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Introducing the knee: Anatomy and biomechanics

Posted By Elsa Urmston and Jonathan George on behalf of the IADMS Education Committee, Monday, November 28, 2016

As dancers, educators and clinicians, we know that knees cope with a lot!  Over the last decade or so, the demands placed on the dancer’s body has increased exponentially and ever more complexly.  Acrobatic movement is becoming evident and the effect to the joints of the limbs can often mean greater incidence of injury.  As Liane Simmel points out “pirouettes on the knees, knee drops, and even a plié in fourth position require particular leg stability and optimal mobility in the knee.”1  In reviewing the literature, Russell2 identifies the lower extremity to repeatedly be the most commonly injured region of the body amongst dancers.

 

 

The knee joint is hugely complex and as Teitz (in Solomon et al, 2005)3 explain there is no bony stability in its structure.  A modified hinge joint, the knee comprises articulations between the femur and tibia, and the patella and femur, held together by a fibrous capsule and connected via a network of ligaments.  It’s this lack of potential stability which makes the knee prone to injury, often through misalignment and poor mechanics, although as well through sudden trauma or overuse.  Over the next couple of weeks we have a series of posts which focus on the knee; today we zone in on the structure, anatomy and mechanics of the knee itself.  Part 2 provides an overview of common knee injuries amongst dancing populations, and in Part 3 we focus on two case studies of young men who have experienced knee issues during their training and have been successfully rehabilitated to class and performance via a joined-up clinical and educative rehab programme.

 

 

The tibio-femoral joint is a hinge joint, capable of flexion (bending) and extension (straightening).  The screw-home mechanism allows the knee to slightly internally and externally rotate too.  During the last 30° of knee extension, the tibia (open-chain movement such as rond de jambe en l’air) or femur (closed-chain movements such as ascending from a demi-plié) must externally or internally rotate respectively by about 10°.  This determines the knee as a modified hinge joint.  You can see Rosalie O’Connor from American Ballet Theater demonstrating the screw-home mechanism in a rond de jambe action here!

 

The patellar-femoral joint serves to heighten stability in the joint.  The patella (knee cap) is a sesamoid bone which sits in the quadriceps muscle, and during flexion and extension undergoes complex gliding movements. The fairly unanimous consensus as to the function of the patella is to effectively increase the movement arm of the patella tendon about the tibio-femoral joint, thereby magnifying the movement and force of the quadriceps muscle group about the knee.4

 

 

The stability offered by the joint capsule is complemented by numerous, strong ligaments and more than any other joint in the body, these ligaments are vital in guiding the aligned movements of the bones as they come together to form the joint.  Yet, they are arranged in such a way that the stability is not always constant; some remain taut to ensure stability when the knee is extended and others slacken to ensure mobility when the knee is flexed5.

 

The medial and lateral collateral ligaments

The collateral ligaments are located on either side of the knee joint (collateral means side by side).  The medial collateral ligament – the one on the inside of the knee – is taut in knee extension and external rotation.  It controls the knee if the knee rotates inwards and in fact when the knee bends in a demi-plie, it controls approximately 80% of the medial stress on the knee (Besier et al, 2001)6.  The lateral collateral ligament – located on the outside of the knee – becomes taut with knee extension and provides lateral stability to the knee.  It controls approximately 70% of the lateral stress of the knee for example when the knees bow out on flexion and cause the feet to roll outwards (Besier et al).

 

The cruciate ligaments

The cruciate ligaments join the tibia and femur to one another within the internal structure of the knee.  The cruciate ligaments prevent any forward/ backward motion of the femur and tibia in relation to one another.  The anterior cruciate ligament also has another role in aiding rotation of the knee and controlling hyperextension in the joint.  It also plays a role when deceleration from jumping, floor work and quick changes of direction are required. It is now also widely accepted that the anterior cruciate ligament provides up to 40% of medial knee stability7.

 

The menisci

The medial and lateral meniscus are two cartilaginous discs which sit on the tibia and deepen the articular surface of the knee joint – they provide a kind of collar in which the bony ends of the femur sit, thereby improving the congruency and stability of the knee joint.  They assist with shock absorption and help to friction thus aiding smooth knee movement. The menisci are critical in the production of synovial fluid-‘the oil’- around the knee joint.

 

Bursae

The knee has the most extensive distribution of bursae in the body. More than 20 bursae are thought to be within the knee joint, with the primary role of reducing friction amongst the structures of the knee joint.  Many are located around the patella to aid its gliding function within the muscle and over the top of the joint itself.

 

Iliotibial Band

The iliotibial band is an adaptation of erect posture and provides key lateral support to the knee and hip; it runs down the side of the upper leg from the rim of the pelvis, to the outer edge of the femur and tibia.

 

This super video really provides a great introduction to the anatomy and ligament structure of the knee joint – take a look!

