Exp Brain Res
DOI 10.1007/s00221-009-1854-x

R ES EA R C H A R TI CLE

Flexion reXex modulation during stepping in human spinal
cord injury
Maria Knikou · Claudia A. Angeli ·Christie K. Ferreira · Susan J. Harkema

Received: 18 January 2009 / Accepted: 8 May 2009
© Springer-Verlag 2009

Abstract The Xexion reXex modulation pattern wasinvestigated in nine people with a chronic spinal cord injury
during stepping using body weight support on a treadmill
and manual assistance by therapists. Body weight support
was provided by an upper body harness and was adjusted
for each subject to promote the best stepping pattern withthe least manual assistance required by the therapists. The
Xexion reXex was elicited by sural nerve stimulation with a
30 ms pulse train at 1.2–2 times the tibialis anterior reXex
threshold. During stepping, stimuli were randomly dispersed across the gait cycle which was divided into 16
equal bins. A long latency (>110 ms) Xexion reXex waspresent in all subjects, while a short (>30 ms) and a
medium latency (>70 ms) Xexion reXex were present onlyin three subjects. For each response, the non-stimulated
EMG was subtracted from the stimulated EMG at identical

M. Knikou (&)
Health Sciences Doctoral Programs, The Graduate Center,
City University of New York, 2800 Victory Boulevard,Staten Island, NY 10314, USAe-mail: knikou@mail.csi.cuny.edu; m-knikou@northwestern.edu
M. Knikou
Northwestern University Feinberg School of Medicine,
Chicago, IL, USA
M. Knikou
Sensory Motor Performance Program,Rehabilitation Institute of Chicago, Chicago, IL, USA
C. A. Angeli · C. K. Ferreira · S. J. Harkema
Department of Neurological Surgery,
University of Louisville, Louisville, KY, USA
C. A. Angeli · C. K. Ferreira · S. J. Harkema
Frazier Rehab Institute, Louisville, KY, USA

time windows and bins, normalized to the maximal corresponding EMG, and signiWcant diVerences were establishedwith a Wilcoxon rank-sum test. The long latency Xexion
reXex was facilitated at late stance and during the swing-tostance transition phase. A reXex depression was present
from heel strike until mid-stance and during the swing-tostance transition phase. The short and medium latency Xexion reXexes were depressed during mid-stance followed byfacilitation during the stance-to-swing transition phase.
Regardless of the latency, facilitatory Xexion responses
during the swing phase coincided with decreased activity ofipsilateral ankle extensors. The Xexion reXex was modulated in a phase dependent manner, a behavior that wasabsent for the soleus H-reXex in most of these patients(Knikou et al. in Exp Brain Res 193:397–407, 2009). We
propose that training should selectively target spinal reXex
circuits in which extensor muscles and reXexes are
involved in order to maximize sensorimotor recovery inthese patients.
Keywords Flexion reXex · Human walking · Modulation ·Rehabilitation · Spinal cord injury · Sural nerve

Introduction
Excitation of group II and III muscle aVerents, joint aVerents, and cutaneous aVerents evoke polysynaptic reXex
actions characterized by the classical Xexion and crossed
extension reXexes. The actions of these aVerents (Xexor
reXex aVerents, FRA) (Eccles and Lundberg 1959), aremanifested through alternative excitatory and inhibitory
reXex pathways to Xexor and extensor  motoneurons(Lundberg 1979; Lundberg et al. 1987). FRA pathways
encompass interneurons associated with locomotion. Lundberg

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and Jankowska et al. postulated in spinalized animals
following L-Dopa (a precursor of dopamine and noradrenaline) administration that short latency FRA pathways were
profoundly depressed, while long latency long-lasting
reXex discharges of ipsilateral Xexors and contralateralextensors were released (Anden et al. 1966a, b; Jankowska
et al. 1967a, b; Fu et al. 1975). Because of the strong reciprocal organization and the alternating activity in Xexors andextensors in spinalized animals following short FRA trains,the late-Dopa Xexion reXex was related to stepping(Jankowska et al. 1967a, b; Lundberg 1979). A neuronalorganization similar to that observed in spinalized animals
treated with L-Dopa and Nialamide was postulated for thelong latency Xexion reXex in people with a spinal cord
injury (SCI) (Roby-Brami and Bussel 1987, 1990, 1992).
This comparison was largely based on observations that
FRA volleys induced inhibition of the ipsilateral TA
H-reXex via presynaptic inhibition of Ia aVerents, depressed
contralateral long-latency Xexion reXexes (Roby-Brami andBussel 1990, 1992), and modulated spontaneously occurring step-like movements (Calancie et al. 1994; Bussel
et al. 1988, 1989).
Ongoing cyclic modulation of spinal reXexes during gaitin humans suggests changes in spinal neuronal pathways at
motoneuronal and/or interneuronal levels (Morin et al.
1982; Capaday and Stein 1986). Studying spinal reXexes
during locomotion in people with a SCI can provide information on the physiological state of locomotor-related spinal neural circuits. The short latency reXexes, mediatedlargely by muscle spindle aVerents, during the gait cycle in
SCI subjects vary from a relatively normal pattern, whencompared to non-injured subjects (Morin et al. 1982; Capaday
and Stein 1986), to no modulation (Yang et al. 1991; Faist
et al. 1999; Knikou et al. 2009).
Normalization of spinal locomotor-reXex circuits may
contribute to locomotor recovery following repetitive step

training. This is supported by the recent Wndings reported
for upper motoneuron lesions of cerebral origin in humans,
where intensive step training on a treadmill contributed to
the recovery of the soleus H-reXex depression during theswing phase of gait (Hodapp et al. 2009). Current available
interventions for recovery of walking in people with a SCI
are the Lokomat® Driven Gait Orthosis and locomotor
training using body weight support on a treadmill (BWST)
with therapists providing as needed external manual assistance (Barbeau et al. 1987; Dobkin et al. 1995; Behrman
and Harkema 2000; Wernig and Müller 1992; Wirz et al.
2005). The objective of this study was to investigate the
Xexion reXex modulation pattern in people with a SCI during stepping.

