401. 32, No.
OURNAL
OF APPLIED
4, April
PHYSIOLOGY
1972.
Printed
in U.S.A.
Utilization
H. THYS,
of muscle
T.
FARAGGIANA,
elasticity
AND
R.
in exercise
MARGARIA
Institute of Human Physiology, University of Milan, Milan, Italy
AND R. MARGARIA.
Utilisation
of
1972.
J. Appl. Physiol. 32(4) : 491-494.
-The
exercise of deep bending on the knees from the erect position followed by extension of the legs to return to the upright
posture was performed by man under two different conditions:
the extension (positive work) followed immediately the bending
(rebound exercise), or alternatively a certain interval elapsed between the flexion and the extension to allow the extensors to relax
(no rebound exercise). This exercise was performed on a platform
sensitive to vertical acceleration; the O2 consumption
at steady
state was measured. The maximal speed measured during the extension was higher, the time of positive work was less, the mean
power and the mechanical efficiency were greater in the rebound
exercise. These differences are interpreted as evidence that elastic
potential energy stored in the muscles stretched during the negative work phase of the exercise is utilized for the performance of
positive work.
THYS,
H., T.
muscle elasticity
muscular
legs to return to the upright
position at a frequency
of 20
cycles/min,
as dictated by a metronome.
In one group of
experiments
(no rebound),
the flexion and the extension
were performed
at an interval
of 1.5 s; on the other (re-’
bound),
the extension
movement
took place immediately
after the flexion,
the total number
of cycles per minute
being the same.
The degree of the flexion movement
was approximately
the same in both conditions
and so therefore was also the
work performed.
The exercise was carried on for about 6 min, and the
oxygen consumption
at equilibrium
measured with a closedcircuit
method
(6). In Table
1 the oxygen consumption
values at steady state are given.
The platform
utilized
had strain gauges sensitive to vertical forces; by successive electronic
integrations
the vertical
speed and the displacement
of the center of gravity were
recorded (1). Horizontal
accelerations
(forward
and lateral)
were neglected;
the internal
work, such as that due to viscosity of the body tissues or to contraction
of the muscles
not leading to a displacement
of the center of gravity of the
body (isometric
or symmetrical),
was also disregarded.
Electromyogram
records were taken in some experiments with surface electrodes at the quadriceps
sural level
to insure that the extensor muscles really relaxed between
the flexion and the extension movement.
FARRAGGIANA,
in exercise.
power; efficiency of muscular
contraction
ET AL. (5) have observed
that an exercise consisting of bending the knees followed
by extension of the legs
to return to the erect position is performed
with less energy
expenditure
if the extension immediately
follows the act of
flexion. If, on the contrary,
between the flexion and the
extension phase, a certain interval is allowed, during which
relaxation
of the extensor muscles of the leg takes place,
the performance
of the exercise is more difficult
and the
oxygen consumption
appreciably
greater.
This has been interpreted
as due to the fact that, in the
first, case, the negative work performed
during the flexion
is partly transformed
into elastic energy on the stretched
contracted
extensors of the legs and thus is utilized
for the
performance
of positive work in the extension phase. This
is possible only if the extension takes place very soon after
the stretch phase and no relaxation
takes place between the
two phases.
Since in both cases the number of flexions and extensions
in a minute was the same, it was assumed that also the
mechanical
work performed
was the same and therefore
the efficiency of the exercise appeared to be much higher in
the first instance.
In this paper the mechanical
work was exactly measured
by having the subject perform the exercise on a platform
sensitive to vertical forces, and the actual activity of the
extensor muscles of the limbs recorded
electromyographitally.
MARGARIA
RESULTS
In Fig. 1 the original tracings obtained
from the platform
measurement
on one subject (HT) are shown for the exercise performed
in both ways described above.
When
the extension
follows
immediately
the flexion
(rebound),
the force exerted by the feet on the platform
during the positive work phase attains a much higher value
(250 kg) than when no rebound
takes place (150 kg), in
spite of an apparently
lesser and certainly
shorter electrical
activity. The lift speed is correspondingly
increased to about
1.45 m/s instead of 1.22 m/s. The downward
speed during
the flexion phase is also increased, thus increasing
the work
made on the contracted
muscles.
The EMG records for both the rebound
and the no rebound tests are given in Fig. 2, A and B. From the tracings
it appears clearly that when extension took place immediately after the flexion, no interruption
could be observed in
the electrical
activity of the quadriceps.
