Eccentric strength exercises are a great way to build tendon strength. This is particularly important for the Achilles tendon which absorbs an enormous load while running. Eccentric training requires the use of heavy loads. Athletes need to start with manageable weights and gradually build up over time to allow the tendons to get stronger. 

Not only does it take heavy loads but also a fairly high volume to build up the tendons. Most athletes should build up to 3×15 reps with a 3-second lower eccentric of the heel. Elite-level athletes may need up to 4×80 repetitions. Start with 3 sets of 5 reps adding 2 reps per week until you reach 15 reps per set for 3 sets. Then begin gradually increasing the load by 2 1/2 to 5 pounds per week for 3×15. The goal will be close to achieving body weight on the bar. Be patient as this will take time.