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Original Research

Are Anthropometric, Flexibility, Muscular Strength, and Endurance Variables Related To Clubhead Velocity in Low- And High-Handicap Golfers?

Keogh, Justin WL1; Marnewick, Michel C1; Maulder, Peter S1; Nortje, Jacques P1; Hume, Patria A1; Bradshaw, Elizabeth J2

Author Information
Journal of Strength and Conditioning Research: September 2009 - Volume 23 - Issue 6 - p 1841-1850
doi: 10.1519/JSC.0b013e3181b73cb3
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Abstract

Introduction

Golf has traditionally been viewed as a skill-based sport in which the continual refinement of ball striking and putting skills is emphasised over the development of kinanthropometric (anthropometric and physical fitness) qualities. This is somewhat in contrast to team sports such as Australian rules football (26) and field hockey (27), where regardless of skill level, kinanthropometric characteristics are able to distinguish between players of different ability.

The ability to generate high linear clubhead velocities (and hence achieve large horizontal ball displacements) has long been thought crucial for golf success (10). More recently, clubhead velocity has been shown to correlate with golf handicap (r = 0.95) (19), while driving distance is correlated to performance (prize money) on the American Professional Golfers Association (PGA) tour (r = 0.41-0.88) (20). Therefore, the linear velocity of the clubhead at ball contact (referred to hereafter as clubhead velocity) has been used as golf swing performance measure in many studies assessing factors that contribute to golf performance (13,18,28).

According to Newtonian mechanics, linear velocity is a product of angular velocity and the length of the lever, in this case the arm-club system (22). Thus, an increase in clubhead velocity could occur from an increase in either the angular velocity or arm-club length or through an increase in one factor that is greater than the reduction in the other. The length of the arm and club both have finite limits, with the effective length of the arm-club lever at ball contact controlled by the physical length of each segment and the relative angle between these segments (22). This means that adult golfers are not able to increase the length of the arm-club lever to any great extent and that increases in their clubhead velocity may be best developed by increasing angular velocity. Nevertheless, golfers with longer arms may be at a competitive advantage and thus arm length may be included in talent identification programs for golf.

The angular impulse-momentum relationship states that angular velocity is proportional to the summation of joint torques (resultant torque) produced by the golfer. Although the resultant golf swing torque is dependent on many factors including the magnitude and direction of the ground reaction force (2,41) and the use of the kinetic link principle (37,40), without reasonable levels of overall body strength, golfers would not be able to generate sufficiently high resultant muscular torques (2,37,40). This view appears consistent with the relatively high to very high levels of muscular (electromyographic [EMG]) activity observed in various muscles during the golf swing. These muscles include the hip and knee extensors (3), hip abductors and adductors (3); spinal extensors and abdominals (36,42); and shoulder internal rotators, such as subscapularis, latissimus dorsi and pectoralis major (23,35). For more detail on the EMG activity of the golf swing, the reader is referred to a comprehensive review of this literature by McHardy and Pollard (30). Further support for the importance of total body strength for golf can be found in the results of a number of training studies which reported significantly increased clubhead velocity as a result of resistance-training programs (13,18,28).

While resistance-training appears able to improve clubhead velocity by increasing the resultant muscular torque, the development of flexibility (particularly around the shoulder, trunk, and/or hip joints) has traditionally been emphasized in golf training. The rationale for this emphasis has been that an increase in the range of motion (ROM) in these areas would allow a longer backswing, which then gives the golfer more time to develop high levels of angular velocity (9,44). It is also possible that high levels of wrist abduction-adduction ROM could contribute to the angular velocity of the golf club. By cocking (abducting) the wrists during the early backswing and maintaining this position until late downswing, the moment of inertia can be minimized and the angular velocity maximized. This cocking of the wrists also allows the wrists to more rapidly uncock (adduct) through a greater range of motion during the late downswing, resulting in significantly greater clubhead velocity and ball displacement (38,40).

