A predictive model to quantify joint torques and support reaction forces when using a smartphone while standing with support.

The present study had a dual objective: (1) to present and validate a predictive model of standing posture in the sagittal plane, joint torques and support forces for a smartphone user built from biomechanical principles; (2) propose risk scales for joint torques and reaction forces based on simulations in order to use them into the musculoskeletal disorders prevention. Comparison of the modelled data with experimental measurements (400 tested postures with sample size verification) for calling and texting tasks highlights the model's ability to correctly estimate posture and reaction forces on the ground. The model was able to provide estimates of the range of variation of each parameter for a wide range of environmental conditions as a function of the user body mass index (setting between 12.5 and 50). Joint torques risk scales have been constructed, especially for shoulder and elbow, to characterise the risks incurred by the users. The proposed model enables the postures, joint torques and reaction forces to be estimated from subject's body mass index and environmental configuration without resorting to experimentation, which is relevant in industry. This approach allows the proposition of new scales based on joint torques to reinforce the recommendations for MSDs prevention. BMI: body mass index; LUBA: postural loading on the upper body assessment; MSDs: musculoskeletal disorders; RULA: rapid upper limb assessment; WHO: World Health Organization.