PRESWING

Orientation of ground reaction force vector (GRFV) in sagittal plane

Normal GRF is located

• anterior to ankle joint
• posterior to knee joint
• posterior to hip joint

What effect will this have on joint motion and muscle activation?

PRESWING

Orientation of ground reaction force vector (GRFV) in frontal plane

Normal GRF decreases in magnitude as body weight is shared between two limbs. The "shorter" vector reflects this. The vector passes near the axes of the subtalar, knee, and hip joints.

What effect will this have on joint motion and muscle activation?

During preswing, ground reaction force produces

• a dorsiflexor moment at the ankle joint
• a flexor moment at the knee
• an extensor moment at the hip

The body controls these moments with

• activity in the ankle plantarflexors. The plantar flexors' activity becomes concentric as they develop the important "push off" force for propulsion.
• little or no muscle activity at the knee. The knee flexors are not necessary to initiate knee flexion as long as the ground reaction force is located posterior to the knee. At faster walking speeds, the rectus femoris acts to control excessive knee flexion. At very slow speeds, the ground reaction force is inadequate to flex the knee, and the hamstrings are active.
• continued hip flexor activity; the hip flexors' eccentric terminal stance activity changes to isometric, then concentric activity during preswing and initial swing.

compare to frontal plane

During preswing, ground reaction forces produce very small frontal plane moments around the subtalar, knee, and hip joints. The body need not develop significant muscle activity to respond to these frontal plane moments.