Increase in the force of muscular contractions was demonstrated in 1892
by De Sarlo and Barnardini[289] for caffein and by Kraepelin for tea.
These investigators used the dynamometer as a measure of the force of
contraction; however, most of the subsequent work on motor processes has
been by the ergographic method. Ugolino Mosso[290], Koch[291].
Rossi[292], Sobieranski[293], Hoch and Kraepelin,[294] Destree,[295]
Benedicenti,[296] Schumberg,[297] Hellsten,[298] and Joteyko,[299] have
all observed a stimulating effect of caffein on ergographic performance.
Only one investigation of those reported by Rivers failed to find an
appreciable effect, that of Oseretzkowsky and Kraepelin,[300] while
Fere[301] affirms that the effect is only an acceleration of fatigue.

In spite of the general agreement as to the presence of stimulation
there is some dissension regarding whether only the height of the
contractions or their number or both are affected. As might be expected
from the great diversity of methods employed, the quantitative results
also have varied considerably. Carefully controlled experiments by
Rivers and Webber[302] "confirm in general the conclusion reached by all
previous workers that caffein stimulates the capacity for muscular work;
and it is clear that this increase is not due to the various psychical
factors of interest, sensory stimulation, and suggestion, which the
experiments were especially designed to exclude. The greatest increase
... falls, however, far short of that described by some previous
workers, such as Mosso; and it is probable that part of the effect
described by these workers was due to the factors in question."

Investigations of mental processes under the influence of caffein have
been much less frequent, most notable among which are those of Dietl and
Vintschgau,[303] Dehio,[304] Kraepelin and Hoch,[305] Ach,[306]
Langfeld,[307] and Rivers.[308] Kraepelin[309] observes: "We know that
tea and coffee increase our mental efficiency in a definite way, and we
use these as a means of overcoming mental fatigue ... In the morning
these drinks remove the last traces of sleepiness and in the evening
when we still have intellectual tasks to dispose of they aid in keeping
us awake." Their use induces a greater briskness and clearness of
thought, after which secondary fatigue is either entirely absent or is
very slight.

Tendency toward habituation of the pyschic functions to caffein has been
studie