Trunk muscle activation during exercise with a novel device for neuromuscular lumbo-pelvic reconditioning: The FRED Project

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ORIGINAL RESEARCH
Trunk muscle activation during movement with a new
exercise device for lumbo-pelvic reconditioning
Tobias Weber
1,2
, Doroth
ee Debuse
3
, Sauro E. Salomoni
4
, Edith L. Elgueta Cancino
4
,
Enrico De Martino
2,5
, Nick Caplan
3
, Volker Damann
1
, Jonathan Scott
1,2
& Paul W Hodges
4
1 European Space Agency, European Astronaut Centre, Space Medicine Office (HSO-AM), Cologne, Germany
2 KBRWyle GmbH, Cologne, Germany
3 Faculty of Health and Life Sciences, Northumbria University, Newcastle upon Tyne, United Kingdom
4 The University of Queensland, NHMRC Centre of Clinical Research Excellence in Spinal Pain, Injury and Health, School of Health and
Rehabilitation Sciences, Brisbane, Queensland, Australia
5 Sports Medicine Specialisation School, Medicine, Surgery and Neurosciences Department, University of Siena, Toscana, Italy
Keywords
Deep spinal muscles, exercise device,
fine-wire electromyography, lumbar spine,
rehabilitation.
Correspondence
Tobias Weber, Space Medicine Office,
European Astronaut Centre Department,
Directorate of Human Spaceflight and
Operations (D/HSO), European Space Agency,
Linder Hoehe, Geb. 12, 51147 Cologne,
Germany.
Tel: +49 2203 6001 454
Fax: +49 2203 6001 402
E-mail: tobias.weber@esa.int
Funding Information
This investigation was funded by the
European Space Agency.
Received: 26 January 2017; Accepted:
7 February 2017
doi: 10.14814/phy2.13188
Physiol Rep, 5 (6), 2017, e13188,
doi: 10.14814/phy2.13188
Abstract
Gravitational unloading leads to adaptations of the human body, including
the spine and its adjacent structures, making it more vulnerable to injury and
pain. The Functional Re-adaptive Exercise Device (FRED) has been developed
to activate the deep spinal muscles, lumbar multifidus (LM) and transversus
abdominis (TrA), that provide inter-segmental control and spinal protection.
The FRED provides an unstable base of support and combines weight bearing
in up-right posture with side alternating, elliptical leg movements, without
any resistance to movement. The present study investigated the activation of
LM, TrA, obliquus externus (OE), obliquus internus (OI), abdominis, and
erector spinae (ES) during FRED exercise using intramuscular fine-wire and
surface EMG. Nine healthy male volunteers (27 5 years) have been
recruited for the study. FRED exercise was compared with treadmill walking.
It was confirmed that LM and TrA were continually active during FRED exer-
cise. Compared with walking, FRED exercise resulted in similar mean activa-
tion of LM and TrA, less activation of OE, OI, ES, and greater variability of
lumbo-pelvic muscle activation patterns between individual FRED/gait cycles.
These data suggest that FRED continuously engages LM and TrA, and there-
fore, has the potential as a stationary exercise device to train these muscles.
Introduction
Absence of effects of gravity in Low Earth Orbit, reduces
the magnitude and frequency of mechanical forces acting
on the human body, resulting in profound bone loss in
the lower limb and atrophy of some (in particular the so-
called “anti-gravity”) muscles (Fitts et al. 2001; Chang
et al. 2016). Astronauts also experience flattening of the
spinal curvatures and lower back pain (LBP) in-flight
(Kerstman et al. 2012) and they are at increased risk of
intervertebral disc (IVD) herniation on return to Earth
(Johnston et al. 2010).
Long-term bed-rest (LTBR) is used as a ground-based
analog of microgravity, and has been found to induce
similar changes, including: atrophy of deep spinal mus-
cles, IVD swelling, and a reduced lordotic lumbar spine
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The Physiological Society and the American Physiological Society.
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2017 | Vol. 5 | Iss. 6 | e13188
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Physiological Reports ISSN 2051-817X

(Belavy et al. 2011; Hides et al. 2011b). Furthermore,
spinal extensor muscle activation becomes more phasic in
nature and this persists for at least 6 months following
re-ambulation (Belavy et al. 2007).
But also people with LBP on Earth display atrophy and
altered recruitment of the deep spinal muscles (Hodges
and Richardson 1996; MacDonald et al. 2009). Two dee-
ply situated spinal muscles that make important contribu-
tions to spine control are commonly affected in LBP: the
transversus abdominis (TrA) and lumbar multifidus (LM)
muscles (Hodges 1999). Both contribute to inter-segmen-
tal control of the spine and pelvis via extensive attach-
ments to vertebrae and pelvic segments (Wilke et al.
1995; Hodges et al. 2003), and are activated in various
up-right movements, often in a manner that is tonic (sus-
tained) and not specific to the direction of internal and
external forces (Hodges and Richardson 1997; Moseley
et al. 2002). The morphology and function of these mus-
cles are related to spinal integrity and the development of
LBP (Hodges and Richardson 1996; Belavy et al. 2011;
Hides et al. 2011a), and individuals with LBP display dif-
ferences in the morphology and behavior similar to those
observed after gravitational unloading (i.e. reduced (Fer-
reira et al. 2004), delayed (Hodges and Richardson 1997;
Moseley et al. 2002) and more phasic (Saunders et al.
2004a) activation). Therefore, it is an important aim of
the state-of-the-art exercise interventions to prevent or
treat LBP to improve motor control of LM and TrA
(Hodges et al. 2013c).
Several exercises (Hodges and Richardson 1996; Hodges
1999; Hides et al. 2011a; Hodges et al. 2013c) are known
to activate TrA and LM, and change their recruitment
patterns in terms of activation levels, timing, and inter-
play with other trunk muscles. (Tsao and Hodges 2007,
2008; Tsao et al. 2011; Hodges et al. 2013a). These
exercises train activation of these muscles before their
integration into function during habitual movements.
