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Lower Extremity Work Is Associated with Club Head Velocity during the Golf Swing in Experienced Golfers
DOI 10.1055/s-0034-1367010 Int J Sports Med 2014; 35: 785–788 For personal use only. No commercial use, no depositing in repositories.
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Orthopedics & Biomechanics
Lower Extremity Work Is Associated with Club Head Velocity during the Golf Swing in Experienced Golfers
Authors Affiliations
Key words ▶ kinetics ● ▶ motion capture ● ▶ ground reaction forces ● ▶ golf ●
M. P. McNally1, N. Yontz2, A. M. Chaudhari3 1
Orthopaedics, Ohio State University, Columbus, United States Golf, Nike Inc., Beaverton, United States 3 Department of Orthopaedics and Sport Health & Performance Institute, The Ohio State University, Columbus, United States 2
Abstract
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While the golf swing is a complex whole body movement requiring coordination of all joints to achieve maximum ball velocity, the kinetic contribution of the lower extremities to club head velocity has not been quantified, despite the perception that the legs are a primary source of power during the swing. Mechanical power at the hips, knees, and ankles was estimated during the downswing phase of a full swing with a driver using a passive optical motion capture system and 2 force plates for adult males across a range of age and self-reported skill levels. Total work by the lower extremities was calculated
Introduction
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accepted after revision December 09, 2013 Bibliography DOI http://dx.doi.org/ 10.1055/s-0034-1367010 Published online: February 27, 2014 Int J Sports Med 2014; 35: 785–788 © Georg Thieme Verlag KG Stuttgart · New York ISSN 0172-4622 Correspondence Michael Paul McNally Orthopaedics Ohio State University 2050 Kenny Rd. Suite 3100 43221 Columbus United States Tel.: + 1/614/2932 246 Fax: + 1/614/2932 910
[email protected]
The golf swing is a complex whole body movement, requiring coordination of multiple joints to achieve maximum shot length with accuracy. Generating club head velocity is one of the most important parameters to researchers and coaches alike, because club head velocity has been associated with overall performance (self-reported handicap) [6]. The legs are often regarded by coaches and golf professionals as a primary source of the energy required to maximize club head velocity during an efficient swing [4, 19]. However, despite the perceived importance of the lower extremities in generating velocity during the swing, most biomechanical analyses of the golf swing have focused on the interaction between the pelvis and trunk [1, 2, 11, 13] or the magnitude and timing of weight transfer [9, 18, 21]. Very little evidence exists to explain the kinematic and kinetic contributions of the lower extremity to the golf swing. Lower extremity strength and power influence the development of club head velocity. Recent studies investigating leg strength found significant relationships between club head velocity
by integrating the powers of all 6 joints over the downswing. Regression analyses showed that total lower extremity work was a strong predictor of club head velocity (R = 0.63). Secondary analyses showed different relationships to club head velocity in lead and trail leg lower extremity joints, but none of these were as predictive of club head velocity as the total work performed by the lower extremities. These results provide quantitative evidence that the lower body’s kinetic contribution may be an important factor in achieving greater club head velocity, contributing to greater driving distance and overall golf performance.
and maximum squat strength with correlations between 0.53 and 0.81 [10, 16]. Lower extremity power has also been significantly associated with club head velocity using various tasks of vertical jumping height, with relationships ranging from 0.53 to 0.77 [17, 22]. While these studies do not investigate the biomechanical contribution of the lower extremity to the swing or examine rotational strength or power of the lower extremity, they nevertheless provide preliminary evidence that the legs could be a primary source of club head velocity during the golf swing. Nesbit et al. estimated that lower extremity muscle work accounted for 31.5 % of the work performed by the body in 4 golfers, with the majority of this work being generated by large frontal and transverse plane torques at the hips [15]. Analysis of the work performed by the lower extremity can provide valuable insight into the sources of kinetic energy for generating a golf swing, because mechanical work performed by the legs, calculated as the integral of instantaneous joint power at the ankles, knees and hips, provides a measure of the overall amount of energy that is provided by the legs to accelerate or decelerate the body. This formulation also enables the anal-
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ysis of energy generation (positive work) and energy dissipation (negative work). While Nesbit’s small study observed that the lower extremities perform significant work during the golf swing, it did not investigate the relationship between lower extremity biomechanics and club head velocity. To develop new techniques that improve golf performance and reduce the risk of injury, we first need to better understand the lower extremities’ role in the golf swing. As a first step in this direction, the primary aim of this study was to determine whether work performed by the lower extremity is related to club head velocity during a golf swing. We hypothesized that golfers who performed more net work with the lower body would also achieve greater club head velocity. We also performed secondary exploratory analyses of the contributions of individual leg work and individual joint work to club head velocity.
