J Musculoskelet Neuronal Interact 2016; 16(1):58-62

Hylonome

Original Article

Effects of short term water immersion on peripheral reflex

excitability in hemiplegic and healthy individuals:
A preliminary study
N.J. Cronin1*, A.M. Valtonen2,3*, B. Waller4, T. Pyhnen2, J. Avela1
2

1
University of Jyvaskyla, Neuromuscular Research Center, Department of Biology of Physical Activity, Finland;
Kymenlaakso Central Hospital, Carea, Rehabilitation and Pain Unit, Kotka, Finland; 3Helsinki Metropolia University of Applied Sciences,
Human Movement and Functioning, Helsinki, Finland; 4University of Jyvaskyla, Department of Health Sciences, Finland
*These authors contributed equally to this work
Tapani Pyhnen died prior to publication of this work

Abstract
Background: Reflex excitability is increased in hemiplegic patients compared to healthy controls. One challenge of stroke rehabilitation is to decrease the effects of hyperreflexia, which may be possible with water immersion. Methods/Aims: The present study examined the effects of acute water immersion on electrically-evoked Hmax:Mmax ratios (a measure of reflex excitability) in 7 hyperreflexive
hemiplegic patients and 7 age-matched healthy people. Hmax:Mmax ratios were measured from soleus on dry land (L1), immediately after
(W1) and 5 minutes after immersion (W5), and again after five minutes on land (L5). Results: Water immersion led to an acute increase
in Hmax:Mmax ratio in both groups. However, after returning to dry land, there was a non-significant decrease in the Hmax:Mmax ratio of 8%
in the hemiplegic group and 10% in healthy controls compared to pre-immersion values. Interpretation: A short period of water immersion can decrease peripheral reflex excitability after returning to dry land in both healthy controls and post-stroke patients, although
longer immersion periods may be required for sustainable effects. Water immersion may offer promise as a low-risk, non-invasive and
non-pharmaceutical method of decreasing hyperreflexivity, and could thus support aquatic rehabilitation following stroke.
Keywords: H/M-ratio, Reflex Excitability, Hemiplegia, Water Immersion, Stroke Rehabilitation

Introduction
Stroke is one of the leading causes of death and disability, and
with prevalence of stroke set to increase over the next 20 years,
the financial burden of stroke is also set to escalate globally1,2.
Irrespective of the type of stroke, loss of functional capacity is
one of the major problems, and a main focus of rehabilitation is
the restoration of function using a wide range of methods3. Following an upper motor neuron lesion such as stroke, the performance of activities of daily living is affected by the development

of spasticity and the consequences of decreased physical activity, e.g. muscle weakness, impaired motor control and soft tissue
contracture4. Prevalence of spasticity after stroke ranges from
19-92% and is a common factor restricting restoration of function5,6. Spasticity is characterised by an exaggerated response of
the stretch reflex, or hyperreflexia, to a velocity-dependent stretch
of a muscle at rest7. One challenge of rehabilitation paradigms for
stroke patients is to decrease the impact of spasticity on active
and passive movement, thereby improving rehabilitation participation and completion of activities of daily living (ADL).

The authors have no conflict of interest.

Abbreviations

Corresponding author: Neil Cronin, University of Jyvaskyla, Neuromuscular

Research Center, Department of Biology of Physical Activity, P. O. Box 35, FI40014, University of Jyvaskyla, Finland
E-mail: neil.j.cronin@jyu.fi
Edited by: J. Rittweger
Accepted 26 November 2015
58

L1 Measurements performed on dry land before immersion.

W1 Measurements performed immediately after immersion in water.
W5 Measurements performed five minutes after being immersed in
water.
L5 Measurements performed on dry land five minutes after the water
measurements.

N.J. Cronin et al.: Water immersion and reflex excitability

Sex

Age
Height (cm)
Body mass (kg)
Reason for hemiplegia

Years since stroke

Achilles tendon reflex
Active ankle dorsiflexion
Active ankle plantar flexion
Passive ankle dorsiflexion
Passive ankle plantar flexion

Patients

3 female,
4 male
4910
17111
8323
4 infarction,
3 haemorrhage
78
(range: 3-26)
3.00
2.11.2
2.31.3
3.61.5
1.91.9

Controls
3 female,
4 male
509
17110
7313
-

Values are mean SD. Achilles tendon reflex scores are based on the
NINDS scale23, 1=hyporeflexia, 4=clonus. Active ankle dorsi/plantar flexion scores are based on the modified MRC scale for testing
strength24, 0=plegic, 5=strong movement. Passive scores are based
on the modified modified Ashworth scale25, 1=normal, 5=rigid.

