Reliability of repeat golf club testing sessions with modified club moment of inertia

The moment of inertia of a golf club, quantified about an axis at the butt of the handle, normal to the swing plane, has the potential to influence both clubhead and ball velocity. The purpose of this study was to assess the reliability of clubhead and ball velocity with changes to moment of inertia over repeat testing sessions and, if reliable, to quantify the effect of modifying moment of inertia. Eleven skilled male golfers hit 20 golf shots with three golf clubs, each with a different moment of inertia achieved through adding mass inside the club shaft and repeated this protocol over three sessions. A commercially available launch monitor was used to measure both velocity variables. Test–retest reliability was assessed via (1) limits of agreement, to determine reliability from a change in magnitude perspective and (2) linear-weighted kappa, to determine reliability from a directional perspective. The effect of moment of inertia on clubhead and ball velocity was determined using one-way, repeated measures analysis of variance tests, with partial eta squared being used to quantify the size of the effect. Increasing golf club moment of inertia reliably decreased clubhead and ball velocity, with fair to substantial kappa results revealed between sessions. The magnitude of decrease in these velocities, however, could not be reliably quantified. Statistically, the influence of moment of inertia was considered large (η2 ≥ 0.662 and 0.404) and significant (p < 0.001 and ≤ 0.006) for both clubhead and ball velocity, respectively.