Therefore, the purpose of this study was to determine whether a number of selected kinanthropometric variables (many of relevance to the laws and preferences in the Wiren (44) golf teaching model) are correlated to clubhead velocity and if these variables can distinguish between low-handicap (LHG) and high-handicap golfers (HHG). It was hypothesized that LHG would have a superior kinanthropometric profile (e.g., be more muscular and stronger, have longer arms and greater levels of flexibility) than HHG, and that a number of these variables would be significantly correlated to clubhead velocity.

Methods

Experimental Approach to the Problem

The present study used a cross-sectional approach to examine the relationship between a number of kinanthropometric (anthropometry, flexibility, muscular strength, and endurance) measures to golf clubhead velocity in LHG and HHG golfers. Specifically, correlational and between-group analysis of variance analyses were conducted to examine this relationship.

Subjects

A total of 10 male LHG (age 22.9 ± 3.4 years; height 1.80 ± 0.07 m; mass 76.8 ± 8.8 kg; handicap: 0.3 ± 0.5) and 10 male HHG (age 27.8 ± 7.8 years; height 1.77 ± 0.07 m; mass 73.5 ± 11.8 kg; handicap 20.3 ± 2.4) gave informed consent to participate in this study. These groups were balanced so that no significant intergroup differences were observed for age, height, or body mass (p = 0.087-0.484).

All participants were right-handed golfers, had played golf for at least 2 years, and were members of golf clubs. Over recent weeks, each golfer was currently averaging at least 3 hours of golf practice and at least one full round (18 holes) of golf per week. Approximately half of the golfers in each group had been participating in a regular gym program for 1 year or more, with the emphasis of these programs generally being on the development of core stability, muscular endurance, flexibility, and general (overall body) strengthening. Due to their limited resistance training history, only a few of the golfers had regularly performed any power, (e.g. plyometric, weightlifting, or medicine ball throws) or golf-specific rotational exercises (e.g., golf swing-specific cable woodchop [GSCWC] or resisted golf swings) previously in training. For those golfers with a coach or conditioner, no more than two golfers in either group used the services of the same professional. For the purposes of this study, LHG were defined as either professionals (members of the New Zealand Professional Golfers Association) or amateurs who had a handicap of 1 or less. The HHG were all amateurs and had a handicap between 18 and 25. Ethical approval was obtained for all testing procedures from the Auckland University of Technology Ethics Committee.

Procedures

Testing was conducted in early to mid-spring so that all golfers had sufficient practice and/or tournament play in the month(s) prior to testing. All participants were required to attend two testing sessions. The first session involved assessment of the players' kinanthropometric profile, with the order of tests being anthropometry, flexibility, then muscular strength and endurance. All tests were conducted at a similar time of day to ensure consistency of measurement between the LHG and HHG groups. Measures of golf performance (clubhead velocity and accuracy) were obtained during the second testing session, which was conducted approximately 1 week after the kinanthropometric testing session.

Anthropometry

Anthropometric testing used International Society for the Advancement of Kinanthropometry (ISAK) protocols (33) and was performed by an ISAK level II anthropometrist. Physical dimensions of stretched stature (height), body mass, skinfolds (triceps, subscapular, supraspinale, and medial calf), lengths (acromiale-radiale/upper arm and radiale-stylion/forearm), breadths (bisacromial/shoulder) and girths (chest and relaxed upper arm) were measured. These anthropometric measures were selected as they allowed some estimate of the amount of total or regional muscle mass (of some of the primary golf swing muscles) or an indication of the length of the upper limb levers used in the golf swing.