Table 1. Subject characteristics for the patient and control groups.

Aquatic therapy is one rehabilitation modality that has strong

evidence to show that it is effective at improving muscle strength
of the paretic leg following stroke3, and is one that holds therapeutic promise. The specific properties of water, such as pressure
and relative weightlessness, could be exploited to reduce abnormal side-effects of hemiplegia such as altered muscle tone and
weakness, which are magnified by the effects of gravity8. There
is evidence that water immersion alters normal neuromuscular
function9,10. Electromyographic (EMG) recordings underwater
have generally demonstrated decreased lower limb muscle EMG
amplitudes and EMG/force-ratios for maximal and submaximal
contractions when compared to measurements on dry land9-11
(but see also 12). In particular, in healthy people seated underwater, H-reflexes tend to decrease10. As the H-reflex is an electrical
test of reflex excitability that involves the same pathway as the
stretch reflex but bypasses muscle spindles13, a decrease in Hreflex size indicates a decrease in reflex excitability. If this finding could be replicated in situations where reflex excitability is
abnormally high, water immersion could be a valuable tool for
clinical rehabilitation.
Neurophysiological responses to water immersion have been
largely unexplored in pathological conditions. Therefore, the present study was designed to examine the effects of acute underwater immersion on neuromuscular function in hemiplegic patients
and age-matched healthy people. It was hypothesised that water
immersion would lead to a decrease in reflex excitability in hemiplegic post-stroke patients.

Materials and methods

Subjects

Seven hemiplegic patients and seven healthy age and gender

matched control subjects volunteered to participate in this study

(Table 1). The study received approval from the Ethical Committee of the Kymenlaakso Central Hospital, and subjects gave
written informed consent. Inclusion criteria for the hemiplegic
patients were: (1) stable hemiplegic symptoms caused by hemorrhage or cerebral infarction, (2) hyperreflexia, (3) ability to walk
with or without assistive device, (4) ability to manage activities of
daily living independently. Exclusion criteria were: (1) cognitive
problems, (2) cardio-pulmonary disease, (3) dementia.
Measurements
After shaving, abrading and cleaning the skin with alcohol,
surface EMG electrodes (Medicotest N-OO-S, Denmark) were
positioned bilaterally over soleus below the distal heads of the
gastrocnemius muscles. Inter-electrode distance was 2 cm. After determining the optimal stimulating location, a cathode electrode (1 cm) was placed over the tibial nerve in the popliteal
fossa, and the anode (5 x 8 cm) was positioned superior to the
patella. Single 1 ms square pulses were delivered by a constant
current stimulation unit (Grass Telefactor, Model S48K, Astromed Inc, USA). Stimulation intensity was adjusted in small
increments (4-5 mA) to produce full H-reflex and M-wave recruitment curves, which were sampled by the EMG equipment
(ME6000, Mega Electronics Ltd, Kuopio, Finland) at a frequency of 1000 Hz and recorded using MegaWin software (Mega
Electronics Ltd). Approximately twenty stimuli were collected
at each measurement interval, using a randomised inter-stimulus
interval of at least 10 seconds. The highest H-reflex and M-wave
values were used for further analysis.
Protocol
All data were collected in a single session, which included
identical measurements performed on land (L1), immediately after water immersion (W1) and after five minutes of immersion
(W5) with the water at the level of the mid-sternum and the lower
limb approximately 60 cm below this point. Finally, the measurements were repeated after five minutes on land (L5). The
room and water temperatures were 26C and 33C, respectively.
Throughout the measurements, subjects were seated in a poolside
elevator chair with hip angle at 110, knee angle at 160 and ankle
angle at 90 (180=full extension). In order to guarantee identical position throughout the measurement session, the ankles and
thighs were stabilized with straps, and subjects were instructed to
keep the head still, eyes open, and look forwards at all times. To
protect the recording equipment and to guarantee patient safety,
each subject wore thin waterproof trousers (Vision Flyfishing,
Finland). At each measurement interval, H-reflexes and M-waves
were evoked from the affected hemiplegic leg in the patient group
and the dominant leg in controls. At L1, the non-affected leg of the
patient group was also measured as a reference.
Statistical analysis
Means and standard deviations were calculated for the
peak-to-peak amplitudes of maximal H- and M-responses and
Hmax:Mmax ratios for all subjects and conditions. Of these parameters, statistics were only performed on the Hmax:Mmax ratio be59

