Generalizing movement patterns following shoulder fixation

Abstract

Generalizing newly learned movement patterns beyond the training context is challenging for most motor learning situations. Here we tested whether learning of a new physical property of the arm during self-initiated reaching generalizes to new arm configurations. Human participants performed a single-joint elbow reaching task and/or countered mechanical perturbations that created pure elbow motion with the shoulder joint free to rotate or locked by the manipulandum. With the shoulder free, we found activation of shoulder extensor muscles for pure elbow extension trials, appropriate for countering torques that arise at the shoulder due to forearm rotation. After locking the shoulder joint, we found a partial reduction in shoulder muscle activity, appropriate because locking the shoulder joint cancels the torques that arise at the shoulder due to forearm rotation. In our first three experiments, we tested whether and to what extent this partial reduction in shoulder muscle activity generalizes when reaching in different situations: 1) different initial shoulder orientation, 2) different initial elbow orientation, and 3) different reach distance/speed. We found generalization for the different shoulder orientation and reach distance/speed as measured by a reliable reduction in shoulder activity in these situations but no generalization for the different elbow orientation. In our fourth experiment, we found that generalization is also transferred to feedback control by applying mechanical perturbations and observing reflex responses in a distinct shoulder orientation. These results indicate that partial learning of new intersegmental dynamics is not sufficient for modifying a general internal model of arm dynamics. NEW & NOTEWORTHY Here we show that partially learning to reduce shoulder muscle activity following shoulder fixation generalizes to other movement conditions, but it does not generalize globally. These findings suggest that the partial learning of new intersegmental dynamics is not sufficient for modifying a general internal model of the arm’s dynamics.

Explicit feedback and instruction do not change shoulder muscle activity reduction after shoulder fixation

“…Although no restrictions were placed on movement trajectories, participants did generate their movements with an arc-like trajectory, as a result of moving predominantly the elbow joint Figure 2 A,C,E (Maeda et al 2017(Maeda et al , 2018(Maeda et al , 2020a(Maeda et al , 2020b . We found substantial shoulder agonist muscle activation prior to movement onset in all experiments as required to compensate for the torques that arise at the shoulder when the forearm rotates about the elbow joint Figure 2 B,D,F (Gribble and Ostry 1999;Maeda et al 2017Maeda et al , 2018Maeda et al , 2020aMaeda et al , 2020b . We then mechanically locked the shoulder joint of the robotic manipulandum, which cancelled the torques that arise at the shoulder with forearm rotation and removes the need to activate the shoulder muscles.…”

“…Movement kinematics (i.e., hand position and joint angles) were sampled by the robotic apparatus at 1,000 Hz and then low-pass filtered (12 Hz, 2-pass, 4th-order Butterworth). In all experiments, data were aligned on movement onset, defined as 5% of peak angular velocity of the elbow joint (See Gribble and Ostry 1999;Maeda et al 2017Maeda et al , 2018Maeda et al , 2020b .…”

“…To compare the changes in amplitude of muscle activity over time and across different phases of the protocols, we calculated the mean amplitude of phasic muscle activity across a fixed time-window, -100 ms to +100 ms, relative to movement onset. These windows were chosen to capture the agonist burst of EMG activity in each of the experiments (See Debicki and Gribble 2005;Maeda et al 2017Maeda et al , 2018Maeda et al , 2020aMaeda et al , 2020b .…”

“…Using this protocol, we tested whether the rates and magnitude of learning these altered arm dynamics during reaching movements can be modulated by explicit strategies and feedback. One group of participants performed this protocol but received no instructions about what to do after their shoulder was locked, a replication of our previous studies (Maeda et al 2018(Maeda et al , 2020b . A second group of participants performed this same protocol with visual feedback about t heir shoulder muscle activity and was instructed to reduce their shoulder activity to zero.…”

Motor learning and transfer: from feedback to feedforward control

“…reflexes) to mechanical perturbations (Wang et al, 2001;Wagner and Smith, 2008;Ahmadi-Pajouh et al, 2012;Yousif and Diedrichsen, 2012;Cluff and Scott, 2013;Maeda et al, 2018Maeda et al, , 2019 . For example, after people learn to generate straight reaching movements in the presence of an external force field or learn to reduce shoulder muscle activity when generating pure elbow movements with shoulder fixation, evoked stretch reflex responses to mechanical perturbations reflect (Ahmadi-Pajouh et al, 2012;Maeda et al, 2018Maeda et al, , 2019 and correlate with (Cluff and Scott, 2013) the learning expressed during self-initiated reaching. Such transfer from feedforward motor commands to feedback responses is thought to take place because of shared neural circuits at the level of the spinal cord, brainstem and cerebral cortex (Pruszynski, 2014;Scott, 2016) .…”