In this article I am going to try and explain rotational power in the simplest of terms.
Most articles from strength and conditioning experts are written in terms of linear force and power. Louie Simmons has made the force equation the most popular of all biomechanical equations: Force = Mass x Acceleration. Fly by night strength coaches use this equation (incorrectly most of the time) to explain everything, which is the main motivation for me to develop content explaining all the other elements of biomechanics that are also important.
Torque = Force x Moment Arm
A great starting point when discussing rotational biomechanics is to explain torque. Torque is the ability of a force to cause rotation on a lever.
Torque is the rotational cousin of the force equation. Torque is the driving force for human movement. Muscles in conjunction with bones, ligaments, and tendons are responsible for movement. Muscles shorten, causing the tendons (a tendon is a flexible but inelastic cord of strong fibrous collagen tissue attaching a muscle to a bone) to pull on its corresponding bone. This creates a rotational movement around a joint.
An example would be the quad tendon that crosses the knee becoming continuous with the patellar tendon attaching to the tibia. When the quadriceps shorten, they pull on the quadriceps tendon, causing the patellar tendon to pull on the tibia producing knee extension. All locomotion is created by torque at the corresponding joints.
So even if you are in a sport like weightlifting or powerlifting which are linear in nature, you still have to understand torque and the elements of rotational power. Let’s take a look at the two components of torque: force and moment arm.
Moment Arm: The moment arm of a force system is the perpendicular distance from an axis or rotation to the line of action of a force.
Force: Torque is dependent on the amount of force, angle of application of force, and of course the moment arm.
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We are going to go over several scenarios where rotational power is important. Moving forward, it’s important all of you understand there are a few biomechanical terms that must always be considered in regards to rotational power. Let’s look at them up front, so you will understand the rest of the article more easily:
Length and size of Moment Arm or Moment of Inertia: When it comes to overcoming a resistance, the length from the force to the joint trying to overcome the force is proportional in regards to difficulty. The farther away = more difficult to overcome.
However, when it comes to producing torque, a longer moment arm produces a larger rotational force aka torque. The farther down the tibia that the patellar tendon inserts will proportionally create a greater torque making it easier to overcome an external force. Another example is a longer bat will produce a greater torque when striking a baseball.
Moment of Inertia = ?m*r²: Simply put the moment of inertia arm being rotated during rotational power. In baseball it is the bat and the arm combined. When throwing a punch, it is your arm. During sprinting, it is your leg rotating at the hip. You get it.
With moment of inertia, you have to consider the overall weight aka the mass, but as you can see the length is even more important. I know that because it is squared. We will go over this a bit more – especially in the sprint recovery section. The technique in various sports will take moment of inertia into consideration in regards to increasing and decreasing angular velocity.
Relationship between Impulse and Momentum:
Σ Δt = I* Δω
Σ Δt = I*(ω f − ω i)
impulse = change in momentum
Σ = average net force acting on an object,
Δt = interval of time during which this force acts,
I = moment of Inertia of the object being accelerated,
ω f = final angular velocity of the object at the end of the time interval,
and ω i = initial angular velocity of the object at the beginning of the time interval.
Ok let’s put this in layman’s terms. The longer that I can apply force to something is directly proportional to the angular momentum that I can produce. Since the moment of inertia is a fixed amount, really what impulse is directly affecting is the angular velocity. The longer that I can apply a torque will proportionally increase the angular velocity of the object. That’s why an athlete’s range of motion and technique are so important.
Rate of Force Development (RFD): When it comes to sports outside of powerlifting and strongman, RFD trumps overall ability to produce force. If someone can back squat 227kg/500lb, they are at least producing a little over 2,225N. That’s a lot of force. However, now that we know true rotational power is formed from applying torque over a period of time, we know that only applying that 2,225N of force for a short time isn’t going to produce the angular momentum that we are after. Now if that athlete can recruit the motor units to produce 2,225N of force instantaneously, they have optimal range of motion, and solid technique, then you have a powerhouse.
RFD is King in Weightlifting!
Now if you are an athletic performance coach, I hope you are starting to see that how you train is more important than the exercises chosen. Specificity is key. If you want your athletes improving their rate of force development, velocity and intent are a big part of the equation. There is one more important point that I hope you are starting to understand:
Assessment and Mobility
Assessment is king!
If you are working with a baseball or softball player, you need to assess their scapula movement, shoulder ROM, thoracic spine ROM, hip mobility (especially internal/external rotation and abduction), rate of force development, angular velocity/momentum, and ability to produce overall force. It’s important that you understand how to assess mobility. It’s helpful to have something like GymAware to measure velocity, and force plates to measure force and RFD unilaterally.
