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After repeated jumps over an elastic surface (e.g. a trampoline), subjects usually report a strange sensation when they jump again overground (e.g. they feel unable to jump because their body feels heavy). However, the motor and sensory effects of exposure to an elastic surface are unknown. In the present study, we examined the motor and perceptual effects of repeated jumps over two different surfaces (stiff and elastic), measuring how this affected maximal countermovement vertical jump (CMJ). Fourteen subjects participated in two counterbalanced sessions, 1 week apart. Each experimental session consisted of a series of maximal CMJs over a force plate before and after 1 min of light jumping on an elastic or stiff surface. We measured actual motor performance (height jump and leg stiffness during CMJ) and how that related to perceptual experience (jump height estimation and subjective sensation). After repeated jumps on an elastic surface, the first CMJ showed a significant increase in leg stiffness (P < or = 0.01), decrease in jump height (P < or = 0.01) increase in perceptual misestimation (P < or = 0.05) and abnormal subjective sensation (P < or = 0.001). These changes were not observed after repeated jumps on a rigid surface. In a complementary experiment, continuous surface transitions show that the effects persist across cycles, and the effects over the leg stiffness and subjective experience are minimized (P < or = 0.05). We propose that these aftereffects could be the consequence of an erroneous internal model resulting from the high vertical forces produced by the elastic surface.
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Exp Brain Res (2010) 204:575–584
DOI 10.1007/s00221-010-2324-1
123
RESEARCH ARTICLE
The trampoline aftereVect: the motor and sensory modulations
associated with jumping on an elastic surface
Gonzalo Márquez · Xavier Aguado · Luis M. Alegre ·
Ángel Lago · Rafael M. Acero ·
Miguel Fernández-del-Olmo
Received: 4 February 2010 / Accepted: 2 June 2010 / Published online: 17 June 2010
© Springer-Verlag 2010
Abstract After repeated jumps over an elastic surface
(e.g. a trampoline), subjects usually report a strange sensa-
tion when they jump again overground (e.g. they feel unable
to jump because their body feels heavy). However, the
motor and sensory eVects of exposure to an elastic surface
are unknown. In the present study, we examined the motor
and perceptual eVects of repeated jumps over two diVerent
surfaces (stiV and elastic), measuring how this aVected max-
imal countermovement vertical jump (CMJ). Fourteen sub-
jects participated in two counterbalanced sessions, 1 week
apart. Each experimental session consisted of a series of
maximal CMJs over a force plate before and after 1 min of
light jumping on an elastic or stiV surface. We measured
actual motor performance (height jump and leg stiVness dur-
ing CMJ) and how that related to perceptual experience
(jump height estimation and subjective sensation). After
repeated jumps on an elastic surface, the Wrst CMJ showed a
signiWcant increase in leg stiVness (P·0.01), decrease in
jump height (P·0.01) increase in perceptual misestimation
(P·0.05) and abnormal subjective sensation (P·0.001).
These changes were not observed after repeated jumps on a
rigid surface. In a complementary experiment, continuous
surface transitions show that the eVects persist across cycles,
and the eVects over the leg stiVness and subjective expe-
rience are minimized (P·0.05). We propose that these
aftereVects could be the consequence of an erroneous inter-
nal model resulting from the high vertical forces produced
by the elastic surface.
Keywords StiVness · Internal models · Vertical jump ·
Perceptual illusion
Introduction
When we walk, run or jump, our musculoskeletal system
needs to adapt its stiVness according to the physical fea-
tures of the surfaces, in order to store and restore elastic
energy in the muscles and tendons (Cavagna 1977).
Changes in stiVness have been modeled by a spring-mass
model. According to this model, a single linear “leg spring”
and a point-mass, equivalent to the mass of the body, can
describe stiVness changes (Blickhan 1989). The stiVness of
the leg spring represents the stiVness of the integrated mus-
culoskeletal system (Farley et al. 1991, 1998; Farley and
González 1996; Ferris and Farley 1997; Ferris et al. 1998;
McMahon and Cheng 1990).
Many athletes include trampoline bouncing as part of
their practice regimen in order to improve their balance and
acrobatic skills (e.g. gymnastics, divers). By increasing leg
stiVness on an elastic surface, humans reduce the average
force required for jumping and, as such, increase the
mechanical work done by the surface (Ferris and Farley
1997). Anecdotally, people report an intriguing and strong
illusion when they attempt to perform a jump on the ground
immediately after jumping on the trampoline. They report
that their body is not able to detach itself from the Xoor and
additional muscular eVort is required to produce a jump
from a non-elastic surface. We refer to this illusion as the
trampoline aftereVect.