Stretched stature was measured without shoes using a standard wall mounted stadiometer to the nearest 0.1 cm. Body mass was measured to the nearest 0.1 kg, using a calibrated SECA Atrax 770 electronic scale (Seca, Hamburg, Germany). Skinfolds were measured on the right side of the body using a Slim Guide caliper to the nearest 0.5 mm. Arm segment lengths and shoulder breadth were measured using a Siber-Hegner anthropometer to the nearest 1 mm. Girths of the chest and relaxed upper arm were measured using a Lufkin W606PM flexible steel tape to the nearest 1 mm. Each skinfold, length, breadth, and girth was measured three times with the median result used in data analyses. All anthropometric measures were highly reliable, with the intraclass correlation coefficients (ICC) being: skinfolds (ICC = 0.87-0.99), lengths (ICC = 0.89-0.94), bone breadth (ICC = 0.97) and girths (ICC = 0.86-0.90). Further, the technical error of measurement (TEM) for all anthropometric measures was within ISAK requirements of <5% for skinfolds and 1% for other measurements.

A number of derived anthropometric measures were also calculated. Body mass index (BMI) was equal to body mass (kg) divided by height (m) squared. Body density (BDM4) was calculated using the sum of four skinfolds (Σ4SF: triceps, subscapular, supraspinale and medial calf) in the equation: BDM4 = 1.09736 − 0.00068(Σ4SF) (45). The Siri (39) equation (BF% = (495 / BDM4) - 450) was used to convert BDM4 to body fat percentage (BF%) to allow comparisons with previous literature. Fat-free mass was estimated by subtracting the fat mass from total body mass. Acromiale-stylion (total arm) length was determined by summing the acromiale-radiale and radiale-stylion lengths for each participant.

Flexibility

Trunk rotation, wrist abduction, wrist adduction, and hip internal and external rotation flexibility was assessed using active ROM tests with methods sharing many similarities to Doan et al. (13). These active ROM tests were selected as they may give some estimate of each player's possible backswing length, or their ability to cock and uncock the wrists during the backswing and downswing. The ROM were recorded using either of two Sony Digital Video Cameras (50 Hz, 1/1000 s shutter speed); one camera was suspended 2 m above the ground for the trunk rotation, wrist abduction, and wrist adduction measurements; the other camera was positioned 2.5 m from the front of a plinth on which the golfer sat for the hip internal and external rotation tests. Golfers were given two practice trials prior to three test measurements for all ROM tests. Each ROM trial was separated by 20 seconds of rest with the best trial of each test recorded for subsequent analysis.

The active trunk rotation test was performed in a similar fashion to that of Doan et al. (13). The golfers were seated with their gluteal fold resting on the edge of a stadiometer box, their knees bent to an angle of 90° while holding a 1.5-m wooden pole (3-cm diameter) behind their head across the upper trapezius. A neutral lumbar and thoracic (lordosis/kyphosis) position was maintained at all times during the test with the golfers looking straight ahead at a mark on the wall in front of them positioned at head height. The golfers rotated to their backswing (right) side as far as possible, holding the end range position for 3 seconds, then moved through the neutral position to maximal rotation in their follow-through (left) side, holding this position for 3 seconds. Trunk rotation ROM for both the backswing and follow-through phases was defined as the change in the angle of the wooden pole as assessed using the overhead camera from the initial (neutral) to the final (rotated) position.

For the active wrist abduction and adduction tests, markers were placed on the metacarpal-phalangeal joint of the third digit of the back (right) hand, the center of the wrist joint between the styloid processes of the radius and ulna, and the medial aspect of the distal portion of the biceps brachii tendon. Golfers knelt on the ground with their right forearm placed on the stadiometer box with the hand in a supinated position, the elbow directly under the shoulder so to create a 90° elbow angle. From this position, the golfer abducted the wrist as far as possible and held that position for 3 seconds, before moving through the neutral position to the maximum wrist adduction position, with this position also held for 3 seconds.

