Biomechanical Analysis Of Spine Based On AnyBody During Simulated Half-squat Parachute Landing

Objective: In present experiment,the biomechanical dynamic analysis of spine was carried out during simulated parachute half-squat landing based on the method of the reverse dynamic analysis,which was utilized by a body simulation system(Anybody model system).Combining with the epidemiological investigations,to predict the possible causes of injury and analyze how to reduce them.Methods: Twenty-two male active-service paratroopers(body height: 175.78±2.34cm;body weight: 71.05±4.73 kg)volunteered to participate in this study.All the healthy subjects with no history of low extremity trauma or spinal diseases.All subjects were instructed to jump off a platform at two different heights of 40 cm,120cm,respectively,and land on a force plateform in a standard half-squat posture.While,the horizontal distance of landing was the same.The Vicon 3D motion capture system was used to record kinetic data and the force plateform tested the changes of ground reaction force,then all the data stored in the form of C3 D.The biomechanical simulation model of spine and lower limbs was established by using Any Body Modeling Sysytem based on the body parameters of subjects.All the experimental C3 D data were introduced into the spine and lower limbs model,which drived the analysis of kinematics and kinetics.The indicators were selected in the model including the upper spine(C0-C1),the lower spine(L-S)torque,joint reaction force of cervical spine(C0-C1),joint reaction force of lumbar segments,activation of 7lumbar muscles.Time period was 1 second,for 0.5 seconds before contacting with the ground and 0.5 seconds after that.The frequency of Anybody system was 200 Hz with a interval of 5ms between each analysis.The changes of biomechanical indicators were analyzed during the parachute landing at two heights,and then the maximum of indicators at two heights were compared to show the effect of different heights on the indicators.Results: The biomechanical changes of spine in the model during parachute landing were dynamic.After the feet contacted with the ground,the effects of ground reaction force on the tortue of spine C0-C1 and L-S,the reaction force of cervical spine(C0-C1)and lumbar segments,and the muscle activation of seven lumbar muscles all had two peaks.The ground reaction force had a sequentially reduced effect on the vertical compressive force of each lumbar intervertebral segments(L5-S1,L4-L5,L3-L4,L2-L3,L1-L2,T12-L1);the distribution of anteroposterior shear force was roughly symmetrical;the internal and external lateral force were also reduced sequentially.the flexor and extensor muscles co-regulated to ensure the balance of spine during the landing process.The maximum torques of the upper spine(C0-C1)in three directions at two drpoping heights were all not statistically significant(p >0.05).The lateral torque,flexion and extension torque of lower spine(L-S)at a height of 120 cm were larger than that at a height of 40cm(p < 0.05),while the rotational torque was not statistically significant(p > 0.05)at two heights.Joint reaction force under the direction of maximum three compare two heights,increasing with height,the maximum height of 120 cm C0-C1 joint reaction force in three directions than40 cm height and large,the difference was statistically significant(p <0.05).With the increase of dropping heights,the joint reaction force of cervical spine(C0-C1)and lumbar segments in three directions,activation of 7 lumbar muscles were all increased(p < 0.05).Conclusion: 1.In present study,the simulation model of spine was used to research biomechanical changes during half-squat parachute landing with the method of reverse dynamics analysis.It was found that the reaction force,torque,muscle activation of spine would have two peaks in response to the ground reaction force.The time of the first peak was tens of milliseconds once the feet contacted with the ground,that is,both lower extremities and trunk were not in the buffer of flexion,while,the second peak was at the moment when the lower extremities and trunk had finished the buffer.2.The analysis of torque and joint reaction force of the upper vertebrae(C0-C1),indicated that avoiding the relative movement of the skull and cervical vertebrae could maintain the relative stability,therefore,it was worthy to improve the exercise and strength of neck muscles,and use a suitable neck brace to limit flexion.3.The analysis of the joint reaction of lumbar intervertebral segments,revealed that reduplicative vertical compressive force and anteroposterior shear force can both cause low back pain,and the anteroposterior shear force had a particular effect.Therefore,the paratroops should strictly follow the half-squat posture,such as the neutral stance position of upper body,half-squat buffer.The use of lumbar brace might reduce the flexion of lumbar and the shear force.4.The analysis of muscle activation of lumbar muscles showed that the flexior and extensor muscles plays an important role in the regulation of the trunk posture during the parachute landing.The lumbar muscles should be strengthened,especially the strength of the core muscle.5.With the increase of dropping heights,the reaction force and muscle activation of each joint all increased,thus it was acknowledged that the height was the main factor causing spinal injury.Strengthening the ground training,improving the ability to bear the impact of the ground,training the space proprioception of the soldiers and landing with an appropriate posture,should be taken into consideration.Frequent parachute landing training would cause muscles fatigue,resulting in the spinal injury because of the decreased stability of trunk.