Methods
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This study meets the ethical standards for sport and exercise science research [8]. A consent form was obtained from the Institutional Review Board to use anonymized data previously collected for non-research purposes. Data for 36 male golfers across a range of self-reported skill levels (professional, n = 2; intercollegiate team, n = 6; handicap 0–7, n = 16; handicap 8–15, n = 7; handicap 16–30, n = 5) was used for analysis in this study (age = 36.3 ± 17.3 years, mass = 87.0 ± 12.8 kg). Each golfer was fitted with reflective markers placed bilaterally over the 5th and 2nd metatarsal heads, posterior calcaneus, medial and lateral malleolus, lateral mid-shank, medial and lateral knee joint line, lateral mid-thigh, ASIS, PSIS, acromion process, lateral mid-upper arm, medial and lateral epicondyle of the elbow, mid-forearm, medial and lateral wrist, and 2nd metacarpal joint. Additional markers were placed over the T-10 vertebra, C-7 vertebra and right scapula. A static trial was used to determine joint centers and position of tracking markers relative to joint centers. Hip joint centers were calculated using a previously defined regression model [5], which is commonly used with Vicon’s plug-in-gait model [7]. A marker placed on the top of the club head was used to calculate the instantaneous peak club head velocity magnitude during the downswing, where instantaneous velocity is equal to the distance the marker traveled from one time point to the next divided by the time (1/300th of a second). Each subject was asked to warm up according to his typical preparation to reproduce his on-course swing in the laboratory setting. Each subject was recorded using an 8-camera motion capture system (MX-F40, VICON Motion Systems, Oxford, UK) with 2 force plates (4060-10, Bertec Corporation, Columbus, OH) that synchronously collects kinematic data at 300 Hz and ground reaction force data at 1 500 Hz. Each subject set up with one foot on each force plate, maintaining contact throughout the swing. Subjects were instructed to perform each swing as if executing a normal straight drive on the golf course. Each subject then performed between 8 and 12 shots with a driver and foam practice golf balls into a net until 8 representative shots were collected. Any shots which the golfer determined were mis-hit or which were not recorded with the motion capture system were repeated. Foam practice balls were used to ensure the safety of equipment and personnel in the lab. As this study focused on development of peak club head velocity as an assessment of golf driving performance [6] and no realistic target was present for the golfer to
McNally MP et al. Lower Extremity Work Is … Int J Sports Med 2014; 35: 785–788
focus on, no attempt was made to estimate ball flight or accuracy. The single swing with the greatest peak club head velocity for each subject was chosen for analysis. Marker trajectories were filtered using a fourth order low-pass zero phase-lag Butterworth filter with a cutoff frequency of 20 Hz to eliminate the potential for club head velocity artifacts due to 3D marker reconstruction errors. Kinematics and kinetic variables were calculated from filtered kinematic data and ground reaction forces using standard inverse dynamics calculations performed in Vicon Bodybuilder (VICON Motion Systems, Oxford, UK). The top of the backswing was defined as the point where club head velocity reaches 0 and the club head reverses direction, while ball impact was determined by the club head’s crossing of a vertical reference plane defined by 2 markers placed on the floor on each side of the club head at address. The peak club head velocity and all joint work calculations were performed between the top of the backswing and ball impact, when energy is being generated to be applied to the golf ball. Analysis of this time interval also minimizes any differences due to hitting foam as opposed to real golf balls, as these differences would only be expected after ball contact. Joint powers for the left and right hip, knee and ankle were calculated as the dot product of the joint moment and joint angular velocity [20, 24]. Joint work was then calculated using MATLAB (Mathworks, Natick, MA) by integrating the area beneath the joint power curves. Individual net joint work of the hips, knees and ankles were summed to calculate the net work performed by the lead leg (closer to the target at address), trail leg (farther from the target at address) and the total net work of the lower extremities. 3 separate stepwise regression analyses were performed (SPSS Inc., Chicago, IL) to assess the relationship of variables of interest with peak club head velocity. The primary analysis included total lower extremity work and age as potential inputs and peak club head velocity as the output. Exploratory stepwise regression analyses were performed investigating the relationship between peak club head velocity and lead and trail leg work with age as a covariate, and between peak club head velocity and lead and trail leg hip, knee, and ankle work with age as a covariate. For each of the analyses, a predictor was included in the regression when p < 0.05.
Results
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Ensemble joint power curves for each lower extremity joint dur▶ Fig. 1. Stepwise regression ing the downswing are shown in ● ▶ Table 1. To analyses for each set of input variables are shown in ● test the primary hypothesis that total lower extremity work is related to club head velocity, the stepwise regression analysis for age and total leg work yielded the final model: Peak Club Velocity = 39 277–0.138(Age) + 0.049(TotalLegWork) with R2 = 0.78 and a standard error of 2.14 m/s. Partial R values were − 0.68 and 0.63 for age and total leg work, respectively, in the final model. Additional analyses found that along with age, both lead and trail leg work are independent predictors of club head velocity, and for individual joints, trail leg work and lead hip, lead knee, and lead ankle work are significant predictors of club head velocity.
Orthopedics & Biomechanics
Lead Leg
5
5
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10
Joint Power (W/kg) knee
0 6
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0 0
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Fig. 1 Ensemble power curves for the hips, knees and ankles of the lead and trail legs.