G. Márquez · Á. Lago · R. M. Acero · M. Fernández-del-Olmo (&)
Facultade de Ciencias do Deporte e a Educación Física
(INEF Galicia), Departamento de Educación Física e Deportiva,
Universidade da Coruña, Avd. Ernesto Che Guevara 121,
Pazos-Liáns, 15179 Oleiros, A Coruña, España, Spain
e-mail: mafo@udc.es
X. Aguado · L. M. Alegre
Facultad de Ciencias del Deporte,
Universidad de Castilla-La Mancha, Toledo, Spain
576 Exp Brain Res (2010) 204:575–584
123
Perceptive illusions of gait and posture have been
reported to be due to a sensory mismatch or sensory habitu-
ation (Lackner and Graybiel 1981; Pelah and Barlow 1996).
However, to date, there have been no studies of the trampo-
line aftereVect. Indeed, it remains unclear if the aftereVect
is related to changes in motor control, perception, or a
combination of these factors. To address this question, we
measured leg stiVness and jump height during counter-
movement jumps (CMJs) performed at maximal eVort, as
well as assessed perceptual judgments, after repetitive
jumps on two diVerent surfaces: a stiV surface (ground) and
an elastic surface (trampoline). We investigated the extent
to which motor and sensory adaptation can be inXuenced by
changes in surface stiVness. We hypothesized that repeti-
tive jumps on a trampoline would lead to perceptive as well
as motor aftereVects during subsequent jumps on the
ground.
Methods
Participants and general procedures
Fourteen healthy male subjects participated in this study
[mean age: 19.57 §3.8; mean weight: 70.30 §12.9; mean
height: 174.86 §7.6]. Participants were recruited from the
Faculty of Sport Sciences of University of A Coruña and
provided informed consent prior to participation. The
experimental procedure was approved by the Ethics
Committee of University de A Coruña.
Test jumps
The subjects were instructed to start in an upright position,
rapidly squat, and then jump into the air with maximal
eVort. The hands were positioned on the hips throughout
the test in order to eliminate the eVect of arm swing during
the performance of each jump. During the squat phase
of the movement, the angular displacement of the knee was
standardized so that the subjects were required to bend their
knees to approximately 90 degrees. A 90 degrees knee bend
was merely a reference value and not an excluding crite-
rion. For a more detailed description about Test jumps
(CMJs) performance see Bosco et al. (1983).
Perceptual judgments
After each CMJ performed on the force plate, the subjects
made two perceptual judgments about their performance.
First, they estimated the maximum height achieved (EH).
The estimated height (EH) of the jump was normalized to
the real height (RH). This ratio EH/RH provided quantita-
tive information of accuracy (1 = maximal accuracy).
In addition, the deviations from one indicate an estimation
bias (values lowers than one is an underestimation of the
height while values bigger than one is an overestimation).
Second, the participants were required to give a subjec-
tive rating of their performance. We used a perceptual scale
similar to that used by Flanagan and Beltzner (2000). Used
a 10-point scale, the subjects were asked to give a score,
comparing the performance of the vertical jump to CMJs
performed at the beginning of the session. A value of one
represented a jump with the same perceptual sensations
while 10 represented a jump with completely diVerent per-
ceptual sensations.
Adapting jumps (repetitive jumps)
During the adapting phase on the elastic and stiV surfaces,
the subjects were required to jump keeping their hands on
their hips. In order to equate the number and rate of jumps
in both surfaces, the subjects jumped in synchronization
with a metronome at a rate of 1 Hz. This was of importance
since the jumping frequency has been shown to aVect the
leg stiVness (Farley et al. 1991; Hobara et al. 2010). This
1 Hz rate was chosen from pilot experiments that showed
that this rate approximated that observed when people per-
formed self-paced jumps on the trampoline.
In order to minimize muscular fatigue during the repeti-
tive jumps, the subjects were asked to jump at low intensity.
Protocol
Main experiment
A week prior to the experimental sessions, subjects prac-
ticed the counter movement jump (CMJ). They were also
trained to make the perceptual judgments.
