Co-activation periods of gastrocnemius and vastus lateralis during walking evaluated by surface electromyography. 
"In vivo" studies reported that the co-activation of gastrocnemius and quadriceps femoris (QF) muscles produces ACL strain values greater than those caused by an isolated activation of either muscle.  Aim of this study was to assess the co-activation of gastrocnemius (lateral head, GL) and vastus lateralis (VL) in healthy and young adults during walking.  To this purpose the Statistical Gait Analysis was performed, that allows a characterization of gait considering hundreds of strides belonging to the same walking trial.  Three GL/VL co-activations were detected during a single gait cycle: in foot-contact phase, from 6.8±8.5% to 22.9±23.3% of gait cycle, (FC co-activation), in push-off phase, from 33.0±11.9% to 41.5±13.4% (PO co-activation), and in swing phase, from 86.5±6.7% to 93.2±5.9% (SW co-activation).  FC co-activation was the most recurrent (in 100% of the strides, P<;0.001) and longest (16% of gait cycle) one.  Thus, the ACL strain due to the co-activation between GL and VL is longer and more frequently during FC phase, than in all the others gait phases.  Moreover, the position of the knee and the amount of the weight-bearing on knee, achieved in this gait phase, suggested that FC co-activation is the one that produces a highest strain value of anterior cruciate ligament (ACL).  These findings could help to better understand risk factors of the ACL injuries and to design more focused preventive and rehabilitative strategies.