Bulletproofing The Volleyball Knee

by Greg Dea

Previously, when I wrote about bulletproofing the volleyball shoulder, I stated that what is common in volleyball is not necessarily normal, or good.

Here we find that the same message applies to how volleyball training and competition affect the knee. Just like at the shoulder, there are things that can be modified for the betterment of volleyball knees.

Attention Athletes and Coaches:
According to sports medicine research:

• Up to 43% of you will have jumper’s knee pain at any one point in time, and this may have been or will continue to be present for over two years.
• Your pain will be between 2 and 6 out of 10.
• About 7-10% of you will play with pain killing medication.
• About 20-40% of you will go into a game with knee pain of some sort in the previous week and that this may have been or will continue to be present for up to eight weeks.
• It’s likely that along with knee pain you’ll also have a shoulder or back injury at the same time.
• If you’re earning money as a professional athlete and miss competition due to a knee injury, you’ll be costing thousands of dollars to your team.

These are the common statistics for knee injuries in volleyball [1, 2].

This is how bad it could get

(Chinese outside hitter Yang Fang Xu injuring her right knee in Japan World Grand Prix, July, 2015.)

Injuries like those that happened to Yang Fang Xu are relatively rare, but clearly they can happen and there are warning signals—some of which are just noise.

The common statistics mentioned above are background noise and full of complex issues. From here in, I’m going to categorize the complexities of so many factors that relate to bulletproofing the volleyball knee so we can pay attention to the real signal amongst the noise—what can we modify?

Modifiable risk factors for future knee injury
Knee injuries in volleyball are related to one of three categories (Figure 1):

1. Intrinsic or internal factors:
   1. The aim of this article is to identify the signals amongst intrinsic factors and shut them down. They are those identifiable and modifiable movement pattern limitations or asymmetries or biomechanical impairments of the individual;
2. Extrinsic factors:
   1. Those environmental or extrinsic factors that might be controlled for but are less predictable;
3. Acute training load variations.

Figure 1 Risk factors for injury – Intrinsic movement or biomechanical factors, extrinsic environmental factors and training load factors

Any significant change in any one of these three can shift the risk of knee injury higher until it eventually crosses the threshold of risk, to manifest injury.

Looking further into those three categories, we see that the first and last one— intrinsic factors and training load—are most modifiable by those in the profession of movement coaching, physical therapy and strength and conditioning. These two areas have been extensively identified and further broken down to reveal six of the top modifiable risk factors for future injury.

In no particular order of importance, and with a non-exhaustive reference list, these are:

1. Significant changes in training load [3-8]
2. Pain [9-13]
3. Movement limitations [14-21]
4. Movement asymmetry [14-16, 22-26]
5. Altered motor control [14-16, 22, 23, 27-29]
6. Limited or asymmetrical motor capacity [10, 13, 15, 22, 23, 28, 30-34]

I hear the strength coaches tapping away on their devices now, about to say: “you’ve left out strength! Lack of strength is the biggest risk factor for injury! To bulletproof the volleyball knee you’ve just got to address strength!”
Well, first—it’s not a competition to be the biggest or most important when it comes to identifying risk factors. Second—I didn’t leave it out. If we consider that the last factor, limited motor capacity indeed includes limited outputs of any movement, i.e. limited strength, endurance, power or even limited “fitness,” we can cover all modifiable factors.

So, we know many things are modifiable. We know these “modifables” are training load issues, extrinsic factors or intrinsic factors. Amongst the intrinsic factors are pain, mobility and motor control limitations, asymmetries and capacities. These intrinsic factors we can simplify even more when it comes to what the knee has to do in volleyball.

The knee has to do three things in volleyball:

1. It has to produce force;
2. It has to absorb force; and
3. It has to reuse force very quickly when it has to rebound off the ground.

Let’s look at what we can glean from the scientists who’ve revealed some data. “Laboratory based kinematic and kinetic analyses have demonstrated the knee contributes 49-56% to a vertical FPT [functional performance test], but only 3.9% to a horizontal FPT.” [35]

The knee, then, clearly doesn’t just need to move well and be strong. It needs to move well and be strong differently between forwards, backwards and vertical movements. This is context-specific capability. Further, it doesn’t operate in isolation. If the knee only contributes approximately 4% to horizontal direction force, the other contributors are ankles and hips, right? With so much going on around the knee, we know that it is not inert to the forces that occur at the joints around it.

