I-1 SFVIFR
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Yang-Hwei
Tsuang1r4, Jui-Sheng
Sun’, Ing-Huo
Chen2,
Shang-Hwa
Hsu3, King-Yaw
Tsao 3, Kuan-Yih Wei4, Yi-Shiong
Hang’
‘Department of Orthopedic Surgery, National Taiwan University Hospital, College of Medicine, National Taiwan University, Taipei, Taiwan
*Department of Orthopedic Surgery, Tzu-Chi Buddhist General Hospital, Hua-Lian, Taiwan 3Graduate Institute of Industrial Engineering, National Chiao-Tung University, Hsin-Chu, Taiwan
“Department of Orthopedic Surgery, Taiwan Provincial Tao-Yuan General Hospital, Tao-Yuan, Taiwan
Abstract
Objective. To define the threshold of muscle injury with cyclic passive stretch.
Design. The changes in the load-deformation curve of muscle-tendon unit were monitored until the failure point by an in viva rabbit model.
Background. Muscle injuries range in severity from a simple strain to complete rupture. Although strains occur more frequently than complete failures, only a few studies have investigated the phenomena of these sub-failure injuries. Monitoring of the continuum for stretch-induced injury allows us to define the threshold of stretch injury.
Methods, Thirty rabbits’ triceps surae muscle-tendon unit preparations were used. One of the pairs (control) was stretched until failure; the other (experimental) was first cyclic stretched to either 12, 20 or 25% of the initial length of the muscle-tendon unit and then stretched to failure. Comparisons were made between the load-deformation curves of the experimental and control specimens.
Resulfs. When cyclic stretched to 12 or 20%, there were no significant changes existed in the biomechanical parameters except the deformation at the peak load. In contrast, all the biomechanical parameters except the ration of the energy absorption changed significantly after 25% strain cyclic stretch.
Conclusions. A threshold for stretch-induced injury does exist. This can be reproduced at the 25% strain of the triceps surae muscle-tendon unit.
Relevance
Muscle--tendon injuries, primarily muscle strains or tears, are extremely common in profes- sional and amateur athletes. This experimental study with rabbits, give evidence that changes in the biomechanical properties after sub-failure injury does exist in the muscle tendon unit. Mare importantly, stretching before competition should not exceed 25% strain in the triceps surae muscle-tendon unit. 0 1998 Elsevier Science Ltd. All rights reserved
Key woi-ds: Cyclic stretch, sub-failure, muscle-tendon unit
C/in. Biomech Vol 13, No. 1, 48-53, 1998
Introduction amateur athletes’.2. These injuries account for almost
Muscle-tendon injuries, primarily
tears. are extrcmcly common in
muscle strains or
professional and
half of all injuries in certain sports”. The triceps surae muscle is one of the most common muscles damaged
in the lower extremities”. In the past two decades,
much literature has been devoted to the preven-
tion2.‘. understanding”, and treatment of these
injuries7.
During the activities of daily living, the musculo- skeletal system is subjected to a wide range of joint
motion. Both muscle and tendon components are
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(‘o~~~.\/\porrtl~,~~~[, urmll rcprinf rrrp~t.\ to: Jui-Shcng Sun, Dcpartmcnt (4 Orthopedic Surgery. Nxtional Taiwan IJniversity Hospital, i‘ollege of Medicinc, National Taiwan University, No. 7, Chung- Shari South Road. Taipei. Taiwan, 10002, ROC’. Tel: +88h 2 ?3’i7OX1)0 (ext. 5277): Fax: +880 2 -7392?12.1.
Twang et al.: Threshold of rabbits’skeletal muscle 49
susceptible to trauma or wear and tear. It is believed that warming up before an athletic event is important
for both performance and injury prevention. In sports
medicine, stretching exercise has been recommended
to prevent injuryh,8 and to improve performance”.
Previous experimental studies in animals have shown
that passive warming increases the extensibility of the
musculo-tendinous unit and that warmed muscle has
greater deformation and less stiffness than cold
muscle, and offers support to the theory that warming
up muscles can aid in injury prevention and improve-
ment in athletic performance”‘.
