Extended practice and the force/force variability relationship

Typescript (photocopy).The purposes of this investigation were to determine the: (1) effect of extended practice on the force/force variability relationship proposed by Motor-Output Variability Theory (Schmidt, Zelaznik, Hawkins, Frank, & Quinn, 1979) and (2) effect of extended practice on the accuracy of the production of force. Method. Subjects were 12 undergraduate volunteers at Texas A&M University. Subjects were required to exert "shots" of force against a load cell in an attempt to have a recorder pen deflect to a target line displayed on graph paper. Target lines represented percentages of each subject's maximal voluntary contraction. Subjects participated in 10 testing sessions of 210 trials each. Data from Testing Sessions 1, 5, and 10 were analyzed. Polynomial regression analysis was used to assess the force/force variability relationship over practice. Separate testing session (3) x block (3) x target line (7) analyses of variance with repeated measures on all factors were used to analyze mean force, force variability, relative force variability, absolute error, absolute error/force percent, constant error, constant error/force percent, and variable error. Results. The force/force variability relationship yielded a quadratic function early in practice with the quadratic component dropping out after extended practice. Force variability decreased across testing sessions for the force range such that a linear force/force variability relationship existed at the end of practice. Force production accuracy was increased with a reduction in force variability. Generally, subjects tended to have greater inaccuracy and bias at the target lines having the largest variability. Conclusions. Based upon the results of the present study, the following conclusions were established: (1) Motor-Output Variability Theory (Schmidt et al., 1979) may need to be reevaluated in terms of inclusion of a quadratic component in the relationship between force and force variability. (2) Inaccuracy in tasks requiring the production of near minimal or near miximal amounts of force seems to be due to the increased variability created by the production of those amounts of force. (3) Force production accuracy increases as force variability decreases. (4) Force variability decreases over practice