Training loads were observed to be protective against muscle injury.

Dear Bijan,

I strongly recommend reading these articles from the International Olympic Committee

https://bjsm.bmj.com/content/50/17/1030

https://bjsm.bmj.com/content/50/17/1043

I also recommend the following paper:

https://bjsm.bmj.com/content/50/5/273

The global message could be: Appropriate levels of internal and external training loads optimise performance, while 'too much' or 'too little' loads increase the risk of injury and reduce performance.

Bijan Rajaeian

In general, high intensity of internal and external loads during training increases the risk of injury for athletes of all levels, ages, and sports. Internal loads refer to heart rates and hematological measures in the body while external load refers to loads experienced by an athlete's body during performances such as weight lifting and sprinting.

Having said that, this is true for an isolated training session. But during training, an athlete can gradually increase his/her load-bearing capacities and it has been proven to improve their injury prevention capabilities. Studies have found that an appropriately graded prescription of high training loads can actually improve a player's fitness and protect him/her against injury. I think this is because our musculoskeletal system adapts itself to the gradual load applied and achieves the ability to sustain higher loads during a sport.

If you are asking about the relationship between internal and external load for athletes, I found an article about that -

Article The Relationships between Internal and External Load Measure...

This relates heart rate-derived measures with accelerations and distance traveled in American football athletes. Hope this helps!