| Objective: Hamstring muscles play an important role in controlling the extension of the knee joint and maintaining the margin of stability when walking and running.Studies have shown that imbalance of knee muscle strength,especially decrease of strength in hamstrings relative to that of quadriceps,is associated with soft tissue injury in motor system and falls.And falls often happen in stair-walking,especially in stair descent,but researches on the negative effects of imbalance of knee muscle strength were mainly found in competitive sports instead of stair descent.Therefore,this research aims to explore the effects of imbalance of knee muscle strength on stairdescent,by decreasing the Hamstrings/Quadriceps ratio(H: Q ratio)of lower limb.Methods: 12 healthy females were recruited in this study to decrease their H: Q ratio to 40±2%,by lying leg curl exercise.Before and right after lying leg curl exercise,stair walking tests were conducted on the above subjects with motion capture system(Vicon),3-D force platform(Kistler)and EMG signal acquisition system(Ultium EMG)to obtain biomechanical characteristics of stair descent.Visual 3D system,Matlab 2018 and Origin18 software were used to analyze and process the data collected.According to the data collected,the gait period was divided into swing phase and support phase.The support phase was further divided into the first double support phase,single support phase and the second double support phase.The paired sample t test was used to analyze and compare the changes in margin of stability,kinematics chain and characteristics of muscle activation,with significance level set below 0.05.Results:(1)By applying leg curl exercise to the subjects,the absolute muscle strength value of hamstrings and the H: Q ratio(from 50.5±6.8% to 40.9±2.6%)both decreased very significantly(p<0.01).(2)In the single support phase,the velocity of COM increased and the margin of stability decreased in anterior-posterior direction,but there was no significant difference in the dynamic stability at the end of the single support period.(3)In support phase,the hip,knee and ankle joint angle increased,the above lower limb joint flexed less,the knee joint flexion moment and ankle dorsiflexion moment decreased,and the overall power of the hip,knee and ankle joint decreased.In the second phase of double support,the positive power of ankle increased with its crest appeared earlier.(4)In the support phase,the RMS and i EMG of semitendinosus muscle and biceps femoris muscle were significantly decreased(p < 0.05),and the muscle activation time was significantly earlier,while the RMS and i EMG of medial gastrocnemius muscle increased,and its muscle contribution rate increased,and the co-activation muscle index of thigh and calf increased by22.33% and 18.75%,respectively.Conclusion:(1)Imbalance of knee muscle strength does not have a negative impact on the dynamic stability in in anterior-posterior direction of the single support phase during stair descent,and the decrease of the hamstrings relative to quadriceps does not increase the risk of falling forward during stair descent;(2)Imbalance of knee muscle strength could lead to changes of muscular activation characteristics of the supporting leg during stair descent,and the activation of the medial gastrocnemius muscle could be enhanced to form the functional compensation of the muscle to re-stabilize the knee joint;(3)The basic motion pattern of stair descent was consistent before and after imbalance of knee muscle strength,but the ability of supporting leg to bear weight,absorb energy and alleviate impact decreased,due to changes in lower limb motion chain induced by imbalance of knee muscle strength. |
