When performing the skill of a tennis serve the movement is
first initiated with ground reaction force (GRF), the ball toss follows along
with the drawing back of the racket. The legs flex at both the knee and ankle
with a rapid drive from the lower limbs that follows through the hips. Leading
onto rotation at the trunk which is then followed by extension at the shoulder
and elbow, finishing with peak velocity at impact (ball and racket contact).
The elbow and wrist do not fully extend until the final moments of the skill in
order to maximise the effects of the kinematic chain i.e. the proximal to
distal summation of speed.
Perhaps the key consideration when performing a tennis serve
is to attain maximum speed of the tennis ball. In order to achieve this,
movements at the various joints and bodily segments mentioned above are used to
gain maximum impact velocity of the racket with the ball. The timing of peak
angular velocities must occur in order of proximal to distal (Elliot et al.
1995) and this is referred to as the kinematic chain. The proximal to distal
concept is crucial to the timing of peak joint flexion, velocities and
magnitudes in order to enhance technical performance. This aims to increase the
rotation of each segment, giving it a large angular velocity, which in turn
will make for an amplified tangential velocity. In the case of the tennis serve
you could argue that two kinetic chains are present: Ankle – knee – hip and
shoulder - elbow – wrist. The graph below displays the velocities observed
during a maximal hit or throw (Hay, 1995).
Notice that the peak velocities of each joint occur one
after the other, in relation to injury prevention, the timing of segmental
movement can have a profound effect on overuse injury. When a segment is out of
sync this can put a huge strain on the other links involved in the technique.
Not only this but, it also results in a reduced accumulation of motion and
therefore end point velocity. Furthering this, the observation of different
serving styles by Elliot et al. (2003) suggests that variation in techniques
load the shoulder differently and therefore have implications for injury.
With regard to the areas of interest, a kinematic analysis
can provide information relevant to coaches for technique improvement and
injury prevention. For example, Elliot et al. observed the contribution of each
bodily segment to a serve, with the forearm being of primary importance. This
would highlight to both coaches and athletes the aspects of the skill they
should prioritise. Further to this, observing the timings of each segmental
movement will indicate if the performer is maximizing their potential angular
velocity. Proximal to distal timing of movements will allow for maximum
velocity values to be attained due to enhanced accumulation of motion in the
kinematic chain. Kinematic analysis does however have its inherent limitations,
when using video observation for example, joint centres often rotate out of
plane. Angle measurements are therefore inaccurate as it is assumed that in a
two-dimensional image all the joints and segments line up, when in fact they do
not. In this instance CODA motion tracking analysis would be beneficial as
opposed to human, as it provides three-dimensional angles.
Overall kinematic analysis can provide meaningful evidence
to support technique selection to both improve peak velocities within the
tennis serve and reduce the likelihood of injury. All of which is fundamentally
dependent on the kinematic chain, a principle which is of chief importance when
considering movements that aim to achieve peak angular velocities such as the tennis
serve.
"The depressing thing about tennis is that no matter how good I get, I'll never be as good as a wall." - Mitch Hedberg
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