Table 2.
Experimental studies investigating muscle synergies in Parkinson’s disease.
| [Ref] | Subjects | State of Therapy | Recorded Muscles | Experimental Task | Synergy Extraction | Main Findings | Conclusions | |
|---|---|---|---|---|---|---|---|---|
| [87] | 15 PD and 14 HS |
ON | Eight leg muscles bilaterally: SOL, GM, TA, VM, RF, SM, BF, GLM | 10 minutes walking on a treadmill | NMF and %VAF | 95% of PD require four or fewer muscle synergies, compared to 57% HS. Similar muscle weights but shifted muscle activation profile in PD. Association between walking speed and total %VAF in PD | Altered timing of modular activation may be responsible for abnormal motor control during gait in PD, rather than different muscle weighting vectors | |
| [88] | Nine PD | ON and OFF |
Eight leg muscles bilaterally: SOL, GM, TA, VM, RF, SM, BF, GLM | Overground walking and walking on a treadmill | NMF and %VAF | No differences between ON and OFF therapy for total %VAF, NoS and the muscle weighting vector. Negative correlation between total %VAF and walking speed, but no correlations with other spatiotemporal gait parameters | Dopaminergic therapy does not influence the number, structure or timing of muscle synergies | |
| [71] | 11 PD 11 HS |
ON | 13 leg and trunk muscle of the right side: TA, SOL, GM, GL, BF, ST, RF, VL, VM, TFL, ESL, EST, RA |
Quiet standing, voluntary sway, releasing a load and fast body motion | PCA analysis with Varimax rotation and factor extraction | Four muscle synergies identified using PCA with rotation. Muscle synergies account for a lower amount of variance in PD (71.5±1.74%) than HS (78.3±1.74%). Muscle synergies are predictors of centre of pressure changes in all subjects | Organization of muscles into muscle synergies is less consistent in PD compared with HS | |
| [83] | 10 PD | ON and OFF | 13 leg and trunk muscles of the right side: TA, SOL, GM, GL, BF, ST, RF, VL, VM, TFL, ESL, EST, RA |
Quiet standing, voluntary sway, releasing a load and fast body motion | PCA analysis with Varimax rotation and factor extraction | Four muscle synergies identified using PCA with rotation. Muscle synergies account for a larger amount of variance in PD during ON (74.7±2.4%) than OFF (68.6±2.2%) therapy. Muscle synergies are predictors of centre of pressure changes | In PD, dopaminergic therapy makes the organization of muscles into muscle synergies more consistent during postural tasks | |
| [90] | 10 PD | ON with DBS-OFF or DBS-ON | Three leg and trunk muscles of the right side: TA, SOL, GM, GL, BF, ST, RF, VL, VM, TFL, ESL, EST, RA |
Quiet standing, voluntary sway, releasing a load | PCA analysis with Varimax rotation and factor extraction | In postural tasks, four muscle synergies were identified using PCA with rotation. Muscle synergies account for similar amounts of variance in DBS-OFF (75.3±2.9%) and DBS-ON (75.1±2.9%). Muscle synergies are predictors of centre of pressure changes regardless of DBS status | DBS does not influence the organization of muscles into muscle synergies | |
| [91] | 10 PD and 10 HS |
ON and OFF | six upper body muscles bilaterally: PM, DP, BB, TB, EXOB, ESL | Standing while balancing external yaw perturbation | NMF and %VAF | Higher values of total %VAF in PD than HS for NoS less than 4. Similar total %VAF during OFF and ON therapy. NoS positively correlate with MMSE scores and negatively with sub-item 3.14 of UPDRS-III (“body bradykinesia”) | PD use a lower number of muscle synergies to maintain balance. l-dopa does not influence muscle synergies during yaw postural perturbations. | |
| [93] | 6 PD | ON | 13 lower back and right leg muscle: RA, EXOB, EST, GLM, TFL, BF, VM, GM, GL, SOL, PL | Overground walking trial and standing while balancing a ramp-and-hold external perturbation before and after a rehabilitation program (three weeks of daily adapted tango classes) | NMF and %VAF | No differences in NoS after rehabilitation training. Rehabilitation improves motor module distinctness (i.e., well-defined biomechanical output between modules), consistency (reduced variability within motor modules) and generalizability (increased sharing of motor modules across gait and balance tasks) | Within- and between-module parameters (e.g., consistency, distinctness and generalizability) reflect motor performance in PD better than NoS | |
| [92] | 10 PD and 8 HS | ON | Six right arm and upper body muscles: PM, DP, BB, TB, FR, ER | Resting tremor and reaching task with and without transcutaneous electrical stimulation of the radial nerve | NMF and %VAF | Three muscle synergies were found both in resting tremor and in reaching tasks. Cutaneous stimulation does not alter synergy vectors, but differently change the time profile of muscle synergies during resting tremor and reaching tasks | The different effects of cutaneous electrical stimulation on vector patterns and the time profile of muscle synergies may imply different spinal pathways for these signals | |
%VAF: Variability Account For; BB: biceps brachii; BF: biceps femoris; DBS-OFF: inactive deep brain stimulation; DBS-ON: Active deep brain stimulation; DP: deltoideus – posterior portion; ER: extensor carpi radialis; ESL: erector spinae lumbar region; EST: erector spinae thoracic region; EXOB: external oblique; FR: flexor carpi radialis; GL: gastrocnemius lateralis; GLM: gluteus medius; GM: gastrocnemius medialis; HS: healthy subjects; MMSE: Mini-Mental State Examination; NMF: Non-Negative Matrix Factorization; NoS: Number of Synergies; OFF: not under dopaminergic therapy; ON: under dopaminergic therapy; PCA: Principal Component Analysis; PD: patients with Parkinson’s disease; PL: peroneus longus; PM: pectoralis major; RA: rectus abdominis; RF: rectus femoris; SM: semimembranosus; SOL: soleus; ST: semitendinosus; TA: tibialis anterior; TB: triceps brachii; TFL: tensor fasciae latae; UPDRS-III: Unified Parkinson’s Disease Rating Scale- part III; VL: vastus lateralis; VM: vastus medialis.