blazevich 2006

Sports Med 2006; 36 (12): 1003-1017LEADING ARTICLE 0112-1642/06/0012-1003/$39.95/0

2006 Adis Data Information BV. All rights reserved.

Effects of Physical Training andDetraining, Immobilisation, Growthand Aging on HumanFascicle GeometryAnthony J. Blazevich

Centre for Sports Medicine and Human Performance, Brunel University, Uxbridge,Middlesex, UK

In addition to its size and the extent of its neural activation, a muscle’sAbstractgeometry (the angles and lengths of its fibres or fascicles) strongly influences itsforce production characteristics. As with many other tissues within the body,muscle displays significant plasticity in its geometry. This review summarisesgeometric differences between various athlete populations and describes researchexamining the plasticity of muscle geometry with physical training, immobilisa-tion/detraining, growth and aging. Typically, heavy resistance training in youngadults has been shown to cause significant increases in fascicle angle of vastuslateralis and triceps brachii as measured by ultrasonography, while high-speed/plyometrics training in the absence of weight training has been associated withincreases in fascicle length and a reduction in angles of vastus lateralis fascicles.These changes indicate that differences in geometry between various athleticpopulations might be at least partly attributable to their differing training regimes.Despite some inter-muscular differences, detraining/unloading is associated withdecreases in fascicle angle, although little change was shown in muscles such asvastus lateralis and triceps brachii in studies examining the effects of prolongedbed rest. No research has examined the effects of other interventions such asendurance or chronic stretching training. Few data exist describing geometricadaptation during growth and maturation, although increases in gastrocnemiusfascicle angle and length seem to occur until maturation in late adolescence.Although some evidence suggests that a decrease in both fascicle angle and lengthaccompanies the normal aging process, there is a paucity of data examining theissue; heavy weight training might attenuate the decline, at least in fascicle length.A significant research effort is required to more fully understand geometricadaptation in response to physical training, immobilisation/detraining, growth andaging.

1004 Blazevich

Historically, changes in human fascicle geome- changes after training and detraining interventionsand/or aging and development.try1 have been impossible to examine since the

excision and subsequent study of muscle could onlybe performed in cadavers. More recently, imaging

1. Geometry of Human Musclestechniques such as magnetic resonance (MRI)[1] andultrasound[2,3] imaging (figure 1) have allowed the invivo examination of geometric, i.e. fascicle length Many human muscles contain fascicles that doand angle, changes.[2,4-8] Results of such research not run directly from origin to insertion, but arehave predominately shown geometry to be highly angled and attach to the muscles’ aponeuroses (fig-changeable. An understanding of how different ure 2). These ‘pennate’ muscles can have complexforms of muscle loading, or exercise training, affects fascicular arrangements that differ markedly be-geometry would allow us to bring about deliberate tween muscles. Their fascicle arrangement is proba-and specific changes that would influence the mus- bly a greater determinant of general muscle functioncles’ force-generating properties. There are, howev- than other aspects such as fibre type.[9] As a generaler, a limited number of studies that have examined rule, muscles with large fascicle attachment anglesthe plasticity of fascicle geometry, or that have and correspondingly short fascicles are suited tocompared populations of individuals who have per- contractions involving high forces[10] and/or pro-formed different exercise training. The present reduce forces over a relatively short range of mo-view summarises research that quantifies changes in tion.[11] Examples include the large proximal mus-fascicle geometry with exercise and detraining, and cles of the legs (e.g. vastus lateralis; see table I),development and aging. A greater understanding of smaller muscles such as those involved in mastica-

tion,[10] and those involved in postural control suchfascicle geometric adaptation could inform bothas soleus and tibialis posterior. Large fascicle anglesphysical performance and rehabilitative trainingare also seen in muscles that attach to long tendons,programmes, as well as allowing the development ofsuch as gastrocnemius lateralis, which produce suf-muscle models that predict physical performanceficient force to remain relatively isometric duringthe propulsive phase of stretch-shorten contractionsin walking,[12-14] running[12,14] and jumping with[15,16]

and without[12,14,17] countermovement, and allow op-timum elongation and recoil of the tendon.[12] Mus-cles that commonly participate in movements re-quiring significant length change or a high shorten-ing velocity tend to be characterised by longfascicles attaching at relatively small angles to thetendon.[9,18] Examples include adductor magnus andlongus, whose longer fascicles attaching at smallerangles allows force generation over a large range ofmotion at the hip during flexion/adduction tasks,and the topologically similar long head of bicepsfemoris, which is highly active during sprint run-ning.[19-21]

b

ACSA

a

α

θβ

Muscle length

lf = ∆a − b

PCSA

Fig. 1. Muscle architectural parameters include: fibre length = dis-tance between ends of a fibre (a to b); pennation angle (θ) =fascicle angle (relative to the aponeurosis [α]) minus the aponeuro-sis angle (relative to the tendon [β]); muscle length; and anatomical(ACSA) or physiological (PCSA) cross-sectional area. The PCSAcan be calculated as (V/t) • sinθ for a simple, uni-pennate muscle,where V is the muscle volume, t is the muscle thickness from oneaponeurosis to the other, and θ is the pennation angle. In morecomplex muscles, PCSA is calculated as V/ lf • cosθ where lf is themean fibre/fascicle length.

