Mechanical work as a function of load mass. (A) Average positive COM work, , and summed joint work per stride, (labeled C+ and J+, respectively), along with negative work (C− and J−). (B) Positive work by ankle, knee and hip joints (A+, K+ and H+, respectively; negative work also shown), compared with summed joint work per stride (J+). Linear trends (denoted by lines, N=8) were fit to each measure, and were statistically significant (P<0.05) in all cases except for negative knee work and positive hip work. Open symbols represent normal unloaded walking (with error bars for s.d.). Left-hand vertical axes show dimensionless quantities; right-hand vertical axes show dimensional SI units.