Grip and load force coordination in cyclical isometric manipulation task is not affected by the feedback type

Decrease in force control among older adults under unpredictable conditions

OBJECTIVES

Falls in older adults generally occur during unpredictable situations. Controlling posture through fine-tuned muscle force before and after falls is necessary to avoid serious injuries. However, details regarding force control among older adults during unpredictable situations are unclear. This study determined the features of force control in a random force-tracking task among older adults.

METHODS

Ten healthy older adults (67-76 years) and eight healthy young adults (20-23 years) participated in three force-tracking tasks with ankle plantar flexion: low-range (LR), high-range (HR), and pseudo-random (PR) force tasks. Force control ability was assessed using the root mean square error (RMSE) between the target and muscle forces produced by the participants. Muscle activities from the lateral head of the gastrocnemius and the tibialis anterior during each task were measured using surface electromyography to calculate the co-contraction index (CCI).

RESULTS

In all tasks, older adults (RMSEs: 1.09-3.70, CCIs: 29.4-56.4) had a significantly greater RMSEs and CCIs than young adults (RMSEs: 0.49-1.83, CCIs: 11.7-20.6; all, p < 0.05). The RMSEs during force generation were significantly greater than those during force release (LR: p < 0.01, HR: p < 0.05), except for the random force-tracking task in older adults. CCIs during the force release phase in both groups (older adults: 27.8-56.4, young adults: 15.0-20.6) were consistently greater than those during force generation (older adults: 24.5-50.4, young adults: 11.7-17.2). CCIs in force-tracing tasks differed in older adults, whereas those in the random force-tracing task increased. RMSEs and CCIs in the random and LR force-tracing tasks were significantly negatively correlated with the functional reach test (all: r > 0.5, p < 0.05).

CONCLUSION

Force control in older adults declines in low-band and random muscle force output. Moreover, increased CCIs in older adults are particularly pronounced during unpredictable situations.

Visuomotor processing and hand force coordination in dyslexic children during a visually guided manipulation task

Developmental Dyslexia negatively affects children's reading and writing ability and, in most cases, performance in sensorimotor tasks. These deficits have been associated with structural and functional alterations in the cerebellum and the posterior parietal cortex (PPC). Both neural structures are active during visually guided force control and in the coordination of load force (LF) and grip force (GF) during manipulation tasks. Surprisingly, both phenomena have not been investigated in dyslexic children. Therefore, the aim of this study was to compare dyslexic and non-dyslexic children regarding their visuomotor processing ability and GF-LF coordination during a static manipulation task. Thirteen dyslexic (8-14 YO) and 13 age- and sex-matched non-dyslexic (control) children participated in the study. They were asked to grasp a fixed instrumented handle using the tip of all digits and pull the handle upward exerting isometric force to match a ramp-and-hold force profile displayed in a computer monitor. Task performance (i.e., visuomotor coordination) was assessed by RMSE calculated in both ramp and hold phases. GF-LF coordination was assessed by the ratio between GF and LF (GF/LF) calculated at both phases and the maximum value of a cross-correlation function (rmax) and its respective time lag calculated at ramp phase. The results revealed that the RMSE at both phases was larger in dyslexic than in control children. However, we found that GF/LF, rmax, and time lags were similar between groups. Those findings indicate that dyslexic children have a mild deficit in visuomotor processing but preserved GF-LF coordination. Altogether, these findings suggested that dyslexic children could present mild structural and functional alterations in specific PPC or cerebellum areas that are directly related to visuomotor processing.