Reactive knee stiffening strategies between various conditioning histories.

Optimizing joint stiffness through appropriate muscular activation is crucial for maintaining stability and preventing injury. Conditioning techniques may affect joint stability by increasing joint stiffness and altering neuromuscular control; however no studies have assessed this in a controlled setting. Fifteen endurance athletes, 12 power athletes, and 15 control subjects sat on a stiffness device that generated a rapid knee flexion perturbation and were instructed to react to the perturbation. Main outcome measures included short-range (0-4°) and long range (0-40°) stiffness and muscle activation from quadriceps and hamstring muscles. Stiffness results revealed greater short-range stiffness in endurance athletes (0.057 ± 0.012 Nm/deg/kg) than controls (0.047 ± 0.008 Nm/deg/kg, p = 0.021); while passive long-range stiffness was greater in power (0.0020 ± 0.001 nm/deg/kg) than endurance athletes (0.0016 ± 0.001 nm/deg/kg, p = 0.016). Endurance athletes had greater reactive stiffness (0.051 ± 0.017 nm/deg/kg) than control (0.033 ± 0.011 nm/deg/kg, p = 0.001) and power (0.037 ± 0.015 nm/deg/kg, p = 0.044) groups. Endurance athletes also displayed greater quadriceps activity during passive and reactive conditions (p < 0.050) compared to power athletes and controls. These findings suggest that power-based training history may be associated with greater passive joint stiffness across the full range of motion, while endurance-based training could positively influence reactive muscular characteristics, as well as resting muscle tone. These unique variations in stiffness regulation could be beneficial to programmes for prevention and rehabilitation of joint injury.

Two- and 3-dimensional knee valgus are reduced after an exercise intervention in young adults with demonstrable valgus during squatting.

CONTEXT: Two-dimensional (or medial knee displacement [MKD]) and 3-dimensional (3D) knee valgus are theorized to contribute to anterior cruciate ligament injuries. However, whether these displacements can be improved in the double-legged squat (DLS) after an exercise intervention is unclear. OBJECTIVE: To determine if MKD and 3D knee valgus are improved in a DLS after an exercise intervention. DESIGN: Randomized controlled clinical trial. SETTING: Research laboratory. PATIENTS OR OTHER PARTICIPANTS: A total of 32 participants were enrolled in this study and were randomly assigned to the control (n = 16) or intervention (n = 16) group. During a DLS, all participants demonstrated knee valgus that was corrected with a heel lift. INTERVENTION(S): The intervention group completed 10 sessions of directed exercise that focused on hip and ankle strength and flexibility over a 2- to 3-week period. MAIN OUTCOME MEASURE(S): We assessed MKD and 3D knee valgus during the DLS using an electromagnetic tracking system. Hip strength and ankle-dorsiflexion range of motion were measured. Change scores were calculated for MKD and 3D valgus at 0%, 10%, 20%, 30%, 40%, and 50% phases, and group (2 levels)-by phase (6 levels) repeated-measures analyses of variance were conducted. Independent t tests were used to compare change scores in other variables (α < .05). RESULTS: The MKD decreased from 20% to 50% of the DLS (P = .02) and 3D knee valgus improved from 30% to 50% of the squat phase (P = .001). Ankle-dorsiflexion range of motion (knee extended) increased in the intervention group (P = .009). No other significant findings were observed (P > .05). CONCLUSIONS: The intervention reduced MKD and 3D knee valgus during a DLS. The intervention also increased ankle range of motion. Our inclusion criteria might have limited our ability to observe changes in hip strength.