Age does not affect the relationship between muscle activation and joint work during incline and decline walking

Older compared with younger adults walk with different configurations of mechanical joint work and greater muscle activation but it is unclear if age, walking speed, and slope would each affect the relationship between muscle activation and net joint work. We hypothesized that a unit increase in positive but not negative net joint work requires greater muscle activation in older compared with younger adults. Healthy younger (age: 22.1 yrs, n = 19) and older adults (age: 69.8 yrs, n = 16) ascended and descended a 7° ramp at slow (~1.20 m/s) and moderate (~1.50 m/s) walking speeds while lower-extremity marker positions, electromyography, and ground reaction force data were collected. Compared to younger adults, older adults took 11% (incline) and 8% (decline) shorter strides, and performed 21% less positive ankle plantarflexor work (incline) and 19% less negative knee extensor work (decline) (all p < .05). However, age did not affect (all p > .05) the regression coefficients between the muscle activation integral and positive hip extensor or ankle plantarflexor work during ascent, nor between that and negative knee extensor or ankle dorsiflexor work during descent. With increased walking speed, muscle activation tended to increase in younger but changed little in older adults across ascent (10 ± 12% vs. -1.0 ± 10%) and descent (3.6 ± 10.2% vs. -2.6 ± 7.7%) (p = .006, r = 0.47). Age does not affect the relationship between muscle activation and net joint work during incline and decline walking at freely-chosen step lengths. The electromechanical cost of joint work production does not underlie the age-related reconfiguration of joint work during walking.