 

 

The musculature

As with the skeletal anatomy of the knee, the muscles which act on the knee are complex!  Because the muscles of the thigh also act on the hips, they often have a dual purpose –hip movement is included in brackets for ease of understanding here!  We have provided a simple table of the main muscles which act on the knee to produce movement.

 

Muscle

Action

Anterior/ front of the thigh

Rectus femoris

Knee extension (hip flexion)

Vastus medialis

Knee extension

Vastus intermedius

Knee extension

Vastus lateralis

Knee extension

Sartorius

Knee flexion (hip flexion, hip abduction and hip external rotation)

Posterior/ back of the thigh

Biceps femoris

Knee flexion and external rotation (hip extension and hip external rotation)

Semitendinosus

Knee flexion and internal rotation (hip extension and hip internal rotation)

Semimembranosus

Knee flexion and internal rotation (hip extension and hip internal rotation)

Popliteus

External rotation of femur when foot fixed; internal rotation of tibia when foot free

Medial surface of thigh

Gracilis

Knee flexion (hip adduction and hip flexion)

Posterior/ back of calf

Gastrocnemius

Knee flexion (ankle plantarflexion (pointing))

 

 

As you can see muscles often have more than one role in creating the movement of the limbs – we separate them out to learn about them, but of course they should be seen in their entirety to understand the complexity of the muscular system.  This video really helps us to see the wholeness of this system but understand each individual muscle’s location in relation to each other – take a look.

 

 

 

 

1.    Simmel, L.Alignment of the leg and its impact on the dancer's knee: Clips from the 2014 Annual Meeting
2.    Russell, J. Preventing dance injuries: Current perspectives, Journal of Sports Medicine, 4, 199-210.
3.    Solomon, R., Solomon, J. & Cerny Minton, S. Preventing Dance Injuries.  Champaign, IL: Human Kinetics, 2005.

4.      DeFrate LE, Nha KW, Papannagari R, Moses JM, Gill TJ, et al. The biomechanical function of the patellar tendon during in-vivo weight-bearing flexion. Journal of Biomechanics 40:1716–1722, 2007.

5.      Clippinger, K. Dance anatomy and kinesiology.  Champaign, IL: Human Kinetics, 2016.

6.      Besier, TF., Lloyd, DG.,  Cochrane, JL. and Ackland. TR. External loading of the knee joint during running and cutting maneuvers. Medicine and science in sports and exercise33, no. 7:1168-1175, 2001.

7.      Quatman CE, Kiapour AM, Demetropoulos CK, et al. Preferential loading of the ACL compared with the MCL during landing: a novel in sim approach yields the multiplanar mechanism of dynamic valgus during ACL injuries. American Journal of Sports Medicine, 42:177–186, 2014.

 

More information about the knee’s structure can be found in a variety of dance specific dance anatomy, kinesiology and safe practice books.

 

Elsa Urmston is the Centre for Advanced Training Manager at DanceEast, Ipswich, UK as well as Chair of the IADMS Education Committee and a member of the One Dance UK Expert Panel for Children and Young People.  Jonathan George is a Chartered Physiotherapist at the DanceEast Centre for Advanced Training.

Tags:  anatomy  biomechanics  dancers  knee  teachers 

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Dancers - More Power To You!: Video from the 2014 Annual Meeting

Posted By Jatin P. Ambegaonkar PhD, ATC, Monday, August 31, 2015

IADMS Ambegaonkar from Steven Karageanes on Vimeo.

Watching a dancer perform can be breathtaking and fun. Being a dancer is hard! This is because dancer’s movements often involve jumping and landing. To safely perform these explosive movements, dancers need good power in their lower body (Lower Body Power - LBP). Understandably, 70% of all dance-related injuries are to the lower body. Dancers are also supposed to have better balance than non-dancers. In athletics, sprinters with better strength and power and balance have better performance. In modern dance, aesthetic performance and jump ability are positively correlated to each other. However, research investigating potential interrelationships between LB power and balance among dancers is lacking.

We examined (a) the relationships between LBP measures and balance in dancers, and (b) the relative contributions of LBP to predict balance in 61 female collegiate dancers (18.3 + 0.7 years, 164.7 + 7.3 cm, 61.7 + 9.5 kg and with 11.3 + 4.8 years of dance experience). The dancers performed three vertical jumps on the Just Jump Mat (Probiotics, Huntsville, Alabama USA) and we calculated Peak and Average Power. Dancers also performed the Single leg hop for distance, which examines horizontal LBP. We measured balance using the Star Excursion Balance Test, a commonly used valid and reliable test of balance, which has been found to predict the risk of injury.

Our primary findings were that all balance and LBP measures were positively correlated to each other. We found that the more powerful dancers also had generally better balance.  Our findings thus challenge the often-pervasive myth among that being more powerful equates to being more muscular, which then can worsen dance performance. In fact, our evidence supports the opposite view that for dancers, being more ‘powerful’ also translates as being more ‘balanced’ - a MUCH desirable quality!