Methods
Experiments were conducted after a full Institutional
Review Board (IRB) approval of the experimental protocol(College of Staten Island and University of Louisville IRB
committees), and in compliance with the Declaration of
Helsinki. A written consent form was obtained from eachsubject before testing. Nine adults (two females, sevenmales) with spinal injury ranging from cervical 1 to thoracic 10 (neurological level) participated in the study(Table 1). Subjects participated in a previous study (Knikou
et al. 2009) and are identiWed here with the same code.
Flexion reXex
Electromyographic (EMG) activity was recorded withbipolar surface electrodes of Wxed inter-electrode distanceof 2.5 cm (2DT2, Multi BioSensors, El Paso, TX, USA)
from the ipsilateral (stimulated) and contralateral (nonstimulated) soleus (SOL), medial gastrocnemius (MG), andTable 1 Characteristics of SCI subjects
Subject ID

Neurological level

ASIA score

Age (years)

Post-injury (years)

GenderAnti-spasticity medication

BWS

m/s

A33

C3

A

52

2.4

M

None

60

0.89

B06

C5

B

38

1.8

F

None

40

1.07

D11

C5

D

39

1.1

M

None

30

0.98

D13

T10

D

60

2.5

F

Baclofen

40

0.98

D14

C7

D

22

3.2

M

Baclofen

40

1.07

D15

C1

D

67

46.0

M

None

50

0.49

D16

C3

D

44

6.5

M

Baclofen

20

0.80

D20

C3

D

63

2.0

M

None

30

0.98

C14

C3

C

41

1.0

M

Gabapentin

35

0.98

Body weight support (BWS) was provided at a level that knee and trunk buckling could be avoided during standing and stepping. The neurologic
impairment is identiWed in column 3 based on the American Spinal Injury Association (ASIA) Impairment Scale (Maynard et al. 1997). Treadmill
speed for each subject is indicated in the last column
C cervical, T thoracic, F female, M male123

Exp Brain Res

tibialis anterior (TA) muscles. All EMG signals were
ampliWed, high pass Wltered (cut-oV frequency at 20 Hz),
full-wave rectiWed and then low-pass Wltered (cut-oV frequency 500 Hz).
The Xexion reXex was evoked according to methods previously employed in SCI subjects (Conway and Knikou
2008; Knikou and Conway 2005; Knikou 2007a, b). A
pulse train of 30-ms duration (1-ms pulses at 300 Hz) was
triggered from a computer (controlled by a customized
Labview script) and delivered to the right (ipsilateral) sural
nerve by a constant current stimulator (DS7A, Digitimer
Ltd., Hertfordshire, UK). With subjects seated, a bipolarelectrode was used as a probe to establish the site which
elicited a response in the TA muscle at the lowest stimulus
intensity. The bipolar electrode was then replaced by two
monopolar electrodes (Medicotest, Ølstykkee, Denmark)which were secured throughout the experiment. Each subject was then transferred to the treadmill (Biodex Medical
Systems, NY, USA). While standing, subjects wore an
upper body harness that was attached to pulleys controlled
by a pneumatic closed-loop force body weight support(BWS) system (Innoventor Engineering Inc., Maryland
Heights, MO, USA).
The stimulus intensity was increased progressively while
subjects were standing with BWS, and the response wasmonitored to observe its sensitivity to the stimulus intensity
strength. The stimulus intensity required to elicit an initial
response in the ipsilateral TA muscle was identiWed as
reXex threshold. During stepping, the sural nerve was stimulated between 1.2 and 2 times the reXex threshold andranged from 6.3 to 29 mA across subjects. The criteria for
selecting the stimulation intensity for each subject wereconstant/repetitive TA responses and absent limb movement. In addition, SCI subjects with preserved sensation
reported no pain (but uncomfortable) and sensation within
the cutaneous receptive Weld of the sural nerve e.g. lateralside of the foot towards the heel and the Wfth to fourth
metatarsal. Based on these criteria, stimulation excited
primarily cutaneous and muscle aVerents (Knikou andConway 2005; Conway and Knikou 2008), but high-threshold aVerents cannot be excluded. The intensity was checked
several times during the course of the experiment to ensure
its stability.
During stepping, a therapist positioned behind the subject assisted with pelvis and trunk stabilization, as well as
appropriate weight shift. Therapists positioned at each limb
provided manual assistance using a customized techniquethat promotes standing and toe clearance during swing(Barbeau et al. 1987; Behrman and Harkema 2000; Visintin
and Barbeau 1994). Before data acquisition, each subject
walked on the treadmill for one to three minutes to selectthe most appropriate BWS and treadmill speed. All therapists provided assistance only when needed.