That means that a currently used strategy to train theses
muscles and treat LBP is to first teach patients how to
activate them in isolation and then incrementally inte-
grate the newly learned activation patterns into more
complex-, and finally into habitual everyday movements
(e.g. reaching over head or standing up from a chair)
(Hodges et al. 2013a). However, specific recruitment
strategies such as learning how to activate certain trunk
muscles in isolation and then to integrate isolated con-
tractions into more complex movements, or how to de-
activate certain trunk muscles where disadvantageous
over-activity is present can be difficult to teach and learn,
requiring supervision by a physiotherapist to confirm cor-
rect activation (Van et al. 2006; McPherson and Watson
2014). Availability of a simple approach could aid transla-
tion to practice. The Functional Re-adaptive Exercise
Device (FRED; Fig. 1) (Debuse et al. 2013; Caplan et al.
2015) was designed on the premise that alternating lower
limb movement in an up-right, weight-bearing posture,
combined with an unstable base of support, would
encourage TrA and LM activation. B-mode ultrasound
and surface electromyography (sEMG) studies of FRED
exercise provide data indicative of tonic activation of TrA
and LM (Caplan et al. 2015), and with less pelvic and
spinal motion than over-ground walking (Gibbon et al.
2013). The device also induces greater activation of trunk
extensor muscles and less activation of trunk flexor mus-
cles than walking (Caplan et al. 2015). As these features
are opposite to the changes observed following LTBR
(Belavy et al. 2011) and microgravity (Hides et al. 2016;
Chang et al. 2016) (personal communication, European
Space Agency physiotherapist), FRED exercise might be
used to help correct changes in trunk muscle activation
0.85
0.80
0.75
0.70
0.60
0.55
1.0 1.2
x-position (m)
1.1 1.3 1.4 1.50.9
0.65 Foot path FREDsmall
Foot path FREDmiddle
Foot path FREDlarge
y-position (m)
AB
Figure 1. The FRED. (A) FRED device in use. (B) Foot paths are shown for the three amplitudes investigated in this study generated using a
biomechanical model of the FRED (Lindenroth et al. 2015). The plot shows that the dimensions of the ellipses increase with increasing FRED
amplitudes. FRED, Functional Re-adaptive Exercise Device.
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The Physiological Society and the American Physiological Society.
Trunk Muscle Activation During FRED Exercise T. Weber et al.

following prolonged gravitational unloading (Evetts et al.
2014). Moreover, as the device appears to address muscu-
lar deficits proposed to play a role in the LBP (Hodges
and Richardson 1996; Hides et al. 2011a), FRED exercise
could also be useful for these patients.
Although estimates of muscle activation with FRED
exercise from muscle thickness measures with ultrasound
imaging (Debuse et al. 2013) and surface EMG (Caplan
et al. 2015) are encouraging, both have limitations (e.g.
cross-talk between muscles for surface EMG; non-linear
relationship between muscle thickness and muscle activa-
tion for ultrasound imaging) for interpretation of activa-
tion of the deeply situated TrA and LM (Brown and
McGill 2008). Considering the limitations of previous
studies to investigate the FRED, the present study sought
to illuminate the immediate effects more in-depth using
intramuscular fine-wire EMG. The aims of this investiga-
tion were (1) to compare lumbopelvic muscle activation
patterns during FRED exercise and treadmill walking, and
(2) to assess the effect of different FRED amplitudes (as
shown in Fig. 1) on lumbopelvic muscle activation.
Methods
Participants
Nine healthy male volunteers (mean [SD] age: 27
(5) years; height: 1.74 (0.05) m; mass: 72.8 (10.3) kg,
body mass index: 24.1 [2.7]) with no history of LBP, or
lower limb pain or injury participated in the study. The
study was publicly advertised at the University of Queens-
land, however, only male volunteers responded to the
announcement. The fact that only male volunteers could
be recruited should not have compromised the findings
and generalizability of results given that immediate trunk
muscle activation was selected as the main outcome
parameter and it is not known to be influenced by gen-
der. Risks and procedures of the study were explained
and all participants provided written, informed consent
before participation. The study was approved by the Insti-
tutional Medical Research Ethics Committee and all pro-
cedures were in accordance with the Declaration of
Helsinki.
Instrumentation
Intramuscular electromyography
Before electrode placement, the overlying skin was steril-
ized (Persist Plus sterilization swab sticks, BD, Franklin
Lakes). Intramuscular bipolar fine-wire electrodes (two
Teflon-coated 75 lm stainless-steel wires with 1 mm
insulation removed from the ends, bent back to form
hooks at 2- and 3-mm length, threaded into a hypoder-
mic 0.50 970 or 0.50 932 mm-needle) were inserted
with B-mode ultrasound guidance (Aixplorer, Supersonic
Imagine, Aix-en-Provence, France) into the trunk muscles
on the right-hand side. Electrodes were positioned as
follows:
1TrA, OI, and OE: Midway between the anterior supe-
rior iliac spine (ASIS) and the ribcage at depths deter-
mined by ultrasound imaging;
2LM: Between L4/L5, 30 mm laterally to spinous pro-
cesses until the needle reached the most medial part of
the L4 lamina;
3ES: At L2, 40 mm lateral to the spinous process.
Surface electromyography
Before electrode placement, the skin was prepared using
an abrasive paste (Nuprep, Weaver and Company, Aur-
ora) and cleansed with an alcohol swab. Bipolar surface
electrodes (Blue Sensor N, Ambu, Ballerup, Denmark)
with an inter-electrode distance of 22 mm were placed on
the skin approximately in parallel with the muscle fibers
as follows:
1OI/TrAs: medial to the ASIS in a horizontal orienta-
tion;
2OEs: one electrode on the distal aspect of the 9th rib
and one medial to this at an angle of ~45°from hori-
zontal;
3LMs: adjacent to the L5 spinous process at an angle of
~15°from vertical.
A reference electrode was placed over the iliac crest.
