UNASSIGNED: Smartphone usage has increased rapidly in the last decade due to rapid technological advancements. This extensive usage of smartphones led to physiological problems and musculoskeletal disorders (MSD) due to inappropriate postures.
UNASSIGNED: Past studies have reported the effects/discomfort of smartphone usage for short periods, ranging from 3-20 min, which does not represent the current era of smartphone dependency (approximately 7-hr/day).
UNASSIGNED: This study was performed on fifty-four participants aged 20-28 with 1-hr smartphone texting in a sitting posture. Flexion angles, both sides (dominant and non-dominant) muscle activity (maximum voluntary contraction %) at sternocleidomastoid (SCM), upper-trapezius (TRP), extensor digitorum (EDM) and abductor pollicis brevis (APB) muscles, and subjective discomfort were analyzed.
UNASSIGNED: After 1-hr smartphone texting, the muscle activity at eight upper extremities: SCM (24.80%), TRP (29.45%), EDM (14.44%) and APB (19.87%) significantly (p < 0.001) increased by 1.5-1.9 times with 27.4±3.18° and 82.94±7.03° head and lower-arm flexion angles, respectively. The mechanical loads on the cervical spine increased by 4.6 times, and subjective discomfort by 3 times.
UNASSIGNED: Maintaining the same posture for a long duration causes postural stress, muscular imbalances, and discomfort, leading to MSD with increased cervical intervertebral disc pressure.

This study recruited 30 young participants (15 men and 15 women) to examine the smartphone usage patterns in three postures (standing, supported sitting, and unsupported sitting) and at five head angle (HA) positions (0°-40°). Cervical erector spinae (CES) and upper trapezius (UTZ) muscle activity, neck flexion (NF), gaze angle (GA), viewing distance (VD), and discomfort scores were collected. Results showed that HA and posture almost affected all responses, while CES muscle activity, NF, and VD differed between sexes. Strain in the neck and shoulder region increased with HA increase. Particularly, when the HA exceeded 20°, the discomfort scores considerably increased. Unsupported sitting should be avoided during smartphone use because of relatively poor responses in all variables. However, both standing and supported sitting have their respective benefits. Sex-related differences were typically observed in the standing position, with women tending to have lower NF but higher CES muscle activity compared with men.Practitioner summary: Although smartphones have become daily necessities, the overall quantitative neck and shoulder strain of using smartphones in different postures is rarely evaluated. We suggest that maintaining the HA within 20° is recommended because of relatively low load on the neck and shoulders. An unsupported sitting should be avoided during smartphone use.

This study examined a system-level perspective to investigate the changes in the whole trunk and head postures while sitting with various lower extremity postures.
Sitting biomechanics has focused mainly on the lumbar region only, whereas the anatomy literature has suggested various links from the head and lower extremity.
Seventeen male participants were seated in six lower extremity postures, and the trunk kinematics and muscle activity measures were captured for 5 s.
Changes in the trunk-thigh angle and the knee angle affected the trunk and head postures and muscle recruitment patterns significantly, indicating significant interactions between the lower extremity and trunk while sitting. Specifically, the larger trunk-thigh angle (T135°) showed more neutral lumbar lordosis (4.0° on average), smaller pelvic flexion (1.8°), smaller head flexion (3.3°), and a less rounded shoulder (1.7°) than the smaller one (T90°). The smaller knee angle (K45°) revealed a more neutral lumbar lordosis (6.9°), smaller pelvic flexion (9.2°), smaller head flexion (2.6°), and less rounded shoulder (2.4°) than the larger condition (K180°). The more neutral posture suggested by the kinematic measures confirmed significantly less muscular recruitment in the trunk extensors, except for a significant antagonistic co-contraction.
The lower and upper back postures were more neutral, and back muscle recruitment was lower with a larger trunk-thigh angle and a smaller knee angle, but at the cost of antagonistic co-contraction.
The costs and benefits of each lower extremity posture can be used to design an ergonomic chair and develop an improved sitting strategy.