However,
in the
“no-rebound”
experiments
a period lasting about 0.6 s with
no electrical
activity is clearly seen between the two welldefined periods of electrical
activity caused by the flexion
and the extension. Electrical
activity appears to be maximal
in the first half of the extension phase under both experi-
METHOD
Six subjects (22-29 years old) performed
the exercise of
bending
deeply on the knees followed
by extension of the
491
492
TABLE
THY&
1. Mechanical
characteristics during positive-work
-
I LVt,
Subj
kg
Vertical
Lift
of Center of
Gravity,
m
No reh
.-
&&ma1
speed
of Lift of
Center of
Gravity,
m/s
Diff
%
Reb
1 Jo reb
keb
0.49
0.50
+2
1.28
1.36
+6
LP
80
0.55
0.54
-2
1.47
1.40
-5
SM
69
0.43
0.55
+28
1.03
1.43
+39
AZ
63
0.53
0.56
+6
1.19
1.43
+20
EG
75
0.44
0.49
1.15
1.62
$41
Av of first
subj
75
0.55
0.51
-7
five
0.481 3 0.52'
+9
-
1.14
1.16
+2
TF,
since
No reb
(0.92)
(1.05)
440
(0.59)
459
(0.62)
551
(0.74)
537
(0.72)
654
(0.88)
598
(0.80)
758
Reb
Efficiency
Diff
%
;o reb
Y
Reb
N 0 re1
Reb
+12
0.61
0.56
-8
1.88
1.40
-25
19
25
+31
+14
0.63
0.54
-14
2.05
1.70
-17
21
26
$24
+49
0.68
0.57
-16
1.73
1.33
-23
18
29
+61
+30
0.72
0.57
-21
1.82
1.30
-29
19
27
f42
+38
0.56
0.47
-16
1.97
1.70
-14
17
22
+29
+14
0.75
0.62
-17
1.77
1.70
-4
23
22
-13
1.89
1.49
-22
18.1
25.:
(1.02)
613
(0.82)
+29
I 1
he had
Diff
%
.-
634
(0.85)
783
-
Average
differences
do not include
subject
tained
in a skiing accident
months
before.
Duration
of the
Positive
Work
Phase, s
MARGARIA
--
-
Reb
564
(0.76)
685
+20
AND
exercise, and total energy cost
Diff
%
No reb
---
71
TF
of
Diff
%
HT
$11
phase
FARAGGIANA,
some
mental conditions:
it is appreciably
less in the flexion phase,
during
the negative work performance,
than in the phase
of extension (performance
of positive work).
In Table 1 individual
data are given for all subjects together with the duration
of the positive work phase and
the average power developed in the performance
of positive
work. The latter was calculated
by dividing
the potential
energy gain during the lift by the duration
of the positive
work phase.
Though
the extension of the movement
is little affected
by the rebound
(+9 %), the duration
of the positive work
difficulty
in performing
0
+37
the experiments
due
to a broken
tibia
sus-
phase decreases in all subjects during the rebound
exercise
(avg - 15 70); consequently,
the power developed
increases
constantly
(avg f29 %). The speed of lifting
increases
appreciably
(avg f21 %).
In spite of the fact that the potential
work performed
is
slightly increased
during
the exercise performed
with rebound
(+9 %), the oxygen
consumption
is always less
(averaging
-22 %), leading to an increase in the efficiency
that in the rebound,
reaches values about 40 % higher than
in the no rebound exercise.
DISCUSSION
FID. 1. Top tracing:
vertical
displacement
of center of gravity
in
on right)
or potential
energy
changes in joule
meters
(S,, ordinate
(E,, ordinate
on the left). S, = 0 indicates
position
of center of gravity
of the body while subject
is standing.
Flexion
downward,
extension
upward.
.Middle tracing:
vertical
component
of speed of center
of
gravity
of the body in meters per second, as obtained
by electronic
integration
from the bottom
tracing.
Bottom tracing: force exerted
by
the body on platform
(in kg) ; subject weight is 7 1 kg. Tracings
at left
refer to exercise performed
with a pause being allowed between flexion
and extension
(no rebound);
tracings
at right,
when extension
immediately
followed
flexion
(rebound).
A certain amount of bouncing
before lifting the body in
the no-rebound
performances
takes place spontaneously;
it
took some training
by the subjects to inhibit
completely
this movement
which,
however,
was limited
to an unappreciable
value, as indicated
in the tracing
of Fig. 1. In
spite of it, the differences
described
between
the rebound
and no rebound
exercises are clearly evident.
The greater speed of lifting attained in the rebound movement is presumably
due to two factors that contribute
to it,
the speed of muscle shortening
being, in fact, the sum of the
speed of shortening
of the contractile
elements
plus the
speed of shortening
of the series elastic elements which are
stretched in the negative work phase (1). The load on the
muscles limits the speed of shortening
of the contractile
elements which alone contribute
to the shortening
of the
muscles in the no-rebound
exercise, whereas
the series
elastic elements elongate with developing
tension.