EMG signals were pre-amplified 2000 times, band-pass fil-
tered between 20 and 1000 Hz (Neurolog, Digitimer,
Welwyn Garden City, UK) and recorded at a sampling
rate of 2000 Hz using a Power1401 data acquisition sys-
tem and Spike2 software (Cambridge Electronic Design,
Cambridge, UK).
Familiarization
Participants were familiarized with exercise on FRED and
walking on a motor-driven treadmill (BH, Vitoria-Gas-
teiz, Spain). The 10-min of familiarization with exercise
on FRED included the three amplitude settings (Fig. 1),
starting with the smallest. The paths traced by the feet at
the three different amplitudes have been reported previ-
ously based on a biomechanical model (Lindenroth et al.
2015). Participants were instructed to maintain their feet
in contact with the footplates at all times, hold their
upper body as still as possible in an up-right posture and
maintain a frequency of 0.42 revolutions per second with
ª2017 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
The Physiological Society and the American Physiological Society.
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T. Weber et al.Trunk Muscle Activation During FRED Exercise

a constant angular velocity throughout each complete
rotation. Visual feedback of frequency and angular veloc-
ity was provided on a screen in front of the participant.
For familiarization with treadmill walking, participants
initially walked at 0.83 m sec
1
and the speed was
increased in 0.056 m sec
1
increments until they reported
that they were walking at their estimated “natural” speed.
After 5 min of walking at their natural speed, they walked
for 5 min at a speed (0.75 m sec
1
) and a stride rate of
0.42 Hz (two steps in one stride) that were matched to
the FRED settings in its middle amplitude.
Data collection
Participants completed five exercise conditions: FRED
exercise at three different amplitudes (FRED
small
,
FRED
middle
, FRED
large
), and treadmill walking at their
natural speed (Gait
natural
) and that matched to FRED
middle
(Gait
matched
). The order was randomized using a sequence
generated by www.randomizer.org. Each exercise condi-
tion was performed for 90 sec, with the final 30 sec used
for analysis. Between conditions, participants rested for
120 sec in a standardized standing position on the floor.
During FRED exercise, a trigger signal was recorded from
the internal rotary encoder (RP6010, ifm Electronic
GmbH, Essen, Germany) to provide a marker for each
completed cycle. For treadmill walking, a footswitch (0.5
inch force sensing resistor, Trossen Robotics) was worn
under the heel of the right shoe insole to mark each heel-
strike.
Signal processing
Data were processed off-line using Matlab (Version
2014a, Mathworks, Natick, MA). For each exercise trial,
the trigger signals were used to divide the final 30 sec
into individual revolutions or gait cycles. EMG data were
visually checked for movement artifacts and any revolu-
tions/gait cycles that included artifacts were removed
before further analysis. From a total of 360 recordings, 16
(LMs: 9; OE: 2; OI: 4: OIs: 1) were removed and missing
values were replaced using the expectation maximization
algorithm (Dempster et al. 1977). EMG data were high-
pass filtered to remove any minor residual artifacts (fine-
wire: 50 Hz; surface: 30 Hz), full-wave rectified,
smoothed using a moving average filter with a time con-
stant of 100 msec, time normalized and averaged across
individual cycles.
The processed signals were used to determine mean
(EMG
mean
), peak (EMG
peak
), and minimum (EMG
min
)
amplitudes of the averaged signal (averaged curve of all
individual FRED/gait cycles). The time (percentage of
each revolution/cycle) for which the muscle was active
was calculated. The threshold for activation was defined
as an EMG amplitude in excess of five SDs above mean
baseline EMG (smallest EMG amplitude for 1 sec). The
coefficient of variation between individual revolutions/cy-
cles (Coeff
variation
) was calculated. The Coefficient of vari-
ation indicates how much the signal during each
individual cycle is varying from all other cycles (bounded
by 0 and 1; lower Coeff
variation
values indicate greater
variation between cycles). As the Coeff
variation
were high
(in particular during FRED exercise), mean, peak, and
minimum EMG were also calculated for each separate
FRED/gait cycle, before averaging all cycles (Cycle
mean
;
Cycle
peak
; Cycle
min
, respectively). EMG amplitudes were
normalized to the peak activation of the averaged signal
(EMG
peak
) across all conditions as normalizing to
EMG
peak
appeared to be more reliable than normalizing
to a maximum voluntary contraction (as it was initially
planned), where a high inter-participant variability was
observed. Across all trunk muscles, highest EMG
peak
val-
ues were observed during the gait conditions (typically
during the stance phase), and it is thus a robust reference
for normalization of amplitudes.
Statistical analyses
After examining each variable for normality a repeated
measures analysis of variance (ANOVA) was used to com-
pare the five different conditions. When the main effect
of Condition was significant (Greenhouse-Geisser
P<0.05), pairwise post-hoc comparisons were under-
taken using Fisher’s least significant difference test (Fish-
er’s LSD). Statistical analyses were performed using SPSS
statistics software (Version 19, IBM, Armonk, New York).
The results (P-values) of all pairwise comparisons as well
as the P-values for the main effect Condition of the pre-
sent statistical analysis are listed in Tables 1, 2.
Results
All participants completed the entire data collection with
no adverse events.
General features of EMG during FRED
exercise and treadmill walking
Figure 2 depicts typical EMG recordings from one partici-
pant from the FRED
middle
and the two treadmill condi-
tions. Visual inspection of the signals reveals a high
variability between individual cycles for FRED
middle.
. This
contrasts a more consistent pattern observed during
treadmill walking. With treadmill walking, LM (LMs) and
ES demonstrate typical phasic activation with bursts of
activity aligned to heel-strike, whereas activation during
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The Physiological Society and the American Physiological Society.
Trunk Muscle Activation During FRED Exercise T. Weber et al.

Table 1. ANOVA and pairwise EMG comparisons of FRED exercise in the middle amplitude and treadmill walking.
