| Purpose:The Achilles tendon(AT)is the largest,strongest tendon in the body,which plays a major role in walking,running and jumping through transmitting force from triceps surae.However,the latest epidemiological survey showed that Achilles tendinopathy was one of the most common injuries and was difficult to cure in distance running with an incidence rate of 10%,which is second only to patellofemoral pain syndrome.The present studies have shown that the AT of runners who wear minimalist shoes with forefoot strike pattern have greater cross-sectional area,impulse,stress,strain,and stiffness than those who wear conventional shoes with rearfoot strike pattern,which indicated the better mechanical properties of AT that were important factors to improve motor and competitive performance.However,the mechanism of AT adaptation under different shoe conditions and foot strike patterns is still unclear.In addition,the current method to relieve AT pain and improve function is mainly the eccentric training of the triceps surae,but the training itself is timeconsuming and may cause delayed muscle soreness.Therefore,the aim of the study is to investigate different shoe conditions and foot strike patterns on the biomechanical characteristics of AT(cross-sectional area,length,force,etc.),and further make up a reasonable running transition program to explore the adaptability of the AT in vivo during running.It would help to provide a theoretical and practical basis for further understanding how to control the AT during training and affect the mechanical properties of AT.Methods:In the study 1,30 male participants were recruited in total,including 10 habitual rearfoot strike runners(RFS),10 habitual forefoot strike runners(FFS),and 10 no exercise habit males(NEH).The morphological properties of AT of the three groups were collected by an ultrasound device.The peak plantar flexor moment of the ankle joint of the three groups was collected by an isokinetic dynamometer,was synchronized to the image changes in vivo AT length.The AT loading was collected and calculated by using a three-dimensional instrumented split-belt treadmill for RFS and FFS running at 10km/h with cushioning shoes.The oneway analysis of variance(ANOVA)was used to compare the morphological(AT cross-sectional area,length,and thickness)and mechanical properties(AT force,stiffness,and hysteresis)differences among the three groups.The independent sample t test was used to compare the difference of AT force,impulse,strain between RFS and FFS.The significance level a was set as 0.05.In the study 2,11 male runners who used to wear cushioning shoes with rearfoot strike pattern were recruited.The participants were randomly required to wear cushioning shoes and five-finger minimalist shoes to run with forefoot and rearfoot strike patterns respectively on a three-dimensional instrumented split-belt treadmill at a speed of 10 km/h.The ultrasound probe was fixed at the junction of the AT and medial gastrocnemius muscle by an elastic bandage.The AT loading characteristics and length changes in vivo,sagittal plane kinematics and kinetics of lower limb three joints,vertical ground reaction forces and their loading rates were simultaneously recorded.A 2x2 two-way ANOVA was used to determine different running shoes(cushioning shoes vs.minimalist shoes)and foot strike patterns(rearfoot strike pattern vs.forefoot strike pattern)of AT loading(force,loading rate,impulse,work,etc.),kinematics and kinetics of lower limb.Paired sample t test was used to compare the first peak of ground reaction force and its loading rate when wearing two different running shoes to rearfoot strike.The significance level a was set as 0.05.In the study 3,30 male runners who used to wear cushioning shoes with rearfoot strike pattern were recruited.They were instructed either to run in five-fingers minimalist shoes with a forefoot strike pattern(TRA,n=15)or keep up their strike pattern(CON,n=15).Ultrasound images were captured to determine the morphological properties of AT.The AT loading characteristics and length changes in vivo,sagittal plane kinematics and kinetics of lower limb three joints,vertical ground reaction forces and their loading rates were simultaneously recorded.A 2×2 two-way repeated measures ANOVA was used to determine the effect of AT loading(force,loading rate,impulse,work,etc.),kinematics and kinetics of lower limb before and after 12-week transition program,and the significance level a was set as 0.05.Results:1.There were no significant differences in the length,crosssectional area,and thickness of AT among RFS,FFS,and NEH(p>0.05).The peak ankle plantar flexor moment and AT force during the maximal volitional isometric contraction were significantly increased 21.4%and 17.2%respectively in FFS than NEH(p<0.05).The peak force,average/instantaneous loading rate,stress of AT were significantly greater in FFS compared to RFS(p<0.05).2.A significant shoe-by-foot strike pattern interaction was observed for instantaneous loading rate of AT(p<0.05).Post hoc analysis revealed a significant increase in