Two experimental sessions were conducted, separated
by a 1-week interval. In one session, the repetitive jumps
were performed on an elastic surface (trampoline), and in
the other session, on a stiV surface (ground). The order of
these two sessions was counterbalanced.
Each experimental session started with a standardized
warm-up protocol to ensure that the subject performed the
vertical jumps with maximal eVort without risk of injury.
The warm-up ended with the subject performing three
CMJs at maximal intensity on a force plate installed at
ground level. After each jump, the subject was provided
with feedback regarding the maximum height of the jump.
This was the only feedback given during each experimental
session. The subject then performed three more CMJs with
an inter-trial interval of 30 s. We used the average of the
three CMJs as a baseline (CMJbsl). After a 1-min break, the
subject performed repetitive jumps (at 1 Hz) for 1 min on
the selected surface (trampoline or ground). Immediately
Exp Brain Res (2010) 204:575–584 577
123
after the repetitive jumps, the subject performed six more
maximal CMJs on the force plate (CMJ1 to CMJ6). After
each CMJ, the subject was asked to make two perceptual
judgments, one rating the maximum height achieved, and
one rating the subjective experience. The subjects jumped
inside an indoor facility with their eyes open and with their
body oriented to the same direction at all times. Thus, the
visual cues were kept constant.
Complementary experiment
We conducted a complementary experiment to examine
repetitive eVects of adaptation. Given that adaptation eVects
were most salient on the Wrst CMJ after adaptation, we had
participants alternate between bouts of repetitive jumping
on the trampoline followed by a single CMJ on the ground.
After the warm-up, the subject performed 3 CMJs on the
force plate. We used the average of these as a baseline
(CMJBSL1). The subject then completed 1 min of repetitive
jumping on the trampoline, followed by a single maximal
CMJ on the force plate. This cycle of 1 min adaptation fol-
lowed by a single jump was repeated eight times, with each
cycle separated by 1 min of rest. The CMJs are referred to
as CMJBK1 to CMJBK8. After the Wnal cycle, the subject per-
formed two more CMJs on the force plate without jumping
on the trampoline (CMJBSL2, CMJBSL3). As in the previous
experiment, the subject was asked to make the two percep-
tual judgments after each CMJ.
Apparatus and data analysis
The elastic surface was a trampoline Wtted at Xoor level,
with a surface of 3 £1.5 m connected to 118 springs along
the outer edge, resulting in a linear stiVness of 8.9 kN/m.
The stiVness of the elastic surface was checked using static
load tests (up to 2,000 N, see Arampatzis et al. 2001) in
which weights were placed on the center of surface, and the
displacement of the surface was measured (Ferris and
Farley 1997). The linear regression between force and dis-
placement was high (r2= 0.99). The stiV surface was a
50.8 £46.4 cm force plate (AMTI, Newton, MA. Surface
stiVness = 35,000 kN/m, see Ferris and Farley 1997).
All CMJs were performed on the force plate, and signals
were sampled at 1,000 Hz. We computed vertical accelera-
tion (from the ground reaction forces [GRF]) to obtain the
vertical velocity and displacement of the center of mass
[CoM] using the double integration method (Cavagna
1975). The height of the jump was obtained from the veloc-
ity value at the moment of takeoV using the following equa-
tion: H=v²/2g where v is the takeoV velocity and g the
gravitational acceleration. Leg stiVness during the CMJ was
deWned as Fpeak/L, where Fpeak is the peak GRF (which
correspond to the lowest position of the CoM), and L is
the vertical displacement of the CoM from the starting posi-
tion to the lowest position (Ferris and Farley 1997; Liu
et al. 2006; Fig. 1).
Statistical analysis
Two-way ANOVAs of repeated measures were performed
with surface (elastic or stiV) and trial (CMJbsl, CMJ1, CMJ2,
CMJ3, CMJ4, CMJ5, CMJ6) as factors. The ANOVAs were
performed for the following variables: height, leg stiVness,
Fpeak, L, and EH/RH ratio. For the analysis of the subjec-
tive experience, the trial factor was reduced to six levels
(CMJ1, CMJ2, CMJ3, CMJ4, CMJ5, CMJ6) since CMJbsl was
used as reference (value equal to one for all the subjects).
