Table 2.

Main information of the studies selected in this review. Single session studies, Stroke Patients (SP).

Study	DIS	IL	CS	N Task	N Cond	PAT	HS	MUS	ALGO	ΔN	ΔW	ΔC
Cheung et al., 2009	SP	FMA 18-64	/	7	1 (free)	8 (3 + 5); 60.1	6; 66.7	(12–16) + (12–16)	NMF	Both groups showed a similar number of synergies.	Similar muscle synergies weights across arms and groups were observed.	Cortical impairment altered the activation pattern for downstream muscle synergies.
Kung et al., 2010	SP	FMA 12-56	/	4	1 (robot)	12 (11 + 1); 46.4	8 (6 + 2); 50.5	8 + 8	PCA	/	/	There were abnormal synergies in the affected side of the stroke patients during the tracking movements.
Cheung et al., 2012	SP	FMA 0-66	Yes	12 × 2	1 (free)	31 (17 + 14); 61.6	0	(10–16) + (10–16)	NMF	A reduced or increased number of synergies was observed in the affected arm compared to the unaffected arm.	Preservation, merging, or fractionation of muscle synergies was found in the affected arm.	/
Roh et al., 2013	SP	FMA 12-23	/	54	1 (robot)	10 (5 + 5); 62.3	6 (4 + 2); 63.2	8	NMF	Four synergies were extracted from both groups.	The elbow synergies were typically retained, while shoulder-related synergies were altered in stroke individuals and were correlated with impairment level.	/
García-Cossio et al., 2014	SP	FMA 0–11/24	Yes	6	1 (free)	33 (21 + 12); 55	0	8 + 8	NMF	The number of synergies in the paralyzed limb was slightly reduced compared to the healthy one.	The merging of healthy muscle synergies was a predominant pattern in patients. The merging or fractionation was not related to the cortex integrity.	/
Roh et al., 2015	SP	FMA 12-66	/	54	1 (robot)	24 (14 + 10); 57.5	6 (4 + 2); 63.2	8	NMF	Four synergies were extracted from both groups.	The elbow synergies were typically retained, while shoulder-related synergies were altered in stroke individuals and were correlated with impairment level.	/
Scano et al., 2017	SP	FMA 11-61	Yes	1	1 (free)	22 (15 + 7); 56.6	0	8	NMF	/	Five basic clusters were identified to characterize a group of stroke patients, and synergies clustering did not correlate with the clinical assessments.	/
Li et al., 2017	SP	FMA 18-32	Yes	2	1 (free)	10 (9 + 1); 60.9	9 (5 + 4); 57.8	7	NMF	/	Pathological synergies of patients were altered from the characteristics of baseline synergy with missing or altered weights and time profiles.	The similarity indices correlated to the kinematic performance and FMA score.
Scano et al., 2018	SP	FMA 12-64	/	1	2 (with and without robot)	22 (15 + 7); -	0	8	NMF	/	Six out of seven paired synergies were very strongly similar between free and robot-assisted hand-to-mouth movement.	There was a trend of a reduction in the magnitude of the activation coefficients during interaction with the robot, though it lacked statistical significance.
Pan et al., 2018	SP	FMA 9-51	Yes	1	1 (free)	35 (27 + 8); 60.3	25 (13 + 12); 59.2	7	NMF	Three synergies were extracted from both groups.	Stroke altered the structure of muscle synergies, which could be characterized as the merging of control group synergies.	/
Israely et al., 2018	SP	FMA 50.8	/	9	1 (free)	13 (7 + 6); -	12; -	8	NMF	Four synergies were extracted from both groups.	The synergies of the study group and the control group were similar to the synergies of the representative. Two synergies were not matched between groups, but the difference was not significant.	The control group exhibited a gradual change in the synergy activation in the amplitude, whereas the study group exhibited consistently significant differences between all movement directions and the representative set of synergies.
