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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
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
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
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
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
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.
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
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
578 Exp Brain Res (2010) 204:575–584
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
In both experiments, post hoc analysis was performed
using t test with Bonferroni correction. Statistical signiW-
cance was set at P·0.05.
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
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
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
Exp Brain Res (2010) 204:575–584 581
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).
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
Kubo et al. 1999; Le Pellec and Maton 1999; Voigt et al.
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
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
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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... However, on the reduced surface stiffness, adults lengthen the legs upon foot-strike and shorten them before take-off (Moritz and Farley 2005). The magnitude of surface stiffness that elicited this effect is consistent with what has been reported for mini-trampolines (Márquez et al. 2010). However, there is little knowledge on the biomechanical or neuromuscular strategies employed on a mini-trampoline. ...
... However, the subjects were constrained to hop at a frequency of 2.2 Hz, which is preferred for a stiff surface. On a mini-trampoline with a surface stiffness of 8.9 kNm −1 , adults preferred to hop around 1 Hz (Márquez et al. 2010). Therefore, modulation of movement on a mini-trampoline when hopping at a preferred frequency might correspond to a distinct pattern. ...
... Through pilot testing, we found that children and adults preferred 1.5 Hz on the mini-trampoline. We estimated the mini-trampoline stiffness to be 7.85 kNm −1 (Beerse and Wu submitted for publication), using fifteen known masses and measuring the surface vertical displacement (Arampatzis et al. 2001;Márquez et al. 2010). For comparison, the force plate stiffness was reported to be 70,000 kNm −1 (AMTI, MA, USA). ...
Full-text available
Improved balance control is an often-cited potential benefit for trampoline interventions. However, it is unknown whether the soft, elastic surface of a trampoline elicits different motion and neuromuscular strategies between adults and children. Therefore, the purpose of the study was to evaluate the center-of-mass (COM) dynamics and neuromuscular strategies for hopping on a mini-trampoline in adults and children. Fourteen children aged 7–12 years and 15 adults aged 18–35 years hopped on a stiff surface and a mini-trampoline. We evaluated the vertical displacement of COM and leg length, as well as the horizontal displacements between hops. We also assessed muscle activation from tibialis anterior, lateral gastrocnemius, biceps femoris, and vastus lateralis during time periods surround landing and estimated fatigue across the hopping cycles. Our results indicated both groups used spring-like leg dynamics to regulate the COM movement while hopping on a mini-trampoline. Children increased horizontal displacements between hops on the mini-trampoline, requiring greater muscle activation during time-periods associated with proprioceptive input. Moreover, children might not have developed the adult-like ability to appropriately adjust muscle pre-activation for feedforward control. Hopping on a mini-trampoline might increase proprioceptive information and postural demand compared to a stiff surface while reducing neuromuscular fatigue.
... The participants were 20 healthy, young adults (10 men and 10 women) with mean age = 23.3 ± 1.23 years, mean body mass = 69.7 ± 7.45 kg, and mean height = 173.1 ± 7.5 cm. Márquez et al. (2010) performed a study that investigated motor adaptation (among other variables) in jumping on a stiff surface after hopping on a trampoline with 14 subjects. Based upon their effect size, we included 20 participants in order to improve our likelihood of demonstrating a change in consistency with repetition. ...
... Therefore, the displacement of both extremities will be very close to equal so long as the extremities are of the same approximate length. Márquez et al. (2010) studied motor adaptation of drop jump for healthy, young men after hopping on a trampoline. They found that most biomechanical variables during drop jump after trampoline hopping converged to the same values as hard ground hopping within six trials. ...
... Based upon those findings, we compared the mean CVs of the participants' first five trials to the last five trials (to ensure that sufficient withinsubject trials were averaged to represent an accurate mean) using one-tailed, paired t test. We justified the one-tailed test based upon findings of Emken and Reinkensmeyer (2005) and Márquez et al. (2010) showing that adaptation of biomechanical variables (including ground reaction force) does take place with repetitions. ...
Most studies of high-speed lower body movements include practice repetitions for facilitating consistency between the trials. We investigated whether 20 repetitions of drop landing (from a 30.5-cm platform onto a force plate) could improve consistency in maximum ground reaction force, linear lower body stiffness, depth of landing, and jump height in 20 healthy, young adults. Coefficient of variation was the construct for variability used to compare the first to the last five repetitions for each variable. We found that the practice had the greatest effect on maximum ground reaction force (p = .017), and had smaller and similar effects on lower body stiffness and depth of landing (p values = .074 and .044, respectively), and no measurable effect on jump height. These findings suggest that the effect of practice on drop landing differs depending upon the variable measure and that 20 repetitions significantly improve consistency in ground reaction force.
