Running on packed sand or running in deep sand, both improve your pump and your bellows.
Steve a triathlete / biathlete asks: It is difficult for me to continue training on concrete and pavement. As a result, I try to train on sand or grass. Should I run on soft sand or hard pack? Are there shoes I can buy that would help?
You only need to run about 10 % of your mileage on asphalt to practice running form on the hard surface, ready for racing. Try your 4 miles of tempo running on the road once a week; avoid concrete at all cost because it's about six times harder than tarmac or normal road. You title your question "knees and legs". I suspect you've got aches from increasing your training too fast, or doing too much high intensity running on hard surfaces. Sand, mud, dirt, grass and trails are excellent training surfaces. They force you to run slower for the same heartrate, giving you the main benefit of altitude training. i.e. lower risk of injury...high running intensity is the second best predictor of injury. Remember that half the purpose of 80 % of your running is to develop a big pump and to maximize your bellows. The heart and lungs don't care if you are swimming, biking, or running at 10 minute miles in 6 inches of mud. However, your running and biking muscles do need some training at 1-2 minutes per mile (for running) slower than race pace, at race pace and also at faster than race pace The trouble with deep sand is that it gets in your shoes. It can also mess with your running form. But oh boy, the advantages: It gives you a tough workout with very low mileage; Your back and shoulders get a workout because you have to maintain balance; You can wear very old running shoes...you are only protecting those feet from an occasional stone or cutting object. Packed sand can be so packed that it has minimal give. Mostly though, you'll see your shoe imprints and you can run with your very best economical form. Soft or packed, make sure the slope, or camber is minimal. The low tide bar and the high tide bar are usually the best areas, but many yards shoreside can be good for really deep sand. Sand dunes will give your quads a great test of course. On steady run days, try a mixture of surfaces. For quality days, deep sand is for strength or resistance training. It could include bounding for 20 seconds at a time, or reps of 1-5 minutes at 2 mile to 15K intensity depending upon which training phase you are in. Packed sand can also be used for long reps at 15K effort, but is more typically used for VO2 maximizing sessions at 5K to 2 mile race pace with 1-3 minute efforts, or however long it takes you to run 400-1,000 meters on the track. As to shoes, you don't need anything special for sand or grass. Wet grass would be best done wearing non-road racing shoes because you'll need some decent gripping from the soles. Trail shoes can be very specialized. Few runners use trails rough enough to warrant the expense.
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