The dynamic activity in massive star forming regions prior to the formation of bright protostars is still not fully investigated .
In this work we present observations of HCO ^ { + } J = 1 - 0 and N _ { 2 } H ^ { + } J = 1 - 0 made with the IRAM 30m telescope towards a sample of 16 Herschel -identified massive 70 \mu m quiet clumps associated with infrared dark clouds .
The clumps span a mass range from 300 M _ { \odot } to 2000 M _ { \odot } .
The N _ { 2 } H ^ { + } data show that the regions have significant non-thermal motions with velocity dispersion between 0.28 km s ^ { -1 } and 1.5 km s ^ { -1 } , corresponding to Mach numbers between 2.6 and 11.5 .
The majority of the 70 \mu m quiet clumps have asymmetric HCO ^ { + } line profiles , indicative of significant dynamical activity .
We show that there is a correlation between the degree of line asymmetry and the surface density \Sigma of the clumps , with clumps of \Sigma \gtrsim 0.1 g cm ^ { -2 } having more asymmetric line profiles , and so are more dynamically active , than clumps with lower \Sigma .
We explore the relationship between velocity dispersion , radius and \Sigma and show how it can be interpreted as a relationship between an acceleration generated by the gravitational field a _ { \mathrm { G } } , and the measured kinetic acceleration , a _ { \mathrm { k } } , consistent with the majority of the non-thermal motions originating from self-gravity .
Finally , we consider the role of external pressure and magnetic fields in the interplay of forces .