Let’s not forget the trunk after stroke.

How much should we focus on the trunk in rehabilitation?  The degree to which we spend time working on trunk muscles and trunk movement is a common and long-standing debate in neurology, musculoskeletal and sporting rehabilitation fields. The trunk certainly gets a bad rap in some stroke rehab circles, so thought I’d put together some of my thoughts.

Axial muscles, unlike the limbs, are innervated by bilateral projections descending from brain and brainstem centres. These axial muscles such as erector spinae, multifidus, quadtratus lumborum, internal and external obliques and rectus abdominus (+ more) all provide movement and stability for many important limb functions. Movement of the hand and arm appear to be closely coupled with these trunk muscles for certain tasks (1,2) and also involve muscles around the scapula and shoulder. Of course, muscles around the lumbopelvic-hip areas also provide trunk movement and stability for movements that involve the upper limb, plus we cannot forget the important role for the muscles of respiration for function and even balance (3,4).

So, what is the role of the trunk muscles when we move upper limbs? Trunk muscles provide ‘very, very early’ and then ‘early’ stabilising activity in order to adjust posture to support the movement of the limbs such as when elevate our arms (5). In humans, it is not very clear how our central nervous system controls muscles on both sides of the trunk during these movements, but research tells us that it depends on the type of movement. Is it static, dynamic, unilateral, bilateral, rhythmic? The trunk muscles, in addition to those around the lumbopelvic-hip region can also help with reach by providing distance and trajectory especially when the body moves outside its base of support. Again, the mechanisms for innervation for these movements would be different to those used for stability & balance, but both would likely occur together. Stability for mobility, and mobility for stability.

Pathways that control the trunk muscles can come from the brainstem and include reticulospinal and vestibulospinal projections.  In addition, corticospinal projections from our brain’s cortex are also involved in trunk muscle innervation, but their role is not well understood (5). They involve both inhibitory and facilitatory outputs, plus interhemispheric interactions between the right and left motor cortices (6). It important to remember that ALL of these descending pathways are also involved in upper limb and hand movements too!

So, what happens after stroke? Well, we know that limbs are affected, and upper limbs and hands unfortunately can lose critical corticospinal outputs needed for fine motor hand dexterity and complex arm movements. Trunk muscle activation also changes following stroke in both isolated trunk movements (7), balance tasks and upper limb activity. Over time weakness, asymmetry and atrophy in trunk muscles becomes evident (8–10). After stroke, the interrupted pathways from cortex to brainstem structures (corticobulbar pathways) will also modify the descending outputs to the trunk (11,12). In addition, the corticospinal pathways may be partially or severely interrupted, in addition to important interhemispheric interactions. All of this means that motor control to the trunk can be affected after stroke.

Bilateral innervation of the trunk muscles, even with all its variable context dependent control mechanisms, may theoretically make it an attractive target for retrieving and retraining whole body motor patterns that encourage upper limb and hand recovery. So, stroke movement rehabilitation will need to focus on lower limb, upper limb AND trunk muscles. New movement patterns, motor planning and control of coordination, balance, strength and endurance are all possibilities. I realise that ‘Task specific Training’ is important for the trunk (13), but functional training that involves trunk muscle activity and muscles around the lumbopelvic-hip region, balance and core stability that are not always classified ‘task specific’ are important (14,15).  They certainly complement tasks and can prepare someone to train toward improved performance in functions such as reaching, pulling, leaning, carrying, eating, and propping with upper limbs. If you have any doubt about the role of training the trunk with upper limb activity following stroke, then I would monitor how much trunk activity is improving movement performance and outcomes both within therapy sessions and over time. The effect size needs to be justified! The whole idea of ‘preferred performance’ is an important part of our teaching in Advanced Neuro Education, in addition to coaching tips for constraint led approaches, sensory facilitation, perturbation and functional training approaches that help bridge the gap toward task-oriented practice.

Human movement after stroke is affected in so many ways, and whole-body sensorimotor control and biomechanics both need to be considered when training movement performance. So, my advice is - Let’s not forget the trunk!

Associate Professor James McLoughlin, 2021

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