However, intensive exercise training can also result in muscle damage and muscle soreness. There are a
large number of biomechanical studies of ligaments
or muscles stretched to failure, but only a few studies
were done to determine the effect of sub-failure
stretch on the elastic behavior and failure properties
of the muscle-tendon unit. Our recent experimental
studies of indirect muscle strain injury have concen- trated on the muscles injured in response to excessive
stretch alone” or stretch and activation’2. Previous
studies did not determine a threshold or minimal
force-displacement necessary to create injury.
Measuring joint range of motion is not practical
for the athlete or patient attempting to assess
improvement in flexibility, therefore stretch distance
is a more useful method of measuring flexibility. In
contrast to previous human study using joint range of
motion to determine the effects of stretching of flexi-
bility, we measured stretch distance relative to a fixed
reference point. The present study investigates a
repetitive stretch of muscle-tendon units using the
minimal strain necessary to create injury, and thus,
define a threshold for injury with cyclic passive
stretch. We chose the New Zealand white rabbit for
our investigation, because their architectural design,
including the soleus in the hindlimb, were considered
to be similar to that found in humans’“. Methods
Thirty New Zealand White rabbits (mean weight, 2.5,
SD, 0.2 kg) were divided into three groups. The
preparation was the same as previously reported”.
After subcutaneous ketamine (dosage, 50 mg/kg)
general anesthesia, an incision from the mid calf to
the plantar surface of the foot was made on the
lateral aspect of each hind limb. The Achilles tendon
was isolated taking special care to maintain the
neurovascular bundle and the tendon insertion intact.
For determining the in situ muscle-tendon unit
(MTU) length, dial calipers (accurate to 0.05 mm)
were used to measure the distance between the origin of the triceps surae at the femur and the insertion at calcaneus with the knee and the ankle at 90” angula-
tion. The anesthetized rabbit was placed in a frame
attached to a MTS machine (MTS BionixTM 858 test
system). The hind limb was immobilized with a
K-wire transfixation through the proximal tibia. The
distal tendinous insertion was freed by osteotomiza-
tion at the calcaneal tuberosity and clamped to the
MTS load cell. A 3 N preload was applied on the
muscle, and then the muscle length was again
measured’ ‘.
The muscle-tendon unit of one hind limb was
cyclically loaded for one hour at a rate of 0.5 cm/min
to any of three strain amplitudes (12, 20 and 25%~).
Once the peak stretch amplitude was reached, the
stretching was discontinued and the muscle-tendon
unit returned to its initial resting length. After cyclic
passive stretch, the muscle was stretched at a
constant rate (0.5 cm/min) until a macroscopic tear or
full division of the ruptured muscle fragments
occurred. In the other hind limb, the muscle-tendon unit was stretched at the same rate to failure of the
muscle-tendon unit. The load and deformation
required to deform the muscles were simultaneously
recorded on a PC by the TestlinkTM system Software
(PCLABTM Data Translation, Data Translation Inc.,
Locke Drive, Marlboro, USA). All muscles were kept
moist and at physiologic temperature using warm
normal saline irrigation. Additional anesthesia was
given as needed. This study received prior approval
of the National Taiwan University Medical College’s
Animal Research Committee. After completion of
the experiments, the rabbits were sacrificed at the
conclusion of the study.
For each triceps surae muscle, the load and defor-
mation of the muscle-tendon unit were recorded and
plotted directly using a PC. Deformation of the
muscle-tendon unit was measured when peak load
was evident. The deformation of the muscle-tendon
unit was calculated by muscle length at peak load
minus muscle length before distraction. The area
under the load-deformation curve before the failure
point represents the relative energy absorbed by the
muscle-tendon unit prior to failure. The difference
of the energy absorbed by the muscle-tendon unit
before the point of peak load and the point of full
separation of the ruptured fragments as well as the
differences between the two limbs were evaluated by the paired t-test. Because of the great individual
variation of the triceps surae muscle strength, only
the statistic method of the paired t-test was used to evaluate the difference between the two limbs of the rabbits in each group. The level of statistical signifi- cance was set at 5%.