... Heitkam et al. showed that including a mini-trampoline in a balance circuit training led to a better balance performance after only 6 weeks (Heitkamp et al., 2001). Further, a repetitive jumping program on a trampoline caused an improvement of balance due to a reduced forward translation in the jump (Ross & Hudson, 1997), and according to Marquez, many athletes include trampoline in their practice to improve their balance (Márquez et al., 2010). ...
... According to what was mentioned previously, mini-trampoline training may enhance balance and jump performance and can lead to the improvement of athletic performance. However, it is noteworthy that most studies on trampoline jumping focused on the long-term effects of trampoline training, when there has been little research on its effect acutely (Márquez et al., 2010). If trampoline jumping presents immediate benefits in terms of jump performance and balance, it may be an efficient exercise to include for a shortterm preparation (e.g. ...
... The improvement achieved by both groups might be due to a learning effect. These results contradict the conclusions of a study by Márquez et al. (Márquez et al., 2010) that showed that 1-min of jumping on a trampoline increases leg stiffness and decreased jump height. This might be due to the difference in the number of jumps performed, as in the previous study, the participants jumped on a trampoline for 60 seconds, which may have led to fatigue. ...
Full-text available
Jumping and balance are necessary skills for most athletes, and mini-trampoline training has been shown to improve them. Little is known about the acute effect of mini-trampoline jumping on jump performance and dynamic balance. Objectives: The purpose of this study is to investigate the effect of 6 maximal jumps on a mini-trampoline on countermovement vertical jump (CMVJ) variables and on balance parameters. Methods: Twenty one recreationally trained individuals participated in three testing sessions and were either allocated to a control group (N=10) or a trampoline group (N=11). All the participants performed a dynamic warm up prior to their assessments. Baseline CMVJ and balance assessments were measured. For the jump performance tests, the control group rested for 30s, and the trampoline group performed 6 maximal CMVJs on a mini-trampoline. Immediately following the trampoline jumps or the rest period, participants performed three jump trials. The jumping protocol was repeated every minute up to 5 minutes and balance was reassessed immediately after only. Results: There was no significant interaction of time by group and no group effects in all the jumping parameters, however, there was a significant increase in jump height (p <0.001) post-condition, and a significant decrease in peak power (p= 0.01) at the 4th minute for both groups. There was no significant interaction of time by condition, no time effect and no group effect (p>0.05) on the balance variables. Conclusion: These results do not support our hypothesis and show that trampoline jumping does not improve jump and balance performance acutely.
... [25] They needed to exert muscle force and neuromuscular responses to stiffen their legs in order to overcome the unstable conditions. [28] Different surface hardness can have an impact on motor and perceptual change. [28] When walking or jumping, the human musculoskeletal system can modify its stiffness in response to the physical characteristics of the surface. ...
... [28] Different surface hardness can have an impact on motor and perceptual change. [28] When walking or jumping, the human musculoskeletal system can modify its stiffness in response to the physical characteristics of the surface. [29] Improved leg stiffness due to exercise on an elastic surface could decrease the average muscle force required for exercise activity by increasing the external force produced by the elastic surface. ...
... [29] Improved leg stiffness due to exercise on an elastic surface could decrease the average muscle force required for exercise activity by increasing the external force produced by the elastic surface. [28] The elastic surface of the minitrampoline in the current study was made from polypropylene; such material could support and distribute plantar pressure while subjects were standing and walking on its surface. As a result, subjects could improve their dynamic stability after the exercise training program. ...