Measure Muscle Main effect (P) Post-hoc (P)
EMG
mean
iEMG TrA 0.296 —
LM 0.118 —
OI 0.032 FRED
middle
<Gait
natural
; Gait
matched
–0.02; 0.01
OE 0.006 FRED
middle
<Gait
natural
; Gait
matched
–0.046; 0.019
ES 0.001 FRED
middle
<Gait
natural
; Gait
matched
–0.027; 0.027
sEMG LMs 0.01 FRED
middle
>Gait
natural
–0.045
OIs 0.476 —
OEs 0.17 —
EMG
peak
iEMG TrA 0.107 —
LM 0.001 FRED
middle
<Gait
natural
; Gait
matched
–0.019; 0.024
OI 0.006 FRED
middle
<Gait
natural
; Gait
matched
–0.008; 0.004
OE 0.02 FRED
middle
<Gait
natural
; Gait
matched
–0.033; 0.018
ES <0.001 FRED
middle
<Gait
natural
; Gait
matched
–<0.001; 0.002
sEMG LMs 0.001 FRED
middle
<Gait
natural
–0.005
OIs 0.056 —
OEs 0.017 FRED
middle
<Gait
matched
–0.05
EMG
min
iEMG TrA 0.012 —
LM 0.023 FRED
middle
>Gait
natural
; Gait
matched
–0.028; 0.038
OI 0.3 —
OE 0.55 —
ES 0.2 —
sEMG LMs 0.001 FRED
middle
>Gait
natural
; Gait
matched
–0.004; 0.006
OIs 0.34 —
OEs 0.26 —
Cycle
mean
iEMG TrA 0.18 —
LM 0.23 —
OI 0.083 —
OE 0.006 FRED
middle
<Gait
matched
–0.024
ES 0.003 FRED
middle
<Gait
natural
; Gait
matched
–0.037; 0.048
sEMG LMs 0.05 —
OIs 0.4 —
OEs 0.15 —
Cycle
peak
iEMG TrA 0.065 —
LM 0.007 —
OI 0.065 —
OE 0.002 FRED
middle
<Gait
natural
; Gait
matched
–0.014; 0.003
ES <0.001 FRED
middle
<Gait
natural
; Gait
matched
–<0.001; 0.001
sEMG LMs 0.003 —
OIs 0.115 —
OEs 0.032 —
Cycle
min
iEMG TrA 0.044 —
LM 0.29 —
OI 0.7 —
OE 0.19 —
ES 0.26 —
sEMG LMs 0.003 FRED
middle
>Gait
natural
; Gait
matched
–<0.005; 0.006
OIs 0.67 —
OEs 0.62 —
Coeff
variation
iEMG TrA 0.023 —
LM <0.001 FRED
middle
<Gait
natural
; Gait
matched
–<0.001; <0.001
OI 0.007 FRED
middle
<Gait
natural
–0.041
OE 0.003 FRED
middle
<Gait
natural
; Gait
matched
–<0.001; 0.004
ES <0.001 FRED
middle
<Gait
natural
; Gait
matched
–<0.001; 0.002
(Continued)
ª2017 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
The Physiological Society and the American Physiological Society.
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T. Weber et al.Trunk Muscle Activation During FRED Exercise

FRED
middle
appears more random and not consistently
aligned with any specific cycle event.
Aim 1: Comparison between FRED exercise
and treadmill walking
When data were averaged across consecutive cycles before
analysis, recordings with fine-wire electrodes revealed that
EMG
mean
, EMG
peak
, and EMG
min
for TrA showed no dif-
ference between FRED
middle
and treadmill walking
(Table 1, Fig. 3). LM EMG
mean
was also not different
when comparison was made between FRED
middle
and
walking, but LM EMG
peak
was lower and EMG
min
was
observed higher during FRED
middle
than walking, and
these latter observations imply less fluctuation of activa-
tion (i.e. more “tonic”). Fine-wire recordings of the
superficial muscles OI, OE, and ES showed lower
EMG
mean
and EMG
peak
during FRED
middle
than both
treadmill tasks (Table 1, Fig. 3), but EMG
min
was not sig-
nificantly different.
Analysis of the data separately for each repetition,
revealed similar observations to the analysis of the aver-
aged EMG (Fig. 4). TrA and OI Cycle
mean
, Cycle
peak
, and
Cycle
min
did not differ between FRED
middle
and treadmill
walking. Although LM Cycle
mean
and Cycle
min
did not
differ between FRED
middle
and walking, LM Cycle
peak
was
lower in FRED
middle
than both walking conditions. OE
Cycle
mean
was lower during FRED
middle
than Gait
matched,
OE Cycle
peak
was less during FRED
middle
than both walk-
ing tasks. OE Cycle
min
did not differ between FRED
middle
and the walking conditions. ES Cycle
mean
and Cycle
peak
during FRED
middle
were lower than during both walking
tasks, but ES Cycle
min
did not differ between conditions.
The duration of activation (percentage of FRED/gait
cycle) showed that OE was active for less time during
FRED
middle
than Gait
natural
. There were no difference for
the other muscles.
The coefficient of variation between consecutive move-
ment cycles was lower (i.e. more variable) for LM, OE,
and ES during FRED
middle
than both walking tasks, and
OI Coeff
variation
was lower during FRED
middle
than Gait
nat-
ural only (Table 1, Fig. 5). The TrA Coeffvariation
did not differ
between FRED
middle
and treadmill walking.
Aim 2: Comparison between FRED exercise
amplitudes
FRED exercise amplitude affected some aspects of trunk
muscles activity. Although EMG
mean
of LM and TrA were
unaffected through amplitude changes of FRED, OE
EMG
mean
increased significantly from FRED
small
to
FRED
large
, and ES EMG
mean
increased significantly from
FRED
small
to FRED
middle
, and from FRED
middle
to FRED
large
. OI EMG
mean
increased significantly from
FRED
middle
to FRED
large
.
TrA EMG
peak
and EMG
min
were not significantly differ-
ent among the FRED conditions. LM EMG
peak
increased
from FRED
small
to FRED
large
, whereas EMG
min
of LM, OI,
OE, ES were not different between conditions. OI
EMG
peak
was higher for FRED
large
than FRED
middle.
