Hi guys, if I can add my 2 cents worth on the subject of gravity (I've forgotten most of my Physics degree but still remember a bit of mechanics). Whilst I don't think gravity offers a magic source of energy, it does indirectly assist us when running by using a bit of lean. There's 2 reasons for this, neither of which are unique to running as they apply when we are on a Segway too; the first applies just when when we have net forward acceleration and the second applies increasingly the faster we go (air resistance).
When setting off, to accelerate yourself forward there needs to be a net forward force on your body, which is applied via friction where your feet (or wheels if on a Segway) touch the ground. As this net force does not resolve through your centre of mass, a net torque is applied to your body, tending to rotate you (=make you fall) backwards. You can counteract this by leaning forwards, so the normal reaction force with the ground (equal and opposite to the force of gravity on our body) is no longer in line with your centre of mass, creating an opposing torque. To illustrate this consider what would happen if the Segway didn't require you to lean forward before accelerating (it would leave a big lump on the back of the head and sales would plummet).
This is why sprinters are leant forwards at the start of races (consider what position they start in the blocks in, centre of mass is well ahead of feet). Of course once up to speed then no net accelerating force is required, although air resistance now applies an increasingly decelerating force the faster we go. This force is applied across our body, and needs to be countered at the feet by a net force in the opposite direction (so a force analagous to the one above needed to accelerate). This creates the same problem of torque, and hence the solution of a bit of forward lean to counter.
Net position of our centre of mass relative to feet (aka "net lean") is therefore is something which is dictated to us if we don't want to end up on our a$$es. However how we make up that net lean (eg with body and legs in a single line, or leaning body and straight legs, or straight legs and leaning body) is the space in which different running forms may exist. Net lean required is biggest for sprinters, due to both greater acceleration and greater speed (and therefore air resistance). For ultra runners less net lean is required, which is ironic as they are usually much leaner than sprinters.
When setting off, to accelerate yourself forward there needs to be a net forward force on your body, which is applied via friction where your feet (or wheels if on a Segway) touch the ground. As this net force does not resolve through your centre of mass, a net torque is applied to your body, tending to rotate you (=make you fall) backwards. You can counteract this by leaning forwards, so the normal reaction force with the ground (equal and opposite to the force of gravity on our body) is no longer in line with your centre of mass, creating an opposing torque. To illustrate this consider what would happen if the Segway didn't require you to lean forward before accelerating (it would leave a big lump on the back of the head and sales would plummet).
This is why sprinters are leant forwards at the start of races (consider what position they start in the blocks in, centre of mass is well ahead of feet). Of course once up to speed then no net accelerating force is required, although air resistance now applies an increasingly decelerating force the faster we go. This force is applied across our body, and needs to be countered at the feet by a net force in the opposite direction (so a force analagous to the one above needed to accelerate). This creates the same problem of torque, and hence the solution of a bit of forward lean to counter.
Net position of our centre of mass relative to feet (aka "net lean") is therefore is something which is dictated to us if we don't want to end up on our a$$es. However how we make up that net lean (eg with body and legs in a single line, or leaning body and straight legs, or straight legs and leaning body) is the space in which different running forms may exist. Net lean required is biggest for sprinters, due to both greater acceleration and greater speed (and therefore air resistance). For ultra runners less net lean is required, which is ironic as they are usually much leaner than sprinters.