Haupental (2011) Loading forces in shallow water running in two levels of immersion

Introduction to Aquatic Exercise Biomechanics

Aquatic exercises, such as shallow water running, are integral to rehabilitation and training protocols. However, the biomechanics of water running, especially the ground reaction forces (GRFs) involved, remain underexplored. This study by Haupenthal et al. aims to fill this gap by analyzing the vertical and anteroposterior components of GRFs in shallow water running at different immersion levels.

Methodological Approach

The study involved 22 healthy adults familiar with aquatic exercises. They performed water running at self-selected speeds in two immersion levels: chest and hip. GRFs were measured using an underwater force plate, providing insights into how immersion depth and running speed influence the biomechanical loading on the body.

Key Findings on GRF Components

The research findings revealed that vertical forces were 0.80 and 0.98 times the body weight at chest and hip immersion levels, respectively, while anteroposterior forces were 0.26 and 0.31 times the body weight. Interestingly, subjects ran faster at lower immersion levels, although no significant differences in force values were observed between the two immersion depths, likely due to the variability in self-selected speeds.

Implications for Rehabilitation and Conditioning

These findings highlight the complexity of prescribing aquatic exercises. While immersion level affects buoyancy and, subsequently, the load on the body, running speed significantly influences the anteroposterior force component. This knowledge is crucial for designing safer and more effective aquatic rehabilitation and physical conditioning programs, taking into account both immersion depth and exercise speed.

Conclusion: Advancing Aquatic Exercise Prescription

This study underscores the need for a nuanced understanding of GRFs in water running, considering both vertical and anteroposterior components. By accounting for immersion levels and running speeds, practitioners can better tailor aquatic exercises to individual needs, enhancing the safety and efficacy of rehabilitation and conditioning protocols.

Meta Description:

Keyphrase: J Rehabil Med 42

J Rehabil Med 42
J Rehabil Med 2010; 42: 664669
2010 The Authors. doi: 10.234016501977-0587
Journal Compilation 2010 Foundation of Rehabilitation Information. \ISSN 1650-1977
Objective: To analyse the vertical and anteroposterior com-
ponents of the ground reaction force during shallow water
running at 2 levels of immersion.
Subjects: Twenty-two healthy adults with no gait disorders,
who were familiar with aquatic exercises.
Methods: Subjects performed 6 trials of water running at a
self-selected speed in chest and hip immersion. Force data
were collected through an underwater force plate and run-
ning speed was measured with a photocell timing light sys-
tem. Analysis of covariance was used for data analysis.
Results: Vertical forces corresponded to 0.80 and 0.98 times
the subjects body weight at the chest and hip level, respec-
tively. Anteroposterior forces corresponded to 0.26 and 0.31
times the subjects body weight at the chest and hip level,
respectively. As the water level decreased the subjects ran
faster. No significant differences were found for the force
values between the immersions, probably due to variability
in speed, which was self-selected.
Conclusion: When thinking about load values in water run-
ning professionals should consider not only the immersion
level, but also the speed, as it can affect the force compo-
nents, mainly the anteroposterior one. Quantitative data on
this subject could help professionals to conduct safer aqua tic
rehabilitation and physical conditioning protocols.
Key words: aquatic exercise; aquatic rehabilitation; running;
J Rehabil Med 2010; 42: 664669
Correspondence address: Alessandro Haupenthal, Rua Ivo
Silveira, 177, apto 802, 88085-001, Florianpolis, SC, Brazil.
E-mail: dedsnet@yahoo.com.br
Submitted May 20, 2009; accepted April 14, 2010
Water running, as a fundamental component of most aquatic
rehabilitation and training protocols, has been little explored
in terms of biomechanical variables, which makes the prescrip-
tion of this exercise by physiotherapists and kinesiologists
difficult when there is a need to control the mechanical load
acting on the individual. Many studies have investigated the use of water running
to maintain and improve physical conditioning, based on the
analysis of physiological variables such as blood pressure,
heart rate, oxygen debt and volume, body temperature, etc. (14). Regarding aquatic rehabilitation, investigators have
focused on the treatment of specific pathologies through the
use of this exercise in hydrokinesitherapy sessions (59).
However, studies of biomechanical variation during the per
formance of water exercises are scarce. Although the kinetic
and kinematic characteristics of walking in water have been
studied over the past few years (1015), information regarding
the biomechanics of shallow water running is less common,
considering only kinematic aspects (16) or even speculating
on kinetic aspects based on the interpretation of indirect static
measurements (17). Despite the fact that weight-bearing is facilitated in the water
due to buoyancy, running in shallow water still involves contact
forces, thus the components of the ground reaction force (GRF)
are required for the execution of the exercise. The values of
this component in water should be lower than those observed
during over-ground running but, in some cases, they could be
excessive and even harmful, depending on the condition of the
individual. An alternative, in order to reduce the risk of injury
and to better adjust the exercise conditions in these cases, is
to vary the level of immersion. It is also important to consider that the resistance imposed
on the body during aquatic locomotion is much greater than
that on land (18), as water is approximately 800 times denser
than air (19). Because of this the anteroposterior component
of the GRF is also required to displace the body forwards and
run in the water. The aim of the present study was to analyse the vertical and
anteroposterior components of the GRF during shallow water
running at a self-selected speed at 2 levels of immersion. The values of the GRF components can provide valuable
information concerning the resultant force acting on the indi-
vidual when running in the water. This is crucial for training
prescription and rehabilitation in the aquatic environment.
Twenty-two subjects (11 males, 11 females), who volunteered in
response to an advertisement, participated in this study. Before any
data were acquired, the subjects were analysed to verify gait disorders,
percentage of body fat (which should range from 12% to 16% for men
and from 20% to 25% for women, based on Pollock & Wilmore (20))
and to ascertain whether they were familiar with aquatic exercises.
After that, the selected individuals were asked to participate in this
study and all signed an informed consent form previously approved
by the Committee for Ethics in Research on Humans of the Institu-
Alessandro Haupenthal, MSc, Caroline Ruschel, MSc, Marcel Hubert, MSc, H\eiliane de Brito
Fontana, BSc and Helio Roesler, PhD
From the Aquatic Biomechanics Research Laboratory, Department of Health and Sports Sciences, Santa Catarina State University, Florianpolis, Brazil

