August 22, 2022
Gravity – It’s Complicated
I’ve examined anti-gravity extension and anti-gravity flexion for years as part of my assessment. Specific methods for testing both of these components are included in both the Observations Based on Sensory Integration Theory as well as the new Structured Observations of Sensory Integration – Motor (SOSI-M). Most often clinicians focus on the amount of time the child can hold the position but does this measurement really provide us with useful information? As usual, the answer is “it depends”.
In a classic study, Sellers demonstrated relationships between antigravity control and postural control. Length of time of supine flexion related to children’s quantity and quality of static balance (standing on one foot) as well as the quantity and quality of their dynamic balance (walking on a balance beam). However length of time of prone extension related only to the quality of dynamic balance. So yes, testing these can point to possible issues in static vs. dynamic balance.
However, there are so many systems involved in postural control that contribute to the successful completion of these test items as well as success in the balance challenges discussed in this study. As an example, from a motor point of view, we understand that good quality prone extension requires good quality eccentric flexion as the opposition force, which might arguably be more difficult than the isolated supine flexion task. So perhaps we need to expand our conceptualization of these test items beyond just the amount of time a child can hold them.
In prone extension we can expand our lens to look at the quality of the extension. Is there smooth extension throughout the spine, hips and knees or is there hyperextension at C1C2 or T12L1 or flexion at the knees? These may point to difficulty with specific anti-gravity muscle groups (multifidus or erector spinae or glute max) either because of vestibular challenges (as supplied by the lateral vestibulospinal tract) or weakness due to a different underlying pathology (ex. muscular dystrophy, spinal muscular atrophy or cystic fibrosis). There may be poor use of flexors eccentrically or the overuse of concentric hamstrings for stability. Is the child stable in midline? Is there breath holding? These may indicate difficulty with central stability and poor recruitment of the inner core team. So many possibilities to be explored.Supine flexion as it is classically tested (legs flexed to torso, arms crossed at chest with head and upper trunk lifted off the floor) again is poorly served by looking only at the amount of time this can be held. Is there difficulty with flexing knees to chest, indicating a difficulty particularly with transversus abdominis, psoas and possibly adductor recruitment? Is there difficulty with the head and upper torso flexion/lift off the surface and over-recruitment of neck flexors indicating a difficulty with recruitment of all abdominals? Is there breath holding? Central stability and the inner core team may again be at play here. What is the alignment of the rib cage in supine and does this negatively impact the ability to recruit the abdominals? Should we be testing with legs in table top rather than flexed to chest as this requires more abdominal activity? We also understand that the extensor synergies are fed directly by the lateral vestibulospinal tract while the flexor synergies are wired through somatosensory input so a child who experienced less tummy time may be at a disadvantage. And finally, good recruitment of anti-gravity extension is required to create the optimal alignment for recruitment of the flexors so the systems are interdependent. So much to consider.
As we conceptualize anti-gravity function as it relates to balance, let’s dig deeper into the quality of the movement rather than just the quantity/how long they can hold it.
As I observe the nuances of a child’s ability to recruit anti-gravity extension and flexion in the traditional test positions, I also like to begin to put them together in a modified boat pose to see how these combine as we begin to move further up against gravity. The young girl in picture 1 below has collapsed into flexion as she weight shifts backwards and prepares to unweight the legs.
In picture 2 she is now able to maintain better spinal extension and use only slight lumbar flexion to control the backward weight shift. She is able to change the pattern because we have worked improved alignment, recruitment, central stability and strength. With this strategy, she easily completes the task.
Finally I like to analyze how a client completes a movement sequence that requires coordination of flexors and extensors. In the video below I’ve used the transition from side sitting to side sitting through flexion.
You can see as this girl begins the movement there is a very slight weight shift backwards to unweight the legs. Spinal anti-gravity extension is maintained with just an ever so slight increase in flexion in the lumbar spine in response to the backwards weight shift. Anti-gravity flexion maintains the upright posture while psoas and the hip adductors maintain the legs nicely flexed to the chest throughout a smooth and efficient movement. There is no collapse of the trunk which shifts the centre of mass further backwards with consequential extension of the legs to counterbalance, no struggle to stay upright, no challenge to movement.
So going forward, I suggest we begin to expand how we think of anti-gravity extension and flexion and how we look at these in balance and in movement. Because as it turns out, anti-gravity function is more complicated than it may first appear.
Blanche EI, Reinoso G, Kiefer DB. Structured observations of sensory integration – motor. Academic Therapy Publications, 2021.
Blanche EI. Observations based on sensory integration theory. Pediatric Therapy Network, 2010.
Sellers JS. Relationship between antigravity control and postural control in young children. Phys Ther. 1988; 68(4): 486-90.
Basaldella E, Takeoka A, Sigrist M et al. Multisensory signalling shapes vestibulo-motor circuit specificity. Cell. 2015; 163: 301-12.