The following is a three part series devoted to the female athlete, specifically ACL injuries of the non-contact variety. Guest contributor Anna Napolitano explores some of the reasoning behind this frequent injury and the role of dance as a possible protection against such injuries. Anna is a 1998 graduate of the University of Michigan with a Bachelor of Science in Athletic Training and Movement Science and is licensed as an AT. She has her CSCS from the NSCA and her PES-NASM through the National Academy of Sports Medicine. Anna owns Innovative Training Solutions, LLC (www.mytrain-solutions.com) where she works with middle and high school athletes on sports performance training with her specialty being in ACL prevention and post ACL rehab. I hope you enjoy this series.
Injuries are a possible outcome experienced by athletes participating in sports of all types with some athletes at a greater risk for injuries than others. According to Hughes et al, women competing in the same sport as men are reported to be six to eight times more likely to experience a non-contact anterior cruciate ligament (NCACL) tear than men (Hughes, 2009). Numerous studies have attempted to gain a better understanding of this phenomenon. The majority of female NCACL injuries occur during jump landings as seen in the sports of basketball and volleyball (Pappas and Carpes, 2011). Another study by Hootman et al found female basketball athletes and volleyball athletes experienced NCACL injuries but at different exposure rates (Herrington, 2011). Many more studies have documented and investigated jump landing biomechanics of female basketball and volleyball athletes. However, not all jump landing athletes are prone to NCACL injuries.
Dancers may not be considered athletes as this form of physical activity is often classified under the “arts”. However, their training protocol places them at risk for soft tissue joint injuries similar to those of traditionally defined athletes. “Dancers may perform more than 200 jumping and landing activities throughout their daily training and practice.” (Orishimo, 2009). Training at this caliber could place female dancers at increased risk of NCACL injury. However, the incidence of NCACL injuries among female dancers is much lower than that of female basketball and volleyball athletes (Orishimo, 2009). Biomechanics of jumping styles may play a role in the difference between female basketball and volleyball athletes and dancers. The purpose of this report is to examine the biomechanics of jump landings and determine why basketball and volleyball athletes are a greater risk for NCACL injury than female dancers.
MECHANICS OF NON CONTACT ACL INJURIES:
Before evaluating the biomechanics of jump landings, a thorough understanding of the mechanics of NCACL injuries occur needs to be discussed. Simply explained, NCACL injuries occur when there is too much stress on the ligament (i.e., anterior cruciate ligament, ACL) leading to failure of the tensile strength of the ligament (Hashemi et al, 2011). Stress factors include the magnitude of the stress as well as the rate of loading of this stress. NCACL injuries may occur during practices, games or competitions. Research has identified not one but numerous risk factors associated with NCACL injuries. Factors found to “affect both ACL strength and the loads applied to it” include anterior shear force, hyperextension, axial compressive loads, internal rotation of tibia, valgus collapse mechanism, combination of anterior and valgus shear forces, and combination of external tibia torque and valgus forces (Hashemi et al, 2011). These factors can be found in sport movements especially jump landings.
BIOMECHANICS OF SPORT MOVEMENTS:
The biomechanics of a jump landing involves observation of the ankle, knee and hip joints and the kinetics/kinematics occurring at these structures. At initial contact of the jump landing the body experiences forces from the ground absorbed through the lower extremities. Gravity is pulling the athlete down and ground reaction forces are pushing back on the athlete as the athlete lands from the jump. Great ground reaction forces during landing tasks result in an increase on the load of the passive structures (ACL) of the knee (Hughes, 2009). The muscles of the hip and knee joint become a protective mechanism at this point. Some studies have attempted to determine if a decrease in ground reaction forces can result by having female athletes perform “soft” landing or “stiff” landings. “Soft” landings are defined as maximizing knee flexion and “stiff” landings are defined as minimizing knee flexion (Lauglin et al, 2011). This same study determined that soft landings allowed the lower extremity to absorb the ground reaction forces experienced by the body. The ability to absorb the ground reaction forces may be due to increased muscle activation during the jump landings and may be protective of the passive structures (i.e., ACL). The “soft” landing may provide an increase in posterior shear force by the hamstrings to counter the anterior shear force created by the quadriceps and tibio femoral joint (Laughlin et al, 2011). This counterbalance between the quadriceps and hamstrings protects the ACL from damage. Hughes et al found an increase in ground reaction forces during the first 40% of landing phase and a decrease in ground reaction forces during the final 60% of the landing phase while observing females landing from volleyball block jumps (2009). The higher absorption of the ground reaction forces at initial contact of landing phase coincide with the athlete’s proper body position and alignment as well as joint positions. The majority of the ground reaction forces was absorbed initially and then subsided as the velocity of the jump decreased. Coincidently, most NCACL tears occur immediately upon ground contact, as the athlete lands or makes a sudden change in direction.