Knee pain and complaints are common among personal training clients. A clear understanding of the internal knee and its function supports the knowledge to program restorative knee exercises, progressions, and regressions (Hassebrock, et al., 2021).

This article examines the roles played by ligament and muscle anatomy in the knee for personal trainers. Additionally, common ligament and muscular injuries are discussed, as personal trainers will likely train clients who have experienced these injuries. 

Since personal trainers are not qualified to treat knee pain directly, programming strategies will be discussed, emphasizing finding pain-free variations and modifications of exercises so that clients can continue to train. Six exercises for resilient and strong knees will also be covered that personal trainers can implement.

Knee Kinesiology

The knee joint is the largest in the human body and allows complex movements. It includes the medial and lateral tibiofemoral and patellofemoral joints (Flandry & Hommel, 2010). Unlike ball-and-socket hip joint articulation, the femoral and tibial surfaces of the knee are not a close fit (Harput, 2020)). 

The knee joint movements are mostly linked to the hip and ankle joint movements. The knee joint sustains high forces and moments and pivots between the human body’s two longest bones (femur and tibia), making it susceptible to injury.

The arrangement of the knee ligaments and muscles that cross the joint provides the much-needed stability that counters the considerable biomechanical stress brought upon the joint (Kittl, et al., 2018). The hinge joint primarily allows movement in the sagittal plane through flexion and extension. It also allows slight medial rotation during flexion and the last stage of the knee’s extension, as well as lateral rotation when “unlocking” the knee.

Knee Anatomy

The knee is the junction of the femur, tibia, fibula, and patella bones. Ligaments, tendons, fascia, and muscles connect the joint and allow movement. Technically named a synovial joint, the knee is called a “hinge” joint for its linear and door-like movement patterns. When the knee is flexed, it can also perform internal and external rotation!

The knee joint has four ligaments, including the menisci, a type of hybrid cartilage/ligament that acts as a shock-absorbing barrier between the articular cartilage on the ends of the femur and tibia.

Three groups of muscles (popliteus, quadriceps, and hamstrings) allow movement and stability at the knee joint. Four muscles are in the anterior compartment of the thigh and are responsible for knee extension, and three are in the posterior compartment and are responsible for knee flexion. The popliteus is located behind the knee joint and ” unlocks” the knee by rotating the femur on the tibia, allowing knee flexion.

Ligaments

Lateral Collateral Ligament (LCL)

The “fibular collateral ligament” is called the LCL because it connects the lateral femur and fibula. LCL limits the sideways motion of the knee. Any hyper or excessive movement in which the knee has to over-stabilize against a sudden change of direction that the surrounding musculature can’t control could damage this ligament. Here are a few other important notes on the LCL:

• It is taut during full knee extension and slack during full knee flexion
• It protects the lateral knee from inside (varus) forces

Medial Collateral Ligament (MCL)

The MCL is called the “fibular collateral ligament” because its attachment sites connect the medial femur and tibia. Like the LCL, it limits the knee’s sideways motion and is generally injured when the stress of quickly changing directions overpowers the force and stabilizing abilities of the surrounding musculature (Bates, et al., 2015). Here are a few other important notes on the MCL:

• It is taut during full knee extension and slack during full knee flexion
• It protects the medial knee from outside(valgus) forces

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Anterior Cruciate Ligament (ACL)

The word “cruciate” means crossing from one side to the other and one over the other. The ACL connects the femur to the tibia along the front center part of the knee. Specifically, one end connects to the anterior tibia along the medial side of the tibias’ sagittal line. The other end connects to the deep portion (almost to the rear but not entirely) of the femur along the lateral side of the femur’s sagittal line.

The ACL, one of the most common ligaments injured in sports, controls the following movements in the knee joint (Bates, et al., 2015):

1. Restricts anterior translation of the tibia
2. Prevents hyperextension of the knee
3. Secondary restraint to tibial internal rotation
4. Resists adduction and abduction in full extension 
5. Guides “screw” mechanism of the knee joint as it approaches terminal extension

Posterior Cruciate Ligament (PCL)

The PCL is the strongest ligament and primary stabilizer of the knee. It also connects the femur via the medial femur epicondyle to the tibia via the posterior intercondylar area. The PCL controls the following movements in the knee joint (Bowman & Sekiya, 2010):

1. Prevents posterior dislocation of the tibia while the femur is fixed
2. Prevents anterior dislocation of the femur while the tibia is fixed
3. Limits hyperextension of the knee
4. Controls stability of the knee during rotation

Knee Cartilage

Cartilage, sometimes called “articular cartilage,” can be found at the end of the fibula, tibia, and fibula and behind the patella. It is a connective tissue with no blood vessels or lymphatics, making it very slow to heal. It also has no nerves, so no pain can be felt. A significant function of knee cartilage is to absorb impact, reduce joint friction, cover the tibia and femur’s subchondral surfaces, and painlessly transfer forces.

