Few methods are available for measurement of muscle strength during physical activities. This study addresses the validity of estimating the force of the quadriceps femoris during the forward lunge and squat carried out by 18 healthy men. Data were collected by the utilization of a musculoskeletal model through computer simulation. From the electromyographic (EMG) recordings of the vasti medialis (VM) and lateralis (VL) and rectus femoris (RF) muscles a correlation coefficient (r) was calculated between the estimated value for the muscle force and the root mean square (RMS) value. The median r for VM and VL was found to exceed 0.65 during the flexion and extension phases of forward lunge and below 0.44 for VM and VL during squat for the same phases. For RF, the median r for both tasks was below 0.39 except during the flexion phase of squat. The coefficient of the similarity in the EMG patterns exceeded 0.93 between the estimated muscle force and EMG RMS for both the forward lunge and squat. The value of the estimated muscle force for RF in these 2 tasks was found to be very small. The results show that the musculoskeletal model can be considered a valid measure for estimating muscle force.