In all subjects, sural nerve stimulation was triggered
based on the signal from a foot switch (Noraxon USA Inc.,
Scottsdale, AZ, USA), and was delivered randomly every
three to ten steps. Stimuli were randomly dispersed
throughout the step cycle, which was divided into 16 equalbins. The step cycle phases of the contralateral leg werealso identiWed by a foot switch. A custom written software
program (Labview, National Instruments, Austin, TX,
USA) was used to identify the 16 bins for the stimulated
and non-stimulated steps. Bin 1 corresponds to heel strike.
Bins 8, 9, and 16 correspond approximately to stance-toswing transition, swing phase initiation, and swing-tostance transition, respectively. At each bin, at least Wve
reXexes were randomly recorded. Data were sampled at2,000 Hz, band-pass Wltered at 20–1,000 Hz (National
Instruments, Austin, TX, USA) and stored on a personal
computer for oV-line analysis.
Data analysis
EMG signals during stepping (with and without sural nerve
stimulation) were full-wave rectiWed, high-pass Wltered at
20 Hz and low-pass Wltered at 500 Hz. OV-line analysis
started with identiWcation of the responses in the ipsilateral
TA muscle with a customized Labview script. To quantify
the Xexion reXex three analysis windows were deWned. The
Wrst window started at 30 ms post-stimulus with 40-ms
duration (short-latency), the second window started at
70 ms post-stimulus with 30-ms duration (mediumlatency), and the third window started at 110 ms post-stimulus with 290-ms duration (long-latency) (Fig. 1).
For each bin of the step cycle, the full-wave rectiWed
area for each response (short, medium, and long), were calculated and averaged separately for steps with and withoutsural nerve stimulation. For each subject and Xexion reXex
component, the average EMG of non-stimulated steps (control EMG) was subtracted from the average EMG of stimulated steps (reXex EMG) at identical time windows and bins
to allow estimation of the Xexion reXex modulation patternduring stepping (Zehr et al. 1997, 1998). This subtraction
resulted in negative EMGs when the average reXex EMG
was smaller than the control EMG and positive EMGs
when the average reXex EMG was larger than the control
EMG. For each bin, the average subtracted reXex EMG,
non-subtracted reXex EMG, and control EMG were normalized to the maximal control EMG for comparison
across subjects to be made possible. This was done separately for the responses of the ipsilateral and contralateralTA, MG, and SOL muscles corresponding to short,medium, and long latencies.
For each subject, statistical signiWcance between thesubtracted reXex and control EMGs at each bin was established with a Wilcoxon signed rank-sum test. The average

123

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regression was Wtted to the data. This analysis was conducted separately for each subject and for the pool data. In
all statistical tests, signiWcant diVerences were establishedat 95% of conWdence level. Results are presented as meanvalues along with the standard error of the mean (SEM).

Results

Fig. 1 Examples of short (a), medium (b), and long (c) latency ipsilateral tibialis anterior (TA) Xexion reXexes from one subject (D20).
For each response, the Wrst time-window indicates the latency, and thesecond time-window indicates the duration for which the response wasmeasured

subtracted reXex and control EMGs from each subject werethen grouped by bin. Statistical signiWcant diVerences were
established with a Wilcoxon rank-sum test. This was done
separately for ipsilateral and contralateral responses ofshort, medium, and long latencies.
The background EMG activity of the ipsilateral TA muscle for each bin was estimated from the mean value of therectiWed and Wltered EMG for a duration of 40-ms (highpass Wltered at 20 Hz, rectiWed, and low-pass Wltered at
400 Hz), beginning 80 ms before sural nerve stimulation.
The mean amplitude of the ipsilateral short, medium, andlong latency TA Xexion reXex was plotted on the y-axis
versus the TA background activity (normalized to the maximal control EMG) on the x-axis, and a linear least-square123

The average walking speed was 0.91 § 0.18 m/s. The step
cycle duration was not signiWcantly diVerent between theright and left legs with a mean duration of 1,160 § 40 and
1,185 § 39 ms (P = 0.32), respectively. The long latency
TA Xexion reXex was consistently observed in all SCI subjects, being signiWcantly depressed (P < 0.05) compared tothe control EMG from heel strike until mid stance (bins
1–4), and facilitated during late stance (bins 6 and 7) andduring stance-to-swing transition phase (bin 8) (Fig. 2a).
The long latency TA Xexion reXex was signiWcantly(P < 0.05) depressed during the swing-to-stance transition(bins 15 and 16).
At long latencies, sural nerve stimulation induced a signiWcant depression of the ipsilateral MG muscle from bins
9–14, while the ipsilateral SOL muscle was depressed
throughout the step cycle (Fig. 2b). In the contralateralmuscles (MG, SOL, and TA, USA), a depression of thereXex EMG was evident during the swing phase (Fig. 2b).
The average (from all subjects) control EMG as a functionof the step cycle is indicated in Fig. 2c. Similar to previous
observations, ipsilateral and contralateral MG and SOL
activity occurred during stance with some activity also
observed during swing (Fig. 2c). Ipsilateral and contralateral TA activity generally increased during swing, withactivity also observed during stance.
The medium and short latency TA Xexion reXexes were
present in three out of nine SCI subjects (D13, D15, andD20). The presence of these responses in these subjects was
not correlated to a physiological soleus H-reXex modulation pattern during stepping (Knikou et al. 2009). Figure 3ashows the average medium latency TA Xexion reXex during
stepping from these three SCI subjects. The medium latency
TA Xexion reXex was depressed at mid-stance (bin 5) followed by a strong facilitation during stance-to-swing transition phase (bin 8). The reXex EMG facilitation remained
during early swing reaching maximal amplitude at bin 11(Fig. 3a), and re-appeared at late swing (bin 14) (P < 0.05).
Sural nerve stimulation decreased the subtracted reXex
EMG amplitude of the ipsilateral MG and SOL muscles
during swing phase (Fig. 3b). Similar to that previously
reported for the long latency muscle responses, FRA excitation during stepping resulted in depression of the subtracted
reXex EMG amplitude of the contralateral ankle extensor
muscles (MG, SOL) primarily during swing. The contralateral

