Fig 1.
Positive and negative work generation during walking and during stair negotiation.
Blue represents the phases during which negative work is generated by the right leg (braking). Red represents the phases during which positive work is generated by the right leg (propulsion). In walking, the right leg both brakes and propels within one stride. In stair negotiation, however, the right leg generates predominantly negative work throughout stair descent (TUL), and predominantly positive work throughout stair ascent (TL). The gait cycle during stair negotiation follows the definition in [1].
Fig 2.
Two ERAS modules with the top and bottom landings.
A) Front view, B) Side view with a user ascending the ERAS. Each ERAS module consists of its own set of extension springs, a pressure sensor, linear guide, movable tread and an electromagnetic latch. The two modules are operated by a single Arduino board (not shown). A force plate at the top landing measures the ground reaction forces of a human user.
Fig 3.
Overview of ERAS human user experiment.
A) Schematic of the ERAS setup. The pictured compression springs are physically implemented in hardware using extension springs (see Fig 2). Participants start each trial on the top landing, storing energy in the springs as they descend the steps. Energy stored in the springs is released back to the user as they ascend the steps. L1 and L2 are the electromagnetic latches, whereas S0, S1 and S2 are the pressure sensors. B) Positive work generated by the knee over TL in each trial over an entire experimental session. Each experiment consisted of 10 pre-assist control trials (blue), 40 assist trials (red), 10 post-assist trials (green) and 10 speed-matched control trials (gray). Solid lines denote mean and thin lines denote one standard deviation across all participants.
Table 1.
Participants’ gender, age, weight, total spring constant, and the weight-normalized spring constant of the ERAS used in the experiments.
Fig 4.
Gait phases during stair negotiation.
A) During ascent, TL begins with the leading foot contact (0%) and ends at mid-swing of the trailing leg (∼32%*). The forward-continuance phase, TFCA, begins at mid-swing and ends at the end of the double-support phase (63.6%†). B) During descent, TUL begins at the second mid-swing of the leading leg (∼70%*) and ends at the trailing leg toe-off (100%). The forward-continuance phase, TFCD, begins with the foot contact (38.8%†) and ends at the mid-swing. *: as defined in [1]. †: as defined in [2].
Table 2.
Work metrics and their definitions.
Fig 5.
Step duration in different stair negotiation conditions.
A) During ascent, step duration on the ERAS was significantly longer than in normal stair negotiation (ASSIST versus PRE). No significant difference was observed in the step duration between ERAS and normal stairs with slower, matched gait speed (ASSIST versus MATCH). Thus, we compared the results (ASSIST) against MATCH (instead of PRE) during ascent. B) During descent, step duration was not different on ERAS versus during normal stair negotiation (ASSIST versus PRE). However, the descent steps in the MATCH condition were significantly longer than on ERAS (ASSIST versus MATCH). Thus, we compared the results (ASSIST) against PRE (instead of MATCH). *** refers to p < 0.001, * refers to p < 0.05, and N.S. refers to no significant difference.
Fig 6.
Joint kinematics and center-of-mass trajectories of participant #9.
A) Joint angle, moment, and power for the hip-sagittal, knee and ankle-sagittal DOFs during TL (0%-32% of the gait cycle during ascent) for ASSIST (red) and MATCH (black) conditions. The solid line indicates the mean trajectory and the dashed lines indicate one standard deviation. B) Center-of-mass height over time during TL (top), and during TUL (bottom). Blue trajectories indicate PRE condition.
Fig 7.
Positive and negative work on ERAS.
Work metrics in the ASSIST condition (shaded boxes, red dots) and in speed-matched control conditions (MATCH for ascent, PRE for descent). Thin and dark gray lines connect each participant’s result in blue, red, and black dots. A) Ascent: Between ASSIST and MATCH conditions, ,
and
were reduced by 17.4 ± 6.9%, 17.8 ± 7.3% and 37.7 ± 10.5%, respectively. PRE trials are also shown for reference (white boxes, blue dots). B) Descent: Between ASSIST and PRE conditions,
,
and
were reduced by 21.9 ± 17.8%, 16.9 ± 21.3% and 26.0 ± 15.9%, respectively.
was also significantly reduced, but the absolute reduction is very small. MATCH trials are also shown for reference (white boxes, black dots). Significance of p < 0.001 are noted as ***, p < 0.01 are noted as **.
Fig 8.
Positive work generated during TFCA.
Positive work generated by the hip-sagittal DOF was significantly higher in the ASSIST condition (0.523 ± 0.119 J/kg) versus the MATCH condition (0.381 ± 0.084 J/kg, p < 0.001). The total positive work generated during TL + TFCA was not significantly different, with a trend towards slight reduction in ASSIST from MATCH (0.088 ± 0.101 J/kg, p < 0.2).
Table 3.
Current energy-recycling devices.