Sit-ups are not ideal for your body. Planks are a solid substitute to replace them. From Harvard Health Publications:

One reason is that sit-ups are hard on your back — by pushing your curved spine against the floor and by working your hip flexors, the muscles that run from the thighs to the lumbar vertebrae in the lower back. When hip flexors are too strong or too tight, they tug on the lower spine which can be a source of lower back discomfort.

If you don’t want to do squats, or simply need a change of pace, try the plank to close out your workout. Doing the same thing over and over can become mundane. Swapping an exercise or two changes the way your mind and body respond when you give it a command.

The plank is a balance and core conditioning exercise. It’s used in yoga, often performed as part of the sun salutation sequence or as part of a vinyasa in a yoga flow sequence. My teammates and I practiced the plank as a stand-alone exercise. There are all sorts of variations of this move, including the full plank, where you balance on both arms and the side plank, where you balance on one arm, one hand and one foot. This exercise can challenge you as much as you’d like. No matter the flavor you choose, you can depend on this exercise to strengthen your core. From livestrong.com:

The plank exercise helps strengthen midsection, upper-body and lower-body muscles along the front of your body. Planks also strengthen inner core muscles that support your joints. The rectus abdominis and transverse abdominis that form your outer and inner abdominal muscles, respectively, are primary supporters during plank exercises. The abdominal obliques also stabilize the plank position isometrically. Upper-body stabilizers include the pectoral and serratus muscles. Lower-body stabilizers include the quadriceps, sartorius and tensor fasciae latae.

At the tail end of my career, I trained with a bunch of MLB players at Pepperdine University in Malibu. While we did traditional power lifts, hit in the cage, threw, etc., we used planks as our kickass finisher.

The idea was to re-tighten the tiny muscles of the spine after all the rotation from swings and throws. For a baseball player, the core is essential to health. For a weekend warrior or someone trying to be fit, it can help alleviate or eliminate back pain. From Harvard Health Publications:

No matter where motion starts, it ripples upward and downward to adjoining links of the chain. Thus, weak or inflexible core muscles can impair how well your arms and legs function. And that saps power from many of the moves you make. Properly building up your core cranks up the power. A strong core also enhances balance and stability. Thus, it can help prevent falls and injuries during sports or other activities. In fact, a strong, flexible core underpins almost everything you do.

Here’s an example of what your plank will look like:

And here’s a side plank:

Twice a week, I suggest starting with three sets of both regular and side planks. Begin with 15 seconds for each set. Every week, add 5-10 seconds. When you’ve reached 2 minutes of good, clean form on each, you’ll know your core is becoming strong.

Let me know how it goes,

Kap