OE
EMG
peak
increased significantly from FRED
small
and
FRED
middle
to FRED
large
, and ES EMG
peak
increased sig-
nificantly from FRED
small
to FRED
middle
and FRED
large
.
Surface EMG recordings showed that LMs EMG
mean
increased significantly from FRED
small
to FRED
middle
and from FRED
middle
to FRED
large
, whereas OIs and
OEs EMG
mean
remained unaffected. LMs EMG
min
increased significantly from FRED
small
to FRED
middle
,
whereas OIs and OEs EMG
min
did not change. LMs
EMG
peak
increased significantly from FRED
small
to
Table 1. Continued.
Measure Muscle Main effect (P) Post-hoc (P)
sEMG LMs <0.001 FRED
middle
<Gait
natural
; Gait
matched
–<0.001; 0.003
OIs 0.007 —
OEs <0.001 FRED
middle
<Gait
natural
; Gait
matched
–0.009; 0.004
Time active iEMG TrA 0.3 —
LM 0.1 —
OI 0.15 —
OE 0.031 FRED
middle
<Gait
natural
–0.024
ES 0.08 —
sEMG LMs 0.2 —
OIs 0.35 —
OEs 0.62 —
Post-hoc analyses were performed provided the P-value for main effect (condition) was ≤0.05 while for pairwise comparisons only P≤0.05
are presented.
IEMG, intramuscular EMG; sEMG, surface EMG.
2017 | Vol. 5 | Iss. 6 | e13188
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The Physiological Society and the American Physiological Society.
Trunk Muscle Activation During FRED Exercise T. Weber et al.

Table 2. ANOVA and Pairwise EMG comparisons of FRED exercise in the three different amplitude settings.
Measure Muscle Main effect (P) Post-hoc (P)
EMG
mean
iEMG TrA 0.296 —
LM 0.118 —
OI 0.032 FRED
middle
<FRED
large
–0.012
OE 0.006 FRED
small
<FRED
large
–0.037
ES 0.001 FRED
small
<FRED
middle
; FRED
large
; FRED
middle
<FRED
large
–0.024; 0.005; 0.034
sEMG LMs 0.01 FRED
small
<FRED
middle
; FRED
large
–0.005; 0.018
OIs 0.476 —
OEs 0.17 —
EMG
peak
iEMG TrA 0.107 —
LM 0.001 FRED
small
<FRED
middle
; FRED
large
–0.008; 0.006
OI 0.006 FRED
middle
<FRED
large
–0.033
OE 0.02 FRED
small
; FRED
middle
<FRED
large
–0.012; 0.012
ES <0.001 FRED
small
<FRED
middle
; FRED
large
–0.009; 0.006
sEMG LMs 0.001 FRED
small
<FRED
middle
; FRED
large
; FRED
middle
<FRED
large
–0.005; <0.001; 0.008
OIs 0.056 —
OEs 0.017 FRED
small
<FRED
large
–0.002
EMG
min
iEMG TrA 0.012 —
LM 0.023 —
OI 0.3 —
OE 0.55 —
ES 0.2 —
sEMG LMs 0.001 FRED
small
<FRED
middle
–0.005
OIs 0.34 —
OEs 0.26 —
Cycle
mean
iEMG TrA 0.18 —
LM 0.23 —
OI 0.083 —
OE 0.006 FRED
middle
<FRED
large
–0.038
ES 0.003 FRED
small
<FRED
middle
; FRED
large
–0.025; 0.011
sEMG LMs 0.05 FRED
small
<FRED
middle
; FRED
large
–0.006; 0.017
OIs 0.4 —
OEs 0.15 —
Cycle
peak
iEMG TrA 0.065 —
LM 0.007 FRED
small
<FRED
middle
; FRED
large
–0.008; 0.01
OI 0.065 —
OE 0.002 FRED
small
; FRED
middle
<FRED
large
–0.018; 0.023
ES <0.001 FRED
small
<FRED
middle
; FRED
large
; FRED
middle
<FRED
large
–0.012; 0.001; 0.038
sEMG LMs 0.003 FRED
small
<FRED
middle
; FRED
large
; FRED
middle
<FRED
large
–0.002; <0.001; 0.011
OIs 0.115 —
OEs 0.032 FRED
small
<FRED
large
–0.003
Cycle
min
iEMG TrA 0.044 —
LM 0.29 —
OI 0.7 —
OE 0.19 —
ES 0.26 —
sEMG LMs 0.003 FRED
small
<FRED
middle
; FRED
large
–0.001; 0.037
OIs 0.67 —
OEs 0.62 —
Coeff
variation
iEMG TrA 0.023 FRED
small
<FRED
large
–0.025
LM <0.001 FRED
small
<FRED
middle
; FRED
large
–0.033; 0.006
OI 0.007 —
OE 0.003 —
ES <0.001 FRED
small
<FRED
middle
; FRED
large
–0.036; 0.009
(Continued)
ª2017 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
The Physiological Society and the American Physiological Society.
2017 | Vol. 5 | Iss. 6 | e13188
Page 7
T. Weber et al.Trunk Muscle Activation During FRED Exercise

Table 2. Continued.
Measure Muscle Main effect (P) Post-hoc (P)
sEMG LMs <0.001 FRED
small
; FRED
middle
<FRED
large
–0.01; 0.046
OIs 0.007 FRED
small
<FRED
middle
; FRED
large
–0.031; 0.029
OEs <0.001 FRED
small
<FRED
middle
; FRED
large
–0.015; 0.001
Time active iEMG TrA 0.3 —
LM 0.1 —
OI 0.15 —
OE 0.031 FRED
middle
<FRED
large
–0.025
ES 0.08 —
sEMG LMs 0.2 —
OIs 0.35 —
OEs 0.62 —
Post-hoc analyses were performed provided the P-value for main effect (condition) was ≤0.05 while for pairwise comparisons only P≤0.05
are presented.
