Effects of verbal instruction on maximal isometric force and rate of force development in people with multiple sclerosis and healthy controls

Background: Isometric maximum force (MaxF) and rate of force development (MaxRFD) obtained from sustained contractions are two important outcomes used in clinical and research settings that evaluate strength and power of an individual, respectively. These outcomes are often impaired in people with multiple sclerosis (PwMS) and are associated with functional limitations. The values of these isometric outcomes can be influenced by the verbal instructions given to the participants prior to contraction. Objective: We investigated the effects of two common instructions - “as hard as possible” (AH) and “as fast and as hard as possible” (AFH) - on MaxF and MaxRFD in both upper and lower extremities in people with multiple sclerosis (PwMS) and in controls. Methods: Twenty-seven PwMS and age- and gender-matched healthy controls completed isometric assessments of grip force (GF) and knee extension (KE) under both AH and AFH instructions. Results: Instructions did not significantly affect MaxF in either group or muscle group (p>0.05). However, AFH instruction yielded a significantly higher MaxRFD compared to AH in both GF and KE across both groups (p