1 The term ‘fascicle geometry’ as used here describes the angulation and length of muscle fascicles. The broader term‘muscle architecture’ will be reserved for the description of the whole muscle structure including fascicle geometry,muscle length and muscle volume (or physiological cross-sectional area).

2006 Adis Data Information BV. All rights reserved. Sports Med 2006; 36 (12)

Plasticity of Fascicle Geometry 1005

2. Effects of Geometry onForce Production

2.1 Fibre Length

Muscle-specific geometric differences can be ra-tionalised by considering the effects of differentfascicle arrangements on force production. Musclescontaining long fascicles would produce forces overlarge length ranges and at high shortening speedsbecause they have a large number of simultaneouslycontracting, serially arranged sarcomeres. Moreo-ver, since the shortening speed of each sarcomere ina fibre or fascicle would be slower for a given speedof whole-fibre shortening when there are moresarcomeres in series, sarcomere force would notdecrease as rapidly as fibre-shortening speeds in-crease, according to the force-velocity relationship.Therefore, at high shortening velocities, longer fas-cicles are capable of generating greater force. None-theless, the increase in sarcomere number wouldincrease the energy cost of force production, sinceforce output is not improved with the increase inenergy-consuming sarcomeres. Indeed, it is theoreti-cally possible that energy consumption per sarcom-ere is increased due to the absorption of energy byneighbouring sarcomeres in protein structures suchas titin,[23-26] cross-bridges,[27,28] and actin and myo-sin filaments,[29-31] as well as by the re-arrangementof the z-band lattice,[32] which has been shown to

100

m

Tendon/aponeurosis

Fascicleforce

θ = 5°

θ

Tend

on fo

rce

(% m

uscl

e fo

rce)

80

θ = 5° θ = 25°

θ = 45°

60

40

20

05 15

Fascicle angle (°)25 35 45 55

m

θ = 25°

θ

m

θ = 45°

θ

Fig. 2. The effect of fascicle angle on the quantity of force directedalong the tendon axis. As fascicle angle (θ) increases, the propor-tion of fibre force directed along the tendon decreases (tendonforce = sum of fibre forces × cos[fibre angle]), where the fibre forceis represented by the arrow attached to the tendon/aponeurosis.The tendon is attached to a mass (m) representing the inertia of thesystem on which the muscle-tendon complex does work. The effectof fascicle angulation on the proportion of force directed along thetendon is minimal when fascicle angle is moderate (e.g. <25°), butincreases non-linearly as fascicle angle increases, as shown in thegraph.

extend by approximately 20% in tetanised fibres.[33]

Thus, muscle containing longer fascicles produce angulation will have a greater physiological cross-force over long length ranges and at high shortening section and thus a greater force-generating capaci-speeds, but the relative energy cost of force produc- ty.[34] Essentially, this fascicular geometry allows ation is high. greater amount of contractile tissue to attach to a

given area of tendon or aponeurosis.[7] Second, fas-2.2 Fibre Angle cicle angulation probably increases force by al-

lowing fibres to operate closer to their optimumWhile increases in fascicle angulation, or penna-length. Fibres in pennate muscles rotate as theytion, reduces the proportion of fibre force directedshorten[35] so tendon excursion is greater than thealong the tendon (tendon force = sum of fibre forcesshortening distance of the individual fibres. Accord-× cos[fibre angle]), this effect is minimal whening to the length-tension relationship, there will befascicle angle is moderate (e.g. <25°), as shown inan optimum sarcomere length at which fibres pro-figure 2. However, angulation probably improvesduce their greatest active force. Since optimumforce generation in three main ways. First, for the

same muscle volume, a muscle with larger fascicle sarcomere length seems to occur at lengths where


1006 Blazevich

Table I. Examples of fascicle angles and lengths in human muscles (average of two cadavers)[22]

Force characteristics Action Muscle Fascicle angle Fascicle length(°) (mm)

