It's the moment on any individual muscle that is a measure of how hard it is working - hence an advanced strength-building standing pose in yoga can easily place a bigger moment on each muscle in a relatively small group of muscles cf individual moments in each muscle in a whole-body weight-lifting exercise. It's simply a case of understanding biomechanics.
Hence the effectiveness of yoga for building muscle strength with a relatively low level of injury.
Animals that maintain amazing strength, agility and flexibility do not do so through lifting weights - they achieve that through movements using purely their own body weight. Yoga-like poses can be easily observed in cats and rabbits for example.
F 5 ft 3. HW: 196 lbs. Primal SW (May 2011): 182 lbs (42% BF)... W June '12: 160 lbs (29% BF) (UK size 12, US size 8). GW: ~24% BF - have ditched the scales til I fit into a pair of UK size 10 bootcut jeans. Currently aligning towards 'The Perfect Health Diet' having swapped some fat for potatoes.