Exp Brain Res
Fig. 2 Long-latency Xexion
reXex modulation during stepping in SCI. a The mean (+SEM)
subtracted long-latency tibialis
anterior (TA) Xexion reXex
amplitude, control TA EMG,
and non-subtracted TA reXex
EMG normalized to the maximum locomotor control TA
activity is indicated as a functionof the step cycle. b The mean(+SEM) long-latency subtracted
responses, non-subtracted reXex
and control EMGs of the ipsilateral and contralateral soleus(SOL), medialis gastrocnemius(MG), and contralateral TA muscles are indicated as a functionof the step cycle. c The mean(+SEM) normalized control
EMG activity (non stimulated
steps) as a function of the step
cycle. Asterisks indicate suppressive and facilitatory subtracted reXex EMGs comparedto the control EMG based on the
P value of the Wilcoxon signed
rank-sum test. i ipsilateral, ccontralateral

TA reXex EMG amplitude was signiWcantly (P < 0.05)
decreased compared to control EMG during early stanceand late swing (Fig. 3b).
The short latency TA Xexion reXex was modulated in aphase dependent manner similar to that observed for the
medium latency TA Xexion reXex. This reXex was facilitated during stance-to-swing transition phase (bin 8) andremained facilitated during early swing (P < 0.05). The
reXex was signiWcantly decreased compared to the control
EMG during mid-stance (P < 0.05) (Fig. 4a). Ipsilateral andcontralateral ankle subtracted reXex EMG responses(Fig. 4b) at short latencies were similar to those previously
reported for the medium latency responses.
The background EMG amplitude of the ipsilateral TA
muscle as a function of the gait cycle is illustrated inFig. 5a. The relationship between TA Xexion reXex amplitude for the medium and short latencies were linearly

related to the TA background activity during stepping(R2 = 0.65 and R2 = 0.7, respectively). The long-latency TA
Xexion reXex was not linearly related to the TA EMG background activity (R2 = 0.075).

Discussion
This study provides evidence of modulation of the Xexion
reXex during stepping in people with a chronic SCI. Our
study showed that the Xexion reXex, regardless of latency,
could be modulated in a phase-dependent manner. The
onset of the long latency Xexion reXex facilitation consistently occurred during late stance, while short and medium
latency Xexion reXexes onset facilitation occurred during
the stance-to-swing transition. Long latency Xexion reXex
inhibition was evident primarily during stance or late123

Exp Brain Res
Fig. 3 Medium-latency Xexion
reXex modulation during stepping in SCI. a. The mean(+SEM) (n = 3; subjects D13,
D15, and D20) subtracted medium-latency tibialis anterior (TA)Xexion reXex, control TA EMG,
and non-subtracted reXex TA
EMG normalized to the maximum locomotor control TA
activity is indicated as a functionof the step cycle. b The mean(+SEM) (n = 3; subjects D13,
D15, and D20) medium-latency
subtracted responses, non-subtracted reXex and control EMGs
of the ipsilateral and contralateral soleus (SOL), medialis gastrocnemius (MG), andcontralateral TA muscles areindicated as a function of thestep cycle. Asterisks indicate
suppressive and facilitatory subtracted reXex EMGs comparedto the control EMG based on the
P value of the Wilcoxon signed
rank-sum test. i ipsilateral,c contralateralFig. 4 Short-latency Xexion
reXex modulation during stepping in SCI. a The mean (+SEM)(n = 3; subjects D13, D15, andD20) subtracted short-latency
tibialis anterior (TA) Xexion
reXex, control TA EMG, andnon-subtracted reXex TA EMG
amplitudes normalized to the
maximum control TA activity is
indicated as a function of thestep cycle. b The mean (+SEM)(n = 3; subjects D13, D15, andD20) short latency subtracted responses, non-subtracted reXex
and control EMGs of the ipsilateral and contralateral soleus(SOL), medialis gastrocnemius(MG), and contralateral TA muscles are indicated as a functionof the step cycle. Asterisks indicate suppressive and facilitatory
subtracted reXex EMGs compared to the control EMG basedon the P value of the Wilcoxon
signed rank-sum test. i ipsilateral, c contralateral123

Exp Brain Res
Fig. 5 a Mean (+SEM) normalized tibialis anterior (TA) EMG
background activity from all
subjects as a function of the step
cycle. b The relationship between the TA long (left panel),medium (middle panel), andshort (right panel) latency average TA reXex amplitude normalized to the maximum control TA
activity is indicated as a functionof normalized TA EMG background activity from all subjects.
In each graph, the 16 points correspond to the 16 bins of the gait
cycle

swing. Our Wndings denote that in people with a SCI, thelong latency Xexion reXex is modulated in a physiologicalmanner during stepping, and that the short and medium
latency Xexion reXexes were preserved in three out of ninesubjects.
Expression of the Xexion reXex during stepping in human
SCI
The long latency TA Xexion reXex was modulated in aphase dependent manner during stepping regardless of theneurologic injury level or ASIA score. SpeciWcally, thereXex was facilitated during late stance reaching maximal
amplitude before swing phase initiation, and was depressedat heel-strike, and during early stance and swing-to-stancetransition (Fig. 2a). This denotes a shift between facilitatory
and inhibitory interneuronal circuits that is consistent witha meaningful behavior of the Xexion reXex during stepping.
Further, Xexion reXex facilitation coincided with reduced
activity of the ipsilateral ankle extensor muscles (MG,
SOL) (Fig. 2b), that occurred largely during the swing.
Ipsilateral and contralateral sural nerve stimulation has
been reported to decrease presynaptic inhibition of soleus
Ia aVerent terminals in non-injured subjects (Delwaide
et al. 1981; Iles 1996), a phenomenon that has been
described for people with a motor complete SCI (RobyBrami and Bussel 1992). Most likely, phasic modulation ofpresynaptic inhibition was involved in modifying the Xexion reXex during stepping since the long latency TA Xexion
reXex amplitude was not directly related to the TA background EMG activity (Fig. 5b), and the reXex was phasically modulated regardless of an incomplete or complete
spinal lesion. It has clearly been demonstrated in spinalized

