The relationship between leg stiffness, forces and neural control of the leg musculature during the stretch-shortening cycle is dependent on the anticipation of drop height.

Abstract

This study aimed at investigating how prior knowledge of drop heights affects proactive and reactive motor control in drop jumps (DJ). In 22 subjects, the effect of knowledge of three different drop heights (20, 30, 40cm) during DJs was evaluated in seven conditions: three different drop heights were either known, unknown or cheated (announced 40cm, but actual drop height was 20cm). Peak ground reaction force (Fmax) to body weight (BW) ratio (Fmax/BW) and electromyographic (EMG) activities of three shank and five thigh muscles were assessed 150ms before and during ground contact (GC). Ankle, knee and hip joint kinematics were recorded in the sagittal plane. Leg stiffness, proactive and reactive EMG activity of the leg muscles diminished in unknown and cheat conditions for all drop heights (7-33% and 2-26%, respectively). Antagonistic co-activation increased in unknown (3-37%). At touchdown, increased flexion in knee (~ 5.3° ± 1.9°) and hip extension (~ 2° ± 0.6°) were observed in unknown, followed by an increased angular excursion in hip (~ 2.3° ± 0.2°) and knee joints (~ 5.6° ± 0.2°) during GC (p < 0.05). Correlations between changes in activation intensities, joint kinematics, leg stiffness and Fmax/BW (p < 0.05) indicate that anticipation changes the neuromechanical coupling of DJs. No dropouts were recorded. These findings underline that anticipation influences timing and adjustment of motor responses. It is argued that proactive and reactive modulations associated with diminished activation intensities in leg extensors are functionally relevant in explaining changes in leg stiffness and subsequent decline in performance.