High Force generation Vastus lateralis 13 80.3

Prolonged (time) Postural control Soleus 32 30.3

Tibialis posterior 19 28.6

Rapid/high Stretch-shorten Gastrocnemius lateralis 17.5 60.6

High velocity/large range of motion Long-range force Adductor magnus 4 113.0

Adductor longus 3.5 94.2

Eccentric/large range of motion Long-range force Biceps femoris (long head) 7 72.6

the highest forces are required (rather than at the accepted, it fails to take into account changes inmuscle’s resting length for example; Herring et fascicle geometry as a possible contributing factor toal.[36] [human]; Lutz and Rome[37,38] [frog]), fibres strength adaptation. Since fascicle geometry signifi-that shorten less for a given tendon excursion are cantly impacts on force generation, loading-depen-likely to stay closer to their optimum for force dent geometric changes would influence force pro-generation. Third, since the time-dependent shorten- duction and ultimately influence movement capaci-ing distance of a fibre decreases during a contrac- ty. There are two ways in which informationtion, the shortening speed will also decrease (v = d/t, regarding changes in fascicle geometry can bewhere v = velocity of shortening, d = fibre displace- gained, one is to examine differences between popu-ment and t = time) and force increases as per the lations who perform different training regimes, andforce-velocity relationship for muscle. So in pennate another is to measure its changes in response tomuscle, a more forceful contraction might be possi- training.ble from exploitation of both the force-velocity and

3.1 Measuring Fascicle Geometry in Humanslength-force relationships in addition to the greaterquantity of contractile material that can attach to the

MRI has been scarcely used to measure fascicletendon or aponeurosis. It is clear then that havinggeometry in humans. The striation patterns resultinglarge fibre angles would be an important adaptationfrom inter-fascicular fat and connective tissue ele-to allow high force production.ments can be visualised using MRI scanning sincetheir signal (brightness in the image) differs from3. Plasticity of Human Muscle Geometrythe muscle tissue because of variations in water

Tissues within the human body have a great content of the tissues. The reconstruction of separateability to remodel in response to stress (e.g. Wolff’s two-dimensional (2-D) images, with the striationLaw for bone). At the gross level of muscle, in- patterns visually marked, into three dimensionscreased loading is commonly associated with an (3-D) allows the accurate determination of fascicleearly (<6 weeks) change in their neural activation geometry. Scott et al.[1] found good agreement be-during heavy lifting[3,38-44] (for a review of neural tween MRI and dissection estimates of fibre geome-mechanisms see Enoka[45]). However, subsequent try in the quadriceps of human cadavers, whileperiods of loading result in an increase in muscle Gaudy et al.[49,50] reported the muscle architecturalsize with more contractile tissue being available for arrangements of masseter, temporalis and lateralforce generation.[3,39,42,46,47] After prolonged periods pterygoid, with little variance between cadaver dis-of training, when the rate of hypertrophy has section and MRI measurements. Lam et al.[51] vali-reached a plateau, further small strength increases dated the measurement of fibre angles of the masse-are most likely to occur by neural mechanisms in ter by MRI in a single human cadaver. To date, noresponse to variations in training intensity or vol- studies have examined fascicle geometry in athletes,ume.[48] While this sequence of adaptation is well or determined longitudinal changes in fascicle ge-



ometry using MRI, perhaps because of the difficulty cles, larger errors in fascicle length estimates havebeen observed when compared with 3-D ultrasoundof manually tracing fascicles and then reconstruct-images (2.4–14.0%).[65] Fascicle length is also diffi-ing 3-D images.cult to accurately measure when the width of theMore recently, other MRI-based techniques haveprobe (often 3.8 or 6cm) is insufficient to capture anbeen developed. Diffusion tensor imaging relies onentire fascicle. In these instances, linear approxima-the diffusion of water in the muscle being con-tions are calculated from muscle thickness and fasci-strained by membranes and other cellular constitu-cle angle measures (length = muscle thickness/sin ×

ents, so in muscles the diffusion occurs largely alongfascicle angle). These do not account for fibre curva-

the plane of the fibres. The diffusion tensor can beture that is present in some hypertrophied or con-

calculated from the measurement of the direction oftracting muscles and errors range from approximate-

the diffusion.[52,53] This technique is more efficiently 0% (non-contracted gastrocnemius) to 6.6% (con-

than traditional methods of MRI determination be-tracted gastrocnemius) according to Muramatsu et

cause the fibres can be tracked rapidly by a fibre-al.,[66] or 2.4% in the non-contracted tibialis anterior

tracking algorithm[54,55] and the fibre orientationsmuscle.[67] Fascicle length can also be estimated by

can be quickly reconstructed. While the techniquemeasuring the visible portion directly and then esti-

has been used to determine fascicle geometry inmating the non-visible portion. Errors of 2–7% have

animal muscles,[53,56-59] it has yet to be used to been reported for this method.[68,69] Regardless, ul-quantify human fascicle geometry adaptations. In trasound imaging techniques allow valid and relia-addition to the diffusion tensor technique, magnetic ble measures of fascicle geometry in vivo, and haveresonance elastography has recently been shown to been commonly used to examine population differ-provide information with regard to fascicle geome- ences and longitudinal adaptations.try in humans.[60] After low frequency (<1000Hz)oscillations are induced in the muscle by a pneumat-

3.2 Population Differences inic or mechanical driver, MRI is used to measure theFascicle Geometryspread of shear waves travelling through the muscle.