The degeneration of cartilage causes osteoarthritis, which leads to potential joint replacements, increased sedentary lifestyles, and mortality. Osteoarthritis is the most common form of arthritis in the United States, affecting 13.9% of the population or 26 million adults over 25 (Jacobs). 

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In the knee, there are two types of cartilage:

1. Hyaline covers the end of the femur, tibia, and fibula.
2. Menisci is a specialized hybrid cartilage type that provides cushioning between the femur and tibia. Learn about meniscus anatomy and injuries in this article.

Composition of Knee Ligaments & Cartilage Structures

Ligaments, which connect one bone to another, are composed of approximately 70% water and 30% organic matrix, along with fibrocytes, the specific type of cells that make up tendons and ligaments.1,2

The organic matrix combines ground substance (a combination of protein and carbohydrate complexes forming a gel-like substance) and collagen. In ligaments and tendons, 90% of the Organic Substance is collagen. Collagen comprises 25 to 30% of the body’s protein.

Collagen production in the body can vary from individual to individual, with the aging process and genetics playing the most prominent role in the ability to make adequate amounts for tissue repair and maintenance. 

Type 1 collagen fibers tend to be more rigid than Type 2. Hence, the ability to withstand the forces generated by movement and keep the bones they hold together without a daily injury. Cartilage, which covers the end of all bones that touch each other, is different in composition compared to ligaments and tendons.

Muscles of the Knee

The knee muscles are responsible for knee flexion and extension and work concentrically to control one movement and eccentrically to control the opposite movement. The prime movers for knee extension are the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. They also function as antagonists for knee flexion. The prime movers for knee flexion are the biceps femoris, semitendinosus, and semimembranosus, assisted by the gracilis, gastrocnemis, and sartorius.

Here are some other functions and facts about some of the knee muscles besides their primary actions.

• Vastus intermedius 
  • The most efficient knee extensor 
• Vastus medialis
  • Assists the patella in tracking along the femoral condyles
• Vastus medialis oblique
  • Prevents lateral displacement of the patella and counters the force of the vastus lateralis
• Biceps femoris
  • Externally rotates the tibia relative to the femur
• Poplitius 
  • Weak knee flexor and initiates extension of the knee when the knee is flexed

 

Common injuries

LCL Sprain or Tear

LCL injuries are the complete opposite of MCL tears. They occur when the inside (medial part) of the knee is struck and pushed out or in sports with many quick stops and turns, such as soccer, basketball, and skiing. LCL injuries are also classified by severity, with classifications and symptoms similar to those of the MCL. However, the LCL does not heal as well as the MCL; in most cases, a Grade 3 injury will require surgery.

MCL Sprain or Tear

MCL injuries occur when the knee is struck on the body’s lateral part (Outside). Since the MCL is located in the inside part (Medial) of the knee and resists widening of the inside of the knee joint when the knee is struck from the outside with a force that causes lateral buckling, it simultaneously separates and widens the medial portion of the knee joint causing the injury.

ACL Sprain or Tear

An ACL tear is most often a sports-related injury but can also occur during rough play, auto accidents, falls, and work-related injuries. Most ACL injuries in sports happen when pivoting or landing from a jump. Like meniscus injuries, patients with ACL tears often feel a “pop,” the knee usually gives out underneath them. Subsequent pain and swelling are to be expected. 

ACL tears do not necessarily require surgery. According to Doctor Jonathan Cluett, Board-Certified Orthopedic Surgeon in Massachusetts, your daily activities and demands should be considered before surgery.

For instance, do you regularly perform activities that require a normally functioning ACL? In addition, if the knee is stable, ACL surgery may not be necessary. Many patients with ACL injuries feel better within a few weeks. The only persistent problem may be instability.

PCL Sprain or Tear

PCL injuries are commonly experienced when the knee is bent and an object forcefully strikes the shin backward. This type of injury can also occur in a car collision when the shin strikes the dashboard. Another mechanism of injuring the PCL is in sports when an athlete falls on the front of the knee.

