Relationships between whole-body kinematics and badminton jump smash racket head speed

Abstract

The purpose of this study was to identify kinematic determinants of shuttlecock speed in
the badminton jump smash. Three-dimensional kinematic (400 Hz) data were collected for 18 experienced male badminton players using an 18 camera Vicon Motion Analysis
System. Each participant performed 12 jump smashes. The trial with the fastest shuttlecock speed per participant was analysed using an 18-segment rigid body model. Parameters were calculated describing elements of the badminton jump smash technique. Four kinematic variables were significantly correlated with racket head speed. Greater peak wrist joint centre linear velocity, jump height, shorter acceleration phase, and greater shoulder internal rotation at shuttlecock / racket impact.

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Miller et al.: RELATIONSHIPS BETWEEN WHOLE-BODY KINEMATICS AND BADMINTON JUMP SM
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RELATIONSHIPS BETWEEN WHOLE-BODY KINEMATICS AND BADMINTON
JUMP SMASH RACKET HEAD SPEED
Romanda Miller1,2, Harley Towler1, Stuart McErlain-Naylor1,3, and Mark King, 1
School of Sport, Exercise and Health Sciences, Loughborough University, UK.1
Centre for Student Success, University of East London, UK2
School of Health and Sports Sciences, University of Suffolk, UK3
The purpose of this study was to identify kinematic determinants of shuttlecock speed in
the badminton jump smash. Three-dimensional kinematic (400 Hz) data were collected for
18 experienced male badminton players using an 18 camera Vicon Motion Analysis
System. Each participant performed 12 jump smashes. The trial with the fastest shuttlecock
speed per participant was analysed using an 18-segment rigid body model. Parameters
were calculated describing elements of the badminton jump smash technique. Four
kinematic variables were significantly correlated with racket head speed. Greater peak
wrist joint centre linear velocity, jump height, shorter acceleration phase, and greater
shoulder internal rotation at shuttlecock / racket impact.
KEYWORDS: velocity; technique; overhead; racket; swing; stroke
INTRODUCTION: The forehand smash is an effective attacking shot in badminton, accounting
for 54% of ‘unconditional winner’ and ‘forced failure’ shots in international matches (Tong &
Hong, 2000). A shuttlecock with a greater post-impact speed will give an opponent less time
to return the shuttlecock. Few studies have investigated the kinematic determinants of
shuttlecock speed in the badminton smash by elite players. Ramasamy et al. (2019) revealed
three kinematic variables significantly associated with shuttlecock speed: maximal wrist
angular velocity during the acceleration phase (i.e. start of the forward swing to contact);
maximal racket head speed between backswing and contact; and jump height, defined as
minimum to maximum centre of mass height. Ultimately, racket head speed is the most
important variable for producing shuttlecock speed as shuttlecock speed is determined by the
transfer of momentum during the collision between the racket stringbed and shuttlecock.
Previous research has reported that linear velocities of the distal segments best explain
variation in shuttlecock speed/racket head speed, however, it is unclear how the distal segment
velocities and subsequent racket head and shuttlecock speeds are generated. Rambely et al.
(2005) found that the wrist was the predominant contributor (26.5%), whilst the elbow and
shoulder joints contributed 9.4% and 7.4%, respectively, towards racket tip velocity.
Additionally, use of low frame rates and unclear methodologies for defining both shuttlecock
speed and racket head speed mean that it is difficult to compare results. This study aimed to
identify full-body kinematic parameters that best explain the generation of post-impact racket
head velocities in the badminton jump smash.
METHODS:
Participants
Eighteen male badminton players (24.3 ± 7.1 years; 1.84 ± 0.08 m; 79.6 ± 8.8 kg) of regional
(n = 9), national (n = 4), and international (n = 5) standard participated in this study. Each
performed twelve forehand jump smashes from a racket-fed lift via an international coach /
player, representative of match conditions. Testing procedures were explained to each
participant, and informed written consent was obtained in accordance with the institutional
ethics committee.
