Co., Tokyo,Japan) with a 7.5-MHz linear probe (17,18,21). A water-soluble gel was placed on the probe to allow for acousticcontact, with no depression of the dermal layer (4). After atleast 10 minutes of sitting in a chair, to allow for a fluid shift,each subject was placed in a supine position, with the legsresting on a bench, to measure the muscle thickness andpennation angle of the VL. Measures of the VL were as-sessed at 50% of the distance between the greater trochanterand lateral epicondyle of the femur (8,23). To measure theLG, subjects were in a prone position, with legs fullyextended on the bench, and the probe placed over the LG.Measures of the LG were assessed at two-thirds of the dis-tance between the lateral epicondyle of the femur and thelateral malleolus (8,23). Fascicle length of the VL and LGwere calculated from the equation reported by Fukunagaet al. (12) (fascicle length = muscle thickness 3 [sin penna-tion angle]21). Two images were recorded of the VL and LGfrom the left (LVL and LLG, respectively) and right (RVLand RLG, respectively) legs of each subject (Figures 1 and 2).One image was used to assess muscle thickness with theother for pennation angle and fascicle length. All imageswere analyzed using a free public imaging software (ImageJ1.40g; National Institutes of Health, Bethesda, MD, USA).Each image was assessed 3 times, and the average value formuscle thickness, pennation angle, and fascicle length fromthe 6 measures were used for analysis (4). Furthermore, theintraclass correlation coefficient (ICC) and coefficient ofvariation percent (CV%) were 0.99–1.00 and 0.8–1.2% forLG muscle thickness, 0.87–0.91 and 6.3–6.5% for LGpennation angle, 1.00–1.00 and 0.8–0.9% for VL musclethickness, and 0.94–0.96 and 5.3–6.0% for VL pennationangle, respectively.

Lower-Body Strength and Power. After a 10-minute whole-body warm-up, consisting of squats, lunges, and dynamicmobility movements, subjects completed the physical testing

TABLE 1. Mean (6SD) recorded values for allstrength and power variables of the group(n = 15).

Elite male surfers(n = 15)

CMJPeak force (N) 1,745 6 383Peak velocity (m$s21) 2.96 6 0.21Peak height (m) 0.57 6 0.08Stiffness (N$m) 3,848 6 884

SJPeak force (N) 1,458 6 228Peak velocity (m$s21) 3.03 6 0.24Peak height (m) 0.48 6 0.06Eccentric utilization ratio(CMJ height/SJ height)

1.18 6 0.10

IMTPPeak force (N) 2,407 6 540Relative force (N$BW21) 3.4 6 0.5Dynamic strength deficit ratio(CMJ peak force/IMTP peakforce)

0.75 6 0.10

CMJ = countermovement jump; SJ = squat jump;IMTP = isometric midthigh pull.

TABLE 2. Mean (6SD) recorded values for all muscle structure measures of the group (n = 15).

VL LG

Muscle thickness(cm)

Pennationangle (8)

Fascicle length(cm)

Muscle thickness(cm)

Pennationangle (8)

Fascicle length(cm)

Left 2.28 6 0.40 18.67 6 3.98 7.44 6 2.28 1.49 6 0.18 13.95 6 2.62 6.37 6 1.55Right 2.28 6 0.44 18.60 6 3.21 7.26 6 1.59 1.55 6 0.16 14.42 6 2.41 6.38 6 1.21

VL = vastus lateralis; LG = lateral gastrocnemius.

