Biomechanics and Medicine in Swimming XI

Aquatic Space Activities – Practice Needs Theory
ungerechts, B., Klauck, J².
University of Bielefeld, Germany
2 German Sport University Cologne, Germany
Aquatic Space Activities (ASA) is a collective term for activities in
water such as: Competitive – Recreational – Masters swimming, Finswimming,
Triathlon, Scuba-diving or Water-exercises. ASA must obey
flow physics. Traditionally, flow physics in aquatic sports is confined to
steady conditions assuming that the flow velocity is constant and the
body is rigid. It is not new information that this is not and was never the
case in human swimming. The fact is that the limbs change motion in
all ASA which causes unsteady flow conditions. Unsteady flow demands
closer consideration of water-motion induced by motion of the limbs’.
The purpose of this paper is to introduce some challenging information
concerning unsteady flow conditions and consequences for practise.
Key words: unsteady flow, momentum-induced propulsion, jet-flow
IntroductIon
Aquatic Space Activities (ASA) focuses attention on the interaction
between the actively moving body (or its parts) and water-mass set in
motion. ASA also deals with the mutual effects of buoyancy and the
reaction to induced water-motion. Buoyancy is a natural reaction due to
displacement of water-mass of any body, submerged in water. Induced
water-motion or water flow is a physical reaction due to displacement
of water-mass of any submerged body moving relative to the surrounding
water.
Reactions to induced water-motion have some contra-intuitive features.
A simple drag approach suggested by most books on swimming
does not explain swimming speed sufficiently. The hands’ actions are not
most effective if water resistance is at a maximum (the term effective
focuses on the result of an action). Intuition may tell us that e.g. a hand
is moving the same block of water either back or moving new blocks
by changing hand motion. This is not the complete story. What really
happens is (completely) different. According to the law of conservation
of mass, in a flow, water cannot be pushed away in relation to the surrounding
water-mass. Nevertheless it is true that, where there is one
solid body (like a swimmer’s hand) there cannot be another body. However,
where does mass of displaced water go? The aim of this paper is to
introduce some facts of flow physics, beyond our understanding of the
existing biomechanical aspects, and relevant for a better understanding
of aspects of aquatic space activities and the reaction of water-set-inmotion.
Methods
Swimming text books mostly provide a chapter related to biomechanical
background of activities in aquatic space. It is curious that over the
decades, the content of this chapter, including pictures has not really
changed from the very first publication of the author and coach “Doc”
Counsilman. Obviously these text books were so convincing worldwide
that later authors had no reason to change this. In nearly the same
decades numerous studies have been presented in the congress series of
biomechanics and medicine in swimming, it seems as if their outcome
is of minor relevance.
During the same time period in parallel, much research was done in
the field of swimming animals. Researchers in this field, starting with
similar assumptions to their colleagues in human swimming, soon became
aware that the steady flow approach was not helpful in answering
their questions. The alternative approach, mostly based on concepts published
by Lighthill, opened a quite new route to structuring the research
and to study unknown hydrodynamic features related to self-induced
chaPter2.Biomechanics
propulsion. Some of these results have held and can act as a guideline in
swimming literature, because the myths and the impact on swimming
technique are considerable. Coaches and researchers may mislead their
swimmers when still following the ancient concepts.
results
Let’s consider the properties of water mass. It has long been known that
aquatic space is characterized, like any body, by mass and other features.
Firstly, water has the property to change shape. Any body, e.g. the hands
or feet, even when moving relative to water, displaces some water-mass.
This means the motion of the water (in the vicinity to the moving body)
is changed and flow is created: if water is at rest (before it is displaced)
this is also true: what counts is the relative change of motion. Since the
relative motion between the body and water is essential, research on
aquatic activities executed in a flume also applies to activities in a pool.
Pressure is an important feature of water at rest and in motion: the
weight of the water mass displaced by a body submerged in water exerts
a force due to water pressure in an upward direction, called buoyancy.
A body, submerged and moving relative to water produces a change
of pressure and due to that change “some” particles are set into motion.
Based on the continuity-pressure-concept, the pressure causes and
maintains a flow, a continuous movement of the fluid from one place to
another. According to Euler (1707 - 1783) particles-in-motion (i) possess
mass, weight, acceleration, velocity, etc. and (ii) occupy some space
in the vicinity of the moving body (whereas the rest of the water-mass
remains undisturbed).
This space of particles-in-motion is full of vectors and scalars, forming
vector fields.
Fig 1 Flowing water: sketch by Leonardo Da Vinci, and today:
cm.math.uniuc.edu/242syl.php
Vector fields allow for transitional and rotational motion of the particles
(the particle itself does not rotate, but follows a circular path), indicating
that vorteses form and the effects induced by vortex forms are likely
to occur. By independent variations in time and space, the flow is to a
certain extent “unpredictable” (also when the displacing action causes
“un-steady” flow conditions). From practical experience it is known that
swimmers claim they sometimes feel a “different slip” although using the
same biomechanical actions.
Internal friction exists between particles, measured as viscosity,
which is the cause of pressure change. The change of pressure is mediating
between any displacing action of any body and particles-in-motion;
hence, the interaction of the limbs and water-mass means pressurechange.
In addition, viscosity causes repulsive actions. The reaction to
this repulsive action is mostly measured by the established u²-law of resistance
– provided that the flow remains unaffected by any body (called
a steady situation).
Momentum-induced propulsion in aquatic space ie the act of displacing
a whole body in aquatic space, is related to a change of motion
of water-mass. The change of motion of any mass is called momentum.
Galileo (1565-1642) already pointed out that momentum is the property
by which the motive agency moves and the body resists. Newton
(1642-1727) claimed that displacement means imparting momentum to
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