Fig 1.
Schematic of experimental setup.
A. Schematic of the anatomical arrangement of muscles recorded during the experiment. Abbreviations are as follows: the clavicular head of pectoralis (Pec), teres major (TerM), anterior deltoid (ADel), posterior deltoid (PDel), the long and lateral heads of triceps (TriL and TriS), the short and long heads of biceps (BiS and BiL), brachioradialis (Brd), flexor carpi radialis (FCR), flexor carpi ulnaris (FCU), and extensor carpi radialis (ECR). Blue dots indicate locations of LED markers for motion capture. Target locations illustrate the reaching directions from the central target (green). Drawing is not to scale. B. The target locations and the top view of the starting position in the medial and lateral workspaces for the right arm. The starting position is in magenta, the target – in green, and other targets – in shades of gray. The same reaching movements were also performed with the left arm in the left lateral workspace and the common central workspace.
Fig 2.
Examples of kinematics, dynamics, and muscle activity profiles during reaching.
Each plot shows profiles per condition averaged across repetitions (n = 15) of the same reaching movement in one direction by one participant. A. Profiles (solid lines) and standard deviation (shaded areas) of shoulder flexion-extension angle and angular velocity. B. Shoulder flexion-extension torque (t) profiles that caused the movements in A. C. Normalized electromyography (EMG) profiles of anterior deltoid (ADel), triceps long (TriL), and biceps long (BiL) that accompanied the movements in A. Shaded areas show the standard error for the mean across repetitions (n = 15) of the same movement.
Table 1.
Variance accounted for during reaching with the right and left arm in the lateral and medial workspaces.
Fig 3.
Principal components of EMG and muscle torques.
Data from the four conditions indicated by the pictograms are shown in plots arranged in columns. Solid lines show profiles of features in time, colors indicate individuals. A. Gravity-related postural features in EMG and muscle torques. B. Propulsion-related dynamic features in EMG and muscle torques. EMG V1 and EMG V2 are the 1st and 2nd eigenvectors obtained from EMG, respectively; MT V1 and MT V2 are the 1st and 2nd eigenvectors obtained from muscle torques, respectively; MTP V1 and MTD V1 are the 1st eigenvectors obtained from the postural and dynamic components of muscle torque, respectively.
Fig 4.
Method for calculating linear regressions between PC scores across reaching directions.
A. Normalized electromyography profiles (norm EMG) and the first 2 principal components (EMG V1 and EMG V2) are shown for 4 reaching directions in the right lateral workspace by the same subject as in Fig 2. Arrows within circles illustrate the reaching directions as in Fig 1B. Blue labels show estimates of scores for EMG V1 for two movement directions. B. Circles show scores for posterior deltoid (x axis) and anterior deltoid (y axis) across all movement directions, including outward and inward movements for the same subject. Scores labeled in A are shown with blue arrows. The line shows the linear regression, which was not significant in this instance.
Fig 5.
Correlations between the temporal eigenvectors derived from EMG and muscle torques.
Dots show maximal R2 values from individual cross-correlations between profiles in Fig 3, red lines show mean values, and grey boxes show standard deviations. EMG V1 and EMG V2 are the 1st and 2nd eigenvectors obtained from EMG, respectively; MT V1 and MT V2 are the 1st and 2nd eigenvectors obtained from muscle torques, respectively; MTP V1 and MTD V1 are the 1st eigenvectors obtained from the postural and dynamic components of muscle torque, respectively. Brackets and stars show significant differences summarized in Table 2.
Table 2.
Post-hoc differences in coefficient of determination between reaching workspaces and principal component features.
Table 3.
Post-hoc differences in coefficient of determination between males and females.
Fig 6.
Postural and dynamic scores from PCA on EMG for outward and inward reaches.
Dots show individual scores averaged across muscles and movement directions, red lines show mean values, and grey boxes show standard deviations. EMG V1 and EMG V2 are the 1st and 2nd eigenvectors obtained from EMG, respectively. Llat and Lmed indicate reaching with the left arm in lateral and medial workspaces, respectively; Rlat and Rmed indicate reaching with the right arm in lateral and medial workspaces, respectively.
Fig 7.
Postural scores from PCA on EMG per muscle.