For the hip internal and external rotation tests, markers were placed on the center of the tibial tuberosity of each leg and between the midpoint of the lateral and medial malleoli. Golfers were seated on a plinth (high enough so that the feet could not touch the ground) with their hips and knees bent to 90°. The centers of the knee joints were aligned with the edge of the plinth and the hips and knees were stabilized by tight belts to prevent unwanted movement during the test. A box was also placed behind the golfer so they could not extend or flex their back while performing the movement. The golfers internally rotated their bent leg as far as possible and held this position for 3 seconds, then moved back through the neutral position and externally rotated the same leg as far as possible, and held this position for 3 seconds before moving back to the neutral position. The same movements were completed on the opposite leg.

Joint markers from each of the flexibility tests were digitized using Silicon Coach Professional (Dunedin, New Zealand) to enable calculation of joint ROM (12). Reliability was high for all ROM variables with ICC of 0.97-0.99 and repeated angle measurement error of ∼1°.

Muscular Strength and Endurance

Prior to performing the muscular strength and endurance tests, the golfers completed a standard warm-up of 5 minutes of cycling on a TRUE Z8 stationary bicycle, a series of upper and lower body stretches, and a familiarization set (∼20-40% 1 repetition maximum [RM]) of each of the three muscular strength (GSCWC, bench press, and hack squat) and one endurance (isometric prone hold) tests. Each strength and endurance test was separated by a 2-minute rest.

The muscular strength and endurance tests were selected after reviewing the golf conditioning (13,18,28) and EMG (3,35,36,42) literature. A rotational golf-specific exercise (the GSCWC) was included as previous training studies that reported significant improvements in clubhead velocity and/or driving distance all included resisted trunk rotation exercises (13,18,28). It was felt that the GSCWC was more specific to the golf swing than the medicine ball throws and medicine ball golf swings used by Doan et al. (13) and Fletcher and Hartwell (18), respectively and of comparable specificity to the elastic cord resisted golf swings of Lephart et al. (28). However, golf swings performed with elastic tubing were not a viable option in this study as a simple strength measure (in kg) would not be able to be obtained with such a test.

The three general exercises (bench press, hack squat, and isometric prone) were chosen on the basis of the following factors: a) that they (or very similar derivatives) had been included in training studies that had shown significant improvements in clubhead velocity/ball displacement (13,18,28); b) major agonists in these exercises were highly active in the golf downswing (3,35,36); and c) subjects with limited resistance training backgrounds could safely perform these tests. In contrast to the other tests, the isometric prone assessed trunk muscular endurance not strength. The rationale for this was that the level of trunk muscular endurance and not strength appears to be negatively related to the incidence of lower back pain and injury in golfers (16,17) and positively related to the ability to stabilize the trunk, allowing the efficient transfer of forces through the trunk and arms to the clubhead (8,9).

All golfers completed the series of tests in the same order. For the GSCWC, bench press, and hack squat exercises, the 1RM was estimated from a submaximal (3-6 RM) load using the equation of Brzycki (6). This equation states that predicted 1RM = mass lifted/1.0278 - 0.0278X, where X is the number of repetitions performed. If more than 6 repetitions were performed, the golfer rested for 2 minutes and then performed the same exercise with a heavier load that would likely result in failure in 3-6 repetitions.

The GSCWC test is a rotational exercise that is very similar to the golf swing in terms of posture, range of motion, intended velocity, direction of force (torque) application, and coordination patterns (8). When performing this exercise, the golfers assumed a standing position approximately 1.5 m from the cable weights stack, with their body perpendicular to the weight stack and the cable height adjusted so that it was slightly above shoulder height. In order to achieve a similar position to the setup (address) position of the golf swing, the golfers were asked to stand with their legs approximately shoulder width apart with a small bend in the knees and to lean slightly forward at the hips. The starting position required the golfer to grab the cable with both hands (as they would a golf club) and allow their trunk and hips to rotate towards the weight stack and for the front arm to be adducted to a horizontal position. From this start position, the golfer rotated their trunk and hips away from the weight stack while pulling the cable downwards and forwards towards where they imagined the golf ball to be positioned on the tee (see Figure 1). In keeping with the mechanics of the golf swing and the kinetic-link principle (22), the golfers were advised to lead with the hips, followed quickly by the trunk and arm segments and to keep the front arm straight throughout the movement. A repetition was considered valid if the golfers' front arm started in a horizontal position and finished in a vertical position.

