|The "internal force" depiction or model of human movement; as can be contrasted, related, or connected to the "external force" depiction or model of human movement.|
The notion of an “internal force” is a bit of a blurry conception, one not necessarily known or germane to modern science.
This is a consequence of the fact that modern science is framed around the principle of inertia, having historical roots in the impetus theory of motion of Aristotle (Physics, c.350BC); which was updated into the proto inertia model of Leonardo da Vinci (Codex Atlanticus, c.1500) and the later laws of motion as formulated by Isaac Newton (Principia, 1687), all anchored in the model of external forces or outside forces acting on bodies, the inside nature of the “body” not necessarily of importance.
This outside force based laws of motion model, intern, became integrated into the foundation of thermodynamics works of Rudolf Clausius (The Mechanical Theory of Heat, 1865); which, in guise of modern chemical thermodynamics of Gilbert Lewis (Thermodynamics and the Free Energy of Chemical Substances), have since come to be handed down to us as the “system-based” model of quantitative free energy measured "driving forces" of reactions, processes, and transformations; Gibbs free energy, in particular, being the drive of natural earth-bound freely-running processes and reactions. Hence, the notion of “internal forces”, psychodynamic model aside, are a bit of a foreign subject to modern hard physical science.
In any event, German polymath Johann Goethe’s human chemistry protégé Arthur Schopenhauer was the first to notice a possible inconsistency between the newly proposed external factors randomness-framed evolution model of Charles Darwin (1859) and the older plant and animal metamorphosis change model (1786) and human chemical theory model (1796) of Goethe, along with his own will to power model (1818-1844) of human movement. As summarized by Miguel de Unamuno: 
“Judging Darwin’s theory solely by an extensive extract in The Times, [Schopenhauer] described it, in a letter to Adam Louis von Doss (March 1, 1860), as ‘downright empiricism’ (platter Empirismus). In fact, for a voluntarist like Schopenhauer, a theory so sanely and cautiously empirical and rational as that of Darwin left out of account the inward force, the essential motive, of evolution. For what is, in effect, the hidden force, the ultimate agent, which impels organisms to perpetuate themselves and to fight for their persistence and propagation? Selection, adaptation, heredity, these are only external conditions. This inner, essential force has been called will on the supposition that there exists also in other beings that which we feel in ourselves as a feeling of will, the impulse to be everything, to be others as well as ourselves yet without ceasing to be what we are.”
Here we well see the collision of a number of conflicting theories and points of view: namely, Goethe's theory of change and choice being dictated by the force of chemical affinity, atoms up to humans, and the reinterpretation of this model in regards to the "will to power" theory of Schopenhauer, both colliding with Darwin's theory of natural selection, adaptation, and heredity—all rooted historically in the dominate Aristotle-Galileo-Newton model of external forces moving bodies—all heaped percariously on the four letter word "will", which as Goethe famously stated is not free (as the world-dominating Anunian religion model has led us to believe), but rather derives from previous sensory impact, interaction, or exchange forces, as can be quantified temporally, such as via the readiness potential, among other possibilities—a complex issue to say the least.
Austrian psychologist Sigmund Freud, in all clearness, is probably the only one to have made significant advance in the inner force subject, with is bound energy/free energy view of the id-ego-superego model of repression and conserved forces of the mind.
In 1918, German-born American biological physiologist Jacques Loeb opened his Forced Movements, Tropisms, and Animal Conduct the following position: 
“The analysis of the mechanism of voluntary and instinctive actions of animals is based on the assumption that all these motions are determined by internal and external forces.”
On this basis, Loeb went on to outline a pure materialism view of chnopsological (biological) movement.
1. Unamuno, Miguel de. (1912). Tragic Sense of Life (pg.147). Dover, 1954.
2. (a) Thims, Libb. (2007). Human Chemistry (Volume Two) (Loeb, pgs. 475, 523). Morrisville, NC: LuLu.
(b) Loeb, Jacques. (1918). Forced Movements, Tropisms, and Animal Conduct (pg. 13). J.B. Lippincott Co.; Dover, 1978.