Brazilian Journal of Motor Behavior
Research Articles
Fontana, Herzog
238 of 245
Fascicle shortening upon activation in voluntary human muscle contractions
Morphological Sciences Department, Federal University of Santa Catarina, Florianopolis, SC, Brazil
Human Performance Laboratory, University of Calgary, Calgary, Canada
Correspondence to: Heiliane de Brito Fontana.
VL fascicle shortening as a function of activation/force
depends crucially on muscle length.
Fascicle shortening results from a compromise between
the force generating potential of the muscle and muscle
elasticity at different lengths.
Fascicle kinematics cannot easily be estimated from
joint angle and activation.
EMG Electromyography
MTU Muscle tendon unit
MVC Maximal voluntary contraction
RMS Root mean square
VL Vastus lateralis
Received 25 07 2023
Accepted 29 09 2023
Published 30 09 2023
BACKGROUND: The dependence of fascicle length on complex interactions with joint angle
and force challenges the interpretation of in vivo joint mechanics, muscle mechanical
properties, contractile behavior, and muscle function.
AIM: The purpose of this study was to determine the complex interaction between muscle
activation, joint angle, and fascicle length for isometric contractions of the human vastus
lateralis muscle (VL).
METHOD: Knee extensor torques, joint angles, EMG activation, and fascicle lengths were
determined in nine healthy subjects during maximal and submaximal isometric contractions.
RESULTS: Fascicle shortening during isometric contractions depended on muscle-tendon
unit length/joint angle and activation, reaching a maximum between angles where VL had its
maximum force potential and minimum resistance to fascicle shortening. Maximal fascicle
shortening shifted to shorter muscle-tendon unit lengths with decreasing activation.
CONCLUSION: Fascicle shortening upon activation depends crucially on the force generating
potential and stiffness of the muscle and can reach 30% of the resting fascicle length. Not
accounting for the complex interactions between muscle length, force potential, muscle
structure, and muscle stiffness has led to erroneous interpretations of the function and
properties of healthy and diseased muscles.
KEYWORDS: Series elasticity | Muscle properties | Isometric contraction | Force-length |
Force-velocity | In vivo muscle function | Muscle work
Muscle tendon unit (MTU) length changes can be derived easily from joint angular excursion, if the instantaneous moment arm
is known
. In contrast, length changes of fascicles and fibers depend heavily on complex interactions between muscle architecture,
activation, and the force-dependent elongation of series elastic elements
. Muscle structure, such as the pennation angle and fascicle
length, change during activation and force production, even when the contraction is “isometric” on the MTU level
This dependence of fascicle length on complex interactions with several factors challenges the interpretation of in vivo joint
mechanics, muscle mechanical properties, contractile behavior, and muscle function. One could make an argument that contraction types
concentric, eccentric and isometric - should be defined on the contractile element (fibre/fascicle) level since it is the contractile
element’s conditions (length, rate of change in length, and history of length changes) that affect the basic muscle properties (force-length
, force-velocity relationship
, and history-dependent relationships
A further complication in understanding fascicle mechanics during voluntary contractions is that activation, muscle length/joint
angle, and series elasticity interact and affect fascicle lengths in a complex way. There are at least two factors that need to be considered
when determining fascicle mechanics from joint kinematics: (i) the force potential of muscles, and therefore the capacity to stretch series
elastic elements, depends on the joint angle
, and (ii) the passive resistance to fascicle shortening also depends on joint configuration
because of differences in stiffness and slackness of elastic elements across the operating range of the muscle
However, despite the obvious importance of knowing fascicle, and thus contractile element lengths, little systematic work has
been done to determine the interactions between muscle lengths/joint angle, force potential, muscle structure, and muscle stiffness that
affect fascicle shortening properties and in vivo behavior.
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De Brito Fontana,
239 of 245
Research Articles
The purpose of this study was to determine the interaction between EMG activation, joint angle, and fascicle length for in vivo
isometric contractions of the human vastus lateralis muscle (VL). We hypothesized that a given level of activation produces different
amounts of fascicle shortening depending on knee joint angle, and that the amount of fascicle shortening is greatest in the mid-range of
knee motion, somewhere between the joint angles where VL force is greatest and resistance to fascicle shortening is smallest.
Nine subjects (7 male and 2 female) took part in this study. Subjects gave written, informed consent to participate in this
research, and all procedures were approved by the University of Calgary’s Conjoint Health Research Ethics Board. The subjects were
recreational athletes participating in sports such as running, swimming or soccer. Mean±SD age, height and weight were 27±3 years,
1.76±0.13 m and 69±15 kg, respectively. The maximal and submaximal fascicle force-length relationships obtained from this study have
been published previously
Knee extensor torques and knee joint angles were measured on a Biodex II dynamometer (Biodex Medical Systems, Inc.,
Shirley, NY). Subjects were seated with the back supported and the hip joint flexed at 80˚. Straps across the shoulders, waist and thigh
were used to stabilize subjects and isolate the action of the knee extensor muscles. The lower limb was positioned until the knee joint
rotational axis was aligned with the axis of the dynamometer arm throughout the entire range of motion. Full knee extension was defined
as 180 degrees. The tibia was strapped to the dynamometer arm 3 cm proximal to the lateral malleolus.