F1-30
Figure 1:
Golfer performing the golf swing-specific cable woodchop exercise. a) Starting position. b) Mid position. c) Finishing position.

For the bench press test, golfers lay in a supine position on an exercise bench. At all times, the golfers were required to keep their buttocks and shoulders in contact with the bench and their feet flat on the floor. The bar was gripped with an overhand grip ∼1.5 times shoulder width apart, so that when the elbows were bent to 90°, the hands were directly above the elbows. The barbell was lifted off the rack, lowered until it touched the chest, and then pressed back up until the elbows were fully extended.

When performing the hack squat test, the golfers positioned themselves with their back against the padded surface, shoulders wedged beneath the yokes attached to the backrest, feet placed shoulder width apart on the footplate. The golfers then flexed their knees to 90° before extending the knees back to the starting position. During the movement, the heels were required to maintain contact with the footplate and the gluteals, shoulders, and head with the backrest.

The isometric prone hold required the golfers to assume a prone position on the floor with the only contact between the floor being the feet and the forearms. The shoulder was in a flexed position, with the elbows bent to 90° and positioned directly under the shoulders (see Figure 2). All golfers were required to hold this position for as long as possible, with the time recorded using a stopwatch. The test was terminated when the first of two criteria was met. The first criterion was when the golfer suddenly collapsed to the floor. The second was when the golfer's hips (anterior superior iliac spine) dropped or lifted more than 2 cm from the starting (neutral spine) position. For the second criteria, the golfer was given an initial warning on the first occurrence; when there was a repeat infringement, the test was terminated.

F2-30
Figure 2:
Golfer performing the isometric prone hold exercise.

Golf Swing Performance

Golf swing testing was conducted at Auckland University of Technology's Golf Swing Clinic, which has eight bays (each ∼20 m long) with an artificial grass Astroturf surface and netting. Prior to performing the golf swing testing, each golfer completed a semistandardized warm-up, which consisted of ∼10-15 minutes of progressively more intense golf swings and stretching. During this warm-up, the subjects were asked to aim at the target to familiarize themselves with the experimental setup. The target, which was positioned 15 m in front of the golfer, was 1 m wide and 1.4 m high, with the bottom of the target 4.2 m above the ground to approximate the expected trajectory of the 5-iron.

After completing their warm-up, each golfer performed 10 swings for maximum velocity and accuracy with their own 5-iron golf club. The 5-iron was chosen over a driver for the following reasons: a) to reduce the ball velocity and hence the chance for the ball to pass through the net (as this had occurred when some LHG used a driver in pilot testing); b) to make it easier to determine the accuracy of the shot; and c) to more easily conduct a 50-Hz video analysis of the golf swing (5). While there are some differences in the technique used with a 5-iron compared to other clubs, there appears to be more similarities than differences (11,15). Target accuracy was defined as the percentage of shots that hit the target. Each swing was separated by a rest period of 1 minute to ensure sufficient recovery time.

Maximum clubhead velocity for each golf swing was measured using a Stalker Professional Sports radar gun (Applied Concepts, Plano, Texas, USA) operating at a frequency of 34.7 GHz (29). The Stalker radar gun was held in a 180° posterior position to the golfer as they performed each golf swing. Prior to each testing session, the radar gun was calibrated using the tuning fork as recommended by the manufacturer and performed in previous research (29). Once calibrated, the radar gun had a measuring accuracy of ±0.1 mph (±0.04 m·s−1) for activities up to 300 mph (∼133 m·s−1). The intertrial reliability of the players' clubhead velocity was high as indicated by the coefficient of variation (CV) and intraclass correlation coefficients (CV = 3.1% and ICC = 0.88).

