The importance of neuromuscular rate of force development for physical function in aging and common neurodegenerative disorders - a systematic review.

We systematically reviewed existing literature regarding lower extremity neuromuscular rate of force development (RFD), maximal muscle strength (Fmax), and physical function in neurodegenerative populations, and to what extent these outcomes are affected and/or associated. Following PRISMA guidelines, 4 databases (Pubmed, Embase, SPORTDiscus, Web of Science) were searched. Across aging, Parkinson Disease (PD), Alzheimer's Disease (AD), Multiple Sclerosis (MS), or Stroke, included studies should report (Part 1) deficits in lower extremity RFD, Fmax, and physical function (~ individuals having inferior vs. superior physical function), and/or (Part 2) associations between RFD (or Fmax) and physical function. A total of N=32 studies (n=1087 participants) were included. Part 1: deficits in RFD (-31%, mean; N=22) were comparable to deficits in physical function (-26%; N=7), yet both deficits exceeded that of Fmax (-21%; N=20). Part 2: associations between RFD and physical function (r²=0.13, mean; N=16) were comparable to associations between Fmax and physical function ((r²=0.15; N=12). Lower extremity RFD is (1) particularly sensitive (i.e. adapts earlier and/or more extensively) towards neurodegeneration, and more so than Fmax, and (2) of importance for physical function but apparently not superior to Fmax. RFD could serve as a useful indicator/biomarker of changes in neuromuscular function elicited by neurodegeneration.