Introduction
Welcome to the first lesson in our module on Biomechanics for Hypertrophy Training. In the previous module, we concluded with a powerful principle: training a muscle in its lengthened position is a potent stimulus for growth, a phenomenon we called stretch-mediated hypertrophy. We established that challenging a muscle in a stretched state is important, but we haven't yet discussed how to quantify that challenge.
Today, we will address that by exploring a foundational concept in biomechanics. The learning outcome for this lesson is to define moment arm and explain how it determines the torque a muscle must produce against a given load.
By the end of this lesson, you will understand the physics of why an exercise feels harder at certain points and easier at others. This is the first step toward analyzing movements with a coach's eye, allowing you to see beyond the weight on the bar and understand the true mechanical challenge imposed on a muscle.
1. From Force to Torque: The Language of Rotation
In resistance training, our muscles don't just produce linear force; they create rotation around our joints. The term for a rotational force is torque. Understanding torque is non-negotiable for understanding how muscles are loaded during exercise.
Torque is the product of two things: the force being applied and the distance from the point of force application to the pivot point (the joint). This distance is called the moment arm.
The equation is simple but profoundly important:
To put it simply, the moment arm is the leverage that a force has to cause rotation. A longer moment arm means more leverage, and therefore more torque for the same amount of force.
Now, let's hear this concept explained in a very direct, coaching-focused context.
Understanding Squat Mechanics, Torque, Moment Arms - Nerd Stuff with Hypertrophy Coach Joe Bennett
The video 'Understanding Squat Mechanics, Torque, Moment Arms' by Hypertrophy Coach Joe Bennett provides a clear, practical definition of torque and its relevance to training.
Watch the segment from 02:55 to 04:00. Pay close attention to the formula provided and the key takeaway statement about what muscles do.
As Joe Bennett states, the most accurate description of a muscle's function during exercise is that muscles manage torque at joints. They produce an internal torque to counteract the external torque created by the weight you are lifting. This is the central concept for today's lesson.
2. The Two Moment Arms: Internal vs. External
The torque equation applies to both the weight you are lifting and the muscle that is lifting it. This means we have to consider two different moment arms in every movement:
- External Moment Arm: The perpendicular distance from the joint (fulcrum) to the line of action of the external resistance (e.g., the dumbbell). This determines the demand or load torque placed on the joint.
- Internal Moment Arm: The perpendicular distance from the joint (fulcrum) to the line of action of the muscle's pull (its tendon). This determines the muscle's leverage in producing its own torque to meet the demand.
This distinction is critical. Let's get a formal definition for each.
How can exercise strength curves affect hypertrophy?
The article 'How can exercise strength curves affect hypertrophy?' from S&C Research provides precise, distinct definitions for both internal and external moment arms.
Please read the entire text of this resource. Focus on the two key subsections: 'What determines our capacity for producing a turning force...': Pay attention to the definition of 'Internal moment arm length'. 'What determines the requirement for a turning force...': Pay attention to the definition of 'External moment arm length'.
To visualize this, look at the following image of a bicep curl.
This image reveals a fundamental truth about the human body: our muscles almost always operate with a very short internal moment arm compared to the external moment arm of the weight we are lifting. This is known as a mechanical disadvantage.
What does this mean? Look back at the torque equation. If your muscle's moment arm is very short, how much force must it generate to create the same amount of torque as the external weight? The answer is: a much, much larger force.
Biomechanics and Muscle Leverage | CSCS Chapter 2
Dr. Jacob Goodin explains this concept of mechanical disadvantage clearly in his video on biomechanics.
First, watch from 08:09 to 09:06 to understand how the ratio of moment arms determines mechanical advantage or disadvantage. Then, watch from 16:39 to 17:55 for the key summary point about how this applies to the human body and its implications.
The key takeaway is that the internal forces on our muscles and tendons are many times higher than the actual weight we are lifting. Lifting a 20 kg dumbbell might require your biceps to generate forces equivalent to 100 kg or more, all because of its poor leverage. This has significant implications for understanding stimulus, fatigue, and injury risk.
3. Moment Arms in Motion: Why Exercise Difficulty Changes
The moment arms are not fixed; they change continuously as you move through an exercise's range of motion. This is why a lift feels harder at some points than others.
Let's analyze this in the context of a squat.
Understanding Squat Mechanics, Torque, Moment Arms - Nerd Stuff with Hypertrophy Coach Joe Bennett
We'll return to the Hypertrophy Coach video, as it brilliantly illustrates how the external moment arm changes during a squat and why that matters.
Watch the following segments: 01:33 to 02:55: See how the moment arms at the hip and knee are identified. 04:00 to 05:27: Understand why the moment arm is just as important as the weight on the bar. 05:27 to 06:42: See the practical result—how torque increases dramatically as you squat deeper, making the bottom of the lift the hardest part.
As the video demonstrates, when you perform a squat, the horizontal distance from the barbell to your knee and hip joints increases as you descend. This lengthens the external moment arm, which in turn increases the torque your quads and glutes must produce to overcome the load.
The torque is maximal at the bottom of the squat, where the external moment arms are longest. This is precisely the position where the muscles are most lengthened. This convergence is key: we achieve a high-stretch, high-tension environment at the bottom of a squat because the biomechanics of the movement create the largest torque demand in that lengthened position.
This dynamic interplay occurs in nearly every free-weight exercise. The bicep curl is another classic example, where the external moment arm is longest—and the torque demand is highest—when your forearm is parallel to the floor (a 90-degree elbow angle).
Conclusion
In this lesson, you have moved from a simple understanding of "lifting weights" to a more precise, physics-based model of "managing torque." This framework is essential for the advanced analysis required of an expert coach.
Key Takeaways:
- Torque is the rotational force muscles produce to move joints, calculated as Force × Moment Arm.
- The moment arm is the perpendicular distance from a joint to the line of force, representing the force's leverage.
- We must consider two moment arms: the external (from the weight) which creates the demand, and the internal (from the muscle) which meets that demand.
- Most human muscles operate at a mechanical disadvantage (short internal moment arm), meaning they must generate forces far greater than the external load.
- The external moment arm changes throughout an exercise, causing the torque demand to vary. This explains why certain parts of a lift are harder than others.
Preview for the Next Lesson
Today, we focused on the demand side of the equation: the torque that an exercise requires you to produce. But this is only half the picture. The other half is a muscle's capacity to produce torque, which also changes depending on its length.
In our next lesson, we will explore this other half by analyzing the strength curve of a muscle. You will learn to analyze the interaction between an exercise's resistance profile (how its torque demand changes) and a muscle's strength curve to optimize the stimulus for hypertrophy.