The Importance of Wrist Torque in Driving the Golfball (P1)
A Simulation Study
The speed of the clubhead at impact is the principal factor that determines the distance that a golfball will travel. Clubhead speed is known to be a function of the sequential segment velocities of the chain link that makes up the golf swing (Herring & Chapman, 1992). The wrist joint, being the most distal anatomical joint in this chain, would be expected to play a role in the development of the final clubhead speed. Over the years, golfers have debated whether the action of the wrists should be passive or active in the releasing of the clubhead prior to impact. Most golfers have an intuitive belief that the addition of a properly timed muscular torque at the wrist joint will increase clubhead speed at impact. However, a number of pundits, including the legendary Bobby Jones, believe the opposite is true. Jones (1966) stated that during the swing the club “freewheeled” through the ball. Williams (1967), who worked from a stroboscopic photograph of Bobby Jones swing, likewise concluded that the uncocking torque applied by the hands was negligible. Jorgensen (1994), using a computer simulation of a two segment planar model of the golf swing, provided insight into this freewheeling theory of the golf swing. His simulation work revealed, paradoxically, that anything the golfer does with his wrists during the downswing to decrease the wrist-cock angle results in less clubhead speed at impact than if the golfer allows the wrist joint to open naturally. While Jorgensen (1994) reported that the early onset of muscular wrist torque during the downswing was ill advised, his simulation work revealed that clubhead speed might be increased marginally (0.7%) if the torque was delayed until 0.07 s prior to impact. However, he cautioned that the effectiveness of this delayed wrist torque was very sensitive to its duration time; longer or shorter duration times produced clubhead speeds less than maximal.
he simulation work by Jorgensen, while very instructive, was based on a twosegment golf swing system, comprising an arm and a club segment, with torque generators inserted at the wrist and shoulder joints. The shoulder and wrist torque generators in his model, when activated, were constant in magnitude, with no allowance made for the force-velocity or activation properties of muscle. These modeling limitations suggest that the potential role of wrist torque in the golf swing may still not be fully understood. The purpose of this paper was to re-examine the question as to whether, in theory, a properly timed wrist torque during the downswing can significantly improve clubhead speed without jeopardizing the desired club position at impact.
The golfer was modeled as a three-segment, two dimensional (2-D), linked system with the golfclub, arm, and torso segments moving in a plane tilted 60° to the ground (Figure 1). The assumption of planar movement of these segments during the downward swing is well supported in golf literature (Cochran & Stobbs, 1968; Jorgensen, 1994). The golfclub was modeled as a rigid segment which is consistent with the conclusion of Milne and Davis (1992) that, contrary to popular belief, shaft bending flexibility plays only a minor dynamic role in the golf swing. For the purposes of the 2-D representation, the torso was collapsed along its longitudinal axis so that it lay in the movement plane as a rigid rod with a length equal to the distance from the sternal notch of the sternum to the glenoid fossa of the scapula. Torque generators were inserted at the proximal end of each segment and provided the model with the capability of adding energy to the system. The torque generators used in the simulation were programmed to be constrained by the activation rate and force-velocity properties of human muscle. The force-length property of muscle was expected to play a second-order role in the outcome of the performance (Caldwell, 1995) and, as such, was not included in the simulation model. The activation rate and force-velocity properties associated with human muscle were implemented using the calculated instantaneous isometric torque predicted from a linearized Hill model structure (Niku & Henderson, 1985; Sprigings, 1986) as input to the force-velocity approach described by Alexander (1990).
In equation 1, T is the instantaneous value produced by a torque generator, T is the maximum isometric torque of the torque generator; oma is the maximum angular velocity of the associated joint; o is the instantaneous joint angular velocity: I is a shape factor controlling the curvature of the torque/velocity relationship; t is the elapsed time from initial torque activation; and T is the activation time constant (Sprigings, 1986; Pandy, Zajac, Sim, & Levine, 1990). For the present study, T was set at 180, 120, 60 Nm for the spine, shoulder, and wrist, respectively (Neal, Burko, Sprigings. & Landeo, 1999); om was set at 20, 30, 60 rad.s’ for the spine, shoulder, and wrist, respectively (Neal et al., 1999); t was set at 40 ms (Sprigings, 1986); and I was assigned a value of 3.0 (Alexander, 1990).