1. Introduction
When considering the physics of sporting implements, a key area of consideration is how the physical
properties of an implement will affect the performance. Therefore, understanding this relationship is of
great importance in order to provide athletes with useful performance information. There are a limited
number of physical properties which could be altered to improve performance and within a single sport;
restrictions on an implement's mass, length and shape can be set by the sport's governing body. For
example, in tennis, the International Tennis Federation state that a racket can have maximal dimensions of 73.7cm in length, 31.7cm in head width and can have a maximal hitting area of 39.4cm long and 29.2 cm
wide [1]. One physical property which is of particular importance is swing weight. Swing weight is a
common term referring to the implement's resistance to rotational acceleration and is also known as the
moment of inertia. Performance is often measured as the outbound velocity of the impacted object [2].
However, the maximum speed at which the implement can be swung directly influences the outbound
object velocity and can therefore also be treated as a key performance parameter.
The concept of swing weight influencing the performance of a sporting implement was first discussed
by Daish in 1972 [3] where an experiment was carried out measuring the club head speed for golf drivers
with varying club head mass. Daish found there was a linear logarithmic relationship between swing
speed and club head mass, however only four players were tested. In 2003 Smith et al. [4] found a similar
relationship in softball with an experiment that varied swing weight and mass separately in a series of bats
and measured the swing speed. It was found that the mass of a softball bat is almost independent of the
swing speed whereas the moment of inertia has a linear relationship with the normalised swing speed for
each player in the experiment. A further study was completed by Cross and Bower in 2006 [5] who
measured the maximum swing speed achieved with restricted motion when swinging rods with different
properties. Cross and Bower converted Daish's club head mass data to show the relationship between
moment of inertia and swing speed for the golf clubs. It was found that Daish's data and their own new
data could be described by equation 1,
ܸ ൌ
ூ (1)
where C is a player constant and n is the gradient of the swing speed - swing weight log plot. This
relationship is the same as in the work by Smith et al. and the three studies found the value of n to be 0.26
±1. A similar study by Crisco et al [6], analysed by Nathan [7], compared angular velocity with moment
of inertia in baseball and found n=0.28 ± 0.04, which is very comparable to Daish, Smith et al and Cross
and Bower's work, which suggests a relationship may exist between different sports as well as within a
single activity.
The research in the literature is very much focussed on the relationships present in a single sport and
very little work has been done describing the relationship between swing weight and swing speed across
multiple sports. The aim of this work is to review the mechanics of multiple swinging sports and to define
the relationship that exists between the moment of inertia and the swing speed across this range of sports.