In order to understand how muscle force is affected by contractile conditions, I will first review some basic principles on muscle structure and innervation. Then I will focus on muscle force production as a function of muscle length, speed of shortening and stretch, and contractile history. While establishing these foundations, I will outline how the distribution of fiber types influences force and power as a function of the speed of muscle contraction, and how it affects fatigability of muscle force production. Similarly, architectural features influencing the contractile speed, the force, the power, and work potential of muscle will be discussed. Finally, I will address briefly the effect of age on muscle force.