"has been firmly linked" is a correct and usable phrase in written English.
You would use it to indicate that there is a strong connection between two things. For example: "The rise in CO2 levels in the atmosphere has been firmly linked to global warming."
The development is a further complication for Pardew, who has been firmly linked with PSV Eindhoven full-back Erik Pieters and Lyon counterpart Aly Cissokho, and a loan move for Manchester City's Wayne Bridge, in recent weeks as well as Maiga and a series of other frontmen.
US military intervention has been firmly linked to the emergence of "inclusive leadership" in Iraq.
The RecQ family of helicases is conserved across multiple species and has been firmly linked to genomic maintenance [ 1– 3].
Although few other studies estimate the magnitude of the effect of food advertising, TV watching as such has been firmly linked to obesity.
Over the last 6 years, genetic variability at >60 genetic loci has been firmly linked to type 2 diabetes risk (68).
Among the various risk factors identified for preterm birth, maternal inflammation, particularly chorioamnionitis, has been firmly linked to preterm birth in epidemiologic and laboratory studies (Romero et al. 2007).
Blocking patterns caused by slow-moving meanders of the jet stream have been firmly linked to some devastating events, including the 2010 summer floods in Pakistan, which killed 2,000 people and affected 20 million, and also the searing heatwave in Russia in the same year, which killed 50,000 people and wiped out $15bn£12bnbn) of crops.
Alteration and/or mutations of the ribonucleoprotein TDP-43 have been firmly linked to human neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD).
Disturbances in any of these processes have been firmly linked to combined OXPHOS deficiency (for a recent review see [ 6]).
A direct link between inflammation and cancer has been firmly established, with NFκB emerging as a key player [51], [52].
The link between glycemic control and microvascular complications has been firmly established (1, 2).