When something occurs at the hip, the force transfers down to the knee. Similarly, when something occurs at the foot and ankle, the force transfers back up to the knee. Indeed, it has been observed that “optimal timing of segmental performances,” i.e. the coordination of hip, knee and ankle, improves average take-off velocity in vertical jumps. [36] When we look further into biomechanics research, we see that:

• In vertical force production, the hip adds approximately 28% to the task and the ankle approximately 23%. [37]
• In vertical force absorption, we see from multiple studies that the ankle, knee and hip contribute 14-43%, 33-47% and 12-42% to the shock absorption respectively. [38]

This data tells us that our chosen or reactive movement patterns dictate how much force we produce. A dash forwards demands much more force from the hip and ankle than it does the knee. A vertical jump demands much more force from the knee. And then a landing could require all sorts of variations on how much each joint absorbs force. We’ve even seen that a run-back landing (that occurs with an unsuccessful block) puts more valgus stress on the knee than a stick or stick-to-controlled step back landing. [39] With so much variation in landing types, strategies, force distribution and joint angular velocities, we see that there needs to be adequate mobility to support reflexive control at each joint.

This research highlights that the knee has to produce, absorb and reuse force in four directions:

1. Horizontally,
2. Vertically,
3. Laterally, and
4. With rotation.

So, to identify what is modifiable, it makes sense to have movement evaluations that capture when a volleyball athlete has difficulty doing those things in those directions, initially at bodyweight and then above bodyweight with speed.

That is the topic of one and two day courses for professionals in strength and conditioning or physical therapy—courses on screening, assessing and testing movement biomechanics. For our needs, let’s jump ahead to some summaries of those evaluations.

It wouldn’t be a stretch of the imagination to guess that there are a few patterns that can reveal intrinsic limitations in movement, motor control and then capacity—the squat and lateral lunge, for example. Both of these demonstrate how the body requires mobility at key joints, the ankle, knee and hips to lower and raise the trunk vertically, like a coiled spring, and decelerate forces in a horizontal, lateral and rotary direction.

The Squat
The squat is the movement that absorbs and produces force vertically. In the element of training where planning and action is directed at developing capacity, loaded squats are a desirable component of training for its relationship to vertical jump height [40, 41] and subsequently acceleration capacity. [42] In the element of training where planning and action is directed at evaluating for weak links in mobility and motor control, the squat is a desirable component for its usefulness in observing such weak links in lower body triple flexion, lower trunk stability and upper quarter extension. The patterns can be similar or very different when loaded in training and unloaded in an evaluation such that loaded squats may hide motor control deficiencies.

The Lunge
The lunge is the movement that absorbs forces horizontally and laterally when decelerating. Given that most of volleyball occurs in accelerations horizontally, accelerations and decelerations laterally, and accelerations and decelerations vertically, the squat and lateral lunge hold most relevance—as exercises, as well as evaluations. One might even argue that a lunge is the same as a squat but with asymmetrical stance. Both require triple flexion at the hip, knee and ankle, with a stable foot and trunk below and above these three joints. This triple flexion doesn’t always occur in isolation—it occurs with rotations and lateral shearing. The rotary forces, coupled with lateral forces that occur in volleyball movements stress the functional envelope of the knee to a great extent, repeatedly. So much so that preparation demands nothing less than full movement at the hip and ankle.

An important element of the lunge over the squat is what happens in the rear leg. A person who lunges with a forward trunk lean either has limited mobility or poor control of trunk and pelvis position on the hip. Sound familiar? Two of the risk factors for future injury are limited mobility or limited control of mobility. If the trailing, rear, or down leg, with its hip extension and knee flexion combination, has limited mobility, leaning forward at the trunk is the only option. Spotting this dysfunction in a lunge leads us to evaluate mobility first via the Modified Thomas Test. A normal MTT sees the thigh parallel with the ground and knee flexed to 90 degrees. Deviations from that (lateral alignment of the thigh, higher than parallel thigh, knee flexed less than 90 degrees) indicate a tissue extensibility dysfunction of the lower anterior or lateral chain, or an anterior hip joint mobility dysfunction. With jumper’s knee being so prevalent in volleyball, [1] having limited mobility of the anterior lower chain sets the passive elastic tissues up for more strain.

