WAC Fitness Instructor Alice Harrington shows the Spiderman Push-Up, a great exercise for core, upper back, deltoids, triceps, and oblique muscles.
Start in a push-up position with your back straight and your core tight.
Lower into a push-up and pause at the bottom. During the pause bring your right knee up and touch it to your right elbow. Make sure you keep your core tight.
(Try to move your leg around your hip rather than under your torso.)
As you return your leg to the starting position, push yourself back into the straight-arm position. Repeat on the left side.
Complete three sets of 10–50 reps depending on ability.