N.J. Cronin et al.: Water immersion and reflex excitability

Hemiplegic group

H-reflex
M-wave

(mV) (mV) Hmax:Mmax

Control group

H-reflex
M-wave
(mV) (mV) Hmax:Mmax

L1
Non-affected
3.46 (1.53)
5.70 (1.39)
0.60 (0.21)
3.65 (1.67)
7.23 (2.30)
0.52 (0.16)
Affected
4.83 (2.24)
7.46 (2.49)
0.64 (0.21)
W1
5.70 (2.56)
7.75 (2.29)
0.73 (0.24)
4.32 (2.41)
7.80 (3.78)
0.58 (0.17)
W5
5.11 (2.67)
7.56 (2.17)
0.66 (0.28)
4.16 (2.72)
7.28 (3.72)
0.58 (0.20)
L5
4.31 (2.26)
7.06 (1.95)
0.59 (0.26)
3.36 (1.81)
7.16 (2.45)
0.47 (0.14)
Table 2. Absolute values for H-reflex, M-wave and Hmax:Mmax ratios on dry land (L1), immediately after water immersion (W1), after 5 minutes of
immersion (W5) and after 5 minutes on land (L5). Values represent group mean (SD).

cause this is a normalized value that allows group comparisons.

Subject characteristics were compared between groups using independent samples t-tests where appropriate. A mixed ANOVA
(group x time) was used to examine differences between groups
(between-subjects factor) and experimental time points (withinsubjects factor), and bonferroni post hoc tests were used as appropriate. Within-group differences in maximal M-wave amplitude across time points were assessed using repeated measures
ANOVA. For all ANOVAs, Mauchlys sphericity test was performed, and where this assumption was violated, GreenhouseGeisser adjustments were used. Healthy and affected limbs were
compared at baseline using dependent samples t-tests in the patient group. Significance was accepted for p<0.05.

Results
There were no statistical differences between groups in terms
of age (t=-0.086, p=0.933), height (t=-0.051, p=0.960) or body
mass (t=1.001, p=0.337). The values of maximal H-reflexes,
M-waves and Hmax:Mmax ratios are shown in Table 2. At L1, independent samples t-tests revealed no statistical difference between the Hmax:Mmax ratio of the control group and the affected
patient limb (t=1.194, p=0.257) or the non-affected limb (t=0.817,
p=0.431). A mixed ANOVA revealed a significant main effect
of time (F=9.166, p<0.001) but not group (F=1.174, p=0.302) on
the Hmax:Mmax ratio. The group x time interaction was also not
significant (F=0.686, p=0.567). Pairwise comparisons based on
the significant effect of time revealed that the Hmax:Mmax ratio increased between L1 and W1 (p<0.05; Mean Difference: 0.074,
95% CI: 0.013-0.135). Between W5 and L5, there was a significant decrease in the Hmax:Mmax ratio (p<0.05; Mean Difference:
0.096, 95% CI: 0.004-0.189). When comparing the pre- and postimmersion values on land (L1 and L5), there was no significant
difference in the Hmax:Mmax ratio (p= 0.449). To explore the effects
of water immersion further, relative changes in Hmax:Mmax ratio
relative to the values at L1 are shown in Figure 1. Changes in
H/M ratios were largely due to changes in H-reflex amplitude,
as M-wave amplitudes did not differ across time points in the
patient group (F=0.344, p=0.794) or the controls (GreenhouseGeisser adjusted F=0.715, p=0.475; see also Table 2).
Within the hemiplegic group, the affected and non-affected
60

Figure 1. Group mean changes in the Hmax:Mmax ratio across all time
points. Values are expressed as percentage change relative to the respective values at L1.

limbs were compared using the value recorded at L1 for the

non-affected limb, and the values from each measurement interval for the affected limb. The Hmax:Mmax ratio did not differ
significantly between the affected and non-affected limbs at L1
(t=-1.307, p=0.261), W1 (t=-2.062, p=0.108) or W5 (t=-1.392,
p=0.236). Similarly, at L5, after the subjects had been on dry
land for 5 minutes, there was no significant difference between
limbs (t=-0.688, p=0.529).