Ankle plantarflexion seems to have been forgotten in the world of physical therapy. While there’s no doubt limitations in dorsiflexion are a significant risk factor for injury in volleyball players [21, 43] and must be addressed, the plantarflexion range is important for another reason, especially in jumping athletes. Restricting ankle plantarflexion has been shown to significantly increase the work done at the knee in vertical jump by approximately 50%. [44]
Key modifiables:

• Modified Thomas Test with thigh parallel and in alignment with trunk (not laterally aligned) with knee flexed to 90 degrees.
• Ankle plantarflexion should be 30-40 degrees.

Let’s get volleyball-mobility-specific. In my experience, the more common movements in volleyball that affect the knee that a) we should be aware of, and b) require optimal mobility, include:

The Lateral Lunge – The Lay-Down of The Tibia
The lay-down of the tibia occurs in the pass or dig. When a ball is received to the side, the player shifts their weight to that side, trailing a leg behind. To lower the body to receive the ball, the leading leg goes into triple flexion (hip, knee flexion and ankle dorsiflexion—like a one-side-biased single-leg squat) while the trailing leg goes into hip internal rotation, relative extension (from the base flexed position) and tibial external rotation. This trailing leg combination lays the shin down to the ground at a rapid rate.

The combination of hip internal rotation and flexion is a pattern that varies in range between individuals, but when we examine the hip we want to see a minimum range of motion of 30 degrees of internal rotation at 90 degrees of flexion with neutral hip abduction. An athlete who lays the tibia down flat will typically achieve more than 30 degrees of internal rotation but needs to gain hip abduction.

With such a high frequency of movements demanding tibial external rotation this needs to be offset by strategies to ensure tibial internal rotation patterns and hip external rotation patterns are not lost.

Key modifiables:

• Hip internal rotation mobility—minimum 30 degrees
• Tibio-femoral external rotation mobility—minimum 20 degrees, but expect up to 40 degrees.
• Hip external rotation mobility—minimum 40 degrees
• Tibio-femoral internal rotation mobility—minimum 20 degrees.

The Lateral Lunge—Leading Leg Triple Flexion.
The leading leg in a lateral lunge has to flex at the hip, knee and ankle. It’s optimal for all three to be aligned, to minimize shearing forces, lateral or rotary.

I mentioned above how maintaining hip and tibio-femoral rotation mobility can play a role in offsetting frequent hip internal rotation and external tibial rotation in the trailing leg.

If the leading leg has limited ankle dorsiflexion, the deep triple flexion of the leading leg mobility requirements will be made up elsewhere. For example, leading mid-foot pronation, tibial internal rotation, hip adduction/internal rotation—end result = valgus collapse and increased risk of knee injury. There’s little doubt that allowing valgus collapse with flexion can be a leading association with medial meniscus injuries. [45, 46]

Key modifiables:

• The ankle should be able to achieve 40 degrees or better of closed-chain dorsiflexion with the knee moving over the 3^rd
• The knee should have at least 150 degrees of flexion.
• The hip should have at least 120 degrees of flexion.

In this video, we can see how valgus collapse with flexion might be taught as an intentional strategy in clearing the leg from under the trunk when rolling after a dig. I think this is a teaching point that should be eradicated.

The Single-Leg Squat
The lateral lunge, with its triple flexion, is a supported single-leg squat. Having a trailing leg can hide competency on the triple flexion side. Checking the single-leg squat is required to check for competency of the pattern. Missing dysfunctions in a single-leg squat because a lateral lunge “looks ok” is like saying “your gait is fine because you don’t limp when you use a stick to help you.”

The pattern is deemed competent when the athlete achieves a stable, grounded foot, with the hips dropping below the knee and trunk parallel to the tibia, or better. Failure to achieve this position leads the coach or therapist to examine the parts (ankle, knee and hip) for appropriate flexion range of motion. The presence of full motion in non-weightbearing positions (see key modifiables in hip and tibio-femoral joint above) indicates that failure of the single-leg squat is not due to restricted movement, only limited control of movement. That requires progressive training, not more mobility.