For the complementary experiment, one-way ANOVAs
were performed with trial (CMJBSL1, CMJBK1, CMJBK2,
Fig. 1 Spring-mass model during vertical countermovement jump
performed over a force plate. The Wgure shows diVerent phases of the
CMJ: a Initial position; b End of the squat movement; c Take oV. Leg
stiVness is calculated using the ratio between peak GRF (Fpeak) and the
vertical displacement of the CoM (L) in the moment of its lower
position
578 Exp Brain Res (2010) 204:575–584
123
CMJBK3, CMJBK4, CMJBK5, CMJBK6, CMJBK7, CMJBK8,
CMJBSL2, CMJBSL3) as the mean factor. The ANOVAs
were performed for the following variables: height, leg
stiVness, Fpeak, L, EH/RH ratio, and perceptive judgement
scale.
In both experiments, post hoc analysis was performed
using t test with Bonferroni correction. Statistical signiW-
cance was set at P·0.05.
Results
Main experiment
Measures of motor performance
The ANOVA showed a main eVect of trial (F=2.82,
P= 0.016) and a signiWcant surface * trial interaction
(F=2.96, P= 0.013) for leg stiVness (Fig. 2a). After repet-
itive jumps on the trampoline, leg stiVness increased sig-
niWcantly on CMJ1 in comparison with CMJbsl (P= 0.002).
Leg stiVness returned to baseline values by CMJ2 (CMJbsl
vs. CMJ2: P> 0.05). Further evidence of the rapid return to
baseline is supported by the observation that leg stiVness
was also higher on CMJ1 compared to the last four CMJs
(CMJ3 to CMJ6: P·0.05 for all comparisons). In contrast,
adaptation on the rigid surface produced no changes in leg
stiVness across the series of test jumps. Finally, in a com-
parison across surfaces, leg stiVness on CMJ1 was greater
following adaptation on the elastic surface compared to the
rigid surface (P=0.039).
Regarding the L, the ANOVA showed a main eVect of
trial (F= 3.192, P= 0.028) and a signiWcant surface * trial
interaction (F=3.00, P= 0.012; Fig. 2d). After repetitive
jumps on the trampoline, L measures decreased signiW-
cantly on CMJ1 and CMJ2 in comparison with CMJbsl
(P·0.05) and returned to baseline values by CMJ3
(CMJbsl vs. CMJ3 to CMJ6, P> 0.05). L was also lower on
CMJ1 and CMJ2 compared to the last four CMJs (CMJ3 to
CMJ6: P·0.05 for all comparisons). However, adaptation
on the rigid surface produced no changes in the L dynam-
ics across the series of test jumps. Moreover, in a compari-
son across surfaces, L on CMJ1 and CMJ2 was lower after
the elastic surface compared to the rigid surface (P= 0.010
and P= 0.049 for CMJ1 and CMJ2, respectively). In rela-
tion to the peak force data, this parameter did not show any
signiWcant changes across the jumps and conditions
(Fig. 2c).
Turning to jump height, the ANOVA showed a main
eVect of trial (F= 10.78, P·0.0001) and a signiWcant
surface * trial interaction (F=2.42, P= 0.035). Repetitive
jumps on the trampoline led to a signiWcant decrease in height
on CMJ1 in comparison with CMJbsl (P= 0.005, see Fig. 2b).
Height increased over subsequent jumps, resulting in
signiWcant diVerences between CMJ1 and CMJ4 to CMJ6
(P·0.05 for all comparisons). CMJ3 to CMJ6 did not diVer
from CMJbsl. As with stiVness, adaptation on the rigid sur-
face produced no changes in jump height. A comparison
across the two adapting surfaces revealed that the height
reached in CMJ1 after jumping on the trampoline was
signiWcantly lower than that reached in CMJ1 after repeti-
tive jumps on the stiV surface (P= 0.04).
Measures of perceptual judgments
The analysis of ratio EH/RH showed a signiWcant main
eVect for surface (F= 17.62, P= 0.001), trial (F= 6.41,
P= 0.01), and a surface * trial interaction (F=7.21 P=
0.007). Adaptation on the elastic surface led to a signiWcant
decrease of EH/RH (Fig. 2e). That is, the subjects underes-
timated how high they jumped. This eVect was limited to
the Wrst jump (P·0.05 for all paired comparisons). Thus,
after the elastic surface, the subjects were jumping lower
than before and they underestimated their jump height.
For the stiV surface, there were no changes in EH/RH. The
EH/RH ratio in CMJ1 and CMJ2 after jumping on the
trampoline was signiWcantly lower in comparison with
repetitive jumps on the stiV surface (P=0.006 and
P= 0.012, respectively).