Trail Leg
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–1
%of downswing
%of downswing
Table 1 Summary of the stepwise regression analysis for each set of input variables. Model
Candidate variables
Final model
1
TotalLegWork, Age
Peak Club Velocity = 39.277 – 0.138(Age) + 0.049(TotalLegWork)
2
LeadLegWork, TrailLegWork, Age
Peak Club Velocity = 39.088–0.134(Age) + 0.061(LeadLegWork) + 0.039(TrailLegWork)
3
LeadHipWork, LeadKneeWork, LeadAnkleWork, TrailHipWork, TrailKneeWork, TrailAnkleWork Age
Peak Club Velocity = 43.431–0.189(Age) + 0.105 (RKneeWork) + 0.091(LeadAnkleWork) + 0.081 (LeadKneeWork) + 0.049(LeadHipWork)
Predictors
Partial R
Age TotalLegWork Age LeadLegWork TrailLegWork Age RKneeWork LeadAnkleWork LeadKneeWork LeadHipWork
− 0.679 0.633 − 0.669 0.571 0.443 − 0.824 0.615 0.505 0.489 0.375
number
Discussion and Implications
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Total leg work was observed to be a significant predictor of peak club head velocity during the golf swing across a wide range of age and skill levels, supporting our primary hypothesis. Secondary exploratory analyses of side-to-side differences showed that both lead and trail leg work were related to club head velocity, with lead leg work showing a stronger relationship than the trail leg. Additionally, an individual joint analysis showed that the trail leg knee joint was the greatest joint predictor of peak club head velocity, followed by the lead ankle, lead knee and lead hip. These results suggest that the primary contributions from the trail leg come from the knee joint, while the lead leg uses more of a combined effort of the hip, knee, and ankle joints to generate energy. However, while the lead and trail leg each play an important role in generating energy during the golf swing, their combined efforts may be most important in the development of club head velocity. While this study focused on the positive work performed (i.e., energy generated) by the legs and its association to club head velocity, several potential mechanisms exist for energy genera-
tion to cause acceleration of the club head. One possible way the energy generated by the lower extremities could be used to increase club head velocity is through upward bodily movement. Miura found that having an upward pull at impact helped to increase swing velocity through “parametric acceleration” in which the centripetal force acting on the moving club and the upward pull velocity of the club act together to increase the total kinetic energy of the club [14]. Chu et al. suggests that this upward path at impact should involve the entire body, potentially driven by the legs, rather than the arms and wrists acting alone [3]. Previous studies investigating baseball pitching and javelin throwing, which also have a significant rotational component, have also suggested lead leg extension as a potential bracing mechanism allowing for a greater transfer of energy to the upper extremities [12, 23]. The lack of previously published data on lower extremity biomechanics during the golf swing limits our ability to compare the work we calculated with other studies. The individual net joint work reported in this study is significantly less in magnitude than previously reported by Nesbit et al. [15]. Nesbit also reported negative work performed in the lead knee and ankle, as McNally MP et al. Lower Extremity Work Is … Int J Sports Med 2014; 35: 785–788
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opposed to positive work observed in this study. Given that Nesbit only included 4 subjects, it is not possible to discern between differences due to methods for calculating work and those due to the 4 individuals being potential outliers rather than representative subjects. However, Nesbit found identical results when comparing inverse dynamics calculations to their model estimates and very similar ground reaction forces between actual and their estimates, which suggests that differences between Nesbit and the current study most likely are due to the populations tested. While this study found lower extremity energy contributions to club head velocity to be independent of age, other factors such as mass, swing style, experience, gender or previous injury could affect an individual’s lower extremity contribution to club head velocity. Skill level is comprised of multiple factors unrelated to driving performance (i.e. accuracy, short game, mental preparation) and was not accounted for in the analysis of this study. This lumping of all skill levels together limits the applicability of this study within specific skill groups. Accuracy of the shot is another important consideration in golf, but estimating the flight path of the shot was beyond the scope of the study due to the use of foam practice balls for safety reasons. A single shot that was determined to be each individual’s best performance was used for analysis. In the future, averaging multiple trials for each subject may provide more generalizable results by determining how consistently these lower extremity power variables can explain club head velocity even in the face of technique errors experienced during individual swings. Despite this limitation, the primary finding of this study shows that the total amount of work performed by the legs has a significant effect on the development of club head velocity during the golf swing across a wide range of ages and skill levels. Based on results of this study, golfers may be able to significantly increase their club head velocity by utilizing the lower extremities to a greater extent. The optimal technique for maximizing transfer of energy from the lower extremities to the club head and the best training methods for achieving such technique merits further study. Ultimately, this could lead to specific techniques and training programs which may help to improve a golfers overall golf performance.
Acknowledgements
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The authors would like to thank Lindsay Becker, DPT for her assistance in the recruitment and collection of participants.
McNally MP et al. Lower Extremity Work Is … Int J Sports Med 2014; 35: 785–788
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