You will also need to have a way to measure posture and the strength of his or her decelerators. If an athlete is powerful, mobile, and technically sound and yet hurt all the time, they are of no value to any team. Therefore another key to understand as athletic performance practitioners is this:
Availability is king!
I had a chance to work with Dr. Lawrence Gray, D.C. early in my career – and to date this was the best thing I could have done to improve as a practitioner. Up until this point, I had only focused on performance, mainly my own. He had been my go-to sports medicine doctor during my entire powerlifting career, which led to multiple world records and world championships. Learning assessment and treatment with him is a tool that every athlete I have worked with since has benefitted from. Personally, I wish the strength and conditioning industry would practice an apprenticeship format with part of the career path being time spent with a doctor like Dr. Gray.
Durability and availability will get you on a field of play quicker than any other attribute. The opposite will get you a quick exit from the sport. Rotational sports are riddled with injury. If you want to look like an expert, spend some time understanding deceleration, posture, and correction, then you will immediately be in the top 1% in my experience. That’s why so many high profile athletic performance practitioners seem to be lacking in their ability to increase power and strength. You don’t have to be that good at those things if you can keep athletes healthy and on the field. However, if you can do both, then you become invaluable.
Here are a few simple videos that you can use to start your assessment game:
https://youtu.be/OwsMB1B6-kE (Forward head)
https://youtu.be/fb7gOr0edwM (Shoulder Internal Rotation)
https://youtu.be/tgim4WWeAy0 (Rounded Back aka Kyphosis)
https://youtu.be/mi1dPRlmRoA (Anterior Pelvic Tilt)
Now let’s put a few of these principles to action!
Pitching a Baseball: Why do you think that the majority of pitchers are long and lanky? Remember, Impulse is the ability to produce a force over a period of time. The longer that you can produce a force will lead to greater velocities on the baseball. That’s why the wind up and delivery are so important. Check out this video:
After watching that video, you will understand my little section on assessment. By the way, my next article is going to be all about assessment, so get ready. The video shows the importance of ROM on impulse and torque. It also shows the importance of force development – and yes, the importance of being strong. Hopefully you are starting to understand the importance of traits like ground contact time, true plyometrics, and velocity. That’s why my relationship with GymAware is so important. (Use Code: ‘MASH5’ to get a 5% discount on either of their products)
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Lifting Technique: When it comes to lifting weights, you have to consider the external torque you will be overcoming. You will also need to consider the internal torque your body is capable of producing at the individual joints. When it comes to the external torques on the body, there are three important points to consider:
- The force produced by the mass of the object you are lifting and gravity
- The direction of that force which is always vertical in weightlifting and powerlifting
- The perpendicular distance from that vertical line of force and the axis of rotation of the joint being considered
This is why it’s so important to keep joints resisting the external load as close to the line of action as possible. If you have ever performed a clean or deadlift, then you probably know how much heavier the barbell seems after allowing your butt to fly up faster than your shoulders. This biomechanical mistake in weightlifting or powerlifting increases the demand at the hip and any intervertebral joint of the back. Here’s a video that will clarify: https://youtu.be/2wyAVMt3ng4
Leg Recovery During Sprinting Mechanics: I hear coaches debate sprinting about as often as I hear coaches debate lifting technique in weightlifting. They talk about the start position, use of blocks, shin angle during the acceleration phase, arm action, and so much more. One of the big keys that I believe to be low hanging fruit is action of the leg during the recovery phase. I am talking about what happens when the foot has struck the ground propelling the body through the air, and the active leg has complete hip, ankle, and knee extension behind the body.
Now it’s time to recover the leg and start the process all over again. A major key is to shorten that moment of inertia as much as possible. If you watch the Olympic level sprinters, you will see their foot brush their butt shortening the moment of inertia as much as possible. This action will maximally limit the resistance at the hip, which now we know will increase velocity. During angular momentum, angular velocity is inversely proportional to the moment of inertia meaning the moment of inertia goes up and velocity goes down and vice versa. Here’s an image to further clarify:
Image Courtesy of Spikes Only
I hope that I have made rotational power a bit simpler for all of you. As athletic performance professionals, we really do have the opportunity to make huge impacts if we understand a few principles in biomechanics. If we perfect our abilities to assess, we can find low hanging fruit that can have massive impact with little stress on the body. The key is putting a little time up front understanding these principles – and then like anything else practice, practice, and practice some more. The cool thing is that once you grasp the concept of biomechanics, all of it becomes proportionally easier. It becomes a game much like a puzzle, but this game will lead to gains.