by Dennis Zacharkow, PT
Mandal (1976, 1981) has been the main advocate of a chair where the seat can slope forward from 15° to 20°. According to Mandal, most work takes place in a forward-leaning posture without use of a backrest, and with excessive flexion of the lumbar spine. Mandal believes the forward-sloping seat will minimize the hip flexion and spinal flexion needed to lean over a desk or table.
Contrary to Mandal's claims that forward-sloping seats will improve one's sitting posture, in practice these chair designs actually destabilize the body and cause the pelvis to slide forward on the seat. An increase in leg muscle activity is then necessary to counteract this forward thrust on the seat, along with friction between the skin, clothes, and seat. This drag on clothing from the relative movement between the sitter and the seat becomes another source of discomfort (Kroemer and Robinette, 1968; Branton, 1970; Ward, 1976; Oborne, 1982; Corlett and Eklund, 1984).
Due to the forward displacement of the center of gravity with a forward-sloping seat, the individual will eventually slump forward and achieve trunk stabilization by leaning heavily on the armrests of the chair, the front edge of the desk, or the wrist rest of the keyboard. In other words, the arms end up functioning to support the collapsed trunk.
Another overlooked issue with forward-sloping seats is the tendency for the ischial tuberosities (sitting bones) to slide forward rather than backward on these seats (Bennett, 1928). When leaning forward over a desk, this sliding forward of the ischial tuberosities will promote spinal flexion, instead of facilitating a flexion at the hips.
In contrast to Mandal's theory is YogaBack inventor Dennis Zacharkow's forward-leaning concept involving a slight backward-sloping seat of approximately 5° along with an adjustable sacral support. The tendency for the ischial tuberosities and pelvis to slide backward rather than forward on a backward-sloping seat will facilitate a flexion at the hips, rather than the lumbar spine, as one leans forward with the pelvis rocking over the ischial tuberosities (Cotton, 1904; Bennett, 1928).
To facilitate the backward sliding of the pelvis, the sitter should concentrate on pressing back against the sacral support while leaning forward (Figure 1). As a result, pelvic stabilization through sacral support can still be provided in a forward-leaning posture over a desk or table. Maintaining gentle pressure against the sacral support in a forward-leaning posture will promote activation of the transversus abdominis and an elongated lumbar spine. The resulting pelvic stabilization will free the arms from functioning to support a collapsed trunk, as frequently occurs with the use of forward-sloping seats.
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