Statistical Analyses

Standard descriptive statistics (mean, SD, and range) were calculated for the golf performance (clubhead velocity and accuracy), anthropometric, flexibility (ROM), and muscular strength and endurance test variables. Normality of data (for each group) was assessed using the critical appraisal approach (34). If the mean and median (of each variable) differed by less than 10%, then normality was assumed. However, if this preliminary criterion was breached, two additional criteria had to be breached for the data to be described as displaying a nonnormal distribution. These criterions were a) mean and SD test (2SD < mean), b) Shapiro-Wilks statistics (p > 0.05), c) skewness and kurtosis statistics (within 1), and d) skewness or kurtosis/SE (within 1.96). Individual data that breached these criterions were excluded from further analysis.

Significant intergroup differences in these measures were assessed using a one-way analysis of variance. Cohen effect sizes (d) were also calculated in order to quantify the magnitude of the between-group differences. In accordance with the revised effect size (ES) magnitudes of Drinkwater et al. (12) for sport science research, effect sizes were defined as trivial (<0.2), small (0.2-0.6), moderate (0.6-1.2), or large (>1.2).

Relationships between clubhead velocity and the dependent variables for the entire sample of 20 golfers were determined using Spearman's rank correlation coefficients. This test was chosen over the more common and powerful Pearson product moment correlation because the significant between-group differences in a number of the dependent variables made the total group data bimodal in nature. The bimodality of this data violates the assumptions of normality underlying the Pearson product moment correlation (4). All statistical analyses were conducted using SPSS v11.5 for Windows with significance set at p < 0.01 to account for the relatively high number of between-group comparisons.

Power analyses were conducted to determine the number of subjects required in this study. According to Hopkins (20), a correlation of r = 0.5 can be considered high and equates to an effect size of 1.2. Twenty-two subjects in total would be required for a correlation coefficient of r = 0.5 to be significant, with 80% power and a risk of type I error of 5%. Using the methods of Hopkins (19) and data from Barrentine et al. (2) for 20 professional LHG and 20 HHG (handicap >15), 12 subjects would be required in each group to show significant between-group differences in clubhead velocity with a power of 80% and a risk of type I error of 5%.

Results

Compared to HHG, LHG generated significantly greater (12%) clubhead velocity and were significantly more accurate as evidenced by their ability to hit the target more than twice as often (Table 1). Low-handicap golfers also had significantly greater GSCWC (28%) strength than the HHG (Table 1). Although not statistically significant, LHG also tended to have significantly greater (5%) acromiale-radiale (upper arm) and (4%) acromiale-stylion (total arm) lengths and (30%) bench press strength, but less (24%) right hip internal rotation than HHG (ES = 0.98-1.12, p = 0.023-0.042). No statistically significant intergroup differences were observed for any of the other kinanthropometric variables.

T1-30
Table 1:
Golf swing performance, anthropometry, flexibility (range of motion), and predicted 1RM muscular strength measures of low handicap and high handicap golfers, and correlations between the selected variables and maximum clubhead velocity.

Significant correlations were observed between clubhead velocity and that of handicap, target accuracy and GSCWC strength (Table 1). Although not statistically significant, clubhead velocity also tended to be positively correlated to bench press and hack squat strength and to acromiale-radiale and acromiale-stylion length (r = 0.45-0.52, p = 0.019-0.045). No significant correlation was observed between clubhead velocity and any ROM variable.

Discussion

The purpose of the present study was to determine if selected kinanthropometric variables were correlated to clubhead velocity and if these variables could distinguish between LHG and HHG. Results were somewhat consistent with the initial hypotheses, with a small number of kinanthropometric variables significantly (or tending to be) correlated to clubhead velocity and significantly (or tending to be) different between LHG and HHG. Such results suggest that a golfer's performance may be partially related to their kinanthropometric profile.