The analysis of the scores of the subjective ratings
showed a signiWcant main eVect for surface (F= 27.6,
P·0.0001), trial (F=40.52, P·0.0001), and a
surface * trial interaction (F=31.17, P·0.0001). Post hoc
analysis revealed signiWcantly larger scores for the elastic
than for the stiV surfaces for CMJ1, CMJ2, and CMJ3 trials
(P·0.0001, P·0.0001, and P·0.05, respectively). The
altered perception of the jumps after the trampoline
decreased signiWcantly from CMJ1 to the CMJ4 trial (CMJ1
vs. CMJ2, P·0.0001; CMJ2 vs. CMJ3, P·0.0001; CMJ3
vs. CMJ4, P·0.01). No diVerences were found between
the last three trials (CMJ4, CMJ5, and CMJ6; Fig. 2f).
Complementary experiment
The ANOVA showed a signiWcant main eVect of trial
(F=8.96, P·0.0001) for leg stiVness (Fig. 3a). Leg stiV-
ness increased on jumps performed after immediately each
block of repetitive jumps on the trampoline in comparison
with CMJBSL1 (P·0.05 for all paired comparisons). How-
ever, the leg stiVness for CMJBSL1 was not diVerent than the
CMJBSL2 and CMJBSL3, indicating that leg stiVness recov-
ered quickly after the last block of jumps on the trampoline.
Interestingly, we found signiWcantly greater leg stiVness for
the CMJBK1 in comparison with CMJBK4, CMJBK5,
CMJBK6, CMJBK7, and CMJBK8 (P·0.05 for all these
comparisons). These results indicate that the eVects of
Exp Brain Res (2010) 204:575–584 579
123
adaptation from trampoline jumping were reduced with
successive inducing cycles of adaptation. Similar results
were found for L parameter. In this parameter, the
ANOVA showed a signiWcant main eVect of trial
(F= 9.817, P·0.0001; Fig. 3d). L decreased on jumps
performed after each block of repetitive jumps on the tram-
poline in comparison with CMJBSL1, CMJBSL2, CMJBSL3,
(P·0.05 for all paired comparisons). There were no
signiWcant diVerences between CMJBSL1, CMJBSL2, and
CMJBSL3, indicating a total recovery after the last block of
Fig. 2 Mean (§SEM) of leg stiVness (a), jump height (b), peak force
(c), L (d), ratio EH/RH (e), and perceptual scale scores (f) of CMJs
before and after 1 min of repetitive jumps (shaded square) over stiV or
elastic surfaces. (#) DiVerences between surfaces. (*) DiVerences be-
tween jumps on the elastic surface. Note that no changes were found
between jumps on the rigid surface. (#), (*) P·0.05; (##), (**)
P·0.01; (###), (***) P·0.001
580 Exp Brain Res (2010) 204:575–584
123
jumps on the trampoline. In addition, we did not Wnd sig-
niWcant changes for L dynamics across blocks. Regarding
the peak force the ANOVA did not show any signiWcant
changes across trials (Fig. 3c).
The height of the jump showed a main eVect of trial
(F=7.42; P·0.0001; Fig. 3b). Jump height was reduced
after each block of repetitive jumps in comparison with the
CMJBSL1 (P·0.05 for all paired comparisons). No diVer-
ences were found between the initial baseline and the Wnal
two jumps (CMJBSL2 and CMJBSL3). Unlike the stiVness
measure, we did not see a change in the modulation of jump
height across successive adapting cycles.
The analysis of EH/RH ratio showed similar results to
that of the jump height (Fig. 3e). There was a main eVect of
trial (F= 10.00, P·0.0001) and the post hoc tests showed
that the EH/RH ratio after each block of repetitive jumps
was signiWcantly lower than CMJBSL1 (P·0.05 for all
paired comparisons). There were no diVerences between
CMJBSL1 and CMJBSL2 and CMJBSL3, showing a total
recovery of the EH/RH ratio after the initial post-adaptation
jump following the last cycle. The underestimation of jump
height persisted across the eight cycles of adaptation.
