Effect Of Plyometric Training Under Different Surface On Military Cadets’ Agility

Objective:The research first explored the optimal height range of Plyometric training.Then,the long-term effects of stable and unstable surface plyometric training twice a week for 4 weeks on the agility of military cadets were studied.In order to provide theoretical support and training reference for modern military physical training,the internal mechanism was expounded from three aspects of anantomy,performance and neuro-muscular electrical changes.Methods:According to the results of 30m×2 snake running,40cadets from National University of Defense Technology were randomly divided into stable surface Plyometric training group(SP group;N=13),unstable surface Plyometric training group(UP group;N=14)and traditional military training group(T group;N=13).The reflexively jumping height and EMG data were collected from 15cm,30cm,45cm and 60cm by height groper and wireless surface EMG instrument Delsys Trigno,and the optimal training height was obtained,which was applied in the 4-week training intervention.The results of vertical jump,Illinois agility and 30m×2 snake running before and after training were tested.The thickness and electromyography changes of four muscles in rectus femoris,biceps femoris long head,tibialis anterior muscle and gastrocnemius muscle were measured by Ultra-portable Diagnostic Ultrasound System and wireless surface EMG instrument Delsys Trigno.Paired-sample t test and one-way analysis of variance were used to compare the differences before and after training and between groups,and P<0.05 was considered as significant difference.Results:1)When the drop jump height was lower than 45cm,the reflexive jump height and the degree of muscle activation increased with the increase of the fall height.When the jump box height was 60cm,the reflexive jump height decreased significantly,and there was a significant difference compared with 45cm(P<0.05).2)After 4 weeks of training intervention,the vertical jump performance of SP group and UP group was 2.94cm and 1.37cm higher than that of T group,and SP group was 1.57cm higher than that of UP group(P<0.05).Illinois:the performance of SP group and UP group was significantly improved compared with the previous test,and the time was 0.48s and 0.52s lower than that of T group,respectively(P<0.01).30m×2 snake running:the performance of SP group and UP group was significantly improved compared with the previous test,and the time was 0.65s and 0.52s lower than that of T group,respectively(P<0.01).3)After 4 weeks of training intervention,the calf circumference of SP group and UP group was significantly increased compared with the previous test(P<0.05),but no significant differences in thigh circumference;The thickness of rectus femoris muscle,medial head of gastrocnemius muscle,long head of biceps femoris and tibialis anterior muscle in SP group and UP group were significantly increased compared with the previous test.The thickness of rectus femoris and biceps femoris long head in T group was significantly increased compared with the previous test(P<0.05).4)After 4 weeks of training intervention,the vertical jump EMG results showed that there were significant increased in RMS of medial gastrocnemius,sEMGmean,IEMG and rectus femoris RMS between SP group and UP group(P<0.05).In 30m×2 snake running the RMS of the medial head of the gastrocnemius muscle in the SP and UP groups at the first and second steps of the second pole and in the UP group at the second step of the 11th pole were significantly increased compared with the pretest(P<0.05).The activity of the T group and other muscle groups was generally increased,but there was no significant difference compared with the pretest(P>0.05).sEMGmean of medial head of gastrocnemius muscle in SP group and UP group was increased at step 1 of the 2 pole and at step 1 and 2 of the 11 pole of the UP group(P<0.05).The IEMG of the gastrocnemius medial head of SP group and UP group was significantly increased at the first step of the second pole,and the first and second steps of the 11th pole of UP group compared with the previous test.The recruitment ability of rectus femoris muscle at the first and second steps of the second pole in UP group was increased(P<0.05).Biceps femoris,long head muscle and tibialis anterior muscle showed an upward trend in the same time,but there was no significant difference compared with the pre-test(P>0.05).Conclusion:1)The height of 45cm is the best for cadets to Plyometric training represented by drop jump.2)Four weeks of Plyometric training in different surface can effectively improve the cadets’ performance in vertical jump,Illinois agility and 30m×2 snake running.3)The training period of different surface Plyometric improves the cadets’ agility by shortening the eccentric-concentric time during speed change and direction change.Stable surface is more beneficial to improve eccentric braking ability.The unstable surface is more favorable to the development of concentric pushing force.