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in the following order: CMJ, SJ, and IMTP. To perform theCMJ, subjects were required to stand in an upright positionon a portable force plate (400 Series Performance ForcePlate; Fitness Technology, Adelaide, Australia) witha wooden dowel placed across their backs. The force platewas connected to a portable laptop, running an analysissoftware package (Ballistic Measurement System; FitnessTechnology), and sampled at 600 Hz. Subjects performed 3trials of the CMJ, from a self-selected depth, with instruc-tions to jump as high and quickly as possible (31). Subjectswere then required to perform 3 trials of the SJ. Subjects heldthe wooden dowel across their upper back, with a linearposition transducer (PT9510; Fitness Technology) attachedto the dowel. Subjects were instructed to be in a position,whereby the top of thighs were parallel with the ground, andwere required to hold the position for 3 seconds beforejumping as high as possible on the command “go”(15,22,29). The linear position transducer was interfacedwith the portable force plate and was attached to the porta-ble laptop running the analysis software package. Each sub-ject’s best trial was used for analysis. The best trial for theCMJ and SJ were determined by the vertical jump height.Additionally, for the SJ, trials were omitted in the event ofa small amplitude countermovement of greater than 2 cm, asobserved by the displacement-time trace on the analysissoftware package (15,29). All jumps were analyzed for the

following variables: peak force (PF), peak velocity (PV), andjump height. The ICC and CV% for these variables of theCMJ and SJ were as follows: PF (0.98 and 3.9%, and 0.97 and2.1%, respectively), PV (0.99 and 1.0%, and 0.92 and 3.2%,respectively), and jump height (0.94 and 4.0%, and 0.82 and6.8%, respectively). Furthermore, a measure reflective oflower-body MTC stiffness was calculated from the CMJ,with the equation of Fpeak/DL, whereby Fpeak is the peakground reaction force and DL is the vertical displacement ofthe center of mass (9,25,30). The ICC and CV% for lower-body MTC stiffness was 0.96 and 4.5%, respectively. Addi-tionally, to discriminate the effect of the SSC, the eccentricutilization ratio (EUR) was determined with the followingequation: EUR = CMJ jump height/SJ jump height (22).

To perform the IMTP, subjects were required to stand onthe portable force plate, gripping a customized pull rack,with their shoulders placed over the bar, in a position similarto that of the second pull of a power clean (13). Subjectsperformed 2 trials of the IMTP, with 2 minutes of restbetween each trial. In the event of a difference in the PFbetween the 2 trials of greater than 250 N, a third trial wasperformed (20). Knee angle was measured with a goniometerto ensure a range of 125–1408 (14), with subjects instructedto push as hard as possible into the force plate (27,31). Thislarge range in knee angle was used to account for individualdifferences in relative limb length, with Comfort et al. (5)

TABLE 3. Correlation coefficients (r) (90% CI), explained variance, and interpreted strength of relationships betweenLVL and RVL thickness, and PF, PV, jump height and rPF for the CMJ, SJ, and IMTP.

Variable 1 Variable 2 rExplained

variance (%) p90% CIlower

90% CIupper

Strength ofrelationship

LVLthickness

CMJ PF 0.54 29 0.04 0.13 0.96 Large

CMJ PV 0.66 43 ,0.01 0.29 1.00 LargeCMJ jumpheight

0.63 39 0.01 0.24 1.00 Large

SJ PF 0.77 60 ,0.01 0.46 1.00 Very largeSJ PV 0.83 69 ,0.01 0.56 1.00 Very largeSJ jump height 0.72 51 ,0.01 0.37 1.00 Very largeIMTP PF 0.53 28 0.04 0.11 0.94 LargeIMTP rPF 0.24 — 0.40 — — Small

RVLthickness

CMJ PF 0.63 40 0.01 0.25 1.00 Large

CMJ PV 0.81 65 ,0.01 0.51 1.00 Very largeCMJ jumpheight

0.80 64 ,0.01 0.51 1.00 Very large

SJ PF 0.75 56 ,0.01 0.42 1.00 Very largeSJ PV 0.78 60 ,0.01 0.47 1.00 Very largeSJ jump height 0.70 49 ,0.01 0.35 1.00 Very largeIMTP PF 0.60 36 0.02 0.21 0.99 LargeIMTP rPF 0.36 — 0.19 — — Moderate

CI = Confidence interval; LVL = left vastus lateralis; RVL = left vastus lateralis; PF = peak force; PV = peak velocity; rPK = relativepeak force; CMJ = countermovement jump; SJ = squat jump; IMTP = isometric midthigh pull.




recently identifying that changes in knee angle do not sig-nificantly alter PF. The force plate was connected to a por-table laptop, running the analysis software package, andsampled at 600 Hz. Each subjects’ best trial, as determinedby the trial with the highest PF, was used to determine PFand relative PF (rPF) (N$BW21). The ICC and CV% for PFfor this cohort were 0.97 and 5.4%, respectively. To reflect anathlete’s ability to rapidly apply force during a dynamic

movement, relative to their maximal force capacity, thedynamic strength deficit (DSD) ratio was calculated usingthe following formula: DSD = CMJ PF/IMTP PF (32).