Full-text available
Objective: Foot and ankle exercise has been advocated as a preventative approach in reducing the risk of foot ulceration. However, knowledge about the appropriate types and intensity of exercise program for diabetic foot ulcer prevention is still limited. The current study aimed to examine the effects of an eight-week mini-trampoline exercise on improving foot mobility, plantar pressure and sensation of diabetic neuropathic feet. Methods: Twenty-one people with diabetic peripheral neuropathy who had impaired sensation perception were divided into two groups. The exercise group received a foot-care education program plus an eight-week home exercise program using the mini-trampoline (n = 11); whereas a control group received a foot-care education only (n = 10). Measurements were undertaken at the beginning, at the completion of the eight-week program and at a 20-week follow-up. Results: Both groups were similar prior to the study. Subjects in the exercise group significantly increased the range of the first metatarsophalangeal joint in flexion (left: p = 0.040, right: p = 0.012) and extension (left: p = 0.013) of both feet more than controlled subjects. There was a trend for peak plantar pressure at the medial forefoot to decrease in the exercise group (p = 0.016), but not in the control group. At week 20, the number of subjects in the exercise group who improved their vibration perception in their feet notably increased when compared to the control group (left: p = 0.043; right: p = 0.004). Conclusions: This is a preliminary study to document the improvements in foot mobility, plantar pressure and sensation following weight-bearing exercise on a flexible surface in people with diabetic neuropathic feet. Mini-trampoline exercise may be used as an adjunct to other interventions to reduce risk of foot ulceration. A larger sample size is needed to verify these findings. This trial is registered with COA No. 097.2/55.
... In recent years, trampoline exercise has been investigated as a means of preventing lower extremity injuries 12) , as well as effects on gait in cerebral palsy infants 13) . In generally, it has been known that the change of surface stiffness influences the stiffness of the leg spring during ground contact in hopping and running [14][15][16] . ...
... On the other hand, there is also evidence of negative effects of trampoline exercise. For example, Márquez et al. reported that jumping on an elastic surface such as a trampoline increased leg stiffness, decreased jump height, and generated less stored and returned energy 12,19) . In particular, Ferris et al. has claimed that landings on an elastic surface might increase the leg stiffness 14) . ...
Full-text available
[Purpose] To determine whether repetitive trampoline or hard surface jumping affects lower extremity alignment on jump landing. [Subjects and Methods] Twenty healthy females participated in this study. All subjects performed a drop vertical jump before and after repeated maximum effort trampoline or hard surface jumping. A three-dimensional motion analysis system and two force plates were used to record lower extremity angles, moments, and vertical ground reaction force during drop vertical jumps. [Results] Knee extensor moment after trampoline jumping was greater than that after hard surface jumping. There were no significant differences between trials in vertical ground reaction force and lower extremity joint angles following each form of exercise. Repeated jumping on a trampoline increased peak vertical ground reaction force, hip extensor, knee extensor moments, and hip adduction angle, while decreasing hip flexion angle during drop vertical jumps. In contrast, repeated jumping on a hard surface increased peak vertical ground reaction force, ankle dorsiflexion angle, and hip extensor moment during drop vertical jumps. [Conclusion] Repeated jumping on the trampoline compared to jumping on a hard surface has different effects on lower limb kinetics and kinematics. Knowledge of these effects may be useful in designing exercise programs for different clinical presentations.
... Vertical toe marker displacement represented the mini-trampoline surface displacement. The stiffness of the mini-trampoline was estimated using known masses and measuring the mini-trampoline vertical displacement for each mass [23,24]. We plotted a linear regression line, estimating the stiffness of the surface as the slope (7.85 kN m − 1 ). ...
Background While mini-trampolines have been used among a variety of groups including children as an intervention tool, the motor behavior children adopt while hopping on this soft, elastic surface is unknown. Identifying coordinative structures and their stability for hopping on a mini-trampoline is imperative for recommending future interventions and determining appropriateness to populations with motor dysfunctions. Research question Do children demonstrate similar biomechanical and coordination patterns as adults while hopping on a mini-trampoline? Methods Fifteen adults aged 18–35 years and 14 children aged 7–12 years completed bouts of continuous two-legged hopping in-place on a stiff surface for 10 s at a time and on a mini-trampoline for 30 s at a time. 3-D motion capture tracked whole-body movement. We evaluated whole-body vertical stiffness as a ratio of peak vertical force and peak vertical displacement, as well as spatiotemporal parameters of hopping. Coordinative structures were evaluated as continuous relative phase angles of the foot, shank, thigh, and pelvis segments. Results and significance Adults did not modify whole-body vertical stiffness on a mini-trampoline, while children increased whole-body vertical stiffness to compensate for the reduced surface stiffness. Both groups conserved the coordinative structure for hopping on a mini-trampoline by modulating hopping cycle timing. Moreover, children hopped with an adult-like coordinative structure, but required greater shank-thigh and thigh-pelvis out-of-phase motion. However, the consistency of their coordination was diminished compared to adults. Children aged 7–12 years old have formed a stable coordinative structure for spring-mass center-of-mass dynamics that is preserved on this soft, elastic surface. However, children might be developing control strategies for preferred whole-body vertical stiffness, particularly when required to dampen peak vertical forces. These results highlight the importance of evaluating the emerging motor behavior to manipulated environmental constraints, particularly when considering the utility and appropriateness of mini-trampoline interventions for children with motor dysfunctions.
