Lower limb muscle activation and biomechanics during single-leg hopping in different directions

Single-leg forward and vertical hopping are commonly employed to evaluate knee neuromuscular function following anterior cruciate ligament reconstruction. However, similar hopping performance between limbs does not ensure full knee recovery. Single-leg backward hopping has been suggested to impose greater knee kinetics, but its effects on lower limb muscle activation and kinematics remain unclear.

To quantify the effect of hopping directions on lower limb muscle activation and biomechanics during jumping, focusing on the knee joint.

Forty-eight injury-free participants performed single-leg forward/vertical/backward hopping with motion, force, and surface electromyography data collected. Peak and mean muscle activation of quadriceps, hamstrings, and triceps surae, peak trunk/hip/knee/ankle angles, and hip/knee/ankle moments in the sagittal plane during the jumping phase were calculated. Hopping performance was also recorded. One-by-three repeated-measures ANOVAs were conducted to quantify the effects of hopping directions.

Forward hopping demonstrated greater hopping performance, trunk/ankle angles, hip/ankle moments, and hamstring activations, while smaller hip/knee angles, knee moments, and quadriceps activations compared to hopping in other directions. Vertical hopping showed the greatest knee angle compared to forward and backward hopping. Backward hopping exhibited the smallest trunk/ankle angles, hip/ankle moments, and muscle activations of biceps femoris/gastrocnemius medialis/soleus with the greatest knee moment among hopping directions.

Forward hopping may serve as a general performance exercise but might underrepresent knee-specific measurements. Vertical hopping may be more appropriate for monitoring quadriceps function and knee control. Backward hopping imposes the greatest knee mechanical demands with limited hip and ankle involvement, likely making it a promising metric for identifying deficits in dynamic knee control.