How To Physics Your ACL Tear

I recently started shadowing a doctor at Hamilton Orthopedics.
Although I have seen a large handful of middle aged people with arthritis
in their knees and hips, my greatest field of interest is athletic injuries.
Playing soccer, basketball and running track in high school and sitting
in front of the TV watching UEFA Champions League matches and Knicks
games with my brother and my dad, sports have played a huge role in my
life since a very young age. Fortunately, I haven’t had any injuries requiring
an operation and a long recovery. Although, I’ve still had the typical shin splints
and sprained fingers. In competitive sports, played at a high level, injuries are
inevitable, many of which occur at joints. Our skeletal muscles exert forces through our joints.
The seven forces that act at the knee joint are the tibiofemoral joint force, the patellar tendon force,
the hamstring muscle force, the gastrocnemius muscle force, the anterior shear force,
the posterior shear force and the ground reaction force. 

It’s when there is a force greater than the forces from normal activities
that ligaments, tendons and bones can be injured.
One of the most common knee injuries is an ACL (anterior cruciate ligament) tear.
ACL tears tend to occur when there is a rapid deceleration (velocity and
acceleration in opposite directions) and low velocity in one’s movement,
therefore also decreasing one’s momentum.
An example of this is when one suddenly stops or changes directions in sports.

https://youtu.be/jo1UlzDeY-o

When one tears their ACL, the forces that break the ligament are greater than the ligament forces. 

The forces that break the ligament include:

1. A large anterior shear force - A large anterior shear force on the ACL tends to occur
when there is hyperextension of the leg. 

2. A ground reaction force (GRF)-The ground reaction force can contribute to the
ACL injury of an athlete when there is ligament dominance. This means that the
quadricep and hamstring muscles are not able to absorb the GRF like they should
and so the knee ligament ends up absorbing the GRF. An example of this is when
an athlete plants their foot or jumps.

3. An internal rotation force - The adduction internal rotation of the hip is a force moving clockwise
along the femur and the tibial rotation is a force moving counterclockwise along the tibia.
Therefore, these rotational forces are moving in opposite directions. 

That’s why strengthening exercises, such as squats and lunges are often critical

in preventing injuries to the ACL. The shorter time given to decelerate results in these

greater impulse forces, and all these forces can work individually or together to place

a large amount of stress on the knee resulting in injury. So if you want to take

some preventative measures, you can start by hitting the gym and improving strength,

flexibility and balance to decrease the likelihood of the ligament breaking forces acting

on the knee. Working on properly executing jumping movements, sudden stopping movements

and changing direction movements is very important for reducing your risk of an ACL tear.