= Biomechanics
{wiki}
<Biomechanics> applies <mechanics> to living organisms, from the <forces> exerted by a <muscle> to the deformation of a <biological cell>. It connects geometry, motion, material response and biological activity. A <bone> supporting a load, blood moving through a vessel, and a <muscle> producing work are all mechanical systems, but their living components can remodel, consume chemical energy and change their properties.
A simple <torque> balance explains why an internal <muscle> <force> can greatly exceed an external load. In an idealized stationary forearm, suppose a downward load $W=20\,\mathrm N$ acts with perpendicular <moment arm> $r_W=0.30\,\mathrm m$ about the elbow and the balancing <tendon> <force> has <muscle moment arm> $r_m=0.030\,\mathrm m$. Neglect the forearm’s own <gravitational weight> and other <muscles>. Zero net <torque> requires
$$
F_mr_m=Wr_W,\qquad F_m=\frac{20\times0.30}{0.030}=200\,\mathrm N.
$$
These are perpendicular <moment arms>, not necessarily distances measured along the <bone>. Force balance also requires a joint reaction; the <torque> equation alone does not determine every internal <force>.
Geometry must also be separated from material behavior. A <tendon> with sectional area $20\,\mathrm{mm}^2$ carrying that illustrative $200\,\mathrm N$ has mean axial <stress> $10\,\mathrm{MPa}$. Its extension depends on its length, <constitutive equation> and loading history, not just the applied <force>. Measurements of <stress> against <strain> therefore answer a different question from measurements of <force> against extension. For a living specimen, hydration, temperature and the rate of deformation can all be controlled experimental variables.
An influential motivating experiment is <Archibald Vivian Hill>’s measurement of heat and work during <muscle contraction>. In his 1938 primary paper, a <thermopile> detected tiny temperature changes; an optically amplified <galvanometer> made the response fast enough to study shortening. Comparing contractions under different loads connected shortening speed, mechanical work and liberated heat. Such measurements constrain an active <muscle> model in a way that simply stretching a passive spring cannot.<The heat of shortening and the dynamic constants of muscle by A. V. Hill>[Part I, printed pp. 136–137, apparatus; Parts II–III, load-dependent shortening and contractile/elastic components]{ref} The example captures a central aim of <biomechanics>: use measurable <forces>, motion and energy transfers to test how a biological mechanism works.
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