Kinematic synergies and equilibrium control during trunk movement under loaded and unloaded conditions

The aim of the present investigation was to study the adaptation of the kin ematic synergy responsible for equilibrium control during upper trunk movem ents to a 10-kg load added to the subject's shoulders. Five adult subjects were asked to bend their upper trunk forward to an angle of 35 degrees and then to hold the final position for 3 s, first without any load and then wi th a 10-kg load fixed to their shoulders. The final anteroposterior CM posi tions 400 ms after the movement offset, the time course of the anteroposter ior center of mass (CM) shift during the movement, the EMG pattern of the m ain muscles involved in the movement and the initial CP shift were studied under both unloaded and loaded conditions. The kinematic synergy was quanti fied by performing a principal components analysis on the hip, knee and ank le angle changes occurring during the movement. The results indicate that: (1) the final anteroposterior position of the CM changed little if at all i n the presence of the additional load, and that the anteroposterior CM shif t was minimized throughout the duration of the movement; (2) the kinematic synergy was still characterized, in the presence of the additional load, by a strong coupling between the angle changes, as indicated by the fact that the first principal component (PC1) accounted for more than 99% of the hip , knee and ankle joint movements. A change was observed, however, in the ra tio between the angles: the ankle extension increased, thus compensating fo r the additional theoretical forward CM shift that the additional load coul d be expected to cause; (3) the lack of change in the initial backward CP s hift observed under loaded condition as well as the lack of change of the i nitial agonist EMG bursts suggest that the initial feedforward control of t he kinematic synergy was not affected in the presence of the additional loa d. An increase in the antagonist bursts, presumably reflecting an adaptatio n of the kinematic synergy, was observed during the late phase of the movem ent; and (4) it is concluded that the adaptation of the kinematic synergy t o the load was due to a specific change in the feedback control during the braking phase of the movement which presumably increases the ankle joint ex tension and consequently causes an increased backward shift of the hip whic h compensates for the forward shift due to the load.