Statistical Analyses

Mean and SD were reported for all muscle structure measuresand lower-body strength and power variables (Tables 1 and 2).Normality of data was assessed with the Shapiro-Wilk statistic.

TABLE 4. Correlation coefficients (r) (90% CI), explained variance, and interpreted strength of relationships betweenLLG and RLG pennation angle, and PF, PV, jump height, and rPF for the CMJ, SJ, and IMTP.

Variable 1 Variable 2 rExplained

variance (%) p90% CIlower

90% CIupper

Strength ofrelationship

LLGpennation

CMJ PF 0.38 — 0.17 — — Moderate

CMJ PV 0.63 40 0.01 0.25 1.00 LargeCMJ jumpheight

0.51 — 0.06 — — Large

SJ PF 0.53 28 0.04 0.12 0.95 LargeSJ PV 0.28 — 0.31 — — SmallSJ jump height 0.33 — 0.23 — — ModerateIMTP PF 0.70 48 ,0.01 0.34 1.00 Very largeIMTP rPF 0.63 40 0.01 0.25 1.00 Large

RLGpennation

CMJ PF 0.33 — 0.23 — — Moderate

CMJ PV 0.39 — 0.15 — — ModerateCMJ jumpheight

0.24 — 0.39 — — Small

SJ PF 0.32 — 0.24 — — ModerateSJ PV 0.26 — 0.34 — — SmallSJ jump height 20.01 — 0.97 — — TrivialIMTP PF 0.26 — 0.35 — — Small

CI = Confidence interval; LLG = left lateral gastrocnemius; RLG = right lateral gastrocnemius; PF = peak force; PV = peakvelocity; rPK = relative peak force; CMJ = countermovement jump; SJ = squat jump; IMTP = isometric midthigh pull.

Figure 3. Relationship between isometric midthigh pull (IMTP) peakforce and peak jump height in the countermovement jump (CMJ).

Figure 4. Relationship between isometric midthigh pull (IMTP) peakforce and peak jump height in the squat jump (SJ).


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In the event of the assumption of normality being violated,the data was log-transformed for analysis. Pearsonproduct-moment correlation coefficients (r) were per-formed on all measures to identify whether any significantrelationships were present between the muscle structuremeasures and lower-body strength and power variables, aswell as within the strength and power variables. To inter-pret the magnitude of relationships, the strength of thePearson correlation coefficients were classified as 0.0–0.1(trivial), 0.1–0.3 (small), 0.3–0.5 (moderate), 0.5–0.7(large), 0.7–0.9 (very large), and 0.9–1.0 (near perfect)(16). Furthermore, the coefficient of determination (r2)was calculated for all significant relationships to demon-strate the explained variance. Additionally, 90% confi-dence intervals were calculated for all statisticallysignificant relationships. All statistical analyses were

performed using a statistical analysis package (SPSS, ver-sion 22.0; IBM, Chicago, IL, USA), with statistical signif-icance set at p # 0.05.

RESULTS

Significant relationships were identified between LVL andRVL thickness and PF, PV, and jump height in the CMJ andSJ, as well as with PF in the IMTP (Table 3). Similarly,significant relationships were found between LLG pennationangle and PV in the CMJ, PF in the SJ, and PF and relativePF in the IMTP (Table 4). However, no significant relation-ships were identified between RLG pennation angle and anylower-body strength and power variable (Table 4). Further-more, IMTP PF demonstrated a very large relationship withPF in the CMJ (r = 0.76, r2 = 0.57, p , 0.01) and SJ (r = 0.81,r2 = 0.65, p , 0.01), and large relationships with jump heightin the CMJ and SJ (p , 0.01 and p = 0.02, respectively)(Figures 3 and 4). Additionally, lower-body MTC stiffnessexhibited large relationships with DSD ratio (p , 0.01),RLG pennation angle (p , 0.01) (Figures 5 and 6), PF(r = 0.60, r2 = 0.36, p = 0.02), and jump height (r = 0.53,r2 = 0.28, p = 0.04) in the CMJ.