Discussion
The present study was among the first to examine reflex responses both in and out of water, and this technique may open
new possibilities for aquatic research. The results revealed that
immediately after immersion in water, the Hmax:Mmax ratio increased in the affected limb of hemiplegic stroke patients and
healthy controls. This is in contrast to the results of a previ-

N.J. Cronin et al.: Water immersion and reflex excitability

ous study that reported an approximately 30% reduction in the

Hmax:Mmax ratio during water immersion10. This apparent difference may be due to the fact that waterproof trousers were worn
in the present study but not in the previous study. Previous research using compressive joint supports and taping suggests a
modulation of proprioception following their application14,15,
which may be mediated by pressure and/or cutaneous pathways.
Alternatively, it has been suggested that water immersion leads
to a rapid decrease in afferent firing of some pathways16, and this
may be briefly over-compensated by other pathways. In addition
to the use of waterproof trousers, the period of immersion was
also longer in the study of Pyhnen and Avela (>10 minutes versus 5 minutes), which may also have contributed to the observed
differences between studies. This suggestion is somewhat supported by the finding that the Hmax:Mmax ratio almost returned
to pre-immersion levels after just 5 minutes of immersion in the
patient group, and may thus have continued to decrease with a
longer immersion period. The longer-term effects of immersion
are of clinical relevance as it is currently unknown how long
these effects last and how they impact function in activities of
daily living.
Although not reflected in the group level statistics, after returning to dry land, reflex excitability was generally lower at L5
compared to L1 in both groups (Figure 1). This is in agreement
with the results of Pyhnen and Avela10. It is also noteworthy
that while the control group showed no change in Hmax:Mmax ratio during immersion (i.e. between W1 and W5), the ratio dramatically decreased in the patient group. Conversely, there was
a large decrease in Hmax:Mmax ratio between W5 and L5 in controls, and this difference was much smaller in the patient group.
Interestingly, both groups showed similar relative decreases at
L5, although the mechanisms of these changes may be different
between groups, a topic that warrants further research. For example, stroke may lead to changes in numerous neural pathways,
and it cannot be determined from our data whether and how such
changes affect reflex responses during and after immersion.
The finding that water immersion can alter reflex excitability
over a short timescale may have important clinical implications.
From our data it is not possible to determine whether functionally relevant decreases in reflex excitability would best be elicited
by performing rehabilitation during water immersion, or whether
to perform immersion followed by rehabilitation on land. Both of
these rehabilitation methods have shown promise for restoration
of function post-stroke (e.g. 3; see also 17 regarding dry immersion), but it remains to be determined which of these modalities
is more effective and under which conditions.
Certain limitations should be considered when interpreting
these results. For example, in our design, subjects were strapped
into a chair and their feet did not contact the pool floor. Although necessary to enable posture to be matched between test
conditions, this differs from standard underwater rehabilitation protocols, where subjects stand or walk on the pool floor.
This may be clinically relevant, since cutaneous receptors in the
foot sole, as well as muscle spindles and Golgi tendon organs,
likely respond differently to weight bearing compared to our
study design. The small sample size may limit generalizability
of the results, since reflex response amplitudes are highly vari-

able between trials and individuals18, and may be influenced by

subtle changes in numerous peripheral, spinal and supraspinal
pathways19. Moreover, although H-reflex inter-session repeatability is good on land20, we currently do not have repeatability
data concerning underwater measurements. Finally, the stroke
patients in this study were independently mobile and capable
of performing ADLs, and it remains to be seen how immersion
would influence reflex responses in patients with more severe
spasticity and functional impairments. Previous research has
shown that on land, the Hmax:Mmax ratio is positively correlated
with spasticity scores21. This suggests that immersion-induced
decreases in Hmax:Mmax ratio may be smaller in more severely
spastic patients, although this remains to be tested, especially
as the correlation between Hmax:Mmax ratio and spasticity is not
necessarily seen in all individuals22.
In conclusion, a short period of water immersion generally led
to a reduction in peripheral reflex excitability after returning to
dry land in both healthy controls and post-stroke patients. However, the data highlight the need for research into the immediate
and long-term effects of immersion for longer periods of time, i.e.
reproducing clinically used immersion durations. This method,
when combined with appropriate rehabilitation techniques, may
offer promise as a low-risk, non-invasive, non-pharmaceutical
training environment.

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