animals through direct recordings that FRA evoke primary
aVerent inhibition in their own terminals (Eccles et al.
1962). However, because brain inXuences FRA transmission, and FRA interact with group I inhibitory interneurons(Lundberg and Voorhoeve 1962), postsynaptic mechanisms
cannot be excluded.
The spinal interneuronal circuits engaged in the longlatency Xexion reXex have been correlated to the interneuronal circuitry generating stepping (Sherrington 1910;
Jankowska et al. 1967b). This is substantiated by recent
studies which showed that FRA volleys reset the locomotor
rhythm by terminating the ongoing extensor activity andinitiating a new Xexor burst (Perreault et al. 1995; Schomburg
et al. 1998; Stecina et al. 2005). If we assume that spinal
interneuronal networks for locomotion exist in humans, we
can suggest that the Xexor pattern generator was engaged in afunctional way in people with SCI during stepping.
The phase dependent Xexion reXex modulation we
observed might have been driven by movement related
aVerent feedback interacting with spinal neuronal circuitsthat mediated the shift between facilitatory and inhibitory
interneuronal circuits within the FRA pathway. Plantarpressure, muscle contraction, and hip position modulate thelong latency Xexion reXex in motor complete and incomplete SCI subjects at rest (Conway and Knikou 2008;
Knikou and Conway 2005; Knikou 2007b). Recent studies
have shown that changes in hip position entrain the step
cycle presumably via activity of hip Xexor group II muscle
aVerents (Kriellaars et al. 1994; McVea et al. 2005; Perreault
et al. 1995), which are a part of the FRA system. Our current observations are consistent with the modulation patternof the long latency Xexion reXex in SCI subjects by hip
stretches, which is facilitated during hip extension and123

Exp Brain Res

depressed during hip Xexion (Knikou et al. 2006, 2007;
Knikou 2007a). In this study, Xexion reXexes were facilitated at the step cycle phases at which the hip was extended
and depressed when the hip was Xexed as occurs during theswing-to-stance transition. Further research is needed, however, to understand the role of sensory feedback on thephase dependent modulation of spinal reXexes in both SCI
and non-injured subjects.
The long latency Xexion reXex was present despite thepresence or absence of short and medium latency Xexion
reXexes. These responses were observed in three (D13,
D15, and D20) motor incomplete (ASIA D) SCI subjects,
with injuries at cervical and thoracic levels (Table 1). Flexion reXexes at latencies shorter than 100 ms are not commonly observed in complete or incomplete SCI subjects(Roby-Brami and Bussel 1987; Knikou and Conway 2005;
Knikou 2007b). This may be related to the stimulation
intensity being sub-threshold for these responses (Meinck
et al. 1985), the need for presence of descending input, andengagement of diVerent interneuronal pathways. The Xexion reXex at short and medium latencies was facilitated during the stance-to-swing transition phase (Figs. 3, 4), anddepressed during mid-stance. Jones and Yang (1994) postulated that in people with an incomplete SCI the medium
latency TA Xexion reXex was facilitated during swing andoccasionally during stance. The diVerence between ourstudy and that of Jones and Yang (1994) may be attributed
to excitation of diVerent aVerent groups based on diVerent
stimulation sites (posterior tibial nerve at the medial malleolus vs. sural nerve) and intensities.
The Xexion reXex depends largely on the stimulus
strength and site of stimulation (Kugelberg 1948; Andersen
et al. 1999, 2001; Spaich et al. 2004). This dependency has
resulted in nociceptive Xexion reXexes observed at latencies of 40–60 and 110–400 ms (Shahani and Young 1971;
Hagbarth and Finer 1963), and even beyond 2,000 ms(Kugelberg 1948) in non-injured subjects at rest. Regardless the latency, the nociceptive Xexion reXex is modulated
in a phase-dependent manner in non-injured subjects(Crenna and Frigo 1984; Spaich et al. 2004, 2009), similar
to our observations.
Sural nerve stimulation at non-nociceptive levels in noninjured subjects at rest does not evoke long latency
responses (Knikou 2007b). The long latency Xexion reXex
we observed here appears to be related to the patient population, and our Wndings are in line with those reported inchronic spinal cats (Forssberg et al. 1977). The responses
observed in non-injured subjects following non-nociceptive
stimulation of skin, sensory or mixed peripheral nerves arealso modulated in a phase-dependent manner during walking (Duysens et al. 1993; Tax et al. 1995; Zehr et al. 1997,
1998; Van Wezel et al. 1997). However, foot innocuous
stimulation induced responses were reported to be phase123