The spread of waves is constrained by the stiffnessA muscle’s phenotype is at least partly reflectiveof muscle-based tissues, where lateral (with respect

of its long-term activity patterns. Although a per-to the muscle fibres) transmission is affected byson’s genetic predisposition might play a role, it isparallel layers of connective tissue but longitudinal

transmission is relatively unimpeded along the fib-res. Again, this technique has not yet been used toquantify muscle fascicle geometric changes longitu-dinally; however, both diffusion tensor imaging andelastography show great promise as fascicle imag-ing techniques.

The most common method of measuring fasciclegeometry is via the acquisition of 2-D longitudinalimages of the muscle using ultrasound[2,4-7,61,62] (fig-ure 3). When the ultrasound transducer (probe) isoriented in the plane of the fascicles, the fascicleangles and lengths can be accurately measured.Measures of fascicle angle (≤1.5°)[2,63,64] and fasci-cle length (≤1.5mm)[2] have been shown to be simi-lar to those measured directly in cadavers. When thetransducer is not oriented in the plane of the fasci-

Skin/fat layer

Vastuslateralis

Vastusintermedius

Bone

ba

Fig. 3. Images of human muscles can be obtained using ultrasoundimaging. In this longitudinal section of the thigh, the skin/fat layer,femur (bone), and vastus lateralis and vastus intermedius musclesare clearly visible. The aponeurosis (apo) and line of fascicles (fas)are also visible, from which fascicle angle can be determined, andfascicle length can be estimated using standard trigonometric pro-cedures.[5-7,68,69] Physical training can alter a muscle’s fascicle ge-ometry. Here, a period of heavy strength training has resulted in anincrease in fascicle angle concomitant with the increase in musclethickness (a vs b).


1008 Blazevich

expected that the physical work capacity of athletes, relatively regardless of cost. These data would sug-gest that there are significant differences in thewho have consistently trained with specific move-fascicle geometry of well trained sprinters comparedment patterns for a period of many years, wouldwith lesser-trained sprinters, endurance runners orvary according to the adaptations elicited by theircontrol subjects.training. Indeed, significant, training-specific differ-

Unique geometry is also seen in weight-trainedences in work capacity have been shown amongindividuals. As expected, muscle thickness mea-groups of athletes with varied training histo-sured at 13 sites on muscles including tricepsries.[70-76] Thus, one method of considering training-brachii, vastus lateralis and gastrocnemius lateralisdependent fascicle geometric adaptations is to com-of powerlifters was significantly correlated withpare populations with different physical activity his-squat, bench-press and deadlift performance (r =tories.0.63–0.91[80]). However, fascicle length of the vas-A significant quantity of research has examinedtus lateralis was also positively correlated with squatdifferences in fascicle geometry between differentlift (r = 0.50) and deadlift (r = 0.54) strength, andathlete populations. For example, both vastus later-triceps fascicle length was correlated with bench-

alis and gastrocnemius in highly trained male sprint-press strength (r = 0.52). While muscle thickness

ers (100m in <11.0 seconds) were found to containand fascicle angle were positively correlated (triceps

longer fascicles attached at smaller angles than inbrachii r = 0.64, p < 0.01; gastrocnemius r = 0.48, p

lesser-trained sprinters (100m in >11.0 seconds[77]).< 0.05), triceps brachii fascicle angle was negatively

In females, sprinters have been shown to have small-correlated with bench-press performance (relative to

er fascicle angles in vastus lateralis than non-trained fat-free mass, r = –0.45) and gastrocnemius fasciclecontrols (with no differences in gastrocnemius) and angle was negatively correlated with deadlift per-longer fascicles in vastus lateralis and gastrocnemi- formance (relative to fat-free mass, r = –0.56). Inus lateralis, which were significantly correlated with another study, bodybuilders were shown to have100m best time (r = –0.51 and r = –0.44, respective- greater muscle thickness and fascicle angles thanly[78]). A comparison of male and female sprinters non-weight-trained controls.[2] There is also somerevealed no differences in absolute or relative (to leg evidence that muscle size, fascicle angle and fasci-length) fascicle length.[78] When the fascicle geome- cle length are simultaneously greater in athletes whotry of sprinters was compared with well trained require a large body mass and physical strength asendurance runners, differences were also seen.[79]

well as fast movement velocities (e.g. Sumo wres-While endurance runners (13.5–14.5 minutes for tlers[81]), than they are in the normal population.5000m) had a lesser muscle thickness in the vastuslateralis and gastrocnemius muscles than sprinters 3.3 Training Effects on Fascicle Geometry(10.0–10.9 seconds for 100m), their fascicles were