Loading forces in water running
tion. Mean (standard deviation; SD) age, height and mass for the
male subjects were 24.0 (3.0) years, 1.80 (0.05) m and 74.6 (6.8) kg,
respectively. For the female subjects, the mean (SD) age, height and
body mass were and 23.0 (2.5) years, 1.67 (0.05) m and 56.3 (3.8)
kg, respectively.
All tests and procedures were carried out at the swimming pool and
at the Aquatic Biomechanics Research Laboratory of Santa Catarina
State University, Florianpolis, Brazil. In order to measure the vertical
and anteroposterior components of the GRF (Fy and Fx, respectively),
an 8.0 m long walkway platform (covered with non-slip material) con-
taining an underwater force plate was placed at the bottom of a heated
swimming pool (30 1 C). The acquisition system was composed of
the underwater force plate (dimensions 400 mm 400 mm 100 mm,
sensitivity of 2 N, error lower than 1% and 300 Hz natural frequency),
the ADS2000-IP for signal conditioning and AD conversion and the
software AqDados 7.02 for signal analysis and editing (Lynx Tecno-
logia Eletrnica LTDA, So Paulo, SP, Brazil). Data collection procedures began with measuring the anthropometric
data as follows: (i ) body mass of the subjects using an electronic scale
(Plenna, model MEA-08128, scale 0.1 kg, So Paulo, Brazil); ( ii)
height of the subjects using a stadiometer (Sanny American Medical
do Brasil LTDA, scale 0.01 m, So Bernardo do Campo, SP, Brazil);
and (iii) subjects cutaneous folds using a scientific calliper (CES-
CORF Equipamentos Antropomtricos LTDA, scale 0.1 mm, Porto
Alegre, RS, Brazil). The percentage of body fat was determined through calculation of
subjects body density (21). For the male subjects, body density was
calculated via a regression equation using the sum of the thoracic,
abdominal and thigh skin folds (22). In women, the regression equation
uses the sum of the tricipital, supra-iliac and thigh skin folds (23). After the anthropometrical measurements, the subjects were asked to
enter the pool. Each subject underwent an adaptation period in order to
get used to the equipment and data collection conditions. Subsequently,
the subjects were instructed to perform 6 trials of water running at
a self-selected speed at each of the 2 levels of immersion: (i ) chest
level, which corresponds to the subjects xiphoid process sterni; and
( ii) hip level, which corresponds to the subjects iliac crest (Fig. 1).
The walkway platform was adjusted according to each subjects
height and these levels of immersion were chosen by the researchers because they correspond to anatomical points that can be identified
easily by professionals who often prescribe aquatic exercises in their
daily work routines.
The trials were considered valid when the subjects touched one of
their feet to the force plate, presenting a flight phase (with no double
support phase), without looking downwards and without reducing the
rhythm of movement (Fig. 2). Running speed was measured with a system composed of an
electronic synchronized stopwatch and 2 photocell timing lights,
positioned 2.5 m apart. The total number of analysed force curves was 528 (6 Fy and 6 Fx
curves per subject at each level of immersion). The curves were ex-
ported and analysed through a processing routine created with Scilab
4.1.2 software (Institut Nationale de Recherche en Informatique et en
Automatique INRIA, Ecole Nationale des Ponts et Chauss ENPC,
France), which consisted of the following phases: (i ) offset correction;
( ii) application of the calibration coefficient and filters (Butterworth
with a low-pass cut-off frequency of 20 Hz, order 3, determined from
analysis of the spectral density of the signal strength); (iii) normali-
zation by the body weight measured outside the water (in order to
observe the percentage of load reduction in comparison with the values
obtained outside the water); (iv) verification of the vertical force peak
(Fy peak) and of the anteroposterior force peak (Fx peak). For this
study, the peaks were defined as the maximum value presented by the
components, normalized by body weight, occurring at any period of
time from the beginning until the end of the GRF curve; (v ) verifica-
tion of contact time during the support phase (time elapsed between
the first and the last contact of the foot with the force platform); (vi)
calculation of loading rate (LR). The LR was calculated from the
linear slope, from initial contact to the onset of maximum force, for
the vertical component of the GRF; (vii) time normalization by the
percentage of support; and (viii) calculation of the Fy and Fx mean
curves, in units of body weight (BW). Data were analysed through the statistical package SPSS version
13.0. Descriptive statistics (mean and SD) and inferential statistics
(analysis of covariance (ANCOVA), with speed as a covariate) were
used, once the normality and homogeneity of data had been confirmed
through the Shapiro Wilk and Levene tests, respectively. The level of
significance was set at p 0.05.
Fig. 3 shows the average curves for the vertical (Fy) and the
anteroposterior (Fx) components of the ground reaction force
during shallow water running at the chest and hip levels of
Fig. 1. Immersion levels used in the study: chest level (left) and hip
level (right). Fig. 2. Example of a valid trial: (1) approach, (2) foot contact, (3)
propulsion, and (4) flight phase.
J Rehabil Med 42

