The relative capabilities of natural and artificial muscles for lifting a weight can also be compared, but must be normalized to muscle mass per muscle length to provide a fair comparison. For instance, human soleus muscle (a powerful muscle in the back of the lower leg) has an average resting length of 0.361 m (31). At a 1,037 kg/m3 density and a 0.35 MPa maximum stress (19) this natural muscle provides 337 N·m/kg as the performance metric for weight lifting. In contrast, an 8.49 mm length of coiled, 127 μm diameter nylon 6,6 monofilament (C=1.1) weighing 228 μg provided reversible work against a 1.06 N tensile force, thereby yielding 39.5 kN·m/kg as the performance metric for weight lifting. Hence, based on the ratio of these performance figures, the coiled nylon can lift 117 times higher weight than can be lifted by natural soleus muscle having the same length and mass.