Surface EMG was recorded from VL using bipolar electrodes (Norotrode 20, inter electrode spacing 22 mm). The skin was
shaved and cleaned with alcohol before placing the electrodes approximately two-thirds along a line from the anterior superior iliac spine
to the proximal end of the patella. A ground electrode was placed on the tibial tuberosity. Skin impedance was checked and pronounced
acceptable once it was less than 5 kΩ. EMG, torque and joint position were recorded at 1000 Hz using the Windaq data acquisition
system (Dataq Instruments, Akron, OH).
VL fascicles were imaged at 37-49 Hz using a 12.5-MHz linear array ultrasound probe (50mm, Philips Envisor, Philips
Healthcare, Eindhoven, The Netherlands). The ultrasound probe was attached to the skin on the lateral mid-thigh region using a custom-
built holder and was supported by the examiner during testing. Probe orientation and location were adjusted until the best image of VL
fascicles was obtained. An external function generator (B-K Precision 3010, Dynascan Corp., Chicago, IL) was used for synchronization
of all signals.
Knee joint angle, VL EMG activity, and VL mid-portion fascicle lengths were obtained continuously. The ultrasound images
were also used to determine the distance between the deep and superficial aponeuroses and the angle pennation using freely available
software (MicroDicom version 0.7.8). When fascicles were not visible along their entire lengths, fascicle lengths (Fl) were calculated using
standard trigonometry
. Fascicles were assumed to be straight lines and an error of 2-7% has been ascribed to this assumption
Subjects executed a standard warm up consisting of ten submaximal knee extension and flexion repetitions. For the actual
testing, subjects performed two maximum isometric knee extensor contractions at ten different knee angles (from 80° to 170° at 10°
increments) in a randomized order, and the trial with the greater torque was used for analysis. Subjects were asked to build up to the
maximal torque over 5 seconds and then hold the maximal torque for another 2 seconds. Verbal encouragement and visual feedback of
the knee extensor torque were provided during all contractions. A rest period of at least 2 min was strictly enforced and was extended
upon a subject’s request.
Knee angle, EMG, fascicle length, and torque were exported to Microsoft Office Excel (Microsoft Corp. Redmond, WA) and
analyzed using Matlab (Math-Works, Natick, MA). Root mean square values (RMS) of the EMG signal were calculated (100 ms window)
throughout the entire contraction. Patellar tendon moment arms were calculated using the regression equation defined by Herzog and
. Knee extensor force was calculated by dividing torque by the patellar tendon moment arm. VL force was approximated by
multiplying the total knee extensor force by 0.34
Fascicle lengths at 0 to 100% of maximum activation (in 10% increments, normalized to the maximum EMG at each knee
angle), and at 10, 20, and 30% of the maximum force at optimal length were determined at all knee angles. Mean fascicle shortening as a
function of activation and force was then calculated across all joint angles.
The interaction between activation and joint angle on fascicle lengths were analyzed based on a repeated measures ANOVA
comparing fascicle shortening (passive active) across the 10 different angles (α=0.05). The effect of activation was further analyzed
using regression analyses relating fascicle lengths and activation at long (80˚ knee angle), mid-range (130˚), and short muscle lengths
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De Brito Fontana,
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Research Articles
Mean VL fascicle lengths ranged from 13.5±1.2 cm for passive VL at its longest position to 6.3±0.7 cm at maximum activation
and its shortest position. VL maximal force production varied from approximately 523 N at 170° of knee extension to an optimal force
production of 1588 N at 100° of knee extension. For contractions producing the same level of force, fascicle shortening was greater at
more extended knee positions, and consequently shorter VL MTU, compared to more flexed knee positions and longer muscle lengths.
The solid lines in Figure 1 show the mean values for fascicle shortening at 10% and 30% of MVC at the optimal knee joint angle. The
dashed line shows the maximum VL force angle curve shape.
The effect of activation on fascicle length (passive – active) was significantly different across joint angles (F = 2.651; p = 0.048).
For a given activation, fascicle shortening was greatest in the mid-range, decreasing towards the flexed and extended knee joint angles
(Figure 1, columns).
Figure 1. Fascicle shortening at different levels of activation and different knee joint angles (180° =full knee extension = shortest VL length). Fascicle shortening was
greatest in the mid-range of knee joint angles because of the interaction between muscle force generating potential at the different joint angles/MTU lengths (illustrated by
the VL force angle relationship - dashed line) and the decrease in resistance to fascicle shortening at more extended knee angles/short MTU Lengths (as illustrated by the
increase in fascicle shortening for a given amount of force: solid lines, triangles = 30% of MVC force and squares = 10% of MVC force). In addition, fascicle shortening for
10% increments in activation were greater at low compared to high levels of activation, especially at the more extended knee positions/short MTU lengths. Values are
means across subjects and error bars represent standard errors of the mean for the total fascicle shortening distances for the full activation conditions.
Fascicle lengths decreased at all joint angles as activation increased. Since the effect of activation on fascicle shortening was
shown to depend on joint angle, regression analyses of activation-dependent absolute fascicle lengths were performed separately at an
intermediate length (knee angle of 130°) and at the longest (80°) and shortest VL lengths (170°) (Table 1).