Results
All of the triceps surae muscle-tendon units under
distraction had a similar curve pattern, as shown
previously”. The load-deformation curve began with
an initial increasing slope and ultimately reached the
steep cirop followed by a curve with gradual increasing and decreasing of the forces. The slopes of thrse curves were measured a1 every linear portion of the r‘urve. The means of peak tensile force. dispiacc- mcnt at peak tensile force and slope are summarized i tI T;rhi~, j
Afrct- cyclically stretched ,tt either 12 or 20%
-train. the shape of the load-deformation curve of
triceps surac muscle-tendon unit did not show any
Ggnitjcant change [Figure I (A)]. When the muscic-
tendon unit wan cyclically stretched after 259; strain,
o statistically significant difference in the biomechan-
ical parameters of the triceps surae muscle-tendon
rmit between the control and experimentally cyclic
rtretchcd group” can be noted [Figure l(B)]. In the
Froup ol‘ i A ? and 20C;i~ cyclic stretch, ail biomechanicai
parameters between the control and experimental
cyclic stretched muscle-tendon unit showed no signi-
ficant difference except in deformation at the peak
!oad (‘I’abies I and 3). In the group of 25% cyclic
~trctch. ail biomechanicai parameters between the
control and experimental cyclic stretched muscie-
tendon unit showed a statistically significant differ-
c)ncc. except for the ratio of energy absorption
(Tables I and 2 j. After 60 min cyclic stretch to 2SC4.
the peak load decreased IO.i%. deformation at peak
load decreased iO.Y,% and slope of the ioad-defor- mation increased 27.9% (Table I ).
The energy absorption before complete separation
of the disrupted fragments ctf the muscle-tendon unit
was expressed bv the area beneath the ioad-deforma- iion curve. The means of total energy absorption,
energy absorption before peak tensile force and the
ratio of energy absorption before peak tensile force
arc shown in the Table 2. In the rabbits group after cithcr I? or 20% cyclic stretch, the means of the total
energy absorption and the energy absorption before
peak load remained constant: while after 25% cyclic
stretch. the means of the total energy absorption and
the energy absorption before peak load decreased
significantly. The differences were statistically signifi-
cant (,r = 0.0025 and 0.0017 respectively). No statis-
tically significant difference appeared between the
ratio of energy absorption in both groups (P > (LOS).
The total energy absorption before muscle-tendon
unit failure decreased 33.1%; the energy absorption before peak load decreased 35.75% (Table 2).
The sites of failure were within 0.1-1.0 mm from
the distal musculo-tendinous junction for soieus
muscle and within S-10 mm from the distal muscuio-
tendinous junction in the lateral head of gastro-
cnemius muscle. While in the medial head of
gastrocnemius muscle, failure occurred within
15-30 mm from the distal muscuio-tendinous
junction as previous reported”.
Discussion
Muscuio-tendinous strain injuries have been cited as
the most common injury in competitive athletics’.“.‘J.
Their f’rcquency and disabling effects have been
documented in many epidemiologic studies” Is.
Strains not only cause a significant loss of time from
sports, but are also a common source of pain. and
impair performance following return to competition.
Despite their common occurrence, there have been
relatively few studies investigating the effects of these
injuries. Possibly as a result of the incomplete study
of these injuries, treatment is extremely variable,
ranging from complete rest and immobilization of the
injured muscle to immediate return to athletic
competition, sometimes after local injections into the
injury site’“. Previous studies involving stretching
injury have used large total displacements (beyond
the muscle’s physiologic range of motion) to create
injury’ ‘-?“. These studies did not determine a
threshold or minimal force-displacement necessi-
tated to create injury. Muscle injuries can range in
severity from a simple strain to a complete rupture.
However, strain or sub-failure comprise 80% or more of ail muscle injuries and are much more frequent
than complete failures. Only a few studies have
investigated the phenomena of these sub-failure
injuries”. Despite this prevalence, there is a paucity
of information on the mechanics of muscle strains.
