Thigh strength and activation as predictors of knee biomechanics during a drop jump task.
OBJECTIVE
To examine whether normalized quadriceps and hamstring strength would predict quadriceps and hamstring muscle activation amplitudes and whether these neuromuscular factors would predict knee kinematics and kinetics during a drop jump task.


METHODS
Thirty-nine females and 39 males were measured for isometric quadriceps and hamstring strength and were instrumented to obtain surface electromyography, kinematic, and kinetic measures during the initial landing of a drop jump. Multiple linear regressions first examined the relationship between thigh strength and activation then examined whether these neuromuscular variables were predictive of hip and knee flexion excursions, knee extensor moments (KEM), and anterior knee shear forces during the deceleration phase of the drop jump.


RESULTS
Females versus males produced lower normalized thigh strength and demonstrated greater quadriceps and hamstring activation amplitudes during the drop jump. Lower thigh muscle strength was a weak (males) to moderate (females) predictor of greater quadriceps activation amplitudes. However, thigh strength and activation were poor predictors of hip and knee joint excursions and KEM. Regardless of sex and thigh strength, anterior shear forces were greater in individuals who demonstrated less hip flexion and greater knee flexion excursions and greater peak quadriceps activation and internal KEM during the landing.


CONCLUSIONS
Although thigh muscle strength explained some of the variance in quadriceps and hamstring activation levels as measured with surface electromyography, we failed to support the hypothesis that these neuromuscular factors are strong predictors of sagittal plane hip and knee flexion excursions or KEM. Although greater quadriceps activation amplitude was a significant predictor of greater anterior tibial shear forces, its contribution was relatively small compared with kinematic and kinetic variables.
Shultz
Sandra J
SJ
Department of Exercise and Sport Science, University of North Carolina at Greensboro, NC 27412, USA. sjshultz@uncg.edu
Nguyen
Anh-Dung
AD
Leonard
Michael D
MD
Schmitz
Randy J
RJ
D000070598:Anterior Cruciate Ligament Injuries; D001696:Biomechanical Phenomena; D001835:Body Weight; D005080:Exercise Test; D005260:Female; D006801:Humans; D007717:Knee / Q000502:physiology*; D008297:Male; D009068:Movement; D053580:Muscle Strength / Q000502:physiology*; D012307:Risk Factors; D013848:Thigh; D019415:Torque; D016474:Weight-Bearing; D055815:Young Adult