Passive static stretching alters the characteristics of the force-velocity curvature differently for fast and slow muscle groups—A practical application of Hill's equation

Studies showed fast muscle fibers have a greater constant b value of Hill's equation than that of slow muscle fibers, and the changing ratio of b/Vmax indicates the altered characteristics of muscles under certain conditions such as static stretching. This study was to investigate the effect of acute passive static stretching on the curvature of force-velocity curve in people with different muscle fiber types. A two-step work was conducted in current study through using Hill's equation: 1) calculated b values for each subject at different conditions (non-stretched and stretched) to determine muscle groups, and 2) examined the effect of static stretching on different muscle groups. Sixty-five college students performed isokinetic leg extensions at 5 speeds to test peak torque, following either a non-stretching or two passive static quadriceps stretching exercises. The peak torque and corresponding velocity were used to calculate the b constant. Data reduction consisted of calculating a Z score for each non-stretched and stretched b values. Individuals, whose non-stretched b constant was above or below one standard deviation of the Z score, were designated as the less curved (fast) and more curved (slow) groups, respectively. A paired t-test was used to analyze the pre and post intervention effect on b values for each group (p < 0.05). This study found passive static stretching significantly altered the b constant of the fast group, but no effect on slow group. Therefore, we suggest static stretching should be avoided immediately before fast or explosive activities in individuals using predominantly fast muscle fibers.