Effect of engagement of postural musculature on energy cost of upper limb exercise

Aim: When performing associated movements of the upper limbs in the horizontal plane, the postural engagement is much higher during isodirectional (ISO) than antidirectional (ANTI) coupling. It may be assumed that, being the moving masses identical in the two cases, any difference in the metabolic cost between the two coupling conditions may be attributed to the involvement of postural musculature. Oxygen uptake (VO2) was therefore measured during ISO and ANTI coupling. Methods: Six participants underwent 4 exercises each consisting of intermittent trials (12s movement, 12s rest) of cyclic adduction-abduction with both arms in the horizontal plane, either in ISO or ANTI coupling and at both 1.4 and 2.0 Hz. Each exercise lasted 5 min. During exercises we recorded the torque (Tz) about the trunk vertical axis exerted on ground, as a global index of postural engagement, and breath-by-breath gas exchanges and heart rate (fH). Results: At steady-state (last minute of each exercise) it resulted that: i) the amplitude of Tz ANTI (Nm±SE) was 0.5±0.2 at both 1.4 and 2,0 Hz, indicating a minimum postural engagement; while Tz ISO raised to about 19±2 and 27±3, respectively (ANTI vs ISO: p always <0.0002); ii) VO2 ANTI (ml•min-1) was significantly lower than VO2 ISO at both 1.4 Hz (477±45 vs 781±73 p<0.002) and 2.0 Hz (631±80 vs 1493±160, p<0.0002); iii) fH ANTI (b•min-1) was significantly lower than fH ISO only at 2.0 Hz (106±10 vs 142±8. p<0.0002). Conclusion: The above results show that the large difference in VO2 between ANTI and ISO exercises should be totally ascribed to a change in the energy expenditure related to postural requests, which are negligible in ANTI and conspicuous in ISO coupling.