TrA
FRED middle Gait natural Gait matched
LM
LMs
OI
OIs
OE
OEs
ES
Cycle length (%)
0 20406080100 0 20406080100 0 20406080100
Cycle length (%) Cycle length (%)
Figure 2. Representative processed EMG curves of one participant. Intramuscular and surface EMG recordings from one participant for
FRED
middle
and the two gait conditions. The thick black line depicts the averaged signal of all individual cycles (thin gray lines) as calculated
analyzing the last 30 sec of each task. The light dotted line at the bottom of each plot indicates the zero reference for each channel. Cycle
length represents one complete revolution on the FRED or the time from heel contact to heel contact of the right foot for treadmill walking.
Note that unlike the gait data that begin and end with right foot strike, data for the FRED exercise are temporally organized to a set point in
the smooth foot path.
Figure 3. Mean, peak and min EMG amplitudes of the averaged EMG data. Group mean (SD) of intramuscular and surface EMG signals
during the three FRED conditions (FRED
small
, FRED
middle
, FRED
large
) and treadmill walking at natural speed (Gait
natural
) and at a step frequency
(0.84 Hz) matched to FRED
middle
(Gait
matched
). The figure shows mean (A), peak (B) and minimum (C) amplitude of the averaged curves
normalized to the greatest peak activation of the averaged signal for each muscle. Intramuscular EMG–LM, lumbar multifidus; OI, obliquus
internus abdominis; OE, obliquus externus abdominis; ES, erector spinae; TrA, transversus abdominis; surface EMG-LMs, lumbar multifidus; OIs,
obliquus internus abdominis; OEs, obliquus externus abdominis. *P<0.01 and
#
P<0.05 for pairwise comparisons.
2017 | Vol. 5 | Iss. 6 | e13188
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ª2017 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
The Physiological Society and the American Physiological Society.
Trunk Muscle Activation During FRED Exercise T. Weber et al.

75
LM
LMs
*
**
#
#
#
Ols OEs TrA
OI OE ES
#
##*
*****
##
##
#
###
Condion: P = 0.118
LM OI OE ES
##
##
#
*** * *
**
****
**
#
###
Condion: P = 0.476 Condion: P = 0.17 Condion: P = 0.296
LMs
*** *
**
**
Ols
OlLM
**
*
**
##
LMs Ols OEs TrA
##
ESOE
OEs TrA
Condion: P = 0.056
Condion: P = 0.3
Condion: P = 0.34
FREDsmall FREDmiddle FREDlarge Gaitnatural Gaitmatched
Condion: P = 0.26 Condion: P = 0.12
Condion: P = 0.55 Condion: P = 0.2
Condion: P = 0.107
50
25
0
75
EMG mean (% EMGpeak)
EMG peak (% EMGpeak)
50
25
0
75
100
50
25
0
75
100
50
25
0
EMG min (% EMGpeak)
75
50
25
0
75
50
25
0
A
B
C
ª2017 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
The Physiological Society and the American Physiological Society.
2017 | Vol. 5 | Iss. 6 | e13188
Page 9
T. Weber et al.Trunk Muscle Activation During FRED Exercise

75
LM
LMs
*
**
#
#
Ols OEs TrA
OI OE ES
****
####
##
Condion: P = 0.23 Condion: P = 0.083
LM OI OE ES
#
*****
##
#
#
**
**
##
Condion: P = 0.4
Condion: P = 0.065
Condion: P = 0.15 Condion: P = 0.18
LMs
**
#
** **
Ols
OlLM
*
**
LMs Ols OEs TrA
#
ESOE
OEs TrA
Condion: P = 0.115
Condion: P = 0.29 Condion: P = 0.7
Condion: P = 0.67
FREDsmall FREDmiddle FREDlarge Gaitnatural Gaitmatched
Condion: P = 0.62 Condion: P = 0.44
Condion: P = 0.19 Condion: P = 0.26
Condion: P = 0.065
50
25
0
75
Mean cycle (% EMG
peak
)
Peak cycle (% EMG
peak
)
50
25
0
75
125
175
100
150
50
25
0
75
125
175
100
150
50
25
0
Min cycle (% EMG
peak
)
50
25
0
50
25
0
A
B
C
2017 | Vol. 5 | Iss. 6 | e13188
Page 10
ª2017 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
The Physiological Society and the American Physiological Society.
Trunk Muscle Activation During FRED Exercise T. Weber et al.

FRED
middle
and from FRED
middle
to FRED
large
, OEs
EMG
peak
increased significantly from FRED
small
to FRE-
D
large
, whereas OIs EMG
peak
remained unaffected
(Table 2, Fig. 3). The duration of activation (percentage
of FRED/gait cycle) did not differ between conditions
for any muscle except OE, which was active for a
longer period during FRED
large
than FRED
middle
(Table 2, Fig. 5).
LM
LMs
***
*
*
*
*
**
*
**
**
**
*
*
*
### #
##
#
#
#
Ols OEs TrA
OI OE ES
*
*
*
*
**
*
##
#
LM OI OE ES
##
##
Condion: P = 0.15 Condion: P = 0.08
OlsLMs TrA
OEs
Condion: P = 0.2
Condion: P = 0.1
Condion: P = 0.35
FREDsmall FREDmiddle FREDlarge Gaitnatural Gaitmatched
Condion: P = 0.62 Condion: P = 0.3
1
0.2
0.4
0.6
0.8
0
1
0.2
0.4
0.6
0.8
0
Coefficient of variaon (0–1)
Time acve (% 1FRED/gait cycle)
75
100
50
25
0
75
100
50
25
0
A
B
Figure 5. Coefficient of variation and time active. (A) The coefficient of variation indicates the variation of individual FRED/gait cycles from the
averaged signal. (B) Time active indicates the percentage of time a muscle was active during the task. Intramuscular EMG–LM, lumbar
multifidus; OI, obliquus internus abdominis; OE, obliquus externus abdominis; ES, erector spinae; TrA, transversus abdominis; surface EMG-LMs,
lumbar multifidus; OIs, obliquus internus abdominis; OEs, obliquus externus abdominis. *P<0.01 and
#
P<0.05 for pairwise comparisons.