independently modulated for the TA and phase dependently
modulated for other ipsilateral and contralateral muscles(Belanger and Patla 1987). A cyclic modulation in the TA
responses was observed during running and cycling (Patla
and Belanger 1987). This suggests that the task-dependent
regulation of these responses occurs independently of themuscle activity during movement (Belanger and Patla
1987; Patla and Belanger 1987).
The role of sensory feedback and excitation thresholdafter a SCI in humans may be shifted. Further, because theproportion of aVerents (largely A and A) activated by
the stimulus train play a signiWcant role in the expression of
the long latency Xexion reXex (Conway et al. 1995), a comparison of Xexion reXexes evoked in SCI and non-injured
subjects cannot be made. It should be noted that cutaneomuscular, nociceptive, and non-nociceptive Xexion reXexes
may share common spinal interneuronal circuits and neuronal pathways.
Last, TA impaired function during walking in neurological disorders has been strongly correlated to an absent or
reduced corticospinal drive to TA motoneurons on the basisof absent short-term TA motor unit synchronization during
the swing phase (Hansen et al. 2005; Nielsen et al. 2008).
Despite the impaired TA function due to supraspinal deWcits after neurologic injury and the supraspinal control of
the Xexion reXex (Holmqvist and Lundberg 1961;
Lundberg and Voorhoeve 1962; Hongo et al. 1969), theelectrically induced Xexion reXexes in SCI subjects weremodulated in a phase dependent manner during stepping.
Study limitations
Spinal cord injury was characterized as ASIA D in six outof nine subjects, while people with ASIA A, B, and C participated in the study increasing the variability of the lesion
type within the SCI group. A challenge of studying individuals with SCI is their availability at individual rehabilitation and research centers. Our understanding of spinal
reXex circuitry and neuronal pathways during stepping
would be greatly enhanced by collaborations across centers
to increase the numbers of subjects representing diVerent
levels and completeness of SCI. Three subjects were also
taking anti-spasticity medication at the time of the study.
Baclofen enhances the levels of presynaptic inhibition(Abbruzzese 2002), a mechanism that accounts largely forthe phase dependent modulation of monosynaptic spinal
reXexes during walking in humans (Morin et al. 1982;
Yang and Whelan 1993; Faist et al. 1996). However, theTA Xexion reXex was modulated during stepping in all SCI
subjects, including those under medication.
During stepping using BWST, external manual assistance is provided by therapists so appropriate kinematics
and kinetics of walking to be generated (Wernig and Müller

Exp Brain Res

1992; Behrman and Harkema 2000; Field-Fote et al. 2005).
This manual assistance may have resulted in excitation of
diVerent types of aVerents, including cutaneous aVerents,
which modulate and mediate Xexion reXexes in SCI subjects (Knikou et al. 2007; Knikou 2007b). However, understanding the interaction of spinal interneuronal circuits andsensory feedback related to cutaneous input, stretch, andload after neurologic injury with rehabilitative interventions is important for optimizing strategies so to improve
recovery of walking after SCI.

Conclusion
The Xexion reXex, regardless of latency, could be modulated in a phase dependent manner during stepping using
BWST and manual assistance in people with a chronic SCI.
Flexion reXex facilitation coincided with decreased activityof ipsilateral ankle extensors, supporting a reciprocal organization of the FRA interneuronal circuits during stepping.
Our results suggest that the mechanisms underlying phasedependent TA Xexion reXex modulation are preserved in
SCI subjects and may have been triggered or potentiatedfrom the intervention utilized in this study. Based on ourcurrent Wndings and on the abnormal modulation pattern of
the soleus H-reXex observed in the majority of thesepatients (Knikou et al. 2009), we suggest that training
should selectively target spinal reXex circuits in which
extensor muscles are involved so sensorimotor recovery inthese patients can be maximized.
Acknowledgments This work was supported by the New York State
Spinal Cord Injury Research Trust Fund through Department of HealthContract No. C020933 to Maria Knikou. Authors thank the clinical research team of the Frazier Rehab Institute, the research participants fortheir valuable contribution, and Dr. Yangsheng Chen for his excellent
engineering contribution to this work.

References
Abbruzzese G (2002) The medical management of spasticity. Eur J
Neurol 9:30–34
Anden NE, Jukes MG, Lundberg A, Vyklicky L (1966a) The eVect ofDopa on the spinal cord 1. InXuence on transmission from primary aVerents. Acta Physiol Scand 67:373–386
Anden NE, Jukes MG, Lundberg A, Vyklicky L (1966b) The eVect ofDopa on the spinal cord. 3. Depolarization evoked in the central
terminals of ipsilateral Ia aVerents by volleys in the FRA. Acta
Physiol Scand 68:322–336
Andersen OK, Sonnenborg FA, Arendt-Nielsen L (1999) Modularorganization of human leg withdrawal reXexes elicited by electrical stimulation of the foot sole. Muscle Nerve 22:1520–1530
Andersen OK, Sonnenborg FA, Arendt-Nielsen L (2001) ReXex receptive Welds for human withdrawal reXexes elicited by non-painful
and painful electrical stimulation of the foot sole. Clin Neurophysiol 112:641–649

