Download Transverse mechanical properties of rat skeletal muscle

Transverse mechanical properties of rat skeletal muscle
under in vivo ramp compression
D. GAMET*
Laboratoire Biomécanique et Bioingéniérie, UMR6600 CNRS, Compiègne
*Corresponding author. Email: [email protected]
Keywords: muscle; compression; transverse force
1 Introduction
Studies about effects of external mechanical
strains on muscle aimed to investigate skin or
muscle pathologies by long term pressure
application, impact damage, muscle fatigue (1)…
Few studies concerning mammalian muscles
have investigated twitch contraction under in
vivo compression either on rat muscle fibre
bundles using high hydrostatic pressure (2) or on
human biceps using a compressive cuff (3) but
no data were available on twitch transverse.
In our previous work (4), axial twitch and its
transverse force in rat muscle were studied
during in vivo compression relaxation tests with
short duration (30s). Our present purpose was to
study the effects of ramp compression and its
cumulative effects on axial and transverse force.
2 Methods
Experiments were performed on both the left and
right tibialis anterior (TA) of six Wistar rats
(500±35g). TA was pseudoisolated with a special
care to keep in order nerve and vascular system.
Sciatic nerve was exposed to apply supramaximal electrical stimulations (7V, 10µs
duration). Distal tendon was linked to a force
transducer to measure contraction force or axial
force (FA). Then muscle length was adjusted to
its optimal length (l0 28.4±1.2 mm). Muscle
inner face was placed on a rigid non metallic
plate and carefully in full contact with it. Using a
traction machine (Synergie 400, MTS),
compressions on muscle belly part were applied
by an indenter with 20 mm diameter; an
electrical insulator (celluloid thin layer)
separated surface indenter and muscle. TA
thickness (4.55±0.26mm) was measured with
respect to the rigid plate (height indenter=0 fig
1). Muscle ramp compressions were applied with
a rate of 0.22%.s-1 (or 0.01 mm.s-1) from 0%
(muscle contact) to 50% of TA thickness.
Following the ramp, the compression was released
with the same rate. On the same axis of MTS
indenter, a force transducer was used to measure
indentation force or transverse force (FT).
Experiments consisted in 3 successive cycles (ramp
compression - ramp decompression). Electrical
stimulations were applied for each 2.5% of
compression.
Figure 1: Example of transverse and axial force
kinetics during compression-decompression cycle
applied on rat TA.
Axial and transverse twitch forces (twA, twT) were
quantified by the contraction amplitude (FA-Amp,
FT-Amp), time to peak contraction, and time to
half relaxation.
Axial twitch parameter values were normalized
with respect to the initial axial twitch parameter
values (for example twA-Amp0). Transverse twitch
parameter values were normalized with respect to
the maximal transverse amplitude obtained during
the first compressive ramp. For each parameter,
according to compression level, the mean
(±standard deviation) were calculated over the 12
muscles.
3 Results and Discussion
When compressions were applied on muscle, a
visco-elastic behaviour was described by the
curvilinear changes in the baseline of transverse
force (maximal forces were respectively to cycle
twA-Amp (% twA-Amp0)
Ramp 1
120
100
30%
80
60
40
Ramp 1
Ramp 2
20
Ramp 3
0
0
20
40
60
80
100
Normalized twT-Amp (% twT-Amp max)
Figure 4: Relationship between twT-Amp% and
twA-Amp% in rat TA. Arrow pointed on 30%
compression level.
Ramp 2
100
Ramp 3
80
60
40
20
0
0
Contact
10
20
30
40
50
Compression level (% )
Figure 2: Relationships between compression
level and amplitude of axial twitch expressed in
% with respect to the initial value.
twT-Amp (% twT-Amp max)
maximum was estimated at 30% of compression;
corresponding also to the break point in twA-Amp
changes. The relationships between amplitudes in
twT and twA (fig 4) well exhibited two kinds of
muscle behaviour when compressions were
applied. Before 30% of compression, muscle
generated axial force while transverse force
increased. After 30% of compression, muscle was
altered in both axial and transverse forces.
Normalized twA-Amp (% twA-Amp0)
number 27.35±5.06; 26.74±5.22; 27.07±5.32 N).
In the same way, baseline of axial force was
altered and this was interpreted as a global
muscle shortening.
When twitch contractions were evoked, twT
were surimposed on force transverse and at the
same time as twA. As previously reported (4),
the major difference between normalized twA
and twT shapes was identified on time to halfrelaxation; the times to peak contraction were
similar.
twA-Amp were affected by the cumulative effect
of successive ramps and compression level (fig
2). Highest changes were obtained between
Ramp 1 and Ramp 2 while smallest changes
were obtained between Ramp 2 and Ramp 3.
This illustrated the conditioning phase due to
Ramp 1. For each ramp, a relative stability (resp.
100%; 40%; 30%) followed by a decrease (resp.
38%; 23%; 22%) in twA-Amp was observed.
The common break point was estimated at 30%
of compression.
Ramp 1
100
Ramp 2
Ramp 3
80
60
40
20
The 30% level as a break point confirm our previous
discrete results obtained for 5%, 10%, 20% et 50% of
compression (4).
These results were interpreted according to the
muscle functional alterations as osmotic balance
(Ramp1 as preconditionning ramp) and as global
muscle shortening but also alterations in mechanical
intrinsic properties when compressive levels higher
than 30% were applied.
4 Conclusions
The effects of in vivo compression on rat TA muscle
were dependant of the compression level and of their
repetitive applications on both axial and transverse
force. By using compression, study of transverse
twitch may be able to bring information on transverse
phenomenon in muscle contraction.
0
0
10
Contact
20
30
40
50
Compression level (% )
Figure 3: Relationships between compression
level and amplitude of transverse twitch
expressed in % with respect to the maximal
value obtained during Ramp1.
Cumulative effects of successive ramp were
observed in twT-Amp (fig 3). For each ramp,
when compression level rose, twT-Amp
increased to a maximum (resp. 100%; 48%;
42%) since decreased more rapidly. This
References
[1] Piscione J, Gamet D (2006)
Eur J Applied Physiol, 97, 573-581.
[2] Ranatunga KW, Geeves MA (1991)
J Physiol, 441, 423-431.
[3] Brown T, Galea V, McComas A (1997)
Muscle Nerve, 20, 167-171.
[4] Gamet D, Piscione J (2008)
Comp Methods Biomech Biomed Eng ,11, S1, 97.