666A. Haupenthal et al.
The curve morphology for each GRF component was similar
when comparing the levels of immersion. However, when com-
paring the patterns obtained underwater with the characteristics
of a force curve produced during over-ground running there are
some differences. Regarding the vertical component, the im-
pact peak that usually occurs on land (2427) was not observed
in the water. Considering the anteroposterior component, on
land the Fx curve presents 2 peaks that are clearly identified,
1 negative and 1 positive (2830); in water the negative peak,
for most of the subjects, did not occur and only the positive
peak was observed at the end of the propulsion phase. Table I shows the means and SD of the variables.
When the subjects were immersed to chest level the mag-
nitude of Fy peak varied from 0.39 to 1.24 BW (mean 0.80,
SD 0.24 BW). At the hip level, Fy peak varied from 0.70 to
1.50 BW (mean 0.98, SD 0.18 BW). The Fx peak ranged from
0.12 to 0.36 BW (mean 0.26, SD 0.07 BW) and from 0.19
to 0.47 BW (mean 0.31, SD 0.07 BW) at the chest and hip
levels, respectively. No difference was found between the contact times and load-
ing rates when comparing the levels of immersion (p = 0.083
and 0.616, respectively). However, the subjects ran faster
( p 0.001) when immersed to the hip (mean 0.88, SD 0.10
ms) than to the chest (mean 0.67, SD 0.10 ms). The results of the ANCOVA showed that the difference
between the levels of immersion was not significant both for
Fy peak (F = 2.261, df = 1, p = 0.140) and Fx peak (F = 3.202,
df = 1, p = 0.081). The effect of running speed on force values
was significant only for Fx peak (F = 27.730, df = 1, p 0.001)
and not for Fy peak (F = 0.036, df = 1, p = 0.081).
The results of this study show that there is no significant differ -
ence for the vertical and anteroposterior force peaks between
the hip and chest immersions, probably due to variability in
speed, which was self-selected. When running over-ground, the vertical impact peak occurs
within 50 milliseconds after foot strike, which corresponds to
one spike in the GRF curve. This impact phase has been stud-
ied extensively over the years (2427), mainly because of the
great number of runners all over the world, the development
of shoes, and the risk of injuries. As this impact peak does not
exist during shallow water running, this exercise could be used
as an alternative way to maintain or even improve the physical
conditioning of athletes and patients under treatment with a
lower amount of load during the foot contact. The anteroposterior component of the GRF is mainly related
to the accelerations required to move the body forward and
it is necessary to change the movement speed (2830). The
Fx curves in water running did not present the first negative
peak, which can be related to the lower speed in relation to
over-ground running and by the water resistance acting on the
individuals. Similar results were found by Roesler et al. (15)
when analysing the underwater walking in fast speed situa-
tions. The authors suggested it might be because the subjects
modify the walking support phase in the water in an effort to
gain speed, leaning the body forward and touching the force
plate only when the leg has already passed the longitudinal
body axis. When running in water, subjects must also alter the
mechanics of running (18), modifying their support technique
(the heel does not touch the force plate) and performing only
the propulsion (positive) phase, with the maximum force oc-
curring by the end of the contact (7080% of foot contact) at
both levels of immersion. According to the literature the values of the vertical com-
ponent of the GRF during over-ground running vary from
1.6 to 4.0 BW and for over-ground walking the value is ap-
proximately 1.2 BW (2830). The only information about the
values for water running was given by Andrews et al. (17),
who suggested that the vertical force acting on an individual
who runs immersed to the shoulder corresponds to 0.1 BW.
However, this suggestion was based on the study of Harrison &
Bulstrode (31), who measured the Fy values with the subjects
immersed to several levels in the upright position. Roesler et
al. (15) had already found values higher than 0.1 BW for slow
speed walking in water with subjects immersed to the shoulder.
Thus it was expected that the values for water running would
be higher than those suggested by Andrews et al. (17) and
lower than those presented for over-ground running. The mean
Table I. Vertical force peak, anteroposterior force peak, loading rate,
contact time and running speed in each level of immersion (mean
level Fy
(BWs) CT
Chest 0.80 (0.24) 0.26 (0.07)3.38 (1.40) 0.50 (0.15)0.67 (0.07)
Hip 0.98 0.18) 0.31 0.07)4.25 (1.63) 0.46 (0.11)0.88 (0.10)
GRF: ground reaction force; Fy: vertical component of the GRF; Fx:
anteroposterior component of the GRF; BW: units of body weight;
LR: loading rate; CT: contact time; RS: running speed; SD: standard
Fig. 3.
Vertical (Fy) and anteroposterior (Fx) forces applied by the subjects
during shallow water running (Fy at hip level black solid line; Fy at chest
level grey solid line; Fx at hip level black dotted line; Fx at chest level
grey dotted line). Note: since the average curves were calculated based
on individual curves normalized by the contact time, their interpretation
should be limited to the morphology analysis. For reference values for
Fy and Fx peaks, see Table I.
J Rehabil Med 42

