The big toe joint. Also known as the first metatarsophalangeal joint (MTPJ). It's not something most of us ever really think about.

However, if you're someone who suffers from knee pain, hip pain, lower back pain or tight calf muscles, maybe you should be paying a little more attention to it. This is especially true in runners and even those who walk any distance regularly, for exercise or not. So, really, most people.

What I'm really referring to here is the ability of the first MTPJ to extend — that is, bend backward. A normal range of motion into extension for this joint is around 60 degrees. This amount of movement is required to allow a normal, functional and efficient gait cycle.

As we know, the body is a clever machine. If movement is not available or is painful in one joint, it will find another way of moving to avoid the problem. While this is often fine and dandy for a little while, eventually the altered gait will result in pain elsewhere.

So what happens if you don't have enough first MTPJ extension? There are a few possibilities, but the most common compensations include:

The medial heel whip

When you can't bend the big toe back enough the foot may turn to abduction to get by. What this means is that the foot spins slightly on the ball, so that the heel moves towards the other foot and the toes point out slightly. We then push off on the inside of the foot, rather than straight forward. This can cause a variety of problems, from bunions to SI joint-related lower back pain.

As we know, the body is a chain. An increase in movement in one area translates up the chain (leg) to the next area. An increase in abduction in the foot results in increased external rotation of the lower leg.

The knee tries to correct this resulting in shearing forces across the joint. If no luck here, the increased rotation continues up to the hip joint. If you have enough internal rotation range of motion here, the hip joint may correct the faulty movement from below. If not, the lumbar spine and SI joint pay the price.

Reduced hip extension

A lack of big toe extension can also dramatically reduce your hip extension (ability to move your whole leg backward behind the hip joint). A good range of hip extension is vital to running efficiency as it fires up the hip flexors to then spring into life and propel us forward.

Hip extension occurs in the second half of the stance phase, where the body is moving forward, over the standing leg. Right at the end of stance phase is when both the big toe joint and the hip joint should be maximally extended. If one or the other is restricted, then the stride length is shortened. Try it out yourself:

Stand with a wide stance, left leg in front. Shift your weight on to your left leg as if running, lifting the heel of the back leg up, so only the toes remain on the floor — as happens at the end of stance phase. Pay attention to the range of motion at both the hip and the first MTPJ at the most extended point. Now try to lift your back leg forward, as if going to take your next step. Notice how easy it is to transfer the weight and move forward.

Now try it again, this time imagining your big toe will only extend to half its full range. Notice how your hip can't extend so far back? Again try to lift the back leg and move it forward. It should feel more difficult to do. Notice how you have to use the hip flexors more to achieve this.

The biggest problem with reduced hip extension is the increased requirements on the hip flexors to propel you forward. This soon leads to overworked, tight and tired muscles which increase the anterior pelvic tilt and potentially inhibit the gluteus maximus. This leads to a whole other chain of issues.

A normal range of motion into extension for first metatarsophalangeal joint (MTPJ) is around 60 degrees.

Assessment

So how do you know if you have enough big toe extension? The easiest and most appropriate way to have a look for yourself is in standing. You need to be weight bearing to get a more accurate picture of your range when running or walking.

In a standing position, simply bend over (or get someone else to do it) and physically pull your big toe back with your fingers, as far as it will comfortably go.

Imagine that the position when the toes are rested on the floor is zero degrees. You should be able to achieve at least 60 degrees of movement into that extended position. That's two-thirds of the way to a right angle if that helps you visualize it. The most accurate way of assessing this range is using a goniometer.

Correction

Correction of a hallux limitus (official diagnosis of limited big toe joint extension) depends on the cause of the restriction. A hallux limitus is classed as either functional or structural.

In functional cases, there is usually no pain in passively forcing the big toe into extension, and the range increases in a nonweight-bearing position compared to standing. This is good news because it is easier to fix.

The cause may be due to tension in the flexor hallucis longus muscle found in the back of the calf, which passes a tendon under the sole of the foot, to the big toe. Its job is to flex the first MTPJ, and therefore it stretches when we extend the joint. Tension in this muscle can reduce extension range of motion. This is the easiest cause to fix, as often some soft-tissue work, stretching and self-mobilization exercises will do the trick.

The cause may also be something known as "jamming" between the head of the first metatarsal and the proximal phalange of the big toe. When extending the joint, these two bones should slide against each other nicely, with the phalange moving upward as the metatarsal moves downward.

There are a number of reasons that are thought to be able to contribute to this problem, and it would take a skilled podiatrist to evaluate the foot to determine the exact cause in each case. The treatment here may be mobilizations to encourage more movement between these two bones, or the use of orthotics to encourage greater flexion of the first metatarsal.

In cases of structural hallux limitus, the big toe is not extending sufficiently due to problems in the joint. If this is the case, the restriction will be present equally when weight bearing and nonweight bearing and may be painful.

This condition is usually degenerative and a result of repeated jamming of the first MTPJ, which causes damage to and eventual degeneration of the cartilage between the two bones. Extra bony growths may also develop around the joint due to changes in the forces around the area.

If suspected, an X-ray can confirm the diagnosis. If left untreated, the condition can progress to become a hallux rigidus — where the MTPJ doesn't extend at all. Surgery is usually the best course of action for a structural hallux limitus.

Hopefully, now you have a clearer idea of why big toe joint extension is so important within the gait cycle and can see how so many injuries could be at least in part due to a lack of extension at this joint.