The present study has investigated a repetitive stretch
of muscle-tendon units using the minimal strain
necessary to create injury, and thus, define a
threshold for injury with passive stretch. The
hypothesis of this work was that sub-failure injury
Table 1. Biomechanical data of the slope, peak tensile load and deformation at peak tensile load of the composite triceps surae muscle-
tendon unit (N = 10)
12?/0 20% 25%
-__.
Peak lead (N) Control 482.2 (SD, 68.0) 463.5 (SD, 58.4) 447.4 (SD, 56.9)
Stud\; 485.0 (SD, 63.8) 459.7 (SD, 69.6) 401.4 (SD, 90.2)
P value 0.8290 0.8627 0.0256
Deformation at Control 42.1 (SD, 5.3) 40.1 (SD, 4.6) 43.7 (SD, 5.8)
Peak load (mmi Study 38.5 (SD, 5.7) 36.6 (SD, 4.1) 35.0 (SD, 6.7)
P value 0.0098 0.0058 0.0055
Slope (N/mm) Control 19.9 (SD, 1.6) 18.1 (SD, 2.5) 17.9 (SD, 3.7)
Study 20.1 (SD, 2.0) 19.7 (SD, 2.8) 22.9 (SD, 4.0)
P value 0.7249 0.2677 0.0010
Twang et al.: Threshold of rabbits’skeletal muscle 51
Figure 1. Load-deformation curve of triceps surae muscle-tendon unit after 20 or 25% cyclic stretch. The area below depicts the
relative energy absorbed to failure under various conditions: (- ) control; (- - -) experimentally cyclic stretched to 20 or 25%.(A)
After cyclically stretching to 20% strain, the shape of the load-deformation curve of triceps surae muscle-tendon unit showed no
significant change. (B) When the muscle-tendon unit was cyclically stretched at 25% strain, the peak load, deformation at peak load,
total energy absorption and energy absorption before peak load all significantly decreased, while the slope of the load-deformation
increased significantly.
Table 2. Energy absorption of composite triceps surae muscle-tendon unit (N = IO)
12% 20% 25% Total energy Absorbed (N/mm) Energy absorbed Before peak Load (N/mm) Ratio of energy Absorption WI Control Study P value Control Study P value Control Study P value 13068.2 (SD, 3056.0) 12992.0 (SD, 3513.8) 0.9002 7343.3 (SD, 2138.6) 6934.3 (SD, 1670.1) 0.4730 56.0 (SD, 7.9) 54.2 (SD, 6.5) 0.5913 12245.6 (SD, 4202.0) 10494.1 (SD, 2505.8) 0.1754 6761.3 (SD, 1659.2) 5937.9 (SD, 1525.4) 0.1339 57.3 (SD, 11.4) 57.3 (SD, 10.9) 0.9945 12797.0 (SD, 2879.4) 8564.6 (SD, 3610.1) 0.0025 7102.0 (SD, 2269.5) 4566.9 (SD, 2063.3) 0.0017 55.1 (SD, 10.0) 54.1 (SD, 10.4) 0.7812
tion curve and that the sub-failure injury initial does
not alter the load-deformation behavior above the
icvei of t hc sub-failure injury.