While the assessment of population differencesalso shorter and their fascicle angles greater. Com-provides an insight into possible activity-dependentpared with non-active subjects, young enduranceadaptations in fascicle geometry, a more satisfactoryrunners had larger fascicle angles (13.7%) but amethod of examining the plasticity of fascicle geom-similar fascicle length in gastrocnemius medialis;etry is to directly measure the longitudinal adapta-there were no differences in fascicle geometry in thetions.vastus lateralis.[61] At least for the gastrocnemius,

muscles in endurance runners appear to be adapted 3.3.1 Resistance Trainingto produce forces with minimal metabolic cost (i.e. Early research was suggestive of there being littleshort fibres or fascicles require less energy for a change in geometry in response to resistance train-given level of force production because they have ing. Rutherford and Jones[8] showed no change infewer sarcomeres in series), whereas in sprinters vastus lateralis or intermedius fascicle angles after 3they are adapted for high-speed force development months of quadriceps strength training in seven men



Table II. Muscle architecture changes after strength training in humansa

Study Method Muscles Change

Rutherford and Jones[8] 3mo weight training (previously Vastus lateralis, vastus No change in fascicle angle or lengthuntrained) intermedius

Kawakami et al.[7] 16wk elbow extensor training Triceps brachii (lateral head) 29.1% increase in fascicle angle, no(previously untrained) changes in fascicle length (0.9%)

Aagaard et al.[4] 14wk leg extensor training Vastus lateralis 33.8% (2.7°) increase in fascicle angle(previously untrained)

Blazevich and Giorgi[5] 12wk bench-press and triceps Triceps brachii (lateral head) No change in fascicle angleextension training (well trained)

12wk bench-press and triceps Triceps brachii (lateral head) 39.5% (3.2°) increase in fascicle angleextension training + testosteroneinjections (well trained)

Blazevich et al.[84] 5wk isokinetic leg extension Vastus lateralis, vastus No change in fascicle angle or length(previously untrained) intermedius, vastus medialis

a Significant geometric change seems possible in previously untrained subjects, but these seem more limited in highly trainedathletes.

and five women. There was also no relationship range of motion most influenced the changes (sec-tion 5). Thus, recent research has shown a signifi-between fascicle angle and force-generating capaci-cant adaptability of fascicle geometry in response toty, although the authors reported a moderately highlonger-term resistance training.coefficient of variation (13.5%), making small

changes (<2°) in fascicle angle difficult to detect. Other research has examined changes in wellFurther research showed that significant changes in trained, athletic populations. A small change in tri-fascicle geometry occurred in response to training ceps brachii muscle thickness (+13.6%) after a(see example in figure 3 and a summary in table II). 12-week period of heavy resistance training in pre-Kawakami et al.[7] reported significant increases in viously weight-trained men (n = 5) was shown totriceps brachii physiological cross-sectional area occur without fascicle geometry or strength(PCSA) [+33.3%] and fascicle angle (+29.1%, 4.8°) changes.[5] However, in the same study, anotherwith no changes in fascicle length (–0.9%) after 16 group (n = 4) showed a significant strength increaseweeks of elbow extensor training in five men. Aag- (15.8%, bench-press) accompanied by increases inaard et al.[4] found that fascicle angle (+33.8%, 2.7°; fascicle angle (39.5%, 3.2°) after receiving once-vastus lateralis) increased significantly after 14 weekly intramuscular testosterone enanthate injec-weeks of leg extensor training in 11 men simultane- tions (3.5 mg/kg). Little change in geometry orously with anatomical cross-sectional area (+9.7%; strength appears to occur, therefore, in well trainedquadriceps total area) and muscle fibre cross-sec- lifters who continue to perform similar training,tional area (+12.9%; vastus lateralis). More recently, although the administration of anabolic steroidsGondin et al.[82] reported fascicle angle increases in might allow greater strength increases accompaniedvastus lateralis (14%) after 8 weeks of static electro- by significant geometric adaptation. In anothermyostimulation training of the knee extensors that study,[6] however, two resistance training groupsresulted in an increase of 27% in maximum contrac- comprising competitive athletes (n = 7 and n = 8)tion force and a 6% increase in quadriceps cross with at least 1 year of resistance training experience,section. Also, Alegre et al.[83] reported increases in showed increases in vastus lateralis fascicle anglefascicle length in vastus lateralis after 13 weeks of after 5 weeks of heavy (first session in the week) andstrength training when the concentric phase was explosive (second session) squat lift training thatperformed with maximum speed. These data indi- were significantly greater than a group that omittedcate that fascicle length is also adaptable, although it its strength training, but performed only sprint/jump

training (n = 8; no change). In the same study, vastusis presently unclear whether the training velocity or


