Postural Stability & Bilateral Asymmetry

Recap of Balance and Stability

  • Balance is the ability of an object to remain upright in a static situation, which depends on the position of the Centre of Mass (COM) and the Base of Support (BOS).
    • If the line of gravity falls within the BOS, the object is balanced.
    • If the line of gravity falls outside the BOS, the object is unbalanced and will fall.
  • Factors influencing balance include:
    • Footwear (minimalist, cushioned, motion control, trail shoes).
    • Equipment (bicycles).
    • Modality (resistance training).
  • Balance relies on visual, oral, and proprioceptive systems.

Stability

  • Stability is the ability to maintain the Centre of Gravity (COG) over a BOS with minimal sway.
    • Static stability: Ability to hold balance in a stationary position.
    • Dynamic stability: Ability to maintain balance under changing conditions.
  • Neuromuscular parameters and visual, vestibular, and proprioceptive systems contribute to stability.

Control Systems

  • Vestibular: Stabilizes gaze and adjusts for head movement; not consciously controlled.
  • Visual: Links us with the environment, allowing navigation around obstacles.
  • Proprioceptive: Provides rapid feedback on position in space and status.

Assessment Methods

  • Static assessments:
    • Quiet standing on both legs or one leg.
    • Eyes open and eyes closed conditions.
    • Different stance positions (side-by-side, step, tandem, Romberg, lateral/anteroposterior spacing, internal/external foot rotations).
    • Use of force platforms to measure sway (sway area, sway velocity, time).
    • Use of Inertial Measurement Units (IMUs).
  • Dynamic assessments:
    • Perturbed standing (internal vs. external, expected vs. unexpected, anticipatory vs. compensatory).
    • Multiple single-leg hop-stabilization test.
    • Functional Reach Test, Star Excursion Balance Test (SEBT), Y-Balance Test.
    • Gait and locomotor transitory tasks.

Training Adaptations

  • Posturography parameters during Romberg and Tandem assessments:
    • 95% ellipse area for Romberg and Tandem stances.
    • Sway velocity for Romberg and Tandem stances.
    • Measurements taken with eyes open (REO, TEO) and eyes closed (REC, TEC).
    • Data presented for dancers and control groups.
  • Figure 1 shows significant differences between dancers and control groups (P < 0.01) in posturography parameters.

Postural Stability - Assessment Issues

  • Static assessment using a force platform
  • Dynamic assessment using BSS and Y-balance

Postural Stability – Assessment Issues cont…

  • Both the Balance System Stability (BSS) and Star Excursion Balance Test (SEBT)/Y-balance test claim to measure postural stability.
  • Hypothesis of a strong positive correlation between these tests was rejected.
  • Significant correlations were negative, meaning good performance on one test did not guarantee good performance on the other.
  • Balance tests should be selected based on the specific population (e.g., fall risk, athletic performance).

Bilateral Asymmetry

  • Side-to-side differences between limbs.
  • Includes biomechanical factors such as strength, power, balance, flexibility, proprioception, and muscle activation (EMG).
  • Definitions used in clinical and sporting contexts.
  • Requires determining what constitutes asymmetry and defining a threshold.

Thresholds for Asymmetry

  • 15% difference in clinical settings.
  • 10% difference in maximal torque.
  • 15% for vertical jumping and functional tests (Impellizzeri et al., 2007; Schiltz et al., 2009).
  • Difficulty in determining normal function to quantify deviations (Crowe et al., 2020).

Causes of Bilateral Asymmetry

  • Chronic adaptations to general and sport-specific function.
  • Handedness.
  • Previous injury.
  • Sport-specific demands.

Classifications of Limb Preference

  • Limb used to kick a ball the farthest.
  • Preferred limb for pushing off.
  • Preferred limb for completing a unilateral jump.
  • The limb one is more comfortable landing on from a unilateral jump.
  • The limb perceived to have the best single-leg balance.

Research on Bilateral Asymmetry

  • Bilateral asymmetry has been linked with an increased risk of injury (Myer et al., 2004).
  • Addressing asymmetries has been shown to improve injury outcomes (Croisier et al., 2008; Dalamitros et al., 2015).
  • However, there is no consensus in the literature (Carvalho et al., 2015).
  • Asymmetry can persist after injury and return to play (RTP) (Eagle et al., 2019).
  • Asymmetry can develop two years after bilateral Total Knee Replacement (TKR) (Yang et al., 2015).

Defining Lower Extremity Dominance

  • Studies show variable agreement between preferred leg for kicking and landing.
  • One study found the leg used to kick a ball had 100% agreement between self-reported and observed dominance.

Considerations in Research

  • Postural Stability:
    • Select the most appropriate test for your study with a clear rationale.
    • Ensure the test is suitable for the population (e.g., elderly vs. elite athletes).
  • Bilateral Asymmetry:
    • Define the threshold for asymmetry.
    • Be cautious about using asymmetry as a marker of injury, as you may be comparing two impaired limbs.