Analysis Of Elite Golfers’ Kinematic Sequence In Full And Partial Shots (P2)
Eleven male tournament professionals [height 1.82 ± 0.04 (mean ± 1SD) m; body mass 83 ± 6 kg; age 28 ± 5 years], 23 male amateurs [height 1.81 ± 0.05 m; body mass 74 ± 10 kg; age 17 ± 1 years; handicap 0 ± 2 strokes], and 13 female amateurs [height 1.68 ± 0.07 m; body mass 59 ± 9 kg; age 16 ± 1 years; handicap -2 ± 2 strokes] volunteered to take part in the study. Written informed consent was obtained from parents or guardians of the participants in case of underage, otherwise from the participants themselves.
Measurement device and experimental set-up
Three-dimensional data were collected using a Polhemus Liberty electromagnetic tracking system (Polhemus Inc., Colchester, VT, USA), sampling at 240 Hz. A transmitter, which contains three orthogonal coils (solenoids), generates three different electromagnetic fields in the region of 1–4000×10-9 T. Sensors, which also contain three orthogonal coils, record magnetic flux in the three different fields. Magnetic flux generates proportional currents used to calculate a vector signifying the direction and strength of the magnetic field at the site of the sensor. Dedicated software computes the position and orientation of tracking sensors. According to the manufacturer, the static accuracy is 0.076 mm RMS for sensor position and 0.15° RMS for sensor orientation. In a pilot study, the dynamic accuracy was compared to an optical (infrared) 3D motion analysis system (8-camera ProReflex MCU1000 System, Qualisys AB, GBG, Sweden). In full-swing shots using a five iron, the Bland-Altman mean difference for hand angular speed was -24.0 degrees per second (limits of agreement: -174.7, 126.7). The magnitude of the difference between the systems was variable with changing angular speed, and exceeded the negative limit of agreement for 8 out of 321 positions sampled. At minimum and maximum mean angular speeds the magnitude of the difference between the systems was -4.0 and -17.9 degrees per second respectively.
The orientation of the right-handed orthogonal global frame (G) was such that the positive Xaxis pointed parallel to the direction of the target line, the positive Z-axis pointed vertically upwards, and the positive Y-axis pointed forward from the right-handed golfer. The performance area was a synthetic golf mat positioned right next to the transmitter (see Fig. 1). The experimental setup yielded a measurement space where the distance from transmitter to sensors ranged from 0.164 m to 1.41 m during recordings.
Participants performed a warm-up session of their choice, which also served as a habituation period. Generally, the warm-up lasted approximately 10 min and involved hitting golf balls. Next to the habituation period, three sensors were attached to each participant at the following locations: (1) the lumbo-sacral joint (pelvis); (2) between the shoulders at the level of the third thoracic vertebra (upper torso); and (3) the dorsal part of the leading hand (hand).The pelvic and upper torso sensors were mounted on a harness and the hand sensor was secured with a golf glove.
Following the collection of a static trial, in which the participants were required to stand in the anatomical position (parallel to the target line), spatiotemporal data were collected of the participants’ swings as they performed three trials under each of five different test conditions. These test conditions consisted of hitting a wedge to targets at three discrete distances (40, 55 and 70 m) in addition to full-swing shots with a five iron and a driver in the same direction for maximal distance. The order of test conditions for the wedge was assigned such that participants had to hit the ball progressively further for each shot. This procedure was repeated three times, thus they did not aim for the same target twice in a row except when the ball finished closer to another target (target zone radius = 7.5 m). Such a trial was discarded and followed by a new trial to the same target. Finally, three consecutive full-swing shots were performed using a five iron and driver respectively in the same direction as for the wedge trials.
Participants were allowed to use their own clubs during data collection. Type of wedge was chosen individually and governed by the criteria that it should be the most lofted club the participant repeatedly hit further than 70 meters with a full-swing shot. Data collection took place at four different practise facilities, where submaximal targets consisted of plastic cones or target flags placed at the set distances from the performance area.
The raw data were smoothed using a second-order, bidirectional, low-pass Butterworth filter with a cutoff frequency set at 14 Hz, determined through residual analysis.22 To analyze the motions of the pelvis, upper torso and hand respectively, a local frame was attached to each segment. The directions of the three axes of each frame were assigned so as to approximate the different anatomical axes of rotation for each segment. This was accomplished by determining rotation matrices for the static alignment trial, in which the segment axes were aligned with the global frame. These rotation matrices were then applied for each sampling interval in the movement trials.
In order to examine the sequencing pattern and the angular speed of segment motions in this study, the magnitude of the angular velocity vector of each segment was computed (hereafter defined as angular speed). The angular velocity vector was determined by calculating the finite difference (i.e. central difference method) of the rotation matrix.23 Segment angular speeds for each participant and shot condition were represented by the mean of three successfully performed trials. All kinematic and temporal parameters were calculated using Visual3D v.3.90 Beta and v.3.99 (C-Motion, Inc., Rockville, MD, USA).
Phase and event definitions
The swing was divided into two phases defined by three events (see Fig. 2). The address position was defined as the instant just before the initial movement of the club away from the target. The top of the backswing was determined once the first segment reached minimum angular speed between address and impact. The backswing was defined as the period between the start of the backswing and top of the backswing, the downswing as the period between the top of the backswing and the instant that the left hand returned to its address position (impact). Representative angular velocity curves of the pelvis, upper torso, and hand during the entire period of the golf swing at the shortest and longest shot distance are shown in Figure 3.
Fig.3 The normalized time-history curves for pelvis, upper torso, and hand segment angular velocities (°·s-1) (ωα denotes local frame angular velocity for the bilateral axis, ωβ for the antereo-posterior axis, and ωγ for the axial axis) in one representative participant when hitting shots to 40 m (filled circles) and with driver (open circles) (a-c). The circles in the corresponding normalized time-history curves for angular speed (d-f) indicates minimum angular speed (dotted circles) and maximum angular speed (dashed circles).
Statistics were calculated using SPSS 15.0 (SPSS, Inc., Chicago, IL., USA). Shapiro Wilk´s W test was applied to test normality in the distribution of data. The individual measures in this study were analyzed in separate repeated-measure ANOVAs with group as betweensubjects factor, and test condition and body segment as within-subjects factors. Pre-planned pairwise comparisons were Bonferroni corrected. If data did not conform to the assumption of sphericity P-values were Greenhouse-Geisser adjusted. Significance level was set at P < 0.05.