1010 Blazevich

lateralis fascicle length increased only for the sprint/ no changes in muscle size and geometry of elbowextensors of an untrained limb in five men after 16jump-trained group. Muscle thickness of both vastusweeks of training, although no strength changeslateralis and rectus femoris increased in all trainingwere found either. Blazevich et al.[84] have reportedgroups also, although there were no differences inthat significant increases in contralateral knee exten-performance (sprint, jump and strength tests) be-sor strength after 5 weeks of isokinetic trainingtween the groups after training. Similar to previousoccurred without a change in geometry. Thus, al-studies on untrained subjects,[7,83] these results showthough few data are available, there is no currentthat changes in fascicle geometry occur in responseevidence to suggest that fascicle geometric changesto resistance training in well trained subjects. Theare a factor affecting short-term strength increases inresults may have differed from previous studiesa contralateral limb.where little change was noticed in trained subjects

because the training programme differed sufficient-3.3.2 Endurance Trainingly from that which the athletes had previously per-No research has examined changes in geometryformed. These data were the first to show short-term

after long-term endurance training. It is thereforechanges in geometry in response to training, andnot known whether endurance training can elicitalso show that increases in fascicle length and de-geometric adaptations, or how different modes ofcreases in fascicle angle can occur after high-veloc-training (e.g. cycling, running, lifting) might affectity movement training in humans; it is not known,them. It makes intuitive sense that if a muscle couldhowever, whether other factors such as the traininggenerate force more efficiently, then muscle endur-range of motion influence the architectural adapta-ance would improve. Given that muscles with largetion.pennation and shorter fascicles would generate force

Given that rapid (5-week) changes in geometrywith less metabolic cost, such an adaptation might

have been shown to occur in athletes, one mightbe expected with endurance training. Certainly, effi-

speculate that rapid adaptation is a mechanism byciency during stretch-shortening contractions could

which early strength increases occur in previously be expected to increase following such a geometricuntrained subjects. However, recent data[84] showed adaptation. This reasoning is consistent with theno changes in quadriceps muscle size or geometry in finding that vastus lateralis and gastrocnemius fasci-previously untrained men (n = 7) and women (n = 8) cles were shorter and fascicle angles greater in adespite significant increases in concentric and ec- group of endurance runners compared with sprint-centric isokinetic strength of the quadriceps muscle ers.[79] However, it could also be argued that longergroup after 5 weeks of isokinetic knee extensor fibres would benefit muscles that are commonlytraining. Also, Gondin et al.[82] did not find signifi- recruited during endurance tasks where forces arecant increases in vastus lateralis fascicle angle after produced over large ranges of motion, since such4 weeks of electromyostimulation training, despite fibre geometry is uniquely adapted to perform suchthere being significant increases after 8 weeks. It is work. Also, intramuscular pressure (IMP) is greatesttherefore likely that changes in fascicle geometry in muscles with large fibre angles;[88,89] therefore,are not a significant factor influencing early strength occlusion of blood flow during muscle contractionincreases in previously untrained subjects, although would be greater. This hypothesis is consistent withwhether rapid adaptations occur after training using the decreases in blood volume and oxygen satura-other modes (e.g. isotonic/isoinertial) is not tion seen during exercise (1Hz plantar flexion) in theknown.[7]

distal region of gastrocnemius medialis, where fas-Geometric changes are probably not a factor in- cicle angle and length are greatest.[90] During loco-

volved in strength increases that have been reported motion, IMP increases with movement speed[91] inin the untrained contralateral limb in response to line with the known positive relationship betweenunilateral training.[85-87] Kawakami et al.[7] reported IMP and muscle force,[92-95] so the limitation of



Table III. Muscle architecture changes after detraining/unloading in humansa

Study Method Muscles Change

Abe et al.[103] 20d bed rest Gastrocnemius medialis, No change in vastus lateralis or triceps brachii,vastus lateralis, triceps brachii significant decrease in gastrocnemius (5.5%)

measured standing, but not lying (4.7% decrease)

Kawakami et al.[98] 20d bed rest Vastus lateralis, plantarflexors, No change in fascicle angle or lengthtriceps brachii

Narici and Cerretelli[62] Disuse/injury Gastrocnemius lateralis 16.4% decrease in fascicle angle, 12.7% decrease infascicle length

Bleakney and Maffulli[102] Disuse/injury Vastus lateralis 4.8° decrease in fascicle angle, 7.5mm decrease infascicle length

a Some data are suggestive of decreases in fascicle angle and length in line with muscle atrophy. Differences between studies mightbe related to study duration or the length at which muscles were held during unloading.

