Unilateral strength training with and without a mirror to improve motor function after stroke: Past,present,and future

1 Introduction

The most recent data show that 17 million people suffer a stroke annually [1]. A stroke causes bilateral impairments in strength and sensorimotor functions, where the paretic side is more affected than the non-paretic side [2]. Resistance training after stroke improves muscle strength, gait speed, motor function and quality of life without worsening spasticity [3]. Common clinical practice after a stroke is to target the more-affected side [4]. However, people with a severe hemiparesis are not able to train this side actively due to muscle weakness and limited range of motion [5], which requires a lot of help from the physiotherapist. An alternative and less labour intensive approach is to train the less-affected side and improve muscle strength and motor function of the more-affected side (i.e., cross-education) [6]. The idea is that cross-education can restore the symmetry between the more- and less-affected side by making use of evolutionary conserved bilateral circuits [7].

2 Cross-education without a mirror

The cross-education of muscle force has been known for over 100 years [8] and corresponds to 52% of the force gained on the trained side in neurologically intact persons [9]. The absolute force increase in the untrained side is 9% for upper limb muscles and 16% for lower limb muscles [10]. It has been suggested that the cross-education effect is too small to be of clinical relevance (9) but recent studies in people who have had a stroke prove otherwise [11, 12]. To illustrate, six weeks of isometric dorsiflexion strength training with the less-affected leg improved dorsiflexion strength 34% in the less-affected leg and 31% in the more-affected leg [11]. Four participants who were unable to produce measurable force with the more-affected leg prior to the intervention were able to do so after training the less-affected leg. Such positive cross-education effects are also observed for the upper extremity [12]. The bilateral improvements in strength after unilateral training are likely caused by changes in cortical activation and motor cortical excitability, ultimately increasing the motor output from the primary motor cortex (M1) to the trained and untrained muscles [13]. Interhemispheric inhibition from the trained to untrained (M1) also plays a role in cross-education, as it shows a monotonic decrease over multiple sessions of unilateral strength training [14]. Following a stroke, disinhibition of the unaffected primary motor cortex enhances transcallosal inhibition to the affected primary motor cortex, which impairs the recovery of motor function on the affected side [15]. Cross-education training might improve the recovery after stroke by reducing the amount of inhibition between M1 s. However, the involvement of spinal circuits in cross-education cannot be ruled out, as reciprocal inhibition between antagonist and agonist muscle in the more-affected side was more like the less-affected side following unilateral training [11]. Altogether, strength training the less-affected side induces neural changes at cortical, corticospinal, and spinal level which enhances the recovery of the more affected side after a stroke.

3 Cross-education with a mirror

Mirror therapy is widely used by therapists in the rehabilitation after stroke and uses the cross-education principle to improve motor function, activities of daily living and pain in the more-affected side by training the less-affected side [16]. In mirror therapy, a mirror is placed in the midsagittal plane between the less- and more-affected side, so that viewing the less-affected limb exercising in a mirror creates the visual illusion that the more-affected limb is moving [16]. Motor tasks that are performed in mirror therapy studies are gross, functional and fine motor movements that require low voluntary muscle forces [13, 16]. As people after a stroke suffer from muscle weakness on the more- and less-affected side, mirror therapy should also target muscle weakness using forceful unilateral muscle contractions. A cross-education study in neurologically intact persons illustrates that the cross-education of muscle force is 27% increased when the training was performed with versus without a mirror [17]. This augmented cross-education effect was accompanied by a reduced inhibition of inhibitory interneurons in the M1 [17]. It is suggested that mirror-neurons, which discharge action potentials both during action observation and execution [18], also contributed to the augmented cross-education effect [13, 19]. There is no unified model yet of how mirror training might work but mirror therapy likely requires the integration of three functional networks (i.e., attentional resources, the mirror-neuron system, and the motor network) [20]. Regardless of the underlying mechanisms, mirror therapy while using unilateral forceful contractions might be a successful tool to restore bilateral weakness after a stroke.

The ‘Neuroplasticity Research Group’ in the Institute of Technology Sligo Clinical Health and Nutrition Centre (CHANCE) is currently exploring the benefits of unilateral strength training with and without a mirror post-stroke. Preliminary results from our trials in people with chronic stroke have indicated that mirror aided cross-education training results in significant improvements in the untrained more-affected limb for strength, spasticity, motor function and self-perceived participation. Strength improved in upper limb (21%) and lower limb (10%), spasticity improved in shoulder, elbow, wrist, knee and ankle, motor function improved in upper limb (Chedoke Arm and Hand Activity Inventory), and lower limb (10 m walk test), and self-perceived participation (London Handicap Scale) significantly improved in both upper and lower limb studies. This may not seem very important at first glance but new treatments that use the less affected side of the body to allow patients to independently rehabilitate in their home environment and save exhaustive therapy sessions, are to be welcomed.

The positive outcomes of this feasibility randomised controlled trial have led to the development of the ‘Mirror Strengthening Brace’ which Enterprise Ireland has targeted for commercial funding. This device facilitates our main goal, namely the development of home-based rehabilitation protocols that engage the less affected side of the body to improve strength and motor function of the more affected side. These rehabilitation protocols should save the patient and therapist from exhaustive and expansive therapy sessions.

4 Future

The ‘Neuroplasticity Research Group’ was set up in 2012 by Dr Monaghan and is part of an International collaborative network that focuses on the interaction between limbs to improve rehabilitation outcomes after neurological impairment and orthopaedic injuries. Future studies will elaborate on the cross-education of muscle strength using a mirror in combination with virtual/augmented reality environments. The idea is that the mirror image can be incorporated into the virtual environment while the patient is using the less affected limb. Another research project involves the lower extremities of people with stroke, in which we examine if audio-visual imaging of the more affected side can restore gait asymmetry while walking on a treadmill. We hypothesise that mirror visual feedback works to improve symmetry of step length during gait, and may in fact work by reducing the step length of the less affected side. The usage of evolutionary conserved bilateral circuits is an innovative and promising approach in the rehabilitation of unilateral orthopaedic and neurological impairments. Thus, the application of cross-education is not only relevant for stroke but also for multiple sclerosis [21] and patients recovering from a fracture [22], anterior cruciate ligament reconstruction [23], arthroplasty, fibromyalgia, and arthritis. The results look promising but more convincing evidence is needed before these forms of treatment will be incorporated in the standard protocols for neurologic and musculoskeletal rehabilitation.

Conflict of interest

The authors have no conflicts of interest to disclose.