The principle finding in this investigation was that healthy participants achieved a longer reach distance in the A direction of the SEBT compared with the YBT. Previous authors have reported differences in SEBT reach distances similar to those of the balance tasks we observed after neuromuscular training8,9
and fatigue protocols21
and in injured groups.7,21
These differences have implications for the implementation and interpretation of the YBT. We observed no differences in the other 2 reach directions. These findings may be due to differences in postural-control strategies in completing the dynamic balance tasks.
Postural control may be classified as either static (attempting to maintain a base of support with minimal movement) or dynamic (attempting to maintain a stable base of support while completing a prescribed movement).21
This is a complex process that requires central processing of sensory inputs from visual, vestibular, and somatosensory pathways, as well as a resultant efferent response that controls the precise recruitment of specific motor units.22
The central nervous system generates a “blueprint for movement,” which is delivered to pattern generators at a spinal level, resulting in the patterned activation of α and γ motoneuron systems.23
Feedback and feed-forward control mechanisms play vital roles in the control of movement and the execution of tasks such as the SEBT and YBT. The stimulation of mechanoreceptors located in the periphery (ie, skin, ligaments, muscles, and joints) provides afferent feedback via spinal pathways regarding joint movement and position in various body segments during movement.24,25
The pattern of motoneuron activation is fine tuned by feedback from these mechanoreceptors, which act either monosynaptically or via inhibitory interneurons and provide a corrective response to the action.26
In contrast, feed-forward controls have previously been described as anticipatory actions occurring before the sensory detection occurs.27,28
The mechanism of feedback and feed-forward control may vary between the SEBT and YBT, resulting in differences in reach distances in the A direction. The YBT requires the participant to stand in an elevated position on a central footplate while pushing a sliding block. He or she receives constant proprioceptive feedback throughout the reach excursion from the plantar surface of the reach foot. In the SEBT, the participant places downward pressure through the reach foot only at the end of the reach excursion and, therefore, does not receive a similar level of afferent information throughout the movement, potentially relying on a feed-forward control strategy until contact is made with the tape measure. The postural-control strategy used during the SEBT may mean that the individual does not have the same level of inhibition throughout the movement and, thus, reaches further. An external focus of attention facilitates automaticity in motor control and promotes movement efficiency compared with an internal focus.29
Contact with the sliding block may have induced a more internal focus, thereby resulting in a negative influence on the A direction. In addition, pressure is allowed with the ground by the reach leg, albeit minimally, and actually provides a point of support. In the YBT, the individual may remain in a narrower stance as a result of the feedback received throughout the movement and therefore not reach as far. In upright stance, a stiffening strategy, characterized by decreased amplitude and increased frequency of postural adjustments and leaning back away from the direction of the postural threat, has previously been reported,30,31
resulting in a feeling of less stability when standing on an elevated surface.32
Sabin et al33
recently reported reduced reach distances when performing the SEBT on an unstable surface. The elevated stance-leg position of the YBT, although at a relatively low height, may be perceived as a barrier to reaching further.
Several possible explanations for A reach differences are based on feedback mechanisms. The visual system provides the body with visual cues for use as reference points in orienting the body in space. It is generally agreed that, under normal conditions, the somatosensory and visual subsystems are the primary mediators of balance and postural awareness.25
In the A reach direction, participants receive visual feedback from the reach leg as they move and can observe the scored reach distance on each trial. In the PM and PL directions, visual awareness is reduced, and therefore, the inability of the participants to see their scores may not limit their reach as in the A direction. However, this places an increased demand on somatosensory feedback strategies, meaning that, during the YBT, participants were able to reach similar distances as on the SEBT owing to their contact with the sliding block. Another potential reason for similar reach distances observed in the PM and PL directions relates to the placement of the reach foot on the sliding block of the YBT. Participants were instructed to not place the foot on top of the tubing and in all cases placed the plantar surface of the reach foot on the medial side of the sliding block. This may have resulted in their maintaining support closer to their center of gravity during the YBT than when they reached along the tape measures of the SEBT, again allowing them to reach similar distances in the PM and PL directions. In the A direction, the foot was placed on the lateral side of the sliding block, thus displacing the center of gravity and subsequently reducing reach distances compared with the SEBT.
These factors may also account for the significant differences and relatively large limits of agreement in the A direction, which indicate poor agreement between the tests in this study. Although this poor agreement would have practical implications for the use of these tests together as part of a screening protocol or clinical assessment outcome, it is unlikely that the tests would be used concurrently in these situations. Similarly, previous investigators measuring SEBT performance have indicated that side-to-side differences like those we observed between the SEBT and YBT in the A direction predict lower limb injury4
and indicate deficiency in conditions such as chronic ankle instability6
and anterior cruciate ligament injury.7
Therefore, caution should be exerted when interpreting reach distances from both tests in the A direction in the same individual or group. Based on our findings, the posterolateral direction is the test with the least bias, which may make it more feasible for comparison between the tests. There is a paucity in the literature of studies investigating the relationship between the YBT and injury screening and assessment. Owing to its practicality and ease of use in a clinical setting, further research is warranted.
Finally, the differentiation between the levels of the elevated stance foot on the central footplate and the lower reach foot during the YBT compared with the level base for the SEBT may influence the individual's postural-control strategy. However, the former may be more applicable to everyday situations in which postural control is required on uneven surfaces. The role this strategy may have in the completion of these tests warrants further investigation.