blood flow at increased speeds of locomotion might li[62] found that disuse atrophy of gastrocnemiusbe expected to impact on muscle endurance. Of lateralis (23.1% reduction in anatomical cross-sec-course, during most forms of locomotion, muscle tional area) in an injured leg was associated with acontractions are phasic and blood flow will not be significant decrease in fascicle angle and lengthsignificantly affected when the duty cycle is below a (compared with uninjured leg; time period of injurythreshold limit (e.g. <40%),[93] so the effect of ge- not reported). Also, although measurements wereometry on endurance performance is probably com- taken at varying times after injury, Bleakney andplex. It would seem likely that fibre type is a more Maffulli[102] found significant decreases in fibre an-important muscle-based determinant of muscle en- gle and length of vastus lateralis in legs of subjectsdurance while fascicle geometry affects more the recovering from tibia/femur fracture, compared withlength-tension and force-velocity characteristics of a the non-injured limb (time after fracture, mean = 7.6muscle. months, range = 14 days to 2 years, 7 months). Abe

et al.[103] reported mixed results in subjects who3.3.3 Detraining/Unloading underwent 20 days of bed rest where there was aNumerous studies have investigated the effect of decrease in gastrocnemius medialis fascicle angle

detraining/unloading on muscle size and strength. measured during standing (5.5%) and a non-signifi-For example, significant reductions in strength and cant reduction during lying (4.7%), but no changesmuscle size follow prolonged bed rest[96-98] along

in triceps brachii or vastus lateralis muscles. There-with decreases in fibre size[99] and changes in fibre

fore, while there is some discrepancy in the litera-type (slow- to fast-twitch transformation[100]). How-

ture, there is good evidence that fascicle geometryever, few studies have measured fascicle geometry

changes can occur in response to unloading/de-after a period of detraining/unloading, or have not

training in humans.reported changes in geometry when it has been

The reasons for the disparate results in de-measured for the determination of PCSA. In onetraining/unloading studies are not clear; however,study, Kawakami et al.[98] found no changes in vas-the interaction effects of pre-unloading fascicle ge-tus lateralis, plantarflexors or triceps brachii fascicleometry and the muscle length during immobilisationlengths or angles in subjects (n = 5) after 20 days ofmight be two factors affecting the magnitude ofbed rest despite significant reductions in PCSA (ta-geometric change during detraining. Studies on cat,ble III); a similar finding was reported in four sub-mouse, rat and rabbit muscles have shown signifi-jects in a later publication.[97] These results are simi-cant decreases in fibre length after periods of un-lar to those reported in rat studies where immobilisa-loading when muscles were immobilised at lengthstion of gastrocnemius and soleus muscles atshorter than normal rest length.[94,104-108] These ef-shortened lengths was not associated with changes

in fibre angles.[101] However, Narici and Cerretel- fects have been reduced or reversed when muscles


1012 Blazevich

were immobilised in anatomical[109] or a lengthened 4. Fascicle Geometry through Growthand Agingposition.[93,106,108,109] Immobilisation in a lengthened

position has also been shown to counteract the slow-Fascicle geometric changes have yet to be de-to fast-twitch transformation that often accompanies

scribed in most human muscles through develop-immobilisation[110] and reduces the loss of wholement from infant to adult. Binzoni et al.[112] mea-

muscle mass.[111] These data on animal muscles sured medial gastrocnemius fascicle angle in 134show that the length of immobilisation has an im- subjects (87 men and 47 women) in the age range ofpact on muscle and fibre characteristics, although 0–70 years. Very small fascicle angles at birth in-this has not been fully examined in humans. In creased through childhood and adolescence to reachparticular, given that the fibre angle of a shortened their maxima in early adulthood. A comparison of

bodyweight and height, and tibia length indicated apennate muscle is greater than in a lengthened mus-strong relationship between fascicle angle and phys-cle, one might speculate that immobilisation of mus-ical size, possibly indicating that increases in fasci-cles in a shortened position might maintain the ge-cle angle of the gastrocnemius were related to theometry of highly pennate, short-fibred muscles. Fi-loads imposed on it. No research has examined othernally, given that immobilisation in lengthenedmuscles, making it difficult to predict fascicle geo-

positions,[106] or the application of moderate stretch- metric change with development, particularly ining (30 min/day),[107] has been shown to prevent non-weight-bearing muscles.fibre shortening during disuse, it is also pertinent to More data are available with regard to fascicledetermine whether such interventions might prevent geometry with aging from early adult to old age.the fibre shortening that has sometimes been shown Aging is associated with a loss of muscle size, orto accompany disuse in humans.[62] Certainly the sarcopenia,[113-117] resulting from a loss of fibres/

motor units[118] and a significant reduction in fibreaddition of small amounts of stretching/mobilisa-area, particularly of the fast-twitch fibres.[116,119-121]tion[99] during detraining has been shown to arrestThese morphological changes in muscle have beenmuscle atrophy in the human soleus.implicated, along with possible reductions in muscleactivation,[122,123] in the significant loss of muscle3.3.4 Chronic Stretchingforce[114,115,119,124-126] and power[114,115,126,127] seen in

