Although it's true that the traditional straight leg pushup can be more difficult, performing pushups with bent knees won't prevent you from achieving your strength and fitness goals. If you perform enough repetitions to promote muscle growth, the type of pushups you choose is more a matter of preference than superior effectiveness.
Traditional Vs. Modified Pushups
The traditional pushup, sometimes referred to as the male version, calls for you to keep your body line straight, your legs fully extended, your feet together, your hands shoulder-width apart and your fingers pointing forward or slightly to the inside. The modified bent-knee pushup, sometimes called the female pushup, is similar to the traditional exercise, but is performed with your knees and lower legs resting on the floor. In both pushup exercises, you keep your abdominal muscles tight and your movements slow and controlled.
Upper-Body Factors
Beginners who are unable to perform the regular pushup with good form can strengthen the same muscles by doing modified pushups instead. The same upper-body muscles -- triceps, pectorals and deltoids -- are engaged during traditional and bent-knee pushups. And if you perform enough bent-knee push-ups to fatigue the targeted muscles, the effectiveness is comparable to a set of traditional pushups performed to the point of fatigue.
Lower-Body Factors
Both types of pushups engage the rotator cuff, posterior deltoids, rhomboids, abs and the serratus anterior, although to a lesser degree than the primary targeted muscles. However, the traditional version with straight legs works more muscles in your lower body, involving your glutes and quadriceps as stabilizers throughout the movement. You can balance the differences between the bent-knee and traditional pushups by adding strength exercises that target the muscles not engaged in the modified version.
Keep Challenging Yourself
If you start with the modified pushup, you will eventually build enough strength to switch to traditional pushups. However, you can continue to build strength in your arms, chest and shoulders with the bent-knee pushup as long as you continue to perform enough repetitions to tax the targeted muscles. In any case, ensure balance in your strength training workout by including exercises that target your arms, shoulders, back, abs, glutes and legs at least two times per week. Check with your doctor before starting a strength-training program if you have a history of health problems.