To answer that question in one sentence, the position in which you are in while wearing the ankle weights would have to be known, because the effects of ankle weights will be very different in varying positions. You can use the ankle weights to anchor your feet during sit-ups, or raise them in the air to do isometric movements. You can even do leg raises with them. This is why it is important to know what position you plan on being in when using ankle weights.
Feet Grounded Option
Depending on the weight of the ankle weights, they could keep your feet down rather than seeing them flail in the air every time you attempt a sit-up or crunch. They can also keep you from needing to ask another person to hold your feet during every set. If the ankle weights are substantially heavy, like 20 lbs. each, they would be useful in keeping your feet down while you perform crunches or sit-ups with additional weight on your chest, over your head, or behind your head.
Feet Elevated Option
If you really want to mix it up, occasionally do crunches with your ankle-weighted feet elevated, rather than always having your feet grounded. Keeping your feet elevated while crunching will substantially work your lower abs and hip flexors. Because gravity will be willing your heavy feet to fall to the ground the act of resisting gravity's temptations will put your lower abdominals in a state of intense contraction. The constant tension of holding your weighted feet up isometrically is more focused and can often be more beneficial than moving them up and down for reps.
Think Outside the Box
Crunches are great, but you can get more creative with your ankle weights. Once you've smoked your abs with all the crunches, take a few minutes to recover, then consider some weighted leg lifts from different angles. If you like laying on the ground, keep your entire back -- especially your lower back -- pressed against the floor and raise your legs repeatedly until your form starts to break down and stop. Then repeat for a few sets. You can also do these hanging from a pull-up bar. Bend your legs if holding them straight is too challenging.
More Than Meets the Eye
Using ankle weights for less reps can be more beneficial than simply doing more repetitions without added weight. When you add weight to an exercise, you force the muscles involved to actually perform more volume of work over time due to heavier work loads, and therefore experience better results. Even walking with ankle weights on can be great for your legs, hips and core. The traction decompresses joints that generally never get a break.
- Davis KG, Marras WS.: The Effects of Motion on Trunk Biomechanics.
- Teyhen DS, Williamson JN, Carlson NH, Suttles ST, O'Laughlin SJ, Whittaker JL, Goffar SL, Childs JD.: Ultrasound Characteristics of the Deep Abdominal Muscles During the Active Straight Leg Raise Test. Arch Phys Med Rehabil. 2009 May;90(5):761-7. doi: 10.1016/j.apmr.2008.11.011.
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