Loading forces in water running
Fy peak values found confirmed these expectations and were
even lower than the values of over-ground walking. The results
show that the idea of considering the values of load reduction
based on the subjects static weight when prescribing aquatic
exercises, such as walking and running, as done by Andrews
et al. (17), could be erroneous.
Another interesting variable is the rate of application of
vertical force, which represents the intensity of the load in
relation to time. The greater the loading rate, the greater the
intensity of force acting on the body structures. From Fig. 3
it can be seen that there is a difference in the slope of the Fy
curves, which is a representative characteristic of the loading
rate. In this case, the slope of the curve corresponding to the
level of the hip is steeper, resulting in a shorter time for the
application of the force and, consequently, to higher loading
rates. The differences between the levels were not significant,
although the values obtained for running in the water are much
lower than those found for walking out of the water (from 9 to
11.5 BWs) and correspond to less than one-third of the values
for running on dry land (from 15 to 30 BWs) (25, 32, 33).
In this way, besides the absence of impact peaks, cushioning
of the load in water running occurs more smoothly compared
with walking or running on dry land, representing a further
indicator for recommending this type of activity in cases in
which the workload needs to be reduced. Considering the anteroposterior component of the GRF, the
values range from 0.15 to 0.20 BW and from 0.4 to 0.5 BW for
over-ground walking and running, respectively (2830). No Fx
values for water running have been reported in the literature.
In this study the mean Fx peak in both levels of immersion
was higher than the values for over-ground walking and lower
than those for over-ground running. In relation to speed, the analyses carried out in this study
demonstrate that this variable significantly influences the val-
ues of Fx. On including speed in the statistical analysis, the
differences between the levels of immersion are not significant.
In this way, we should be alert to the risk of mistakenly believ-
ing that by simply increasing the level of immersion we would
be guaranteeing a significant reduction in the vertical loads
acting on the musculoskeletal system of the subjects. In the rehabilitation process, activities in the water can be
started earlier than on land; however, when choosing the exer -
cise, the immersion level, and the movement speedrhythm the
professional should consider the subjects capacity to generate
enough muscular force to perform the exercise against the
resistance of the water. In walking or running the musculature can be recruited in
different ways depending on the manner of execution, the
speed of the movement and the environment in which the
exercise is performed. On changing the level of immersion for
exercise carried out in water, the vertical force opposing grav-
ity is altered, in particular (13, 15). In this way, we prioritize
the work of the musculature that responds to gravity during
exercise. For example, when we reduce the level of immer -
sion we mainly increase the electromyographic response of
the soleus in relation to the gastrocnemius (13, 34). However,
when the running speed is altered, it is mainly the anteropos- terior component of the force that is modified. This increase
in the intensity of Fx may also be related to the increase in the
electromyographic response of the musculature responsible for
the task, as demonstrated by Miyoshi et al. (13), who analysed
walking in water and observed an increase in the activation of
the dynamic musculature (for example, the gastrocnemius in
relation to the soleus) with increased speed.
Furthermore, the increase in speed leads to higher moments
of force on the hip joint (12). According to Kaneda et al. (34),
on increasing running speed in the water there is a marked ac-
tivation of the musculature around the hip, at higher intensities
than those observed with walking on dry land. As a result, de-
spite the magnitude of the vertical component being lower for
water running, the magnitude of the anteroposterior component