DISCUSSION

The purpose of this study was to determine whether anysignificant relationships were present between specificlower-body muscle structures and lower-body strengthand power qualities, as well as within the strength andpower qualities. The results of this study indicate that VLthickness of both the left and right leg was significantlyrelated to performance in the CMJ, SJ and IMTP.Furthermore, LLG pennation angle exhibited significantrelationships with SJ and IMTP PF, and IMTP rPF.Additionally, lower-body MTC stiffness was significantlyrelated to DSD ratio, RLG pennation angle, and PF andjump height in the CMJ.

To the best of our knowledge, this is the first study toreport on the relationships between lower-body musclestructure and performance in the IMTP. Although previousresearch has identified that VL thickness demonstratessignificant relationships with lower-body strength, throughperformance in the squat, it is yet to be reported whethersimilar relationships were present with the IMTP (3,23). Theresults of this study indicate that increased thickness inthe VL is related to greater PF and rPF production duringthe IMTP. It has been well established that the maximalforce that can be produced by a muscle is determined bythe activity of the subunits of the muscle, namely the musclefibers, sarcomeres, and myofibrils (33). Therefore, it is pro-posed that larger thickness of the VL muscles reflects greathypertrophy of the extensors in general and thereby allowsfor a greater production of force, because of a potentiallygreater number of actin and myosin filaments within themuscle (33). This would allow for increased cross-bridgingwithin the muscle fibers, and hence, it is apparent that larger

Figure 6. Relationship between lower-body muscle-tendon complex(MTC) stiffness and right lateral gastrocnemius (RLG) pennation angle.

Figure 5. Relationship between lower-body muscle-tendon complex(MTC) stiffness and the dynamic strength deficit (DSD) ratio.




muscles would be capable of producing greater forces whencompared with smaller muscles (33).

Interestingly, VL thickness was also significantly relatedto all performance variables (PF, PV, jump height) of theCMJ and SJ. Furthermore, the results of this study identifiedthat PF in the IMTP was significantly related to PF, PV,and jump height in the CMJ, as well as PF and jump heightin the SJ. This suggests that the subjects with greatermaximal lower-body strength, as measured with the IMTP,were capable of producing better performances in the CMJand SJ. It appears likely that this is part due to increasedmuscle thickness, allowing for a greater production of force,directly influencing the isometric tests and underpinningperformance in the dynamic tests of power. This isevidenced by VL thickness explaining approximately 30%of the variance in IMTP PF, and 35 and 58% of the variancein CMJ PF and SJ PF, respectively. These results are inagreement with previous research that suggests that lower-body strength underpins lower-body power in a range ofsport relevant activities (6,24).

The LLG pennation angle exhibited significant relation-ships with PV in the CMJ, PF in the SJ, and PF and rPF in theIMTP. The CMJ and SJ data agree with that of Earp et al. (8),which identified that a significant amount of variance wasexplained by LG pennation angle for CMJ jump height andrelative power (b = 0.47, r2 = 0.19, p = 0.02; b = 0.77, r2 =0.42, p , 0.01, respectively) and SJ jump height and relativepower (b = 0.46, r2 = 0.21, p = 0.02; b = 0.42, r2 = 0.17, p =0.03, respectively). However, this is the first time relationshipshave been identified between LG pennation angle and perfor-mance in the IMTP. Furthermore, LLG pennation angle ex-plained 48% of the variance in IMTP PF and 40% in IMTPrPF. Although it is was surprising that significant relationshipswere not also identified between RLG pennation angle andstrength and power variables, it is important to note thatexcept for 2 athletes, all the subjects performed surfing witha “natural” stance (left foot forward). As such, this indicatesthat for most subjects in this study, their dominant foot wastheir left foot, which may help explain the disparities is therelationships between the left and right leg.

