Abstract

INTRODUCTION Football is the most popular sport in the world with more than 260 millions of amateur players and they suffer about 670,000 injuries per year [1. The most common location of injuries is in the lower limb (67-88%). Meniscal rupture and partial or complete ligament sprain are about 19% of the injuries. The normal knee function includes a co-contraction between agonist and antagonist muscle of flexors and extensors to protect the anterior cruciate ligament (ACL) from a sprain or complete rupture [2,3]. The aim of this study was to characterize the co-contraction index of normal amateur football players by the controlled reproduction of the flexor-extensor component of the injury mechanism of ACL. METHODS Twenty male amateur football players (age: 242.14years, height: 1.730.04m, weight: 68.583.94kg, and BMI: 22.891.32) volunteered to take part in the study after signing informed consent forms. The subjects had a training background of almost 5 years and had no health problems (including absence of pain and sprain or knee surgery). This study was approved by the local University Ethics Committee. The subjects walked on a treadmill (PowerJog JW160) for 5 minutes before the test to stablish a comfortable normal speed (3.220.20km/h with treadmill slope of 2%). The EMG activity was recorded sampling at 2000Hz with eight bipolar surface electrodes (Myomonitor® Delsys Inc.) placed bilaterally over vastus medialis oblicous (VMO) and vastus lateralis (VL) of quadriceps femoris and biceps femoris (BF) and semitendinosus (ST) according to SENIAM recommendation. During 10s the speed was maintained and before 60s the treadmill was stopped. The limb on which the treadmill would stop was randomly assigned prior to the test and was established between the heel strike and foot flat of human gait. The EMG raw data were full-wave rectified, low pass filtered at 6Hz and normalized expressed as percentage of EMG value during stand at rest. The Co-contraction index (CCI) was calculated using the method of Winter [4]: ???????????? ???? 2???????????????? ???? 100 2???????????????????? where ???????????????? is the area of total antagonist activity and is calculated by: ???????? ???????? ???????????????? ???? ???? ???????????????????????????? ???????????????????? ???? ???? ???????????????????????? ???????????????????? ???????? ???????? where ????????and ???????? denote the period where the EMG agonist is less than EMG antagonist, whereas ???????? and ???????? denotes the period where the EMG antagonist is less than EMG agonist during the windows analysis: ???????? ???????????????????????? ???? ???? ???????????????????????????????? ???????????????????????????? ???????????????????????? ???????? The windows analysis corresponds to the 20% of the Stance Phase of the Gait. We established two relations for agonist and antagonist: (1)VMO/ST and (2)VL/BF and these were compared before treadmill stop (CCI) and after treadmill stop (CCIA). To evaluate data distribution the Schapiro-Wilk normality test was applied and for the tuples agonist/antagonist before the treadmill detention and after treadmill detention (not before vs after) the Mann-Whitney U test (p<0.05) was applied. RESULTS AND DISCUSSION The mean values were similar for the four groups and were not statistically different (figure 1) but the variance before treadmill detention the VMO/ST (2.60.42) was 2.21 times greater than VL/BF (117.47) and after treadmill detention this relation was 4.05 times greater (VMO/ST: 249.39, VL/BF: 61.48). These phenomena are interesting because the function of the chosen pairs is different. The VMO/ST in addition to flexion and extension control have a role over the control of knee rotation during human gait. For this reason, we believe that a high variability was found. In comparison with other studies we obtained different values of CCI. One of the problems in finding similar values lies in the difference in the windows of analysis and in the choice and allocation of the agonist and antagonist muscles. Many studies use windows of analysis that are larger than ours and include the complete human gait cycle or an entire phase or are not associated with dynamic events. CCI% 100 80 60 40 20 0 VMO/ST CCIB VL/BF VMO/ST VL/BF CCIA Figure 1: Co-contraction index (CCI) before (CCIB) and after (CCIA) detention of treadmill. CONCLUSIONS The knee flexor-extensor muscles play a fundamental role in the protection of ligamentous injury of the anterior cruciate ligament. Currently, there are no reference values for CCI. In this study, we establish specifically values for the normal knee in amateur football players. REFERENCES 1. Beijsterveldt et al. Injury Prevention. 17:e2. 2. Strazza et al., Gait & Posture. 51: 228-233. 3. Shelburne et al. Journal of Biomechanics. 37: 797-805. 4. Kellis et al. Journal of Electromyography and Kinesiology. 13: 229-238.