Can $F=ma$ be used to calculate ground reaction force of someone walking/running? Can Newton's second law of motion $F=ma$ be used to calculate ground reaction force of someone walking/running?
 A: It would be more convenient to use the fact that $F\Delta t = m\Delta v$. There is a constant force of gravity pulling you down, and an occasional force of the foot pushing you back up. Averaged over time, the two must be equal as there is no net change in vertical velocity.
This means that $F_{strike} \approx \frac{W}{\Delta T}$ where $W$ is the weight and $\Delta T$ is the fraction of time that your foot is in contact with the ground. The shorter the time you are in contact, the larger the force while you are there.
It's quite hard to measure acceleration during the impact accurately - there may be a peak, but good shoes, good technique and good running surfaces will help to minimize that.
See also my earlier answer on a related topic and this other one about the effect of running form on energy expended and this one about the effect of incline
A: Yes, it can. If $\vec a$ represents the acceleration of the person's center of mass, then $\vec F_\text{net}$ represents the external forces acting on the person, which include only the normal & frictional forces by the ground as well as the gravitational for by the Earth. If $\vec a$ is known, you'll get two equations (horizontal and vertical) to solve for your two unknowns.
A: To some extent. When walking there is a phase in every gait cycle that both feet are in contact with the ground, hence 2nd law cannot calculate the distribution of loading between legs. When running on the other hand, the heel strike and the change in the inertia of the lower extremity creates an impact transient, which again cannot be depicted by the 2nd law. Pick force and the unloading phase of the stance phase may be calculated with the acceleration of the COM quite accurately when running. 