As hypothesized, the LHG (37.6 ± 1.0 m·s−1) had greater 5-iron clubhead velocity than the HHG (33.0 ± 1.8 m·s−1). These clubhead velocity values appeared similar to other studies involving a 5-iron (∼34-41 m·s−1) (15,24) and as expected somewhat less than that obtained using a driver (∼43-53 m·s−1) (12,14,17). The finding that LHG were more accurate than HHG was also consistent with our hypotheses and the literature (32).

It was apparent that no anthropometric variables were significantly correlated to clubhead velocity and that no significant between-group anthropometric differences existed. Such results may suggest that anthropometry has little influence on clubhead velocity and that the anthropometric profile of LHG and HHG was quite similar. To our knowledge, Kawashima et al. (25) is the only other peer-reviewed study to examine the anthropometric profile of golfers, with their study involving 11 professional, 24 collegiate, 13 general amateur, and 15 collegiate recreational golfers. The professional golfers were significantly heavier, had greater amounts of fat-free mass, and had larger limb girths than the three groups of amateur golfers, with these between-group differences more pronounced the greater the difference in handicap (25). While no significant between-group difference was observed for these variables in the present study, the between-group effect sizes indicated that LHG tended to have small to moderately greater fat-free mass, chest, and arm girths and arm lengths than HHG. In addition, the correlation of the upper arm and total arm length to clubhead velocity (r = 0.450-0.453) approached significance (0.01 < p < 0.05). The results of this study and that of Kawashima and colleagues (25) therefore suggest that while the anthropometry of golfers of varying standards may not differ as much as that of other sports like Australian Rules football (26), subtle anthropometric differences e.g. muscle mass and girths as well as arm length may still contribute to greater levels of clubhead velocity. However, the astute reader should be careful in extrapolating these anthropometric trends too far, as excessive muscular hypertrophy may restrict the range of motion around the trunk and shoulder and increase body segment moment of inertia, all changes that could result in a decrement in golfing performance. This is important because it is not currently known what would constitute the optimal degree of muscular hypertrophy for golfers.

Consistent with previous studies (13,24), the LHG and HHG typically exhibited high levels of flexibility across all assessed joints. It was, however, a little surprising that no flexibility measure was significantly correlated to clubhead velocity and that the LHG were not significantly more flexible than the HHG on any test. In fact, there was a tendency with a moderate effect size for the LHG to have less right hip internal rotation ROM than the HHG. The lack of any strong associations between flexibility and clubhead velocity as well as any between-group differences in flexibility was surprising because Jones (24) reported that 8 weeks of PNF stretching produced a significant (7.2%) increase in clubhead velocity. Further, other studies that have incorporated flexibility training in an overall golf conditioning program all reported significant increases in clubhead velocity (13,18,28).

While somewhat speculative, the trend for the LHG to have reduced internal hip rotation ROM may reflect a between-group difference in hip and shoulder rotation (turn) during the backswing and early downswing. Previous research has shown that the greater the relative rotation of the trunk compared to the hips at the top of the backswing or early in the downswing (referred to as the X-factor and X-factor stretch, respectively), the greater the clubhead velocity and ball displacement (7,31). Therefore, the tendency for the LHG to have reduced internal hip rotation ROM may reflect (and/or result from) their reduced hip rotation during the golf swing. Such a technique may be performance-enhancing as it allows a greater X-factor/X-factor stretch, which in turn positively influences clubhead velocity and ball displacement (7,31).

Results of the present study indicated that GSCWC strength was significantly greater in LHG than HHG and significantly correlated to clubhead velocity. Trends were also evident where bench press strength may have been greater in LHG than HHG (moderate effect size) and significantly correlated (r = 0.500) to clubhead velocity (0.01 < p < 0.05). Such between-group differences in strength were observed even though the LHG's tendency for longer arms would have placed them at a mechanical disadvantage in these exercises compared to the HHG.