is the same. In this situation, a patient with a hip prosthesis, for
example, could be put at risk if this exercise were prescribed
too early, based only on the obvious information concerning
reduced vertical loads in water. In the case of patients with
muscle injuries, for example, a cautious approach should be
taken due to the need to overcome the resistance to running in
water, which is greater than that encountered during locomotor
activities on dry land. Therefore, it is important to consider
not just the behaviour of vertical loads for the appropriate
prescription of exercise in water, since anteroposterior loads
also have a considerable magnitude and can demand raised
levels of muscle activity. On the other hand, there are some situations of less risk for
which water running can be recommended with greater safety
and without further concerns. For example, as this exercise
is performed with the head out of the water, it is a feasible
alternative for physical training for low-risk cardiac patients
(35, 36). Furthermore, elderly, obese individuals and rheumatic
patients respond positively to exercise in water (1, 3739). For
these individuals, exercises on dry land generally cause joint
pain as a result of the load. In these cases the water provides a
safe (less chance of falls or traumatic events) and comfortable
medium in which to exercise, without further risks in relation
to joint loading. Exercises in water are also indicated in processes of rehabili -
tation from fractures. For example, during rehabilitation after
a stress fracture, which is a very common injury in runners,
running in water guarantees the individual maintenance or even
improvement in physical conditioning, besides maintaining
the specificity of the activity. In the process of rehabilitation
after fractures, it is common to require control of loading on
commencing activities with the patient. Traditional methods
for the control of loading, such as crutches and parallel bars, or
even control based simply on the perception of the individual,
do not always guarantee unloading of the joint, and may lead
to injury. Some authors have demonstrated that, even with
training, patients are not capable of detecting and controlling
the load imposed on the affected limb during more intense
exercise (40, 41), and for this reason physiotherapists have
opted for exercises in the water (2, 3, 8), where the support of
the body weight provided by the buoyancy facilitates control
of the load. At the same time, the size of the load acting on
the individual exercising in water is often determined from
J Rehabil Med 42

668A. Haupenthal et al.
practical experience acquired during the working routine, since
published data on this theme are scarce.
In conclusion, the results of this study show that there are
differences in the pattern (morphology) of vertical and an-
teroposterior force curves during shallow water running com-
pared with the curves for over-ground running. Regarding the
comparison between the levels of immersion, though it seems
quite logical that the deeper water would result in lower ground
reactions forces, the difference between the force peaks was
not significant, probably because running speed was not the
same at both levels of immersion. Consequently, not only the
level of immersion but also the speed of displacement should
be considered when running in water. Furthermore, despite
the values of Fy being lower than those found in literature,
both for running and walking on dry land, attention should be
given to the values of Fx, since they are similar to the values
for over-ground running and higher than those reported for
over-ground walking. An analysis of the components of the GRF at different
speeds of movement, combined with its analysis at different
levels of immersion, would provide a better understanding of
the loads applied during shallow water running, supporting
the use of this exercise in different populations. Knowing the
values of force acting on an individual during shallow water
running could help professionals to conduct safer protocols
of aquatic rehabilitation and physical conditioning, consid-
ering not only the physiological alterations that occur in the
aquatic environment, but also the load patterns in different
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