PCL injury can also occur when the knee is hyperflexed and the foot points downward. Symptoms of PCL injuries are similar to those of ACL injuries. In the weeks following the injury, patients state that they can’t trust their knee or that it feels as if it is going to give out.

Ligament and muscle tears are classified by their severity into three categories.

• Grade 1 is a parietal tear. The tendon or ligament is still in continuity, with minimal symptoms. The symptoms are pain with minimal downtime, and most can return to their normal activities or sports within a few weeks.
• Grade 2 is an incomplete grade two tear with more aggravated symptoms, such as more intense swelling, pain, and instability. At least three to four weeks of rest is usually necessary.
• Grade 3 is a complete tear or separation of the ligament, tendon, or muscle. There is significant swelling and pain, and it is difficult to bend the knee. Instability or the knee giving out are common findings. Healing takes at least six weeks or longer. Having a knee brace with lateral and medial stabilizers is recommended. Due to good blood supply and the fact that it usually responds well to non-surgical treatments, it is rarely treated with surgery.

Patellar Tendinitis

Chondromalacia Patella is the degeneration of the cartilage between your patella and femur. Your kneecap, which sits over the front of the knee joint, glides over the Femur as your knee bends or extends. Chondromalacia Patella (also called “Patellofemoral Syndrome”, “Runners Knee”, “Chondromalacia Patella” or “Jumpers Knee”) begins when the kneecap does not move properly and rubs against the lower part of the femur (Dan, et al., 2018).

Causes of chondromalacia patella:

• The kneecap is in an abnormal position(also called poor alignment of the Patellofemoral joint)
• Tightness or weakness of the muscles on the front or back of the thigh
• Flat feet
• Too much physical activity that places extra stress on the kneecap

Symptoms of Chondromalacia Patella are pain behind, below, or on the sides of the kneecap, being more noticeable while climbing up or down stairs, performing deep knee bends, standing for long periods, and running downhill (Nunes-Matinez & Hernandez-Guillan, 2022).

Prepatellar Bursitis

Prepatellar Bursitis is the common cause of swelling and pain on top of the kneecap. The bursa are thin sacks filled with the body’s natural lubricating fluid. They are situated around our joints to prevent muscles, tendons, and skin from catching on bony surfaces throughout the body.

When the knee is traumatized, subjected to repetitive use, or injured, the bursa can swell and fill with blood or fluid, which in turn causes pain and swelling (Rishor-Olney, et al., 2024). If the trauma is associated with a tear in the skin, the bursa can become infected; this is called infected bursitis.

Kneeling daily for extended periods of time or being sedentary increases the risk for prepatellar bursitis.

Symptoms of prepatellar bursitis are:

• Swelling over the kneecap
• Limited motion of the knee
• Painful movement of the knee

Bursitis of the knee can be treated by draining the bursa sac. In cases where infection is possible, an antibiotic is prescribed. In mild cases, resting the site with ice therapy and anti-inflammatory medication may work fine.

Muscle Strains

A muscle strain is an injury that affects the muscle or the tendon. The severity can range from a mild stretch to a partial or complete muscle or tendon tear. These strains often occur where the muscle and tendon meet, known as the musculotendinous junction. Common areas for muscle strains in the hip and thigh include the hip flexors, groin, adductors, quadriceps, and hamstrings.

Hip and thigh muscle strains can result from a sudden, unanticipated movement or repetitive overuse over time. When the injury occurs, you may feel a “pop” or “tear.” The likelihood of reinjury is higher with a previous strain or injury, especially if it wasn’t fully healed (Opar, et al., 2012). Other factors that increase risk include tight muscles, poor motor control, and poor conditioning.

Symptoms of a muscle strain in the hip or thigh include pain, bruising, swelling, reduced mobility, weakness, and trouble walking. Hamstring strains are one of the most common muscles to be strained (Hickey, et al., 2022).  

How Personal Trainers Can Program For Knee Pain

Personal trainers cannot treat pain directly, but you can strengthen and lengthen the muscles supporting the joint. Rather than trying to pick exercises specifically “for knee pain,” personal trainers should help their clients find pain-free exercises and modifications to painful exercises. More often than not, a specific exercise is not to blame for the pain, but rather particular variables within the exercise, such as:

• Range of motion
• Load
• Volume
• Speed
• Stance or limb positioning

By avoiding painful movements or stimuli and training pain-free, positive performance adaptations can continue while simultaneously giving the knee time to rest and recover. When training specific muscles for knee pain, such as the vastus medialis oblique, research shows that training a specific muscle for knee pain is not more effective than a general approach. Therefore, make sure that all of the muscles of the leg are being trained. 