Regarding the subjective experience scores, there was a
main eVect of trial (F=45.66, P·0.0001). SigniWcantly
Fig. 3 Mean (§SEM) of leg stiVness (a), jump height (b), peak force
(c), L (d), ratio EH/RH (e), and perceptual scale scores (f) of CMJs
before and after blocks of 1 min repetitive jumps (shaded square) over
elastic surface. With exception of the peak force, the baseline values
(CMJBSL1, CMJBSL2, CMJBSL3) were always signiWcantly diVerent
to the CMJs performed immediately after the repetitive jumps
(from CMJBK1 to CMJBK8). SigniWcant changes were found across
CMJBK for leg stiVness and perceptual scale scores (statistic symbols
not included in the Wgure), indicating a partial adaptation to the elastic
surface. (*) indicates signiWcant diVerences between CMJBSL and
CMJBK1—CMJBK8
Exp Brain Res (2010) 204:575–584 581
123
higher scores were observed for jumps after each block in
comparison with the CMJ baseline (P·0.001 for all paired
comparisons). The score in the CMJBK1 was signiWcantly
higher than that for CMJBK6, CMJBK7, and CMJBK8
(P= 0.013, P=0.05, and P= 0.04, respectively). These
results indicate that subjects’ impression of their jumps
improved across the cycles (Fig. 3f).
Discussion
Following repetitive jumps on a trampoline, people exhib-
ited increased leg stiVness and decreased jump height when
asked to perform a single jump on a stiV surface (i.e. the
ground). Moreover, these changes in motor performance
were also associated with changes in the subjects’ subjec-
tive experience of their performance.
It is important to emphasize that no previous studies
have assessed sensorimotor aftereVects that involve a glo-
bal body high-speed movement. Although we are aware of
the diYculty of such an approach, we also believe that this
study can contribute to a better understanding of how the
current knowledge about sensorimotor adaptation applies to
skills that are habitual in the Weld of sports.
The motor aftereVects
The increase of the leg stiVness during the Wrst vertical
jump after repetitive jumps on the trampoline was a conse-
quence of the exposure to the elastic surface since there
were no changes in the stiVness after repetitive jumps over
the ground. Humans can adjust their musculoskeletal sys-
tem to accommodate changes in surface stiVness, allowing
them to maintain similar mechanics on diVerent surfaces
(Farley et al. 1998; Ferris and Farley 1997; Ferris et al.
1998; Moritz and Farley 2005). Most of these studies refer
to rhythmic movements (hopping and running) rather than a
discrete task such as the CMJ. Using the CMJ was impor-
tant in this study for two main reasons. First, it allowed us
to obtain an objective measurement of the motor perfor-
mance. Second, the subjects were able to report the subjec-
tive perception of their performance for each jump. When
we jump on an elastic surface, leg stiVness increases; the
opposite is true for jumping on rigid surfaces (Ferris and
Farley 1997). The current results show that the increased
stiVness induced during trampoline jumping was main-
tained when the subjects Wrst jumped on the rigid ground
surface. This increased stiVness after the jumps on the elas-
tic surface was the result of a decrease in the displacement
of CoM, while the peak force remained unaVected. This
modiWcation of the displacement of CoM may be related to
feedback from spindle muscles. However, recordings of
neurophysiologic parameters need to be made in order to
evaluate possible mechanism underlying the observed stiV-
ness changes.
Our results are similar to the aftereVects observed in
response to the Coriolis or inertial perturbation during
movements of the arms (Lackner and DiZio 1994), legs
(DiZio and Lackner 1997), and head (DiZio and Lackner
1995). These aftereVects are not context dependent since
they occur when subjects carry over their adaptation from a
rotating environment into a stationary one, indicating that
the form of adaptation involves an internal model of the
anticipated perturbation (DiZio and Lackner 2003). In line
with this analogy, when subjects performed a new vertical
jump over the ground after jumping on the elastic surface,
an aftereVect was observed as a consequence of the errone-
ous predictions of the motor controller, resulting in a stiVer
limb than was actually needed.
Several studies have focused on the inXuence of surface
stiVness on leg stiVness during single events, such as a drop
jump or landing from a jump (Moritz and Farley 2004; van
der Krogt et al. 2009). These studies reported immediate
changes in the leg stiVness during the landing on both
expected and unexpected surfaces. This rapid change in leg
stiVness (52 ms after the landing) may be due to a passive
mechanism and not due to neural feedback (Moritz and
Farley 2004). Passive adaptation may be critical when
negotiating disturbances during locomotion across a vari-
able terrain (van der Krogt et al. 2009). However, in our
study, one trial was suYcient to recover the correct leg stiV-
ness. Thus, a passive mechanism does not seem appropriate
for accounting for our results.