Dots show individual scores averaged across movement directions, red lines show mean values and grey boxes show standard deviations. EMG V1 and EMG V2 are the 1st and 2nd eigenvectors obtained from EMG, respectively. Llat and Lmed indicate reaching with left arm in lateral and medial workspaces, respectively; Rlat and Rmed indicate reaching with right arm in lateral and medial workspaces, respectively. Muscle abbreviations: the clavicular head of pectoralis (Pec), teres major (TerM), anterior deltoid (ADel), posterior deltoid (PDel), the long and lateral heads of triceps (TriL and TriS), the short and long heads of biceps (BiS and BiL), brachioradialis (Brd), flexor carpi radialis (FCR), flexor carpi ulnaris (FCU), and extensor carpi radialis (ECR).
Table 4.
Post-hoc differences in postural EMG V1 score between muscles.
Table 5.
Post-hoc differences in postural V1 score between reaching directions.
Fig 8.
Postural scores from PCA on EMG per reaching direction.
Data from the four conditions indicated by the pictograms are shown in plots arranged in rows. Dots show individual scores averaged across muscles, lines show mean values, and shaded boxes show standard deviations, blue indicates outward reaches, and grey indicates inward reaches. Circles with arrows show reaching directions; black circles indicate reaching in the horizontal plane; blue circles indicate reaching downwards with gravity in the vertical plane; red circles indicate reaching upwards against gravity in the vertical plane.
Fig 9.
Example regression matrix between postural scores from PCA on EMG.
The coordinates of each dot represent principal component scores for two muscles for a single reaching direction, as in Fig 4B. Muscle abbreviations: the clavicular head of pectoralis (Pec), teres major (TerM), anterior deltoid (ADel), posterior deltoid (PDel), the long and lateral heads of triceps (TriL and TriS), the short and long heads of biceps (BiS and BiL), brachioradialis (Brd), flexor carpi radialis (FCR), flexor carpi ulnaris (FCU), and extensor carpi radialis (ECR). Histograms along the diagonal show the distribution of the scores for a given muscle across reaching directions. Solid lines show least-squares linear regression, and red lines indicate significant relationships with correction for family-wise error, the adjusted alpha = 0.0008.
Fig 10.
Gravity-related muscle co-contraction.
Heatmaps show coefficients of determination (R2) from regressions shown in Fig 10 averaged across 9 participants. Red and orange colors represent strong and moderate relationships, respectively, between EMG V1 coefficients across reaching directions. Pictograms indicate conditions for left reaching in lateral (A) or medial workspace (B) and for right reaching in lateral (C) and medial workspace (D). Muscle abbreviations: the clavicular head of pectoralis (Pec), teres major (TerM), anterior deltoid (ADel), posterior deltoid (PDel), the long and lateral heads of triceps (TriL and TriS), the short and long heads of biceps (BiS and BiL), brachioradialis (Brd), flexor carpi radialis (FCR), flexor carpi ulnaris (FCU), and extensor carpi radialis (ECR).
Fig 11.
Propulsion-related muscle co-contraction.
Heatmaps show coefficients of determination (R2) from regressions shown in Fig 4 averaged across 9 participants. Red, orange and darker blue colors represent moderate and strong relationships between EMG V2 coefficients across reaching directions. Pictograms indicate conditions for left reaching in lateral (A) or medial workspace (B) and for right reaching in lateral (C) and medial workspace (D). Muscle abbreviations: the clavicular head of pectoralis (Pec), teres major (TerM), anterior deltoid (ADel), posterior deltoid (PDel), the long and lateral heads of triceps (TriL and TriS), the short and long heads of biceps (BiS and BiL), brachioradialis (Brd), flexor carpi radialis (FCR), flexor carpi ulnaris (FCU), and extensor carpi radialis (ECR).
Fig 12.
The number of muscle pairs with correlated scores.
Dots show the number of moderate or strong (coefficient of determination R2 > 0.5) linear regressions between pairs of different muscles out of 66 muscle pairs for each participant. Red lines and shaded areas show means and standard deviations, respectively. The left plot is based on regressions between EMG V1 scores; the right plot is based on regressions between EMG V2 scores. Llat and Lmed indicate reaching with the left arm in the lateral and medial workspaces, respectively; Rlat and Rmed indicate reaching with the right arm in the lateral and medial workspaces, respectively.