Data Collection
Forty-seven 14 mm retro-reflective markers were attached to the participant (Figure 1). An 18
camera Vicon Motion Analysis System (400 Hz; OMG Plc, Oxford, UK) collected 3D kinematic
data of the participant, racket and shuttlecock on a badminton court. Joint centres were
calculated from a pair of markers placed across the joint so that their midpoint coincided with
the joint centre (McErlain-Naylor et al., 2014). Hip, thorax, neck and head joint centres were
calculated according to Worthington et al. (2013). A further marker was placed on the bottom
of the racket handle, seven pieces of 3M Scotchlite reflective tape were attached to the racket
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frame, and a single piece of reflective tape was attached around the base of the cork of the
shuttlecock (Figure 1). Participants used their own racket and new Yonex AS40 shuttlecocks
throughout.
Figure 1. Marker locations for the participants; racket and shuttlecock marker locations.
Data Reduction
Position data were labelled within Vicon Nexus 1.7.1 and imported into Matlab v.2018b for all
further processing. Position data of all body markers were filtered using a fourth-order, zero-
phase, low-pass Butterworth filter with a cut-off frequency of 30 Hz, determined through
residual analysis. Six angles were calculated for each trial: shoulder internal / external rotation;
elbow flexion / extension; elbow pronation / supination; wrist palmar flexion / extension; wrist
ulnar / radial deviation; and X-factor. Joint angles were calculated using three-dimensional
rotation matrices, defining the rotation applied to the proximal segment coordinate system in
order to bring it into coincidence with the coordinate system of the distal segment. An XYZ
rotation sequence was used, representing flexion-extension, adduction-abduction and
longitudinal axis rotation, respectively. When describing humerothoracic motion, a YZY
rotation sequence was used as recommended by ISB (Wu et al. 2005). Wrist angles were
normalised based on the player adopting their normal grip within a static trial, which was
considered the neutral position. X-factor referred to the separation angle between vectors
connecting the right and left shoulder joint centres and, the right and left hip joint centres,
respectively, in the global transverse plane. Instantaneous post-impact shuttlecock speed and
racket- shuttlecock contact timing were determined using a logarithmic curve-fitting
methodology (Peploe et al., 2018) with minor adjustments for application to the badminton
smash. Pre- impact racket head centre velocity was interpolated to the calculated time of initial
shuttlecock contact. Due to the focus on investigating maximal performance each player’s trial
with the greatest shuttlecock speed was used for further analysis.
The movement was defined around the backswing and acceleration phases, divided by five
discrete instants: preparation was the point at which the centre of mass height was minimal;
end of retraction was the point at which the racket was most medio-laterally positioned towards
the non-dominant side of the participant within the global coordinate system; racket lowest
point was when the racket tip was at its lowest vertical point (Martin et al. 2012); turning point
was the point, after minimum (most negative), at which the racket head velocity normal to
stringbed became positive; and shuttlecock contact was the closest motion capture frame to
the previously defined instant of racket-shuttlecock contact. The backswing phase was the
time between preparation and turning point, whilst the acceleration phase was the time
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Miller et al.: RELATIONSHIPS BETWEEN WHOLE-BODY KINEMATICS AND BADMINTON JUMP SM
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between turning point and shuttlecock contact. Joint angles were defined at each key instant
and their maximum range of motion through to contact calculated. Furthermore, post-impact
shuttlecock speed, racket head speed at impact, jump height and peak shoulder, elbow, wrist
joint centre linear velocities were calculated for each trial. Duration of phases, as well as total
swing time were also calculated for each trial.
Statistical Analysis
All statistical analyses were performed in Matlab v.2018b. Pearson product moment correlation
analyses, including 90% confidence intervals (CI) were performed between each kinematic
(independent) variable and racket head speed. An alpha of 0.1 determined significance.