The fact that GSCWC strength was more highly related to clubhead velocity than the other muscular strength and endurance tests was consistent with the specificity principle (1,43) as the GSCWC and the golf swing appear highly comparable in terms of their posture, range of motion, intended velocity, direction of force (torque) application, and coordination patterns (8). The between-group differences, and the significant correlation between GSCWC strength and clubhead velocity, also appeared consistent with the conclusions drawn from a number of training studies (13,18,28), which suggested that specific rotational exercises like the GSCWC need to be performed if clubhead velocity is to be improved.

With respect to the between-group differences in clubhead velocity seen in the present study, it is however unclear if it was the LHG's greater GSCWC strength (and hence ability to generate angular velocity) or arm (lever) length that contributed more to their significantly greater clubhead velocity than the HHG. Regardless, golfers with shorter arms need to generate greater joint angular velocities than players with longer arms in order to achieve comparable clubhead velocities and hence ball displacements (22). Thus, the possible improvements in clubhead velocity with the performance of golf-specific resistance training may be even more pronounced for golfers with shorter than longer arms. However, additional research will need to be conducted to support such a statement.

High levels of trunk muscular endurance have been proposed to stabilize the spine during the swing, thereby allowing the efficient transfer of ground reaction forces through the trunk and arms to the clubhead (8,9). The rate of lower back pain in golfers may also be greater in golfers with low levels of, or in imbalance in, abdominal and/or lower back muscular endurance (16,17). On this basis, it may be expected that LHG would have greater levels of isometric prone hold endurance than HHG and that performance in this test would correlate to clubhead velocity. While neither of these comparisons reached statistical significance, there were some tendencies for the LHG to perform better (moderate effect size) than the HHG on this test and for this test to correlate to clubhead velocity (r = 0.434). Although not definitive, the results of the present and previous studies (13,16-18) suggest that core stability/muscular endurance training exercises may have some benefit for golfers.

There were a number of limitations within the present study. These include the inability to recruit sufficient subjects to obtain 80% power for all dependent measures. Therefore, some variables that should have been significantly different between groups or correlated to clubhead velocity may not have obtained statistical significance. The relatively low sample size (n = 20) would also mean that the confidence limits of the correlations would be somewhat broad. Thus, a larger sample (n > 50) of golfers would need to be tested to reduce the uncertainty in the true value of the correlations reported in this study. In addition, the present study assessed golf performance with one club and two groups of male golfers of relatively narrow handicap ranges, age, and anthropometric profiles. Thus, further research is required to confirm that similar results would be found when using other clubs (e.g., a driver) and for a different sample, such as female golfers or male golfers of different standards, age, and/or anthropometric profile to that assessed in the present study.

Practical Applications

Results of this study suggest that LHG and HHG have a relatively similar kinanthropometric profile. However, LHG had significantly greater GSCWC strength and trends for greater bench press strength and longer arms than HHG. While GSCWC strength was the only kinanthropometric variable that was statistically correlated to clubhead velocity, bench press and hack squat strength as well as upper arm and total arm length also tended to be correlated to clubhead velocity. These results suggest that the kinanthropometric profile of LHG may confer a competitive advantage and contribute to their greater clubhead velocity. Talent identification programs may wish to select players with higher GSCWC and/or bench press strength as well as longer arms, while conditioners may seek to further improve trunk rotation strength and power via exercises like the GSCWC and/or other plyometric rotational exercises. These conditioning programs should probably not overemphasize the development of muscular hypertrophy because excessive hypertrophy may negatively affect performance by limiting joint ROM and/or by increasing the moment of inertia of particular body segments. The astute conditioner should also include preventative (prehabilitation) exercises that minimize the risk of injury to at-risk areas, such as the lower back and shoulder.

Acknowledgments

We thank Sam Aickin for assisting with data collection and analysis and the subjects who gave up their time for this project. This project was supported by a Faculty of Health and Environmental Sciences, Auckland University of Technology Contestable Research Grant.

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    Keywords:

    anthropometry; golf; range of motion; resistance training; specificity; talent identification

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