... Concomitantly, adaptations in muscle pre-activation prior to ground contact [11,12,19] and subsequent concentric phase have been reported [11,13]. As manifested in dif- ferent investigations involving neurophysiological and biomechanical methodologies, the motor pattern is adapted surface-specifically, with leg muscle activation depending on the par- ticular attributes of the SGS [14,20]. This is true for proactive (prior to touch down) and reac- tive (after touch down) motor strategies, clustered in subdivisions known as the pre-activation, short-, medium-and long-latency responses (SLR, MLR and LLR; [11,13,19]. ...
Full-text available
With an emphasis on ballistic movements, an accurately anticipated neural control is an essential prerequisite to deliver a motor response coincidentally with the event of ground contact. This study investigated how previous knowledge of the ground condition affects proactive and reactive motor control in drop jumps (DJ). Thereby, human anticipatory capacity of muscle activation was investigated regarding neuromuscular activation, joint kinematics and peak forces associated with DJ performance. In 18 subjects, the effect of knowledge of two different surface conditions during DJs was evaluated. Peak force, ground-contact-time (GCT), rate of force development (RFD) and jump height were assessed. Electromyographic (EMG) activities of the m. soleus (SOL) and gastrocnemius medialis (GM) were assessed for 150ms before (PRE) and during ground contact (GC) for the short-, medium-, and long-latency responses. Ankle and knee joint kinematics were recorded in the sagittal plane.In the unknown conditions peak force, RFD and jump height declined, GCT was prolonged, proactive EMG activity (PRE) in SOL and GM was diminished (P
Full-text available
When mammals run, the overall musculoskeletal system behaves as a single linear "leg spring". We used force platform and kinematic measurements to determine whether leg spring stiffness (k(leg)) is adjusted to accommodate changes in surface stiffness (ksurf) when humans hoop in place, a good experimental model for examining adjustments to k(leg) in bouncing gaits. We found that k(leg) was greatly increased to accommodate surfaces of lower stiffnesses. The series combination of k(leg) and ksurf [total stiffness (ktot)] was independent of ksurf at a given hopping frequency. For example, when humans hopped at a frequency of 2 Hz, they tripled their k(leg) on the least stiff surface (ksurf = 26.1 kN/m; k(leg) = 53.3 kN/m) compared with the most stiff surface (ksurf = 35,000 kN/m; k(leg) = 17.8 kN/m). Values for ktot were not significantly different on the least stiff surface (16.7 kN/m) and the most stiff surface (17.8 kN/m). Because of the k(leg) adjustment, many aspects of the hopping mechanics (e.g., ground-contact time and center of mass vertical displacement) remained remarkably similar despite a > 1,000-fold change in ksurf. This study provides insight into how k(leg) adjustments can allow similar locomotion mechanics on the variety of terrains encountered by runners in the natural world.
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When human hoppers are surprised by a change in surface stiffness, they adapt almost instantly by changing leg stiffness, implying that neural feedback is not necessary. The goal of this simulation study was first to investigate whether leg stiffness can change without neural control adjustment when landing on an unexpected hard or unexpected compliant (soft) surface, and second to determine what underlying mechanisms are responsible for this change in leg stiffness. The muscle stimulation pattern of a forward dynamic musculoskeletal model was optimized to make the model match experimental hopping kinematics on hard and soft surfaces. Next, only surface stiffness was changed to determine how the mechanical interaction of the musculoskeletal model with the unexpected surface affected leg stiffness. It was found that leg stiffness adapted passively to both unexpected surfaces. On the unexpected hard surface, leg stiffness was lower than on the soft surface, resulting in close-to-normal center of mass displacement. This reduction in leg stiffness was a result of reduced joint stiffness caused by lower effective muscle stiffness. Faster flexion of the joints due to the interaction with the hard surface led to larger changes in muscle length, while the prescribed increase in active state and resulting muscle force remained nearly constant in time. Opposite effects were found on the unexpected soft surface, demonstrating the bidirectional stabilizing properties of passive dynamics. These passive adaptations to unexpected surfaces may be critical when negotiating disturbances during locomotion across variable terrain.
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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.