It has previously been reported that larger pennationangles within a muscle allow for a greater physiologicalcross-sectional area (PCSA) (17,19). Because of the higherPCSA, there is a greater concentration of muscle subunits,and hence an increase in the maximal magnitude of forcethat can be produced (8,19). In combination with the rela-tionships identified between VL thickness and CMJ, SJ, andIMTP performance, this study suggests that the athletes withgreater thickness in the VL and increased pennation in theLG exhibit higher levels of lower-body strength and power.These findings, in combination with the previous researchthat has shown that changes in VL thickness explains 64% ofchanges in speed performance in highly trained athletes (23),indicate that future research should determine whetherincreases in VL thickness and LG pennation angle can also

transfer to associated improvements in CMJ, SJ, and IMTPperformance.

Lower-body MTC stiffness was identified to exhibitsignificant relationships with DSD ratio, RLG pennationangle, and PF and jump height in the CMJ. Interestingly, thisstudy is the first to identify significant relationships betweenlower-body MTC stiffness and DSD ratio. Furthermore, thedata of this study suggest that 46% of the variance in DSDratio is explained by lower-body MTC stiffness. Thisindicates that athletes with greater lower-body MTC stiff-ness have the ability to use a greater proportion of theirmaximal isometric force during a dynamic movement. Incombination with the relationships identified between LGpennation angle and performance in the CMJ, this studyprovides further support to research that has suggested thatthe performance of dynamic lower-body activities arestrongly related to the stiffness of the lower-body muscula-ture (2). Although the concept that lower-body stiffnessunderpins performance in dynamic lower-body movementsis not novel, to the best of our knowledge, this is the firststudy to report relationships between these variables usingthe equations presented in this study.

Importantly, the relationship between RLG pennationangle and lower-body MTC stiffness increasingly supportsthe data suggesting that increases in pennation angle result ingreater passive resistance, and therefore increased stiffness andisometric-like qualities during lengthening (8). It is well estab-lished that during a CMJ, the tendon produces and stores themajority of the elastic strain energy. The greater the inherentstiffness of the LG muscle, the more deformation is producedwithin the Achilles tendon during an SSC. When there is anincrease in the tendon deformation, there is greater produc-tion, storage, and redistribution of elastic strain energy (7,11).Therefore, it is proposed that increases in LG pennation anglemay underpin the relationships between MTC stiffness, DSDratio, and CMJ performance. Future research should aim toidentify whether training-specific adaptations in LG penna-tion angle and lower-body MTC stiffness are related tochanges in DSD ratio and CMJ performance.

Although a limitation of this study is the small sample size,the identification of the muscle structures that are related toa greater expression of lower-body strength and powerqualities in this study should still assist practitioners withtalent identification and to provide a sound basis of for futuretraining studies. Such training studies should investigatewhether changes in these specific muscle structures areassociated with a concomitant change in the strength andpower qualities. Furthermore, as a result of the significantrelationships between muscle structure variables and lower-body MTC stiffness, changes in DSD with training should befurther investigated. These analyses would provide strengthand conditioning practitioners with the ability to prescribeeffective training programs and to evoke specific structuralchanges, as opposed to merely mimicking movementvelocities and patterns (7).


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PRACTICAL APPLICATIONS

The results of this study suggest that greater thickness in theVL and increased pennation in the LG muscles may berelated to improved performance in the CMJ, SJ, and IMTP.It is proposed that together, these specific structures are theresult of increased hypertrophy within the muscles, whichimprove the force producing capabilities. The stiffness of thelower-body MTC was related to DSD ratio, RLG pennationangle, and CMJ performance. This indicates that increasedpennation in the RLG appears to be related to greater lower-body MTC stiffness, which allows the athlete to applya greater magnitude of force in a dynamic movement, inrelation to their maximal strength.

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