Figure 4. Mean, peak and min amplitudes determined from individual FRED/gait cycles. (A) Mean, (B) peak, and (C) minimum EMG recorded
from all recorded intramuscular and surface EMG signals from individual FRED/gait cycles. EMG amplitudes were normalizd to the greatest peak
activation of the averaged signal for each muscle. Intramuscular EMG–LM, lumbar multifidus; OI, obliquus internus abdominis; OE, obliquus
externus abdominis; ES, erector spinae; TrA, transversus abdominis; surface EMG-LMs, lumbar multifidus; OIs, obliquus internus abdominis; OEs,
obliquus externus abdominis. *P<0.01 and
#
P<0.05 for pairwise comparisons.
ª2017 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
The Physiological Society and the American Physiological Society.
2017 | Vol. 5 | Iss. 6 | e13188
Page 11
T. Weber et al.Trunk Muscle Activation During FRED Exercise

Analysis of the data separately for each cycle were simi-
lar to the findings of the averaged data. Some minor dif-
ferences were observed for OI, ES, and LMs (Table 2,
Figs. 3, 4). Between-cycle variation was affected by FRED
amplitude. For intramuscular EMG recordings, LM, and
ES Coeff
variation
were lower for FRED
small
than
for FRED
middle
and FRED
large
, and TrA Coeff
variation
was lower for FRED
small
than for FRED
large
only (Fig. 5,
Panel a). For surface EMG recordings, LMs Coeff
variation
for FRED
small
and FRED
middle
exercise was lower than
for FRED
large
, whereas OIs and OEs Coeff
variation
were
lower for FRED
small
than for FRED
middle
and FRED
large
(Fig. 5).
Discussion
This study presents novel results about the immediate
effects of FRED exercise on lumbo-pelvic muscle recruit-
ment and adds important knowledge to the investigation
process of a device that claims to be helpful in the recov-
ery of LBP and in the rehabilitation phase after gravita-
tional unloading (i.e. space flight, bed rest). Consistent
with the proposed objective of FRED exercise, these
results provide evidence that TrA and LM are activated
continuously throughout cycles on the device. FRED exer-
cise differed from treadmill in several respects, including
more “tonic” pattern of activation of LM and lower acti-
vation of several superficial trunk muscles. These data
highlight that FRED exercise may have therapeutic bene-
fits for LBP patients and for individuals after prolonged
gravitational unloading.
Trunk muscle activity difference between
FRED exercise and treadmill walking
Selective EMG recordings of trunk muscles with fine-wire
intramuscular electrodes during FRED exercise and tread-
mill walking revealed differences between these tasks, with
some similarities and differences to previous non-invasive
recordings (Caplan et al. 2015). Previous studies of acute
exercise with FRED reported the activation (surface
EMG) of deep spinal muscles, greater trunk extensor
muscle activation, less trunk flexor muscle activation, and
a phasic-to-tonic shift of LM activation when compared
with walking (Debuse et al. 2013; Caplan et al. 2015).
Using selective fine-wire recordings, the present data con-
firm sustained activation of LM and TrA during FRED
exercise. Although no difference observed in the mean
activation between FRED exercise and treadmill walking,
consistent with the phasic-to-tonic shift in LM reported
by Caplan et al. (2015), the pattern of intramuscular LM
EMG during FRED exercise was characterized by less fluc-
tuating continuous activation (greater minimum
activation, lesser peak activation). This was observed for
both surface and fine-wire LM recordings in the present
study.
The observation of less variation in LM EMG ampli-
tude (lower peaks, greater minima) during FRED exercise
than walking is likely to be explained by the absence of
ground impacts at foot contact in FRED, which are
known to lead to high peaks of LM activation in walking
(Saunders et al. 2004b). It follows that there would be less
difference in the pattern of TrA between FRED and walk-
ing as activation of that muscle is less dominated by
peaks at foot contact in walking (Saunders et al. 2004b).
FRED exercise also aims to reduce the activation of
more superficial trunk muscles that tend to have
enhanced activation in the LBP (Hodges et al. 2013b). As
reported from the surface EMG recordings (Caplan et al.
2015), mean trunk flexor (OI and OE) muscles activation
was less in FRED exercise than over-ground walking. In
the present study, we also observed shorter duration of
OE EMG bursts during FRED. Comparison of surface
and fine-wire recordings indicated that differences
between tasks were more readily observed with selective
fine-wire electrodes, as surface recordings failed to show
differences in some parameters. A departure from the
observations of Caplan et al. (2015) is that trunk extensor
activation (ES EMG) was less, rather than more during
FRED. This difference is best explained by EMG cross-
talk, whereby each EMG recording site reflects the activa-
tion of multiple muscles within the recording field.
Greater recording zone size for surface electrodes means
those recordings will be more compromised by adjacent
muscle activity. For ES, the previously used surface elec-
trodes (Caplan et al. 2015) may have reflected activation
of the superficially placed latissimus dorsi or thoroa-
columbar erector spinae muscles, which we did not
record. Similar to the argument presented for LM above,
the lower mean and peak activation of the superficial
muscles (OI, OE, and ES) during FRED is likely to be
explained by removal of the high demand for trunk con-
trol related to foot strike.
A new observation was that activation of all trunk
muscles was more variable between cycles (i.e. lower coef-
ficient of variation) during FRED exercise. This contrasts
the highly regular pattern of phasic modulation of activa-
tion of most muscles at consistent time points of each
cycle in treadmill walking. There are several possible
explanations. First, greater between-cycle variation might
reflect the novelty of this exercise, and participants’ lack
of familiarity. Analysis of habitual activities shows that
motor units tend to fire more synchronously and more
predictably when a movement is repeatedly performed
(Enoka 1997). When quantified with the coefficient of
variation (amount of variation of individual cycles from
2017 | Vol. 5 | Iss. 6 | e13188
Page 12
ª2017 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
The Physiological Society and the American Physiological Society.
Trunk Muscle Activation During FRED Exercise T. Weber et al.

