Hip Biomechanics: First-Class Lever & Single-Leg Standing

The hip joint acts as a first-class lever: the joint itself is the fulcrum, muscle force is the effort, and body weight is the load.

During single-leg standing in mid-stance, hip joint force normally averages three times body weight, rising to four times body weight when the affected leg is longer, increasing osteoarthritis risk.

In this post, we’ll discuss the biomechanics of the hip joint. In the previous episode, we covered the kinematics of the hip joint. The biomechanics of the hip joint is quite simple. I’ll try to explain this topic to you in a very simple way so that you’ll have complete clarity on the concept.

What topics will we cover?

We’ll cover single-leg standing,
Single-leg standing with the affected side long,
Single-leg standing with the shorter affected side leg,
The effect of the cane by lever approach.

So, let’s get started.

📋 Table of Contents

Understanding the hip by lever approach
Single-leg standing
Single-leg standing with affected side longer
Standing on the affected shorter leg
FAQ

Understanding the hip by lever approach

The hip joint is a first-class lever. You know what a first-class lever is? I’ve already discussed it in detail in one of my posts. However, let me brush up here.

A lever consists of a fulcrum, a weight, and an effort. When the fulcrum is between the weight and the effort, it’s called a first-class lever. So, our hip joint is also a first-class lever.

Let me explain it to you with this diagram.

Fulcrum: Our hip joint is our fulcrum.
Effort: For any motion at the hip joint, let’s say abduction motion, this muscle pull is necessary. In the diagram, you can see the muscle force that is exerting its effort at the insertion point on the pelvis bone.
Weight: Then, our body weight, which passes through our spine, this body weight is our weight of the lever.

If I join the effort with the fulcrum, and join the fulcrum with the weight, as you can see in the figure, the fulcrum lies between the effort and the weight. That’s why we call it a first-class lever.

So, first, let’s try to understand how this concept affects our hips in single-leg standing.

Single-leg standing

You can see the diagram of the hip and first-class lever. The muscle force (MF) designates the effort. The body weight passing through the spine is designated as BW.

We connect body weight with the fulcrum, which becomes the weight arm. We also connect effort and fulcrum, so this becomes our effort arm.

So, the equation of mechanical advantage becomes:

MA = EA X WA

As we know, weight arm means the distance of the weight from the fulcrum. So, let’s find out the mechanical advantage using the equation. So, you can see here that our effort arm is smaller than the weight arm.

That is, if I put this mathematically, weight arm equals 2 times effort arm.

WA = 2EA

So, if we put this into the formula:

MA = EA X 2EA
MF = 2 × BW

So, here, effort arm and effort arm cancel out, so muscle force is equal to body weight, two times body weight.

So, what does this mean? What do we understand by this?
We understand that when we stand in single-leg standing, our muscles have to act with twice the force of our body weight to stabilize our posture. When we are in the midstance phase of the gait cycle, our entire body weight will be on a single leg.

So, during stance phase there are two forces acting simultaneously on the hip joint.

The muscle force, and
the body weight.

Using this formula, we now know that muscle force is two times body weight. So, we add another amount of body weight, making it three times body weight.

This means that during mid-stance, three times body weight pressure is exerted on our hip joint. This often increases the risk of arthritis in our hip joint. Knee alignment problems such as bow legs (genu varum) can similarly alter lower-limb loading patterns.

The next condition is when we stand in single-leg standing, but the affected side is longer.

Single-leg standing with affected side longer

hip joint biomechanics of standing on affected side longer

This means that there is a leg length discrepancy. The affected leg is slightly longer than the normal leg. This is seen in congenital conditions like coxa valga. It can also occur in other conditions.
For example, suppose you have had an accident or suffered a fracture. Even then, you may still see limb length discrepancy.

So, let’s say the leg we’re standing on is longer than a normal leg. So, how can we explain its biomechanics and mechanical advantage?

Let’s start.

You can see in the diagram the fulcrum, the muscle force and the effort. When standing on the longer affected leg, the centre of gravity will shift slightly towards the normal side. Usually, in the normal standing position, it passes through the centre.

This makes the length of your weight arm more than the effort arm.
But, in the present situation, the weight arm becomes three times the effort arm.

So, let me explain this by putting it into a formula.

Muscle force multiplied by the effort arm is equal to the weight arm multiplied by the body weight. So, what happens here is that the weight arm is three times the effort arm, okay.

