Characterization of age-related modifications of upper limb motor control strategies in a new dynamic environment

Abstract

Background: In the past, several research groups have shown that when a velocity dependent force field is applied during upper limb movements subjects are able to deal with this external perturbation after some training. This adaptation is achieved by creating a new internal model which is included in the normal unperturbed motor commands to achieve good performance. The efficiency of this motor control mechanism can be compromised by pathological disorders or by muscular-skeletal modifications such as the ones due to the natural aging process. In this respect, the present study aimed at identifying the age-related modifications of upper limb motor control strategies during adaptation and de-adaptation processes in velocity dependent force fields.

Methods: Eight young and eight elderly healthy subjects were included in the experiment. Subjects were instructed to perform pointing movements in the horizontal plane both in a null field and in a velocity dependent force field. The evolution of smoothness and hand path were used to characterize the performance of the subjects. Furthermore, the ability of modulating the interactive torque has been used as a paradigm to explain the observed discoordinated patterns during the adaptation process.

Results: The evolution of the kinematics during the experiments highlights important behavioural differences between the two groups during the adaptation and de-adaptation processes. In young subjects the improvement of movement smoothness was in accordance with the expected learning trend related to the consolidation of the internal model. On the contrary, elders did not show a coherent learning process. The kinetic analysis pointed out the presence of different strategies for the compensation of the external perturbation: older people required an increased involvement of the shoulder with a different modulation of joint torque components during the evolution of the experiments.

Conclusion: The results obtained with the present study seem to confirm the presence of different adaptation mechanisms in young and senior subjects. The strategy adopted by young subjects was to first minimize hand path errors with a secondary process that is consistent with the optimization of the effort. Elderly subjects instead, seemed to shift the importance of the two processes involved in the control loop slowing the mechanism optimizing kinematic performance and enabling more the dynamic adaptation mechanism.

Figure 1

Evolution of the of the smoothness parameters N.Jerk throughout the experiment in one of the eight direction. Blue line = young group; red line = elderly group.

Figure 2

Hand path trajectories traced by elderly subjects. a) soon after the field application (exercise 3). b) when the field was turned off (exercise 5).

Figure 3

Comparison between the two different experimental protocols. Red line is relative to the first adopted experiment protocol. Blue line shows the behaviour in the second version of the experiment protocol, when subjects prolonged de adaptation phase in exercise 5.

Figure 4

Individual torque profiles at the shoulder and at the elbow of relative to motion toward right direction. Positive values correspond to flexion torques and negative values to extension. Upper side: NF condition; Bottom side: VF field condition.

Figure 5

Torque sign analysis. Mean percentage of movement duration for the elbow and shoulder during which MUSC or IT coincided in sign with NT. The asterisks indicate when the differences between young and elders are significant.

Figure 6

MT _E values for elderly and young groups in adaptation and de- adaptation phases. Bottom side: after the removal of the field (exercise 6) young subjects continued to move with a MUSC _E torque higher than necessary: differences between exercise 2 and 6 are significant in all direction except W; upper side: elders soon restored the more economic solution in terms of effort.