As shown in Table I, WC find that the deformation
at the peak toad diminished at’tcr cyclic stretching at
ail ranges of strain. Howtlver. both the I2 and 20%
~‘roupx demonstrated no biomechanical evidence of
tr
injury in muscle-tendon unit injury, whiic muscles of
the 3ii’; strain group demonstrated evidence ot
injury by diminished peak toad, total energy absorp-
ti09. and energy absorption before peak toad
(Tabicx 1 and 2). These data suggest that a threshold for injury with passive stretch does exist. The finding
rhnr xl’tcr 35”: cyclic stretching the muscle-tendon
unit ixxame more stiff than that of control muscie--
tendrrrr unit supports the possibility oi’ 25% stretched
llltlSCl~ being injured. ‘This difference in stiffness
impiic~ that, i’ctr A given increase in deformation, 25%
cyclic \tretchcd muscle deceiops more force when
cornpaled with I?.‘? or 20% stretched muscle (Figure
i j. Further cvidcncc of the muscle-tendon unit being
injurtxi is obvious by observing that the peak toad,
iota1 ~ncrgy absorption. and energy absorption before
pcah toad wcrc all diminished after 25% cyclic stretch
i’rablr~ I atxi ‘1. It assumes that a critical strain
t~ccls to IV reached before strain injury can bc initi-
aled. i3clorc cucrcise. stretching has been recom-
mcndcd tii prcvcnt injury anti to improve
p”rf(~rlI1;JIicc”.Y ” T’hc widespread clinical impressions
regarding the protective effect of passive stretch on
nlurclc@ are certainly not against these data. In fact, it I‘ vicar that iC a critical strain is not reached, ;I
cictr~mcntal cfi’~ct of cvciic stretching would not
appear
Recently. Panjabi c’t L[/.‘~ elucidated that stretching
i t:c ligament iii X0’+ of the failure subsequently
incrc;tsed deformations below X0%, but had no effect
r)!l anv mechanical propertics above 80%, including
{he peak toad and deformation. This may result in
increased joint laxity, additional toads will be applied
tli other- joint structures and to the joint. In this
ctudv. we demonstrated that when the muscle-tendon
rmit’was cyclically stretched. a statistically significant
decrease in thcb deformation at the peak load of the
1 riceps surac muscle--tendon unit was noted, even
though ail other hiomechanicai parameters showed
no significant change (Tables I and 2). The anesthetic
aed in this study was ketaminc and this drug has no
muscle-relaxant cffecP. The ncrvc function and
related muscle tone were well preservedz3. This fact
suggests that the presence of nerve function can
respond to the cyclic stretch by increasing muscle
!one and then the deformation at the peak toad is
decreased. This suggestion is further consolidated by
the fact that after 25% cyclic stretch, the slope of the
tc,~td-deformatioI1 curve in the muscle-tendon unit is
significantly increased before the peak deformation.
eters between the control and experimental cyclic
stretched muscle-tendon unit showed a statistically
significant difference except in the ratio of energy
absorption. The peak toad, deformation at peak toad,
the means of total energy absorption, energy absorp-
tion before peak toad decreased (Table I and 2).
These biomechanical parameters indicated that
disruption in the muscle-tendon unit has occurred to
some extent.
Cyclic stretching of muscle-tendon units above a
threshold would drastically alter both ioad-deforma-
tion and failure properties. Using an in IJ~VU rabbit
model, we have demonstrated that the 2Oci;
sub-failure stretch does not alter the failure point of
the muscle-tendon unit, but significantly alters the
deformation at the peak toad, possibly by the nerve
reflex. Above 25% strain, the biomechanicai param-
eters are all significantly changed. The fact that there
is significant fiber damage without gross changes in
the mechanical behavior below the sub-failure point
has some intriguing consequences. The most promi-
nent morphological changes in the injured muscle
fibers were the loss of desmin staining occurred in the
absence of contractile or metabolic protein disrup-
tion. The disruption of the cytoskeletal network and
an inflammatory response could further deteriorate
the contractile response”“. Clinically, it is well
observed that mild strains can cause significant pain
without altering muscle activity. The results of this
study arc. then, in general agreement with those
observations, assuming pain is associated with
damage. We postulate that fiber failure without
accompanying change in the gross load-deformation
behavior is a potential mechanism for stimulating the
biological maintenance and repair of the muscle-
tendon unit. This suggests that this model can be
further validated by documenting a biological/inflam-
matory response to a sub-failure injury, which would
not notably alter the load-deformation behavior.
In summary, this study shows that a threshold and
continuum for stretch-induced injury does exist. It is
possible that the muscle fiber disruption occurred
initially and connective tissue disruption occurred
only with larger muscle displacementP. More
importantly, in our model, injury can be reproduced
at 25% strain of the triceps surae muscle-tendon unit.
Acknowledgements
The authors sincerely thank the National Science
Council (ROC) for their financial support of this
research.
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