RESULTS: Maximal shuttlecock speeds for the cohort were 89.6 ± 5.3 m·s-1 (range: 80.1 - 99.8
m·s-1). Racket head speeds were 56.3 ± 4.0 m·s-1 (range: 46.7 - 64.6 m·s-1). Four kinematic
variables were significantly correlated with racket head speed. Greater peak wrist joint centre
linear velocity (r = 0.712; CI: 0.507, 0.917; p < 0.001), jump height (r = 0.494; CI: 0.035, 0.781;
p = 0.037), shorter acceleration phase (r = -0.600; CI:-0.833, -0.184; p = 0.009) and greater
shoulder internal rotation at shuttlecock contact (r = 0.563; CI: 0.131, 0.816; p = 0.015) were
associated with greater shuttlecock speeds. It should be noted that the elbow pronation angle
at preparation (r = 0.495; CI: 0.036, 0.781; p = 0.037) was found to be significantly correlated
with racket head speed, however upon inspection of the data it was evident that this correlation
was entirely due to one player. This player had a very large pronation angle at this key instant,
which was coincidentally the player with both the slowest shuttlecock and racket head speed.
This result was therefore discounted and ignored.
DISCUSSION: Racket head speeds achieved by the participants showed good agreement with
previously reported values by elite players (Ramasamy et al., 2019; Kwan. M., 2010; Kwan et
al., 2011). Racket head speed at impact, normal to the stringbed plane, correlated very highly
with instantaneous post-impact shuttlecock speed (r = 0.903; CI: 0.753, 0.964; p < 0.001)
justifying the use of racket head speed as a performance variable. Whilst jump height was
significantly correlated with racket head speed and Ramasamy et al. (2019) also found that our
thoughts are that jump height in the smash is a characteristic of more able players attempting
to produce steeper smash strokes, rather than being a causal variable of faster racket head
speeds and shuttle speeds.
Distal linear velocities (wrist) explained the variation in racket head speed, yet proximal angles
of the trunk and shoulder also explained the variation in racket head speed. This is suggestive
of the kinetic link principle whereby movements of a proximal-to-distal nature generate and
conserve angular momentum to produce high distal end-point velocities. Important longitudinal
axis rotations, typically difficult to measure and observe, may not always follow this strict
sequence with regards to timing, however the proximal-to-distal nature of overhead strokes
provides a good general understanding of how high distal end-point velocities can be generated
(Marshall, 2000; Fleisig et al., 2003). A more negative X-factor at end of retraction causing
greater racket head speed endorses the idea of the stretch–shortening cycle, whereby more
elastic energy is stored and recovered to enhance the concentric phase. The stretch–
shortening cycle has been linked to greater velocities in throwing actions due to enhancement
of the concentric phase (Elliott, 2006; Elliot et al., 1999). Finally, no ranges of motion were
found to significantly correlate with racket head speed although Lees et al. (2008) previously
suggested that increasing the range of motion can improve performance (racket head speed)
by increasing the acceleration path of the racket, allowing more muscular force to be generated
and applied to accelerate the racket.
The velocity principle, not explored in this study, may also be critical to understanding the
development of greater distal velocities in the badminton smash motion (Lees et al. 2008). This
principle relates to how the angle between segments influences the effect of proximal rotations.
For example, if the elbow angle is 90 the effect of shoulder internal rotation is maximised.
Similarly, if the racket is held at 90 to the forearm segment, the contribution from radio-ulnar
pronation is maximised. This is a difficult principle to observe, due to the complex nature of the
badminton smash motion, where multiple joint rotations are responsible for generating racket
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head speed (Liu et al. 2002). Tang et al. (1995) reported that amongst four elite players, the
forearm-racket angle was 147, which suggests a compromise between the height and speed
at contact between racket and shuttlecock.
Potential limitations existed during this study, including lack of experimental control over the
racket and therefore the impact mechanics between the racket and shuttlecock, as well as the
effect that the racket properties may cause to the technique and kinematics of the player
(Whiteside et al., 2014).
CONCLUSION: This study has shown that proximal kinematic joint angles caused greater
linear velocities at distal joint centres and greater racket head speeds. Greater peak wrist joint
centre linear velocity, jump height, shorter acceleration phase, and greater shoulder internal
rotation at shuttlecock contact were all found to have a significant correlation to racket head
speed. It is suggested that players and/or coaches should aim to decrease the length of the
acceleration phase of the racket though technique/strength to increase racket head speed.
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