Nonlinear Characteristics In Lower Extremity Biomechanics During Fatiguing Run

PurposeIn the long-term,high-intensity running exercise will inevitably induce lower limb neuromuscular fatigue,and exercise fatigue caused by lower limb musculoskeletal system,especially lower limb joint control ability decline,which is the important cause of running injury.However,the previous study only focused on the two stages before and after fatigue,and did not analyze and compare the changes of lower limb mechanics in fatigue process.The relationship between fatigue and biomechanical characteristics is less common among researchers,who have not identified critical points for possible changes,such as how the hip,knee and ankle joints of the lower extremities will be adjusted during different stages of the fatigue process,and it will affect the attitude control of the lower limbs.This study aimed to determine the changes in lower extremity biomechanics during running-induced fatigue intervention,including vertical/anterior-posterior ground reaction force(GRF),vertical GRF loading rate,joint mechanics and stiffness,etc.Methods19 male recreational runners were recruited.GRF and marker trajectories were recorded for 15s intermittently every 2 min to quantify the GRF,the lower extremity kinematics and kinetics during the fatiguing run by Bertec instrumented treadmill(sample rate:1000Hz)and Vicon 3D(sample rate:100Hz)motion capture system.Participants were required to wear unified cushion shoes and run at 3.33 m/s until they could not continue running.They were considered to have experienced fatigue,and intervention was terminated when both following criteria were met:(1)the HR of the participants reached 90%of their age-calculated maximum HR,and(2)the participants could not continue running.Analysis and processing of at least 10 steps of dominant side data at 5 moments(0,25%,50%,75%and 100%)The impact force,joint angles at initial contact,maximum joint angles,joint ranges of motion,change in joint angle,joint extension torque,and stiffness in five phases of fatigue running were compared.Blood lactate concentration(BLa)was also collected before,immediately after fatigue,4 and 9 minutes after fatigue.ResultsCompared with before fatigue,1)the concentration of blood lactate was significantly increased immediately after fatigue,4 min after fatigue and 9 min after fatigue(P<0.05);2)the parameters such as vertical/horizontal GRF and vertical GRF loading rate were not significantly observed during fatigue intervention;3)the hip joint ranges of motion was significantly increased by 50%and 100%during fatigue(P<0.05),while knee joint activity increased significantly at 50%and 100%(P<0.05);4)ankle ROM and knee moment of joint were not observed;5)the vertical extension of hip and knee force was not observed;75%decreased at 100%of the vertical stiffness(P<0.05),The vertical displacement change of the center of gravity significantly increased time of 25%?50?75%and 100%(P<0.05).ConclusionGRF characteristics did not vary significantly through fatiguing run.However,nonlinear adaptations in lower extremity kinematics and kinetics were observed.In particular,a“soft landing" strategy,achieved by an increased ?ROM at the hip and knee joints and a decreased vertical stiffness,was initiated from the mid-stage of a fatiguing run to potentially maintain similar impact forces and reduce the potential risk of injury in prolonged running.