One trial adaptation eVects have also been observed for
unexpected changes in surface friction (Johansson and
Westling 1988) and object shape (Jenmalm and Johansson
1997). Reynolds and Bronstein (2003) showed a quick
adaptation in the gait kinematics when subjects walked
onto a stationary escalator. The recovery of the leg stiVness
could be explained by a forward model of the forthcoming
vertical jump (Bobbert et al. 2008). Such models can be
trained and updated using prediction errors by comparing
the predicted and actual outcome of a motor command
(Wolpert and Flanagan 2001). Thus, the subjects could use
the error between the predicted and actual sensory feedback
occurred in the Wrst CMJ after the trampoline, in order to
update their internal model for the next jump.
After repetitive jumps on the trampoline, there was a
decrease in the jump height in comparison with the previ-
ous jumps over the ground. This decrease in height could
be a direct consequence of the change in leg stiVness
(Bojsen-Møller et al. 2005; Farley et al. 1991; Liu et al.
2006). However, we cannot rule out other factors since
jumping height is inXuenced by a combination of biome-
chanical, neural, metabolic, and morphological factors
(Asmussen and Bonde-Petersen 1974; Bosco et al. 1982;
582 Exp Brain Res (2010) 204:575–584
123
Kubo et al. 1999; Le Pellec and Maton 1999; Voigt et al.
1995).
Perceptive illusion
After repetitive jumps on the trampoline, the subjects
reported a strong alteration in their subjective experience.
When asked at the end of the experiment, to describe their
subjective perception during the Wrst post-adaptation jump,
subjects usually reported very strong perceptions such as
“I felt that I was not able to push my body upwards” or
“I felt very heavy, as my legs suddenly increased their
weight”. Others’ studies have described perceptive illusions
in the Weld of gait and posture (Lackner and Graybiel 1981;
Pelah and Barlow 1996; Hashiba 1998). Our results show
that whole body fast movements (such as vertical jumps)
can also result in strong perceptual illusions after a brief
exposure to a new environment.
In addition to their qualitative impressions, the subjects
also underestimated the actual height of their post-adapta-
tion jumps. Note that this underestimation is relative to the
actual height achieved. Thus, they not only achieve a lower
maximum jump height, but their experience magniWes this
reduction. Although the mechanisms underlying such per-
ceptual illusions are not well understood, it has been pro-
posed that they are produced by a sensory conXict between
the visual, vestibular and somatosensory signals and the
eVerence copy (Hashiba 1998). Jumps on the trampoline
could lead to a new recalibration of sensory inputs due to
continuous high acceleration of the body in the push-oV
phase of the jumps (around 0–4 g acceleration; Bhattach-
arya et al. 1980; Sovelius et al. 2008). This new recalibra-
tion between sensory modalities could disturb the
relationship between self-induced motion and the expected
sensory inputs when the subjects jump again on the ground.
This could explain the erroneous estimation of the jump
height reported by the subjects.
Lackner and Graybiel (1981) demonstrated an illusion of
self-motion when subjects executed deep knee bends in the
high force phase (2 g acceleration) of a parabolic Xight. The
subjects reported a perception of having moved downwards
too rapidly and the support surface moving upwards under
their feet. When they returned to 1 g acceleration level,
subject experienced their movements as abnormal: the
substrate of support and the visual world seemed unstable.
The occurrence of such illusions could indicate that motor
control is actively calibrated to a 1 g reference level. In the
present study, subjects reported feeling the rigid surface
stationary rather than moving upwards, after repetitive
jumps on the trampoline. This suggests that departures
from a 1 g reference level aVect both the execution and
appreciation of voluntary movement (Lackner and Graybiel
1981; Lackner and DiZio 2000).
Notably, in our study, repetitive jumps on the ground did
not lead to perceptual distortions of subsequent, singular
jumps on the ground. According to the sensory conXict
hypothesis, this would be due to the fact that both repetitive
and CMJ jumps were performed over the same surface and
thus, there were no changes in the gravitational Weld that
would require a new recalibration.
Persistence of the aftereVects
The second experiment explored how the motor and per-
ceptive aftereVects were aVected by repeated exposure to
elastic-stiV surface transitions. The results show that con-
tinuous elastic-stiV surface transitions did not aVect these
aftereVects, although they slightly minimized the eVects of
the stiVness and subjective perception.