A mathematical model for terrestrial running is presented, based on a leg with the properties of a simple spring. Experimental force-platform evidence is reviewed justifying the formulation of the model. The governing differential equations are given in dimensionless form to make the results representative of animals of all body sizes. The dimensionless input parameters are: U, a horizontal Froude number based on forward speed and leg length; V, a vertical Froude number based on vertical landing velocity and leg length, and KLEG, a dimensionless stiffness for the leg-spring. Results show that at high forward speed, KLEG is a nearly linear function of both U and V, while the effective vertical stiffness is a quadratic function of U. For each U, V pair, the simulation shows that the vertical force at mid-step may be minimized by the choice of a particular step length. A particularly useful specification of the theory occurs when both KLEG and V are assumed fixed. When KLEG = 15 and V = 0.18, the model makes predictions of relative stride length S and initial leg angle θ0 that are in good agreement with experimental data obtained from the literature.
The question, if muscles can absorb and temporarily store mechanical energy in the form of elastic energy for later re-use, was studied by having subjects perform maximal vertical jumps on a registering force-platform. The jumps were performed 1) from a semi-squatting position, 2) after a natural counter-movement from a standing position, or 3) in continuation of jumps down from heights of 0.23, 0.40, or 0.69 m. The heights of the jumps were calculated from the registered flight times. The maximum energy level, Eneg, of the jumpers prior to the upward movement in the jump, was considered to be zero in condition 1. In condition 2 it was calculated from the force-time record of the force-platform; and in condition 3 it was calculated from the height of the downward jump and the weight of the subject. The maximum energy level after take-off, Ep0s, was calculated from the height of the jump and the jumper's weight. It was found that the height of the jump and Epos increased with increasing amounts of Eneg, up to a certain level (jumping down from 0.40 m). The gains in Epos over that in condition 1, were expressed as a percentage of Eneg and found to be 22.9 % in condition 2, and 13.2, 10.5, and 3.3 % in the three situations of condition 3. It is suggested that the elastic energy is stored in the active muscles, and it is demonstrated that the muscles of the legs are activated in the downward jumps before contact with the platform is established.
Illusions of self motion and aircraft motion are experienced when executing deep knee bends in the high force phases of parabolic flight. The occurrence of such illusions indicates that skeletomotor control is actively calibrated to a 1 g reference level and that departures from this level affect the execution and appreciation of voluntary movements. The origin of the illusory patterns is shown to be understandable in terms of mismatches between efferent control signals and expected patterns of associated muscle spindle activity. It is shown, too, that spindle activity is interpreted within an entire context of spatial information about ongoing and intended motion of the body and whether the body is laden.
A simple test for the measurement of mechanical power during a vertical rebound jump series has been devised. The test consists of measuring the flight time with a digital timer (0.001 s) and counting the number of jumps performed during a certain period of time (e.g., 15–60 s). Formulae for calculation of mechanical power from the measured parameters were derived. The relationship between this mechanical power and a modification of the Wingate test (r=0.87, n=12 ) and 60 m dash (r=0.84, n=12 ) were very close. The mechanical power in a 60 s jumping test demonstrated higher values (20 WkgBW–1) than the power in a modified (60 s) Wingate test (7 WkgBW–1) and a Margaria test (14 WkgBW–1). The estimated powers demonstrated different values because both bicycle riding and the Margaria test reflect primarily chemo-mechanical conversion during muscle contraction, whereas in the jumping test elastic energy is also utilized. Therefore the new jumping test seems suitable to evaluate the power output of leg extensor muscles during natural motion. Because of its high reproducibility (r=0.95) and simplicity, the test is suitable for laboratory and field conditions.
The purpose of the present study was to determine how humans adjust leg stiffness over a range of hopping frequencies. Ten male subjects performed in place hopping on two legs, at three frequencies (1.5, 2.2, and 3.0Hz). Leg stiffness, joint stiffness and touchdown joint angles were calculated from kinetic and/or kinematics data. Electromyographic activity (EMG) was recorded from six leg muscles. Leg stiffness increased with an increase in hopping frequency. Hip and knee stiffnesses were significantly greater at 3.0Hz than at 1.5Hz. There was no significant difference in ankle stiffness among the three hopping frequencies. Although there were significant differences in EMG activity among the three hopping frequencies, the largest was the 1.5Hz, followed by the 2.2Hz and then 3.0Hz. The subjects landed with a straighter leg (both hip and knee were extended more) with increased hopping frequency. These results suggest that over the range of hopping frequencies we evaluated, humans adjust leg stiffness by altering hip and knee stiffness. This is accomplished by extending the touchdown joint angles rather than by altering neural activity.