the instantaneous loading rate of AT with rearfoot strike pattern when acute wearing five-finger minimalist shoes compared to cushioning shoes,as well as a significant increase when acute wearing cushioning shoes with forefoot strike pattern compared to rearfoot strike pattern.In addition,there was no interaction among the other parameters,but there were significant shoe or foot strike pattern main effects.Specifically,peak fore,average loading rate,impulse,peak stress,elongation and work of AT were significantly increased in acute forefoot striking than rearfoot striking.The peak AT force was significantly increased when acute wearing five-finger minimalist shoes compared to cushioning shoes(p<0.05).3.A significant shoe-by-foot strike pattern interaction was observed for time to active peak of ground reaction force(p<0.05).Post hoc analysis revealed a significant increase in the time to active peak of ground reaction force with rearfoot strike pattern when acute wearing five-finger minimalist shoes compared to cushioning shoes,as well as a significant increase when acute wearing five-finger minimalist shoes with forefoot strike pattern compared to rearfoot strike pattern.In addition,there was no interaction among the other parameters,but there were significant shoe or foot strike pattern main effects.Specifically,compare to rearfoot strike,the range of motion of knee joint,contact angle,and minimal plantar flexor moment of ankle joint were significantly decreased,and the peak plantarflexion angle,range of motion,maximal moment,positive work,negative work,and net work of ankle joint,and knee minimal flexion moment were significantly increased in acute forefoot striking(p<0.05).Compare to acute wear cushioning shoes,range of motion of knee joint,peak plantarflexion angle,range of motion,negative work and net work of ankle joint were significantly decreased when acute wearing five-finger minimalist shoes(p<0.05).4.After 12-week transition program,28 participants completed the training and were included in the final analysis(14 in TRA and 14 in CON).The percentage complete of training was 93.3%(28/30)without any running-related injuries.After 12 weeks of training,8 of the 14 participants in TRA transited to forefoot strike,with a transition rate of 57.1%and a significant increase in length,thickness,and cross-sectional area of AT(p<0.05).The main effect of group was observed in peak AT force(p<0.05).The result showed that peak AT force in TRA was greater than CON.The main effects of time were observed in the time to peak AT force,average/instantaneous loading rate,and elongation of AT(p<0.05).The result showed the time to peak AT force was decreased,and average/instantaneous loading rate,and elongation of AT were increased after 12-week training.5.There were no significant interaction effects in the vertical ground reaction force,kinematics,and kinetics of lower limb three joints,but there were significant time or group main effects.Specifically,average loading rate of impact force of ground reaction force,active force of ground reaction force,maximal flexion angle of knee joint,range of motion,positive work,negative work,net work and peak power of ankle joint were significantly increased after 12-week training.The hip joint stiffness,minimal plantar flexion moment and stiffness of ankle joint were significantly decreased after 12-week training.The contact angle,minimal flexion angle and stiffness of knee joint were significantly increased in TRA compare to CON(p<0.05).Conclusion:The greater force,average/instantaneous loading rate and stress of AT,and lower hysteresis of AT in runners who habitually forefoot strike,suggested that long-term forefoot strike pattern could stimulate the enhanced mechanical properties of AT under high loads,which would enable AT to store and release elastic energy more effectively during running.For runners who were accustomed to rearfoot strike in cushioning shoes,peak AT force was significantly increased immediately with five-finger minimalist shoes or forefoot strike pattern,and more AT work during the stretch phase when immediately forefoot striking,suggesting that running with five-finger minimalist shoes and/or forefoot strike pattern seemed to provide sufficient load intensity to improve AT mechanics.As a safe and effective training program,12-week transition program enabled some runners to change from rearfoot strike pattern to forefoot strike pattern(8/14),and the length,cross-sectional area and thickness of AT were significantly increased,and AT force and average AT loading rate were also increased by 6.7%and 19.6%,respectively.It was indicated that gradually wear five-finger shoes with a forefoot strike pattern could stimulate the morphological characteristics of the AT through a repetitive stretch-shortening cycle,as well as play a positive role in improving the mechanical properties of the AT.They together gave rise to the result of improving the ability of the AT to load,thereby preventing and reducing the risk of AT injury. |
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