We are excited with this finding as, to our knowledge, our results are among the first times in the dance medicine and science that we can say to dance teachers and healthcare professionals that dancers balance and power are positively related to each other. In fact, our data suggest that the three LBP measures collectively combined to predict almost 50% of the change in balance scores. Overall, for dancers being powerful is really a good thing!  Furthermore, prior researchers have noted that training can improve power, strength, and dancer performance. Combining these prior reports with our findings, we can recommend that dance medicine and science professionals need to encourage dancers to incorporate strength and power training outside of dance practice in their training regimens to improve balance and overall dance performance.

 

Reference:
Ambegaonkar, J. P., Caswell, S. V., & Cortes, N. (2014). Relationships among lower body power measures and balance in female collegiate dancers. Paper presented at the 24th Annual Meeting of the International Association for Dance Medicine & Science, Basel, Switzerland.

Tags:  biomechanics  dancers  presentation 

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What do ballet dancers, bar tenders and paramedics have in common?

Posted By Luke Hopper, PhD, Friday, April 3, 2015

Lifting is an important technique across dance styles. With diverse modern choreography, lifting is an increasingly common technique not only for males but for female dancers as well. The act of lifting is often described as giving the partner grace and the impression weightlessness as they float through the air. But what about the lifter below? What about their technique? And more importantly what about their health?

In industry, lifting is the focus of a wealth of attention in the interests of occupational safety and health. This is because so many workers injure themselves performing lifting tasks. And the injuries they experience are commonly to the lower back

Dancers experience lots of leg injuries, but backs, particularly lower back injuries are also way too common in dance. A serious back injury can be a disastrous career ending experience for a dancer. Therefore should lifting in dance be considered from an occupational safety and health perspective and how could it prevent injury? In the US, the National Institute for Occupational Safety and Health (NIOSH) have released a safe lifting equation which lists six key variables (in addition to the weight being lifted) that affect risk of injury to the back:

  1. Horizontal location of the object relative to the body
  2. Vertical location of the object relative to the floor/li>
  3. Distance the object is moved vertically
  4. Asymmetry angle or twisting requirement
  5. Frequency and duration of lifting activity
  6. Coupling or quality of the workers grip on the object

Swap the term ‘object’ in the list for ‘partner’ and I’m sure that you can think of a whole lot of examples where dance breaks most if not all of the rules. But should lifting be banned from all choreography? Certainly not. Should dancers question their partner’s weight? Definitely not. Should dancers be aware of the risks of lifting so they can perform safe dance practices? Absolutely.

Alderson et al. (2009) estimated the lumbar forces in male dancers performing two lifts:

  1. Arabesque or presage: where the female stands in arabesque and is lifted in the arabesque posture by the male above his head into straight arms
  2. Full press: where the female stands in front of the male, jumps into the lift and is lifted above the male’s head

Surprisingly, every male dancer, for every lift examined, experienced the largest forces in the back before his partner had even left the ground in the posture shown below (Figure 1). The forces are likely the highest at is point because the male is generating momentum into the lift which requires more force. Think of pushing a piano across the studio, it requires a more effort at the first push compared to after the piano is moving.


                Figure 1: The position of greatest force in the lift.

In addition, despite the female jumping into the lift and ‘helping’ the male, the maximum back forces in the full press lift (Figure 2) were larger than in the arabesque (Figure 3). The full press was likely to have higher forces because of the dynamic jumping like movements of the lift. The explosive movement at the start of the lift may make the lift feel easier but it is because of the increased forces at the start of the lift.

 
           Figure 2: Full press lift.                                 Figure 3: Arabesque Lift.

Dancers should be careful of their posture right at the start of the lift as the discs of the spine are particularly susceptible to injury during these kind of intense movements and flexed body positions. Maintaining good posture throughout the whole lift is really important but male dancers should be particularly aware of the posture they start the lift with considering the high forces they experience at that point.

The forces measured in Alderson et al. were above the NIOSH recommended safe limits. To add a bit of perspective, the forces measured were more than a paramedic lifting a stretchered patient, but less than a bar tender lifting a beer keg. Lifting remains an important component of modern day choreography, but so do the risks to the dancer. Technique is of course important, but approaching lifting in dance from an occupational safety and health perspective can only serve to make dancers more aware of their bodies, closely consider their lifting technique and the forces they put through their bodies.

Recommended reading and images sourced from:

Alderson J, Hopper L, Elliott B, Ackland T. Risk factors for lower back injury in male dancers performing ballet lifts. J Dance Med Sci. 2009;13(3):83-89.

Full text articles from the Journal of Dance Medicine and Science are available to IADMS members!

Luke Hopper, PhD, is a Postdoctoral Research Fellow at the Western Australian Academy of Performing Arts, Edith Cowan University.
Email: l.hopper@ecu.edu.au

Tags:  biomechanics  dancers  lifting 

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