Barbeau H, Wainberg M, Finch L (1987) Description and applicationof a system for locomotor rehabilitation. Med Biol Eng Comput
25:341–344
Behrman AL, Harkema SJ (2000) Locomotor training after human spinal cord injury: a series of case studies. Phys Ther 80:688–700
Belanger M, Patla AE (1987) Phase-dependent compensatory responses to perturbation applied during walking in humans. J Mot Behav
19:434–453
Bussel B, Roby-Brami A, Azouvi P, Biraben A, YakovleV A, Held JP(1988) Myoclonus in a patient with spinal cord transaction. Possible involvement of the spinal stepping generator. Brain
111:1235–1245
Bussel B, Roby-Brami A, YakovleV A, Bermis N (1989) Late Xexion
reXex in paraplegic patients. Evidence for a spinal stepping generator. Brain Res Bull 22:53–56
Calancie B, Needham-Shropshire B, Jacobs P, Willer K, Zych G,
Green BA (1994) Involuntary stepping after chronic spinal cord
injury. Evidence for a central rhythm generator for locomotion inman. Brain 117:1143–1159
Capaday C, Stein RB (1986) Amplitude modulation of the soleus
H-reXex in the human during walking and standing. J Neurosci
6:1308–1313
Conway BA, Knikou M (2008) The action of plantar pressure on Xexion reXex pathways in the isolated human spinal cord. Clin Neurophysiol 119:892–896
Conway BA, Scott DT, Riddell JS (1995) The eVects of plantar nerve
stimulation on long latency Xexion reXexes in the acute spinal cat.
In: Taylor A, Gladden MN, Durbada R (eds) Alpha and gamma
motor systems. Plenum Press, New York, pp 593–595
Crenna P, Frigo C (1984) Evidence of phase-dependent nociceptive
reXexes during locomotion in man. Exp Neurol 85:336–345
Delwaide PJ, Crenna P, Fleron MH (1981) Cutaneous nerve stimulation and motoneuronal excitability: I. Soleus and tibialis anterior
excitability after ipsilateral and contralateral sural nerve stimulation. J Neurol Neurosurg Psychiatry 44:699–707
Dobkin BH, Harkema S, Requejo P, Edgerton VR (1995) Modulationof locomotor-like EMG activity in subjects with complete andincomplete spinal cord injury. J Neurol Rehab 9:183–190
Duysens J, Tax AA, Trippel M, Dietz V (1993) Increased amplitude of
cutaneous reXexes during human running as compared to standing. Brain Res 613:230–238
Eccles RM, Lundberg A (1959) Synaptic actions in motoneurones by
aVerents which may evoke the Xexion reXex. Arch Ital Biol
97:199–221
Eccles JC, Kostyuk PG, Schmidt RF (1962) Presynaptic inhibition of
the central actions of Xexor reXex aVerents. J Physiol (Lond)
161:248–281
Faist M, Dietz V, Pierrot-Deseilligny E (1996) Modulation, probably
presynaptic in origin, of monosynaptic Ia excitation during
human gait. Exp Brain Res 109:441–449
Faist M, Ertel M, Berger W, Dietz V (1999) Impaired modulation ofquadriceps tendon jerk reXex during spastic gait: diVerences
between spinal and cerebral lesions. Brain 122:567–579
Field-Fote EC, Lindley SD, Sherman AL (2005) Locomotor training
approaches for individuals with spinal cord injury: a preliminary
report of walking-related outcomes. J Neurol Phys Ther 29:127–
137
Forssberg H, Grillner S, Rossignol S (1977) Phasic gain control of reXexes from the dorsum of the paw during spinal locomotion. Brain
Res 132:121–139
Fu TC, Jankowska E, Lundberg A (1975) Reciprocal Ia inhibition during the late reXexes evoked from the Xexor reXex aVerents after
DOPA. Brain Res 85:99–102
Hagbarth KE, Finer BL (1963) The plasticity of human withdrawal
reXexes to noxious skin stimuli in lower limbs. Prog Brain Res
1:65–81123

Exp Brain Res
Hansen NL, Conway BA, Halliday DM, Hansen S, Pyndt HS,
Biering-Sørensen F, Nielsen JB (2005) Reduction of common
synaptic drive to ankle dorsiXexor motoneurons during
walking in patients with spinal cord lesion. J Neurophysiol
94:934–942
Hodapp M, Vry J, Mall V, Faist M (2009) Changes in soleus H-reXex
modulation after treadmill training in children with cerebral
palsy. Brain 132:37–44
Holmqvist B, Lundberg A (1961) DiVerential supraspinal control
of synaptic actions evoked by volleys in the Xexion reXex
aVerents in alpha motoneurones. Acta Physiol Scand Suppl
186:1–15
Hongo T, Jankowska E, Lundberg A (1969) The rubrospinal tract II.
Facilitation of interneuronal transmission in reXex paths to motoneurones. Exp Brain Res 7:369–391
Iles JF (1996) Evidence for cutaneous and corticospinal modulation ofpresynaptic inhibition of Ia aVerents from the human lower limb.
J Physiol (Lond) 491:197–207
Jankowska E, Jukes MG, Lund S, Lundberg A (1967a) The eVect of
DOPA on the spinal cord. 5. Reciprocal organization of pathways
transmitting excitatory action to alpha motoneurones of Xexors
and extensors. Acta Physiol Scand 70:369–388
Jankowska E, Jukes MG, Lund S, Lundberg A (1967b) The eVect of
DOPA on the spinal cord. 6. Half-centre organization of interneurones transmitting eVects from the Xexor reXex aVerents. Acta
Physiol Scand 70:389–402
Jones CA, Yang JF (1994) ReXex behavior during walking in incomplete spinal cord injured subjects. Exp Neurol 128:239–248
Knikou M (2007a) Hip-phase-dependent Xexion reXex modulation andexpression of spasms in patients with spinal cord injury. Exp
Neurol 204:171–181
Knikou M (2007b) Plantar cutaneous input modulates diVerently spinal reXexes in subjects with intact and injured spinal cord. Spinal
Cord 45:69–77
Knikou M, Conway BA (2005) EVects of electrically induced muscle
contraction on Xexion reXex in human spinal cord injury. Spinal
Cord 43:640–648
Knikou M, Kay E, Rymer WZ (2006) Modulation of Xexion reXex induced by hip angle changes in human spinal cord injury. Exp
Brain Res 168:577–586
Knikou M, Kay E, Schmit BD (2007) Parallel facilitatory reXex pathways from the foot and hip to Xexors and extensors in the injured
human spinal cord. Exp Neurol 206:146–158
Knikou M, Angeli CA, Ferreira CK, Harkema SJ (2009) Soleus
H-reXex modulation during body weight support treadmill
walking in spinal cord intact and injured subjects. Exp Brain
Res 193:397–407
Kriellaars DJ, Brownstone RM, Noga BR, Jordan LM (1994) Mechanical entrainment of Wctive locomotion in the decerebrate cat.
J Neurophysiol 71:2074–2086
Kugelberg E (1948) Demonstration of A and C Wbre components in theBabinski plantar response and the pathological Xexion reXex.
Brain 71:304–319
Lundberg A (1979) Multisensory control of spinal reXex pathways. In:
Granit R, Pomeiano O (eds) ReXex control of posture and movement. Elsevier, Amsterdam, pp 11–28
Lundberg A, Voorhoeve PE (1962) EVects from the pyramidal tract on
spinal reXex arcs. Acta Physiol Scand 56:201–219
Lundberg A, Malmgren K, Schomburg ED (1987) ReXex pathways
from group II muscle aVerents. 3. Secondary spindle aVerents andthe FRA: a new hypothesis. Exp Brain Res 65:294–306
Maynard FM Jr, Bracken MB, Creasey G, Ditunno JF Jr, Donovan
WH, Ducker TB, Garber SL, Marino RJ, Stover SL, Tator CH,
Waters RL, Wilberger JE, Young W (1997) International standards for neurological and functional classiWcation of spinal cord
injury. Spinal Cord 35:266–274

