Understanding and testing for stability in the context of power generation in sport

This article was co-authored by Peter Colagiuri with the help of Leigh Egger, colleagues at BioAthletic. Colagiuri will release an app for sports injury diagnosis later this year. You can learn more at Sports Injury Online.

There are various components required to create power in the context of athletic performance. Single leg power tasks include cutting or agility during running, jumping for a ball while running and during sprint take off. These tasks are integral in most sports yet a significant portion of our gym based strength training focuses on double leg strength and power. Squats and deadlifts are great for building muscle function but don’t provide a comprehensive platform for athletic function. In order to successfully train and rehabilitate athletes to full athletic performance, we need to ensure that all aspects of performance are adequately addressed.

Muscle power can be measured in isolation via double leg testing, for example a power clean or max jump height test, but this disguises any loss of stability through ankle and hips. Power generation from muscles involved in propulsion can only effectively transfer force in the presence of a stable base. Any compromise to this base will lead to diminished force transfer and reduced output.

The leaky bucket

The leaky bucket analogy describes it best. The water represents the power aspect of the task and the leaks are the stability deficits. As you pour water into the bucket, the leak begins to allow water to escape. The more the bucket fills, the faster the water escapes. This is relevant in two ways. Firstly, if you’re only testing stability with low level demands, such as a simple single leg squat, the leak is going to be minimal and difficult to observe. Secondly, if you have a small leak with low level demands, the deficit will be amplified as the demand increases.

Stability loss in lower body

For the ankle, pronation is an essential component of energy storage on landing and push off movements. It allows the storage of energy in the connective tissue of the foot and ankle, to be returned as as elastic energy during the propulsion phase of the movement. However excessive or uncontrolled pronation becomes a problem on two fronts. First, the connective tissues are unable to effectively store energy and the joints are poorly aligned to optimize energy transfer back to the ground. In addition, excessive pronation also generates significant rotation at the hip, making hip and pelvis stability moor difficult to control.

The hip joint needs to maintain the position of the pelvis and trunk in three dimensions relative to the centre of mass, most often with a femur in motion. It’s a challenging task indeed! The hip can lose stability with lateral and/or rotational loss of control. If the hip is allowed to rotate excessively during landing, the power output is typically compromised with either additional rotation of the pelvis and trunk, which shifts the centre of mass, or poor joint alignment during the propulsion phase, taking muscles outside their optimal length-tension position.

Movements of the knee and trunk are typically the result of ankle or hip joint movement. Therefore observation of knee movements and trunk rotation or tilting should only be used as an indicator of instability somewhere in the biomechanical chain. Correcting the knee or trunk position won’t fix the underlying cause of the stability deficit. As an example, if there is an uncontrolled movement of the foot and ankle into pronation, this will force the knee inwards and the hip to adduct. Correcting the knee position will typically result in a worsening alignment with the knee shifting outwards to compensate for an ankle shifting inwards.

Testing for stability during athletic performance

A single leg deadlift can be an effective test for lateral and rotational stability loss. During a single leg deadlift, there are a number of indicators of stability deficits under high load or with increased speed. You may observe the bar dipping on one side or rotating during the movement. This can indicate a loss of stability around hip and pelvis, creating a shift in the trunk position. Another indicator is the raised leg drifting around behind the stance leg, indicating a loss of rotation control at the hip. During this test any loss of ankle stability typically results in an overall loss of balance or a significant lateral shift of the pelvis towards the stance leg.

Another effective test is a lateral hop and balance with a kettlebell. A lateral hop and stable landing is a great indicator of the ability to control the lateral shift components of stability. Using a kettlebell, held in front of the chest, adds an additional rotating component as it’s positioned in front of the body’s centre of mass. This will challenge hip rotation control when hopping in the direction of the stance leg and pronation control of the foot and ankle when hopping in the direction of the raised leg.

In this test, a loss of stability will be evident with the raised leg swinging behind the stance leg or inability to stick the landing, resulting in small additional hops, or an overall loss of balance. To test stability in the context of high performance power tasks, a distance can be set for the hop relative to the athlete’s ability. Competitive athletes should be able to hop around 1m in width while holding a kettlebell equal to 15% of their body weight. Ensure the hop is lateral without any forward movement.

Often when it comes to stability we just throw our hands up in the air and don’t even attempt to test it. If you are creative, there are plenty of tests available. It’s worth testing both conditions, remembering our leaky bucket analogy. Coaches can then identify if it is is the water or the holes that are limiting their athletes.