RESULTS: Angular Kinematics

Segment angle. The trunk segment moved through a similar range of motion for both ascent and descent (15 deg and 11 deg, respectively). During ascent the trunk flexed slightly as the motion began (to -7 deg at 2.08 s) and extended as the motion reached completion (7 deg at 1.45 s). Descent began with the trunk extending (maximum extension -12 deg at 1.127 s) and concluded with the trunk almost upright (minimum -1 deg at 1.65s).

Figure 3. Trunk segment angle for ascending stairs (left) and for descending stairs (right). The segment angle was calculated as the angle between the segment and the vertical axis. The vertical axis corresponded to the anatomical position, an angle of zero degrees. In both movements increasing values represent clockwise rotation.

Joint angle 1. The range of motion at the knee was slightly greater for ascent (100 degrees versus 91 degrees). The peak extension (182 deg at 1.155 s) was reached halfway through ascent. The peak flexion (82 deg at 1.881 s) was reached just before the conclusion of the motion. This was comparable to peak flexion (84 deg at 0.99 s), reached halfway through descent, and peak extension (175 deg at 1.65 s), reached at the conclusion of the motion.

Figure 4. Knee joint angles for ascending stairs (left) and for descending stairs (right). The joint angle was calculated as the relative angle between the thigh and the lower leg segment. It corresponded to anatomical position; an angle of 180 degrees. For both movements, extension was represented by increasing angles and flexion by decreasing angles.

Joint angle 2. Ascending stairs caused the hip to go through a larger range of motion than descending stairs (60 deg to 31 deg). Ascent began with the hip in flexion and proceeded to peak extension halfway through the motion (188 deg at 1.155 s). The peak flexion was reached at the end of the motion (128 deg at 2.013 s), far greater motion than descent peak flexion (154 deg at 1.155 s). Descending stairs caused the hip to reach peak extension near the beginning of the motion (185 deg at 0.264 s).

Figure 5. Hip joint angles in ascending stairs (left) and descending stairs (right). The joint angle was calculated as the relative angle between the trunk and the thigh. It corresponds to the anatomical position of 180 degrees. For both movements, extension is represented by increasing angles and flexion by decreasing angles.

Joint velocity. Velocities for the motions were similar in magnitude but opposite in direction. Ascent increased fairly evenly and became a smooth curve as the leg extended. Velocity sharply decreased as the knee flexed, and reached peak flexion velocity (-388 deg/s at 1.683 s) during the swing phase. Peak extension velocity (193 deg/s at 2.046 s) occurred at the conclusion of the motion. Descent also increased fairly evenly (reaching peak flexion velocity (207 deg/s at 0.726 s)). A sharp decrease in velocity was due to rapid knee extension (peak extension velocity (-384 deg/s at 1.188 s)).

Figure 6. Knee joint angular velocity for ascending stairs (left) and descending stairs (right). In ascent (left), positive velocity values represent extension. In descent (right), positive velocity values represent flexion due to the change in direction.

Angle-Angle Plot. Hip and knee motion was more tightly coupled in descent than in ascent. Ascent began with hip and knee flexion. The first change in direction was a sequential change. The knee reached peak flexion and the hip angle did not change. The hip then began extension while the knee angle remained flexed. The second change in direction (extension to flexion) was coupled, as seen by the coordination in joint angles. During descent, change of direction was well coupled and the motions were well synchronized.

Figure 7. Coordination of knee and hip joint angles in ascent (left) and descent (right). The arrows indicate the direction of movement.