Runnalls et all., 2019	SP	FMA 9-66	/	14	3 (weight support at three levels)	13 (9 + 4); 70.8	6 (4 + 2); 65.2	8	NMF	Controls and patients with mild impairment showed more synergies with high-weight support. The control group expressed more synergies compared to patients in the moderate-severe group while there was no difference between the mild and moderate-severe groups.	Co-contraction of three deltoid muscles was found in the stroke group.	/
Kim et al., 2020	SP	BS Ⅱ-Ⅳ	/	8 × 2	1 (free)	11 (5 + 6); 56.82	7; 24.86	4	HALS	The study did not find a significant difference in the number of synergies between groups.	Clustering analysis showed that corresponding clusters of mild and control groups showed the highest similarity. In contrast, only three clusters showed a similarity over 0.9 between severe and control groups.	/
Chou et al., 2020	SP	BS Ⅲ-Ⅳ	/	2	2 (free)	5 (5 + 0); 60	8 (6 + 2); 28.1	7	NMF	/	/	/
Wang et al., 2020	SP	FMA 27-61	Yes	36 × 2	1 (free)	15 (9 + 6); 52.1	15 (10 + 5); 48.5	10	PCA	/	/	/
Pellegrino et al., 2021	SP	FMA 5-63	/	8	3 (three force fields)	25 (16 + 9); 60.4	25; -	16 + 16	NMF	There was no difference in the number of synergies between groups.	In absence of force fields, the right-brain damage group had weight coefficients of muscle synergies less altered than the left-brain damage group in both arms and more similar between the two sides of the body.	The difference between the two arms of each subject in terms of activation coefficients was greater in stroke subjects than in controls for all tasks. The similarity of the activation coefficients with those of the controls was task-dependent, which was slightly higher when interacting with the resistive force field than in the other tasks.
Pan et al., 2021	SP	FMA 11–51/52	Yes	1	1 (free)	34 (26 + 8); 59.8	25 (13 + 12); 59.2	7	NMF	Three synergies were extracted from both groups.	Compared with the control group, muscle synergies were altered in stroke subjects, and synergy patterns in the mild-to-moderate group were more similar to the control group.	/
Irastorza-Landa et al., 2021	SP	FMA 2–33.5/54	Yes	5	1 (free)	18 (12 + 6); 54.7	0	8 + 8	NMF	The paretic limb showed a slightly lower number of optimal clusters. No significant differences were observed in the number of synergies in the paretic limb between pre- and post-therapy.	No significant differences were observed in the three muscle synergy features between pre- and post-therapy. The number of synergies showed a weak but significant correlation with motor performance.	The synergy index significantly increased after therapy and correlated with motor function.
Ma et al., 2021	SP	FMA 22-60	/	1	1 (robot)	5; 67.8	11; 26.7	7	MCR-ALS	Three synergies were extracted from each healthy subject, while three and four synergies were identified from three and two patients, respectively.	/	/
Park et al., 2021	SP	FMA 12-64	Yes	4 × 2	1 (free)	16 (14 + 2); 51.6	8 (3 + 5); 54.3	13	NMF	Four and five muscle synergies in the stroke and control groups were observed, respectively.	The composition of muscle synergies was comparable between the groups, except that the three heads of the deltoid muscle were co-activated and formed one synergy in the stroke group, whereas those muscles formed two synergies in the control group.	The modulation of synergy activation coefficients was altered after a stroke.
Wang et al., 2021	SP	BS Ⅲ-Ⅳ	/	1 × 2	1 (robot)	2 (0 + 2); 53	12 (10 + 2); 25	6	NMF	The number of synergies of patients decreased compared with the healthy group.	/	The activation coefficients of patients were different from the healthy group.
Sheng et al., 2022	SP	FMA 13-51	Yes	1 × 2	1 (robot)	9 (6 + 3); 67.2	7 (4 + 3); 47	8	NMF	Two synergies were extracted from both groups.	Low synergy similarity between groups indicates that the muscle synergies of patients were different from the control group.	Patients had a lower amplitude in activation coefficients than the healthy subjects.
Seo et al., 2022b	SP	FMA 12-23	/	54	1 (robot)	10 (5 + 5); 61.8	6 (4 + 2); 63.2	8	NMF	Four synergies were extracted from both groups.	Stroke groups showed abnormal deltoid modules compared to the control group.	The PCA and the multivariate multiple linear regression analyses showed that the alterations in motor modules were associated with abnormal between-force coupling.
Liu et al., 2022	SP	FMA 12-23	/	54	1 (robot)	10 (5 + 5); 62.3	6 (4 + 2); 63.2	8	NMF	/	/	/
Funato et al., 2022	SP	FMA 6-64	Yes	37	1 (free)	20 (20 + 0); 54.5	7; -	41	NMF	Severe stroke subjects had a significantly smaller number of synergies, while it did not differ significantly between healthy participants and those with mild stroke. The average number of synergies was close between groups.	Some synergies in stroke survivors corresponded to merged standard synergies and the merging rate increased with the impairment of stroke survivors.	/