No research has examined changes in fascicle aged individuals. Changes in fascicle geometry aregeometry after acute or chronic stretching of mus- therefore likely given the positive relationship be-cles in humans. Evidence from animal studies indi- tween fascicle angle and muscle size. Data collectedcates that chronic stretching might have a significant on 229 women from the ages of 20–79 years indicat-effect on fibre length. Immobilisation of muscle or ed that pennation of vastus lateralis (r = –0.50, p <

0.001), but not medial gastrocnemius or tricepsmuscle fibres in a lengthened position for periods ofbrachii, might be reduced with age, although theredays to weeks has resulted in increases in the num-was no evidence for a change in fascicle lengthber of serially arranged sarcomeres and overall fibre(relative to limb length).[128] The lesser pennation oflength in animal muscles.[106,108,110] Interestingly,vastus lateralis in elderly compared with young, butmoderate stretching performed for only 30 min/daylack of differences in gastrocnemius or triceps

was enough to reduce or reverse the fibre shorteningbrachii, or in the length of fascicles in any of these

that accompanied short-length immobilisation in rat muscles, has also been reported for men.[129] In amuscle.[107] It is not known whether flexibility train- smaller (n21–32y = 19, n60–69y = 30), but welling in humans results in an increase in fibre/fascicle matched (anthropometrically) sample, Karamanidislength. It is also not clear what effect, if any, chronic and Arampatzis[61] found no differences in gas-stretching would have on fascicle angle. trocnemius medialis or vastus lateralis fascicle



lengths or angles, regardless of whether the subjects geometry shown with aging are influenced by exer-cise interventions.were habitual endurance runners or not. Nonethe-

less, other data have shown significant differencesin gastrocnemius pennation (young [27–42 years] > 5. Conclusions and Future Directions13.2% [elderly = 70–81 years]) and fascicle length

Human fascicle geometry is highly adaptable.(young >10.2%) in subjects matched for height,Heavy weight training in physically active individu-mass and physical activity.[117] These data suggestals is often associated with increases in fasciclethat the aging process is accompanied by a signifi-angle (with some evidence for fascicle shortening)cant change in fascicle geometry that cannot neces-with these changes amplified by anabolic steroidsarily be explained by physical activity levels. Fur-use. In the elderly, the prominent adaptation seemsthermore, Morse et al.[130] showed that decreases into be an increase in fascicle length, although it is notmuscle volume of soleus and medial and lateralclear how geometry changes in response to longer-gastrocnemius were associated with decreases interm training. Both detraining and aging are associ-both fascicle angle and length. These decreases wereated with decreases in fascicle angle and/or de-similar amongst the three muscles indicating thatcreases in fascicle length, although there appears tothese muscles, which fulfil different functionalbe some inter-muscular differences. These changesroles, are affected similarly during aging. Thus,have important consequences for force generationwhile it seems that fascicle geometry changes fromsince increases in fascicle angle allow higher muscleadult to old age, more research is required to sub-forces, while increases in fascicle length allowstantiate these early findings and further examinehigher shortening speeds and for forces to be gener-inter-muscular differences.ated over larger length ranges.

The significant improvements in both muscle While there are significant data showing the plas-strength[131-133] and power[127] after extended periods ticity of fascicle geometry, more research is requiredof resistance exercise in older individuals are associ- to fully understand the effects of physical training.ated with increases in cross-sectional area or volume Not least, no research has examined the effects ofof muscles.[127-130] This increase in cross-sectional endurance or chronic stretching training. Further-area is often largely explained by increases in areas more, in order to more fully understand how fascicleof both type I and II muscle fibres.[131,134] Few geometry responds to heavy loading (e.g. resistanceresearchers have examined the effects of changing training), a number of research lines need to begeometry on muscle size, or indeed the effects of explored. For example, despite a well establishedgeometry on force development. One study[133] has link between fascicle geometry and function, andshown that an increase in muscle volume was attrib- significant evidence of geometric changes in re-uted partly to an increase in fascicle length of the sponse to resistance training, relatively little re-vastus lateralis. This change largely accounted for search has examined geometric changes in responsethe increase in muscle PCSA, and suggests that to athletic training interventions. Only one researchsignificant changes in geometry might occur with study[6] has examined changes in geometry in re-training in aged subjects. In this case, the change sponse to training in athletic subjects who performtoward longer fascicle lengths is at odds with the mixed training regimens, and only one study[128] hasincreases in fascicle angle (with no change or a examined changes in older individuals, so the ef-slight reduction in fascicle length) seen after training fects of training programme manipulations in ath-in younger subjects. This might reflect an adaptation letes and the aged remain relatively unstudied. Also,in less mobile older subjects to a greater movement no research has specifically investigated the effectsrange of motion used in exercise training compared of training mode, contraction type or range of mo-with their normal daily activities. More research is tion. It is unclear whether, for example, adaptationsrequired to determine how the changes in fascicle to isokinetic training are different to isotonic/


1014 Blazevich

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