Abstract
Objectives: The sit-to-stand (STS) transfer mobilizes an extended part of the kinematic chain throughout a postural phase characterized by a flexion of the trunk and a focal phase consisting of a whole-body extension. The aim of this study was to analyze the variations of the global muscular pattern and the biomechanical parameters in both phases, in relation with seat backrest inclination.Methods: Fifteen participants were asked to stand up from a seat with 5 backrest inclination settings and at 2 execution speeds. The ground reaction forces and the activity levels of fifteen muscles of the trunk and lower limbs were investigated.Results: Backrest-induced modifications were mainly observed in the postural phase: inclining the backrest backward increased the phase duration and the activity level of the sternocleidomastoideus and the rectus abdominis, while it reduced the activity of the tibialis anterior. It also allowed for an increased maximal anteroposterior velocity of the body center of mass. Higher execution speed led to increased and earlier muscular activities of many trunk and lower limbs muscles, predominantly in the postural phase.Discussion: Taken together, these results suggest that a greater backrest inclination increases the demand in the postural phase due to the increase of the upper body gravity torque about the ischial tuberosities, and requires an adaptation of muscular activity levels and timing, but with the same overall pattern. The kinetic energy gained during the longer excursion of the trunk may also require less activation of the lower limbs muscles involved in the generation of propulsive forces of the body.
Highlights
The sit-to-stand (STS) is the demanding and frequent transfer from the seated posture to the standing posture
When no significant interaction effect was found between the two independent variables, namely backrest inclination and velocity, a different table was made for each of these variables containing values averaged over the different modalities of the other variable
A significant effect of backrest inclination was observed on the onset times of 4 muscles, angles higher than 90◦ being associated with delayed onsets for vastus medialis (VM) (p < 0.001), and tibialis anterior (TA) (p < 0.05), and TraS (p < 0.01 but with no significant contrast compared to 90◦), and earlier onset of ES L3 (p < 0.01) (Table 1)
Summary
The sit-to-stand (STS) is the demanding and frequent transfer from the seated posture to the standing posture. Integrating kinetics, Schenkman et al (1990) assumed that trunk flexion moves the body center of mass forward but Backrest Inclination and Sit-to-Stand above all increases the upper body forward momentum This momentum is transferred into a whole-body vertical momentum once the seat is unloaded, allowing for wholebody extension. Quadriceps, together with hamstrings, are activated for seat unloading and lower limb extension, with head (upper trapezius) and spinal extensors guiding the verticalization of the whole body (Munton et al, 1984; Roebroeck et al, 1994; Vander Linden et al, 1994; Rodrigues-de-Paula-Goulart and Valls-Solé, 1999; Bouchouras et al, 2015; Chorin et al, 2016). A recent study exploring the effect of backrest inclination on muscular activity showed an increase in the activity level of upper body flexors (abdominal muscles and SCOM) and ST, together with a decrease in TA activity before seat unloading (Tebbache and Hamaoui, 2020)
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