The Importance Of Wrist Torque In Driving The Golfball (P3)
The simulation sequence was terminated when the clubhead reached a position 20 cm horizontally past the proximal end of the torso segment, or if the simulation time exceeded 0.9 s. Although a small time-step interval of .002 s was used during the simulation runs, the exact time of impact was determined by means of interpolation. To reduce the chance of arriving at a local instead of a global minimum during the optimization process, 200 randomly generated starting conditions for each of the six control variables were examined for each optimization trial. The set of “best” starting conditions, as determined by the magnitude of the penalty summation that accrued during each simulation, was then used as the starting conditions for the POWELL optimization process. At the termination of the POWELL optimization process, the optimized set of control variables was stored in memory. This entire procedure was repeated 50 times, with the “best” set of optimized control variables being saved as a permanent file. The magnitudes of all torque generators were set to zero until activated by the optimization process.
Three simulation conditions for the downward phase of the golf swing were optimized. The first simulation condition (SIM-1) provided for the presence of voluntary wrist torque during the optimization search for maximum clubhead speed at impact. The second condition (SIM-2) prevented any voluntary wrist torque being used during the optimization process, which effectively reduced the wrist to a free hinge during the downward swing. A third simulation (SIM-3) was performed where the force-velocity property of muscle was not included in the joint torque generators. The purpose of this third simulation condition was to examine the effect that not including force-velocity property might have on the segment timing reported by earlier simulation studies.
A qualitative validation test was performed on the simulation results by comparing the corresponding image sequences for the three-segment model with a real-life photographic sequence of an elite professional golfer during his downswing.
For the first simulation condition (SIM-1), which permitted a voluntary wrist torque to be present if it improved clubhead speed at impact, the maximum horizontal clubhead speed at impact was 44.0 m/s (~99 mph; Figure 2). The onset of voluntary muscular torque at the joints (Figure 3) demonstrated a proximal to distal pattern, with the torso segment’s torque generator being turned on immediately, followed by the shoulder torque for the arm segment 0.148 s later, followed finally by the torque at the wrist joint 0.138 s after that. The total time of the downward swing to impact was 0.380 s, which is comparable to a value of 0.34 s measured from video for professional golfer, Nick Faldo, whose club speed at impact was approximately 5 m/s faster. For the SIM-1 simulation condition, removing the constraint that forced the clubshaft to be in the vertical position at impact produced a slight improvement in clubhead speed (2%) but at the expense of good clubface alignment at impact.