When modifying an exercise, here are some examples of regressions or progressions to find a pain-free variation. These are just examples and won’t necessarily hold in every case. 

Variable	Squat	Lunge	Hip Hinge
Range of motion	Full squat -> box squat	Walking lunge -> short box step up	Full conventional deadlift -> Partial RDL
Foot position	Regular stance squat -> a stance that feels better than the pre-injury stance	Same as “squat” column or try different wedge setups to see if that different position helps	Same as “squat” and “lunge” column
Tempo	Emphasize a tempo (such as the eccentric portion) that is pain-free and minimize a tempo that provokes pain
Load Position	Front squat -> low bar back squat	Goblet lunges -> open hex bar lunges	Barbell deadlift -> hex bar deadlift

Six Strength Exercises for Resilient Knees

Exercise 1 – Romanian deadlift

This is an excellent exercise for clients convinced they shouldn’t bend their knees. The RDL is a very hip-dominant movement with little knee movement. It trains the posterior chain (glutes, hamstrings, spinal erectors), and many other muscles in the body. For more in-depth coaching on the deadlift, check out this article.

1. Find a comfortable stance width
2. Hinge the hips back with a soft bend in the knees
3. Stop hinging right before the lumbar spine flexes
4. Drive the hips forward to move the body back to a standing position

You can use any modality: barbell, dumbbell, hex bar, etc. Remember the exercise variables and select pain-free ones. The single-leg variation of this exercise and other bilateral exercises in this list are other great options.

Exercise 2 – Hip thrusts

Another excellent posterior chain exercise mainly targets the glutes and hamstrings, with some quadriceps activation. As a hip-dominant movement, this exercise will experience most of the load and movement in the hips rather than the knees, though the knees and quads will still experience some work. 

Remember, these exercises aren’t about avoiding a specific muscle or movement because of an assumption that they’re bad for the knees. Still, they are modified in a pain-free way and train the muscles around the knees. For more in-depth coaching on the hip thrust, check out this article.

1. Find a comfortable stance/foot position
2. Bring the hips and ribs to parallel with the abs
3. Drive the hips towards the ceiling by squeezing the glutes
4. Maintain the knees in alignment with the hips and feet

Exercise 3 – Step down

Step-downs are the same as step-ups, but they are called step-downs to emphasize the eccentric portion of the exercise. This is a highly modifiable exercise, which makes finding a pain-free version especially doable. Here are some modifications: 

• Decide which joint to emphasize movement in by leaning the torso forward for more hips or remaining more upright for more knees
• Hold onto support or use an overhead band for stability and to reduce the load
• Choose a suitable box height

1. Begin by standing up and removing the non-working leg from the box
2. Slowly lower to the floor by maintaining tension in the lower body
   1. Touch the floor with the toes and then the heel
3. Maintain the knee over the foot and decide if it ok for the knee to track past the toes or not, depending on the tolerance of the movement

If stepping up without assistance causes pain, the non-painful knee can step back up or assistance can be used.

Exercise 4 – Sled pushes and pulls

Sleds are a great tool because they train the body in a gait pattern and many other patterns, such as crawling, walking backward, sideways, etc. The sled push and pull are about efficiently transferring power in the horizontal plane of motion. To learn more about sled exercises, read this article. 

Exercise 5 – Calf and tib raises

It is essential to train the muscles below the knee. The gastrocnemius is a biarticular muscle that contributes to knee flexion. Find a foot stance and angle that is pain-free, and choose your preference for loading. Train the movement in a pain-free range of motion that allows for the biggest stretch possible. 

Restricted ankle range of motion has been linked with a higher risk of injury (Taylor, et al., 2022). Calf and tib raises can help improve ankle range of motion if they are trained in a full range of motion or at least a long-length partial range of motion repetition. Tib raises are less known than calf raises, so here is how you can do them. 

• Wall leaning tib raises- lean back into a wall and pull the toes to your nose by pivoting on the heels
• Use a tib raise device to add load and bring your toes to your nose in the seated position

Exercise 6 – Box squat

The box squat essentially does two unique things that a conventional full squat doesn’t:

1. Limits the range of motion, which usually decreases knee flexion
2. This cues you to sit the hips back, placing the load on the hips and resulting in less forward movement of the knees in flexion and extension.

Usually, these two things have a better chance of providing a pain-free squatting opportunity. Read this article to learn more about squat cueing, techniques, and modifications. 

References

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