I hear the pundits tapping away on their devices to comment how no one ever gets into the single-leg squat in volleyball. Maybe they do, maybe they don’t. The purpose of a test is not what it looks like but what it tells us. An incompetent single-leg squat tells us the athlete has either limited mobility or dysfunctional control of the mobility they have. As you’ll recall from the top of the article, two of the top risk factors for future injury are limited mobility and limited control of movement.

The Single-Leg Landing
Having a competent (aligned and criteria-met) single-leg squat at least tells us the mobility and control of triple flexion is present. The single-leg landing, however, is not a natural progression from single-leg squat. That’s because the single-leg landing shouldn’t get as low as a single-leg squat, and if the athlete is to reuse force to bounce back up again, the hip, knee and ankle joints shouldn’t flex more than 20 degrees. While it’s a contentious issue, the idea of muscle slack plays an important role here. Very few experts in the world would argue that Frans Bosch, Professor of motor learning and elite sprints and jumps coach, knows how to get the most out of his jumping athletes. He claims that a hip/knee/ankle flexion, on landing, of more than 20 degrees, indicates taking up of the slack of muscle tissue, with delayed reflexive isometric contraction. He claims that isometric contractions prepare the hip, knee and ankle and foot tendons to reuse elastic energy. [47] Beyond 20 degrees, the limb becomes a force absorber, not a re-user.

So, the single-leg landing becomes a task that reflects force absorption or force reuse at greater than bodyweight, with speed. This is control of movement capacity. It requires underlying mobility (see key modifiables above), a healthy nervous system unfettered by pain with reflexive control of mobility. Failure to provide minimum mobility, and control of that mobility, will show up in faulty and dangerous single-leg landings. Indeed, it has been shown that in double-leg landings in volleyball athletes, the ankle dorsiflexes between 41 and 58 degrees [38] – having less than 40 degrees of dorsiflexion will shift load absorption up the leg to the knee. What’s even more noticeable from this same study of volleyball athletes is the angular velocity that occurs at the ankle, knee and hip in double-leg landing—females have a much higher angular velocity at all three joints—the control of the change in joint position is faster or delayed in females than males. Combining fast joint movement, under 1.5-3.5 times bodyweight [38] with shearing forces is a recipe for problems.

Let’s not kid ourselves. The single-leg landing is perhaps the single most important movement in volleyball. Not because it wins games, but the catastrophic effects of doing it badly can lose games and careers. Earlier, we saw the trauma that occurs at the knee with a failed single-leg landing. You’ll note how little ankle dorsiflexion occurs on landing.

Without joint gliding, sliding and hinging, the mechanoreceptors in the ankle are deaf to the signals for reflexive control of single-leg landing. Without reflexive control, hip adduction/internal rotation ensues. With a fixed foot, valgus collapse at the knee is inevitable.

Don’t just take my word for it, recent research confirms that a significant reduction in ankle dorsiflexion (less than 17 degrees) leads to more hip adduction and internal rotation in a step down test. [43] The step down test is about force absorption on one leg, at bodyweight. You can then imagine that the ground reaction forces of landing from a height, on a limb with limited ankle dorsiflexion will cause an acceleration of hip adduction and internal rotation. Further, a hip that has limited internal rotation is a risk factor for anterior cruciate ligament injury. [48]

Now. let’s examine the fallacy that single-leg landing in volleyball is a poor strategy. It indeed is a perfectly inevitable and unpredictably appropriate strategy that occurs secondary to the upper quarter biomechanics.

If you pause the same video at 18 seconds, you’ll notice the angle of the trunk and shoulder— lateral flexion of the trunk away from the ball. We can hypothesize why this is the case, or I can tell you why. This athlete had a coexisting left shoulder pain, affecting shoulder elevation range of motion. If shoulder abduction and external rotation is painful, laterally flexing the trunk is the only option to get away from pain and still reach the height of the ball. Inevitably the center of mass moves so far laterally, and a double-leg landing is impossible.

From my previous article, “pain in the shoulder can lead to problems moving through the trunk, which can lead to pain in the low back, which can predict injury as far away as the knee [27, 49]. It’s no wonder the top three injuries in volleyball are the shoulder, back and knee.”

It’s now that we can renew a widely held paradigm that restoring the dysfunctions acquired through the demands of the game involves offsetting the effects of the game on the shoulder, trunk and lower limb.

More resources on knee injury and corrective exercise from OTP:

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