The decrease of the leg stiVness across the transitions
suggests that, with experience, the sensorimotor system can
adjust or anticipate the mechanical properties of diVerent
surfaces in an environment. However, leg stiVness contin-
ued to show some eVects of each round of adaptation on the
trampoline. The same trend was observed for the displace-
ment of CoM while the peak force remained unaVected
across the jumps. It is unclear if this eVect would be totally
abolished if we had increased the number of adaptation
cycles.
In contrast to the leg stiVness, the height of the jump did
not show any changes across successive cycles. As we
mentioned above, the height of a jump is a variable aVected
by multiple factors. Thus, it is possible that the small adap-
tation in the leg stiVness may not be suYcient to revert the
eVect of the trampoline on the jump height.
We also observed a related dissociation between the two
perceptual measures. Although the altered subjective per-
ception produced by the Wrst block of jumps on the trampo-
line did not disappear in the following blocks, it was
progressively diminished. These results can be interpreted
according to the sensory mismatch hypothesis (Wolpert
et al. 1995; Blakemore et al. 1998). The subjective percep-
tion could arise due to an erroneous forward model where
there is a mismatch between predictive and actual sensory
feedback. In contrast, the subjects continued to underesti-
mate their performance across the cycles. This result was
surprising since we would expect a diminution in the esti-
mation error to parallel with the trend observed for the sub-
jective perception.
It may be subjective impressions and judgments of
achieved height are diVerent measures of sensory after-
eVects. The former could be more closely linked to with the
sensorimotor system and the latter with a cognitive/percep-
tual system. While the subjective perception provides infor-
mation about the sensations experienced by the subject, the
height estimation is the result of a cognitive process that
Exp Brain Res (2010) 204:575–584 583
123
translates perceptual and motor components of the move-
ment into a self-performance measurement. In line with this
idea, it has been shown that the sensoriomotor system can
operate independently of the cognitive/perceptual system
(Flanagan and Beltzner 2000), which could explain the
diVerence in results obtained for the subjective perception
and the error estimation.
In summary, the current results show that repetitive
jumps on an elastic surface lead to motor and perceptual
changes in subsequent jumps on a stiV surface. Adaptation
on an elastic surface led to an increase in leg stiVness and a
decrease in jump height. Moreover, the subjects underesti-
mated their jump height and had the subjective impression
that their jumps were performed more poorly than before
adaptation. These changes were stables across the continu-
ous surface transitions, since no changes were observed in
the height estimation nor in the jump height. These after-
eVects likely reXect adjustments in an internal model from
the elastic surface that carry over into movements produces
on the stiV surface.
Acknowledgments We thank Noa Fogelson and Richard Ivry for the
revision of the manuscript. This work was supported by Xunta de
Galicia (2009/002).
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The purpose of this study was to gain insight into the control strategy that humans use in jumping. Eight male gymnasts performed vertical squat jumps from five initial postures that differed in squat depth (P1-P5) while kinematic data, ground reaction forces, and electromyograms (EMGs) of leg muscles were collected; the latter were rectified and smoothed to obtain SREMGs. P3 was the preferred initial posture; in P1, P2, P4, and P5 height of the mass center was +13, +7, -7 and -14 cm, respectively, relative to that in P3. Furthermore, maximum-height jumps from the initial postures observed in the subjects were simulated with a model comprising four body segments and six Hill-type muscles. The only input was the onset of stimulation of each of the muscles (Stim). The subjects were able to perform well-coordinated squat jumps from all postures. Peak SREMG levels did not vary among P1-P5, but SREMG onset of plantarflexors occurred before that of gluteus maximus in P1 and > 90 ms after that in P5 (P < 0.05). In the simulation study, similar systematic shifts occurred in Stim onsets across the optimal control solutions for jumps from P1-P5. Because the adjustments in SREMG onsets to initial posture observed in the subjects were very similar to the adjustments in optimal Stim onsets of the model, it was concluded that the SREMG adjustments were functional, in the sense that they contributed to achieving the greatest jump height possible from each initial posture. For the model, we were able to develop a mapping from initial posture to Stim onsets that generated successful jumps from P1-P5. It appears that to explain how subjects adjust their control to initial posture there is no need to assume that the brain contains an internal dynamics model of the musculoskeletal system.
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