The parallel
elastic elements of the muscle, even if we
consider
such things as stress relaxation
effects, certainly
do not appreciably
come into play in this exercise. A
stretching
of them occurs only at very high muscle lengths
that never occur in the muscle “in situ” but only in isolated
muscle preparations.
The increase of the average power is simply an effect of
the increase in the speed of the whole muscle shortening
and, therefore, of the speed of lifting the body as described.
MUSCLE
the
ELASTICITY
FIG. 2. A:
movement:
rebound
flexion
IN
493
EXERCISE
exercise.
lJ@er tracing:
downward,
extension
mechanical
record
of
upward.
Lower tracing:
The lower energy expenditure
in the rebound
experiments confirms the data by Margaria
et al. (5) and by Thys
(7).
The mechanical
efficiency of the movement
has been
calculated
by arbitrarily
considering
as mechanical
work
only the positive work and disregarding
the negative.
In
the no-rebound
experiments,
the efficiency turns out to be
about 0.19, a figure very similar
to that calculated
by
Margaria
(4) on exercises such as walking
on the level
(0.207) where the positive and negative work performed
are
the same, as the case is for the present experiments.
In the rebound
experiments,
efficiency attains appreciably higher values (avg = 0.26) because the mechanical
work performed
in the lifting phase is not all due to the
activity of the contractile
elements of the muscle, but is
partly accounted
for by the “elastic”
energy stored in the
muscles as an effect of the work done on the muscles during
the negative work phase of the exercise.
The energy cost of the negative work fraction of the exercise must have been very much the same in the two
modalities
of the performance,
since the speed of muscle
stretching
was not very different.
Furthermore,
the cost of
the negative work only amounts to a little more than onefifth the value of the same positive work (3, 4), and even
a considerable
difference
in the cost of the negative work
in the two exercises would not have affected appreciably
the large difference
in the total cost of the exercise. This
difference must therefore be attributed
substantially
to the
cost of the positive work performed
in the two conditions.
It may be questioned
whether
the squatting
posture held
for about 1 s/cycle in the no rebound
exercise requires a
different O2 consumption
than standing for the same time
in the no rebound
exercise. Actually
the O2 consumption
was tested for all subjects in the two postures (erect and
global electromyographic
exercise. Same indications
activity
of quadriceps
as in A.
sural.
B: no-rebound
squatting),
and the values were not appreciably
different, as
expected.
Cavagna et al. (2) found efficiency values even higher
than those reported
above for mechanical
work performed
in running
(0.40-0.50).
Probably
the mechanical
efficiency
in running
is so high because the elastic potential
energy
stored in the elastic elements of the stretched muscles, during the fall of the body on the ground,
is better utilized
than in the present rebound
exercise. The lengthening
of
the contracted
muscles in the stretching
phase of this exercise is presumably
too large, and the time course of the
stretching-shortening
events is not the optimal
one. In
running
the flexion-extension
movement
of the limbs is
very limited, as it is in most common exercises such as jumping, weight throwing,
etc.
Furthermore,
the time of the stretching-shortening
phase
of the active muscles in fast running,
where the number of
steps per second is 4-5, is much shorter than in the present
experiments,
and the utilization
of the elastic energy more
efficient.
The difference
between
the no rebound
and rebound
exercises here described supports the hypothesis that an appreciable
part of the positive work done by the muscles in
some exercises can be performed
by taking advantage
of
the elastic energy stored when stretching
the contracted
muscles. This is possible only if the positive work follows
immediately
the negative work; if the muscle is allowed to
relax the elastic energy is turned into heat.
This research
was supported
by the Italian
National
Research
Council
(CNR) .
H. Thys is on a fellowship
of the 163th district
of the International
Rotary
Club. Present address:
Laboratoire
de Physiologie
Humaine
Appliqute,
Universitt
de Liege, Sart-Tilman,
Belgium.
Received
for publication
27 September
1971.
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CAVAGNA, G. rZ., L. KOMAREK, G. CITTERIO,
Power
output
of the previously
stretched
AND
muscle.
R. MARGARIA.
In: Medicine and
Sport. Biomechanics
159-167.
II. Base1 & New-York:
Karger,
1971,
vol.
6, p.
494
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G. A., F. P. SAIBENE, AND R. MARGARIA.
Mechanical
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J. AppZ. Physiol.
19: 249-256,
1964.
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R. Sulla fisiologia
e specialmente
sul consumo
energetico della marcia
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ed inclinazione
de1
terreno.
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1938.
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339-35 1, 1968.
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R., G. A. CAVAGNA,
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di
THYS,
FARAGGIANA,
AND
MARGARIA
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