Let’s put it into this formula. Muscle force multiplied by the effort arm equals three times the effort arm times the body weight. So, here the effort arm will cancel out. So, here the muscle force equals three times the body weight.

MF × EA = BW × WA
WA = 3 × EA
MF × EA = BW × (3 × EA)
MF = 3 × BW

This means that to stabilise this position, muscle force has to exert three times the body weight to maintain this position. So, what happens in single-leg standing is that the pressure on our hip joint in the mid-stance phase becomes muscle force plus body weight. Right?

Our muscle force here becomes three times body weight plus body weight. So, that becomes four times body weight.

This means that if the affected side is longer, then in the mid-stance phase, our hip joint experiences four times the body weight pressure. So, this also becomes a reason for the development of osteoarthritis.

So, in the third condition, the affected side is shorter.

Standing on the affected shorter leg

hip joint biomechanics of standing on affected side shorted

In the diagram, you can see the fulcrum, muscle force, and effort on the hip joint when standing on the shorter leg.

In this condition, the centre of gravity will shift a little to the affected side. This will shorten the length of the weight arm.

So let’s put this into the formula again.

Muscle force multiplied by effort arm equals weight arm times body weight. Weight arm equals two times effort arm, or that’s less than twice the effort arm. Let’s plug this into the formula.

Muscle force, effort arm equals two-times arm. And here, body weight will be the f term. So, muscle force is equal to or less than twice body weight.

MF × EA = BW × WA
WA ≤ 2 × EA
MF × EA = BW × (2 × EA)
MF ≤ 2 × BW

So, the effect of this on the hip joint in the mid-stride will be pressure that will be equal to or less than three body weights, meaning when we stand on the shorter side, and in the mid-stride, the pressure on the hip joint on our shorter side is less. That’s less than three body weights.

We just learned that in normal single-leg standing, the pressure on the hip joint is equivalent to three times body weight. But in the current situation, it is less than three times body weight. This means the force exerted on the hip joint is less.

But one thing needs to be noted here: this is about the mid-stance. In the mid-stance, there is less pressure on the hip joint than three times body weight.

But when we come to the static position, when we stand on the shorter side in the static position, then there is more pressure on it.

FAQ

What is a lever in hip biomechanics?

In biomechanics, a lever is a rigid structure (such as a bone) that rotates around a fixed point called a fulcrum under the action of forces. In the hip, the pelvis and femur act as the lever, the hip joint is the fulcrum, the muscle pull is the effort, and the body weight transmitted through the spine is the load. This arrangement makes the hip behave like a first-class lever, where the fulcrum lies between the effort and the load.

Why is the hip considered a first-class lever?

A first-class lever is defined by the fulcrum being located between the effort and the load. At the hip, the joint centre acts as the fulcrum, the abductor muscle force (effort) acts laterally on the pelvis, and the body weight (load) passes medially through the spine. When you connect the effort to the fulcrum and the fulcrum to the load, the hip clearly fits the first-class lever pattern.

What is the axis of movement at the hip in single-leg standing?

In single-leg standing, the frontal plane abduction–adduction movement at the hip occurs around an anteroposterior axis passing through the hip joint centre. This axis acts as the pivot about which the pelvis tilts when body weight and muscle forces create moments on either side of the joint.

Why does hip joint force reach about three times body weight in normal single-leg standing?

When you stand on one leg in mid-stance, the body weight creates a moment around the hip joint that must be balanced by the abductor muscles. Because the weight arm is approximately twice the effort arm, the muscle force needs to be about twice the body weight to maintain equilibrium. The joint then experiences both the muscle force and the body weight simultaneously, resulting in a total hip joint reaction force close to three times body weight.

How is mechanical advantage (MA) used to explain hip joint forces?

Mechanical advantage in this context compares the effort arm and weight arm around the hip joint. When the weight arm is longer than the effort arm, the muscles must produce a proportionally greater force to balance the lever. By relating muscle force, body weight, and the respective arm lengths, you can estimate how many times body weight the hip joint experiences in different standing and gait scenarios.

Sunit. S. Ekka

The author is a physiotherapist who has been practising for the last 17 years. He holds a Bachelor's in Physiotherapy (BPT) from SVNIRTAR (Swami Vivekananda National Institute of Rehabilitation and Research), one of the prestigious physiotherapy schools in India.

Whatever he learns dealing with his patient, he shares it with the world through blogs and e-books. He also owns a YouTube channel, "Sunit Physiotherapist" with over 8 lakh active subscribers. Here, he shares everything he gets to learn serving the patient.