Why Plank - Does Core Strength Relate to Extremity Injury?

By Trent Nessler, PT, MPT, DPT and Kerri Whitehead, DPT, MSPT, ATC, CSCS. 

How does the AMI assess cores strength?

The Athletic Movement Index (AMI) module is a powerful screening tool that creates objective data for clinicians to identify movement fault in athletes. The AMI requires the athlete to complete a set of seven research-based tests, two of which determine core stability: 

• Plank: The athlete is scored on how long they maintain a neutral plank position within 10 degrees of the starting position during a one-minute plank. 
• Side Plank (left/right): Again, the athlete is scored on control and ability to maintain their starting position within 10 degrees during a one-minute side plank.

The AMI scores are based on the degree of multidirectional movement during the plank: the greater the movement, the lower the score. 

But why does the AMI use the plank to determine core strength as opposed to crunches or bridge? Most importantly, how can an athlete's core strength relate to extremity injury, such as ACL injuries?

The benefit of core stability to enhance controlled upper and lower extremity mobility is proven. The core is a critical part of sports performance and yet often overlooked as a part of training. The original concept of proximal stability for distal mobility can be traced to Proprioceptive Neuromuscular Facilitation (PNF) founded in the late 1940s by Knott and Kabat. Their research found "in an efficient state the trunk provides appropriate proximal stability or controlled mobility to support optimal task or postural performance" (Basmajian et al Rational Manual Therapies, 1993).

The plank and side plank are two test positions utilized in research that demonstrate higher predictive value for determining ACL risk (DeBlaiser et al AM J Sport Med, 2019). The key factor with both tests is proper positioning. Plank training and assessment require correct form to be valuable: 

• An unsupervised exercise program of dynamic core strength including plank and side planks was found to improve function in dancers (Kline et J Dance Med & Sci, 2013).
• Improvements in dynamic postural stability have been shown in several studies to reduce the risk for concussion (Johnston et al AM J Sport Med, 2019, Garner et al In J Kines Sport Science, 2020). 
• Improvements in core stability has been shown to increase vertical jump in basketball players (Hoshikawa et al J Strength Con Res, 2013) and improve speed in sprinters (Tong et al Phy Ther Sport, 2014).

"Training core muscles can modify biomechanics associated with ACL injuries in a side-step cutting task; thus, core strength training might be considered in ACL injury prevention programs to alter the lower extremity alignment in the frontal plane and muscle activations during sports-related tasks" (Jeong et al Am J Sport Med, 2020). 

Studies as far back as 2013 (Frank et al Am J Sports Med, 2013) have shown a strong correlation to poor core stability and an increase in risk for ACL injury prevention: 

• "Core muscles activity is best understood as the pre-programmed integration of local, single-joint muscles, and multi-joint muscles to provide stability and produce motion. This results in proximal stability for distal mobility, a proximal to distal patterning of generation of force, and the creation of interactive moments that move and protect distal joints" (Kibler et al Sports Med, 2006).
• "Decreased trunk proprioception is predictive of future knee injuries in female collegiate athletes" (Zazulak et al Am J Sports Med, 2007). 
• Research evidence supports "a link between core stability and upper extremity injuries amongst athletes who participate in baseball, football, or swimming" (Silfies et al Brazilian Journal of Physical Therapy, 2015). 

Improved performance on these movements means the athlete will be more equipped to transfer kinetic energy from the hips to the core to the shoulders. Yes, core strength does affect the upper and lower extremity; front and side planks are a critical component for performance.