im at uni now. but a good way to update a client speed is using ham.
Check if user has a knife as current weapon then do your stuff
PHP Code:
new Ham:Ham_Player_ResetMaxSpeed = Ham_Item_PreFrame;
RegisterHam( Ham_Player_ResetMaxSpeed, "player", "bacon_ResetMaxSpeed", 1 );
public bacon_ResetMaxSpeed( id )
{
if( is_user_alive( id ) )
{
new Float:flMaxSpeed = your speed;
engfunc( EngFunc_SetClientMaxspeed, id, flMaxSpeed );
set_pev( id, pev_maxspeed, flMaxSpeed );
}
}