the mean of all cycles), a high value was observed for all
trunk muscles during the walking trials. This is consistent
with the highly familiar and repeatable nature of the task
and associated muscle activity. Gait is a habitual move-
ment for healthy humans that is at least in part controlled
by spinal cord neural circuits (Bussel et al. 1996), and its
interplay of muscle activation is genetically determined
(Andersson et al. 2012) with fine-tuning over decades of
exposure.
Second, as mentioned above, FRED exercise lacks high
ground reaction forces at foot strike. As activation of
many of the trunk muscles is associated with foot strike
(Saunders et al. 2004b) this would tend to constrain the
variation between cycles, leading to a higher coefficient of
variation.
Third, greater variation may reflect greater cycle-to-
cycle variation in task demands. FRED exercise was
designed to continuously challenge the muscles control-
ling lumbo-pelvic posture and alignment. By making the
base of support less stable, the intention was to enforce a
need for the trunk muscles to continuously adjust the
spine and pelvis position. This challenge is likely to vary
between cycles, providing a potential explanation for less
consistent EMG patterns. In the present study, the lowest
correlation coefficient for all trunk muscles was observed
during FRED exercise with the small or middle ampli-
tude, indicating that the challenge may be greater (i.e.
more unstable) in these situations.
Changes in trunk muscle activation with
FRED exercise amplitudes
Trunk muscle activation changed significantly when the
foot-path lengths during FRED exercise were altered
through changes in the movement amplitudes. For most
trunk muscles (LM, OI, OE/OEs, and ES) the greatest
EMG
peak,
EMG
mean
, and/or EMG
min
activities were
recorded during FRED exercise with the large amplitude,
although the specific parameters differed between muscles.
Two features of FRED exercise explain the increase with
FRED amplitude. First, the large amplitude setting
imposes greater excursion of the hips, placing greater
demand for the control of proximal body segments. Sec-
ond, the instability of the base of support is likely to be
more difficult to control with large amplitudes. This will
induce greater challenge for control, particularly for the
participants in this study who were novice users (limited
to 10 min of familiarization). During the large amplitude
exercise it was not uncommon to observe “jerky” move-
ments and associated peaks in trunk muscle activation.
Lower cycle-to-cycle variation of LM/LMs, TrA, OI/OIs),
ES and OEs with longer footpaths could imply that
although this task is more challenging, the points in the
task that were most challenging may be more consistent
between repetitions which may tend to constrain the peri-
ods of most activity between repetitions.
Potential role of FRED in rehabilitation of
astronauts, individuals with LBP and
following LTBR
Present results confirm that FRED exercise induces tonic
activation of deeper trunk muscles, with lower mean acti-
vation of superficial spinal muscles (ES, OI, and OE) than
treadmill walking at similar conditions. These features
highlight the potential role of FRED exercise to counter-
act impaired (delayed and phasic) activation of deep
lumbo-pelvic muscles (Hodges and Richardson 1996; Fer-
reira et al. 2004; Saunders et al. 2004a; Hodges et al.
2013b) and increased activation of more superficial trunk
muscles (van Dieen et al. 2003) observed in the LBP, as
well as after LTBR (Belavy et al. 2007) and in the
decreased size of deep spinal muscles as reported from
astronauts after their missions (Hides et al. 2016; Chang
et al. 2016).
Repeated exposure to postural perturbations can
improve timing and amplitude of postural muscle activa-
tion (Horak and Nashner 1986). Further, repeated postu-
ral challenges in a specific environment developed new
motor control strategies, which were transferrable to
another environment (Horak and Nashner 1986). Taken
together with our observed changes in muscle activation
with FRED exercise, this implies FRED exercise could aid
reversal of compromised neuromotor control and that the
neuromotor control of trunk muscles trained through
FRED exercise might be transferrable to other tasks. Clini-
cal trials are needed to confirm the ability of FRED exer-
cise to alter trunk muscle neuromotor control in the long
term in individuals with deficits in trunk muscle function.
Limitations
This study focused on a limited set of muscles based on
the extensive literature highlighting compromised (LM
and TrA) and augmented (OE, OI, and ES) activation in
the LBP and after bed rest. However, this represents a
subset of the trunk muscles that control the spine. Recent
work highlights high variation between individuals
(Hodges et al. 2013b) and involvement of additional
muscles (e.g. psoas,quadratus lumborum) (Park et al.
2013). The present study shows differences (particularly
for OI and OE) between surface and intramuscular
recordings, which highlights that surface electrodes do
not accurately represent their activation and highlights
that fine-wire electrodes are necessary to study the com-
plex muscle system of the trunk.
ª2017 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of
The Physiological Society and the American Physiological Society.
2017 | Vol. 5 | Iss. 6 | e13188
Page 13
T. Weber et al.Trunk Muscle Activation During FRED Exercise

Our interest in this study was to investigate individuals
with no previous experience with FRED and a standard-
ized period of familiarization (10 min) before data collec-
tion. It is unknown whether muscle activation patterns
would differ with greater familiarity with FRED exercise,
particularly when using the larger amplitudes.
Conclusion
Intramuscular EMG recordings confirm that FRED exer-
cise activates LM and TrA continuously. Moreover, com-
pared with walking, trunk muscle activation during FRED
exercise is associated with less activity of superficial mus-
cles while the deep spinal muscles show similar mean
activities. The patterns of activation during individual
FRED cycles vary more than during walking. These data
support the notion that FRED exercise might be effective
to train the deep spinal muscles for populations where
spinal muscle atrophy and compromised neuromotor
control might be present (LBP, recovery after LTBR and
space flight). Future studies are planned to investigate
whether FRED exercise induces long-term improvement
in functional and morphological parameters of trunk
muscles.
Acknowledgments
Dr Simon Evetts receives special thanks for establishing the
initial cooperation between the European Space Agency
and Northumbria University. Paul Hodges receives a senior
principal research fellowship from the National Health and
Medical Research Council of Australia. We would also like
to acknowledge the support of Markus Kiel from the
University of Queensland and of courses all participants.
Conflict of Interest
The authors have no conflicts of interests.
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