The marker setup and angle definitions. Subjects were marked at the hip (trochanter major), knee, ankle and ball of the foot as well as on the head and the vertebra T1 (head and T1 markers are not shown in the sketch). We calculated the inner angles at the knee (φ_knee) and ankle (φ_ankle) joint. According to the spring–mass model, we defined the leg as the distance between the hip and toe marker. The leg angle (α) is measured clockwise with respect to the negative x-axis.

Leg force and leg length during stance phase of the two subsequent contacts. The solid black lines represent level to level running (track type 0, N=99) and the grey shaded area is ±1 s.d. of this reference run on the undisturbed track, the dotted line from level to 5 cm up (track type 1, N=106), the dashed line from level to 10 cm up (track type 2, N=108) and the dashed-dotted line from level to 15 cm up (track type 3, N=110). (A,B) A quasi-elastic leg operation is observed in both contacts. However, the net energy balances are not zero (see Table 4). (C) The peak leg force is slightly increased in preparation for the consecutive step. (D) However, in the case of a perturbation the maximum leg force decreases in proportion to vertical step height. (E) The leg compression in the first contact is not affected in preparation for the vertical step. (F) Here, the leg length at initial contact (touch-down, TD) is shortened as well as the minimum leg length during contact in proportion to the vertical step height. Thus, leg compression remains almost constant.

Knee (φ_knee) and ankle (φ_ankle) joint angles during the two subsequent stance phases. (A,C) Both knee and ankle do not adapt in preparation for the following step as the mean values are within the mean area ±1 s.d. of the reference run. (B,D) Both adapt in the disturbed second contact as the initial contact joint angle (φ_knee,TD, φ_ankle,TD) as well as the minimum joint angle (φ_knee,min, φ_ankle,min) decrease. For detailed values see Table 2.

Leg stiffness adaptation in the two consecutive contacts. We normalized each leg stiffness value for track types i=1–3 (k̃_leg,i) with a subject-specific reference run on the flat, undisturbed track (X̄_k̃leg,i=0) separately for each contact. Open boxes represent trials for the first contact, grey boxes those for the second contact. Leg stiffness was altered between contacts on bumpy ground (track type 1–3). Significant differences were found for track types 1, 2 and 3 (see Table 3).

Stability plot of a spring–mass simulation dependent on angle of attack (α_TD) and spring stiffness (). Stable running requires a proper adjustment of angle of attack to spring stiffness (Seyfarth et al., 2002). The black J-shaped area guarantees at least 30 following contacts (end of simulation) and is referred to as the self-stable area. The circles (first contact) and squares (second contact) represent the data from the track types i=1–3 of a typical subject running at 4.8±0.16 m s^–1. Two distinct regions of stiffness and angle of attack combinations were found. From the first to the second contact both stiffness and angle of attack decrease in accordance with the results of the simulation. However, in most cases the experimental results do not fit into the area of self-stability but, rather, into an area that guarantees at least five subsequent contacts. Initial parameter of simulation: horizontal component of the initial velocity ν_x,0=4.8 m s^-1, initial apex height y₀=1.0 m, body mass m=80 kg, initial leg length l₀=1 m. The grey scale on the right of the graph indicates how many subsequent steps of stable running can be made with the chosen combination of angle of attack and spring stiffness.

Simulation results of peak spring force (F_spring) and maximum spring compression (Δl) for a 15 cm step in the second contact dependent on a variation of spring stiffness (k̃) and angle of attack (α_TD). All simulations started before the first contact on ground level with identical initial conditions (ν_x,0=4.5 m s^-1, y₀=0.95 m) and system parameters (=35.7, α_TD=68 deg., m=80 kg, l₀=1 m). (A,C) By using a fixed angle of attack and decreasing spring stiffness we found that spring force decreased while spring compression increased. Dash-dotted line, =25.5, α_TD=61 deg.; dotted line =19.1,α _TD=61 deg.; dashed line, =12.7, α_TD=61 deg. (B,D) In the case of varying (steepening) the angle of attack and using a fixed spring stiffness, spring force and spring compression decreased. Dash-dotted line, α_TD=59 deg., =19.1; dotted line,α _TD=61 deg., =19.1; dashed line, α_TD=63 deg., =19.1.