Performing sitting transfers figures among the most demanding activities in manual wheelchair users living with spinal cord injury (MWU_SCI). Surprisingly, little is known about how the built environment may change movement strategies and upper extremity (UE) muscle demands during transfers. This study aims to determine to what extent the use of an armrest changes UE muscle demands during lateral sitting transfers in MWU_SCI.

Twenty MWU_SCI performed lateral transfers toward the left under three experimental conditions: 1. without armrest, 2. with a left armrest at 9cm, 3. with a left armrest at 18cm above seat surface. Bilateral recorded EMG signals of the anterior deltoid, pectoralis major, biceps, and triceps were selected for analysis. Muscular utilization ratio (MUR) was obtained through amplitude-normalization using maximum voluntary contractions. Peak MUR and area under the MUR curve (AUC) of each muscle were compared across conditions using a non-parametric ANOVA.

When transferring using the 9-cm armrest, the peak MUR increased in both triceps and decreased in left anterior deltoid, while the AUC decreased in right biceps compared to transfers without armrest. When transferring using the 18-cm armrest, the peak MUR increased in both triceps and decreased in left anterior deltoid and biceps, while AUC decreased in both biceps compared to transfers without armrest. Furthermore, the peak MUR increased in left pectoralis major compared to transfers using the 9-cm armrest.

The use of an armrest during sitting transfers shifts part of the UE muscle demands from the shoulder flexors to the elbow extensors in MWU_SCI. This adaptation may be linked to a reduced forward trunk flexion during transfer, although a confirmatory kinematic analysis is needed. Nonetheless, the availability of an armrest during transfers provides additional opportunities to vary UE muscle demands and to minimize UE musculoskeletal risk exposure.