123

McVea DA, Donelan JM, Tachibana A, Pearson KG (2005) A role for
hip position in initiating the swing-to-stance transition in walking
cats. J Neurophysiol 94:3497–3508
Meinck HM, Kuster S, Benecke R, Conrad B (1985) The Xexor reXexinXuence of stimulus parameters on the reXex response. Electroencephalogr Clin Neurophysiol 61:287–298
Morin C, Katz R, Mazieres L, Pierrot-Deseilligny E (1982) Comparison of soleus H-reXex facilitation at the onset of soleus contractions produced voluntarily and during the stance phase of human
gait. Neurosci Lett 33:47–53
Nielsen JB, Britain JS, Halliday DM, Marchand-Pauvert V, Mazevet
D, Conway BA (2008) Reduction of common motoneuronal driveon the aVected side during walking in hemiplegic stroke patients.
Clin Neurophysiol 119:2813–2818
Patla AE, Belanger M (1987) Task-dependent compensatory responses
to perturbations applied during rhythmic movement in humans.
J Motor Behav 19:454–475
Perreault MC, Angel MJ, Guertin P, McCrea DA (1995) EVects ofstimulation of hindlimb Xexor group II muscle aVerents during
Wctive locomotion. J Physiol (Lond) 487:211–220
Roby-Brami A, Bussel B (1987) Long-latency spinal reXex in man after Xexor reXex aVerent stimulation. Brain 110:707–725
Roby-Brami A, Bussel B (1990) EVects of FRA stimulation on the soleus H-reXex in patients with a complete spinal cord lesion: evidence for presynaptic inhibition of Ia transmission. Exp Brain Res
81:593–601
Roby-Brami A, Bussel B (1992) Inhibitory eVects on Xexor reXexes inpatients with a complete spinal cord lesion. Exp Brain Res
90:201–208
Schomburg ED, Petersen N, Barajon I, Hultborn H (1998) Flexor reXex aVerents reset the step cycle during Wctive locomotion in thecat. Exp Brain Res 122:339–350
Shahani BT, Young RR (1971) Human Xexor reXexes. J Neurol Neurosurg Psychiatry 34:616–627
Sherrington CS (1910) Flexion-reXex of the limb, crossed extensionreXex and reXex stepping and standing. J Physiol (Lond) 40:28–
121
Spaich EG, Arendt-Nielsen L, Andersen OK (2004) Modulation oflower limb withdrawal reXexes during gait: a topographical study.
J Neurophysiol 91:258–266
Spaich EG, Emborg J, Collet T, Arendt-Nielsen L, Andersen OK(2009) Withdrawal reXex responses evoked by repetitive painfulstimulation delivered on the sole of the foot during late stance:
site, phase, and frequency modulation. Exp Brain Res 194:359–
368
Stecina K, Quevedo J, McCrea DA (2005) Parallel reXex pathways
from Xexor muscle aVerents evoking resetting and Xexion
enhancement during Wctive locomotion and scratch in the cat.
J Physiol (Lond) 569:275–290
Tax AA, Van Wezel BM, Dietz V (1995) Bipedal reXex coordination
to tactile stimulation of the sural nerve during human running.
J Neurophysiol 73:1947–1964
Van Wezel BM, OttenhoV FA, Duysens J (1997) Dynamic control oflocation-speciWc information in tactile cutaneous reXexes fromthe foot during human walking. J Neurosci 17:3804–3814
Visintin M, Barbeau H (1994) The eVects of parallel bars, body weightsupport and speed on the modulation of the locomotor pattern of
spastic paretic gait. A preliminary communication. Paraplegia
32:540–553
Wernig A, Müller S (1992) Laufband locomotion with body weightsupport improved walking in persons with severe spinal cord injuries. Paraplegia 30:229–238
Wirz M, Zemon DH, Rupp R, Scheel A, Colombo G, Dietz V, Hornby
TG (2005) EVectiveness of automated locomotor training in patients with chronic incomplete spinal cord injury: a multicenter
trial. Arch Phys Med Rehabil 86:672–680

Exp Brain Res
Yang JF, Whelan PJ (1993) Neural mechanisms that contribute tocyclical modulation of the soleus H-reXex in walking in humans.
Exp Brain Res 95:547–556
Yang JF, Fung J, Edamura M, Blunt R, Stein RB, Barbeau H (1991)
H-reXex modulation during walking in spastic paretic subjects.
Can J Neurosci 18:443–452

Zehr EP, Komiyama T, Stein RB (1997) Cutaneous reXexes during
human gait: electromyographic and kinematic responses toelectrical stimulation. J Neurophysiol 77:3311–3325
Zehr EP, Stein RB, Komiyama T (1998) Function of sural nerve
reXexes during human walking. J Physiol (Lond) 507:305–314

123