Post by Admin on Jan 24, 2016 19:46:29 GMT
Calculating Joint Torque
When thinking about how much torque is required about a joint to articulate a limb, we must consider the weight, length & centre of mass of the attached limb, & also the weight, centre of gravity & sum of inertial forces acting above/supported by the joint.
Because the force required to move a mass linearly increases the further away from the fulcrum it gets, it is beneficial at this stage to overview our average persons weight & height/length illustration again.
Recapping on our regional weights based on an average 70kg person;
Weight Breakdown of 70kg Person
Head/Neck: 4.97kg (7.1%)
Torso: 33.81kg (48.3%)
Upper Arm (each): 2.31kg (3.3%)
Lower Arm (each): 1.33kg (1.9%)
Hand (each): 0.42kg (0.6kg)
Upper Leg (each): 7.35kg (10.5%)
Lower Leg (each): 3.15kg (4.5%)
Foot (each): 1.05kg (1.5%)
If we also assume the person is 170cm in height;
Height Breakdown of 1.7M Person
Head/Neck: 23.46cm (13.8%)
Torso: 51cm (30%)
Upper Arm (each): 29.24cm (17.2%)
Lower Arm (each): 26.69cm (15.7%)
Hand (each): 17.68cm (10.4%)
Upper Leg (each):39.44cm (23.2%)
Lower Leg (each): 41.99cm (24.7%)
Foot (each): 7.14cm (4.2%)
From here, we able to get an better understanding of the torque acting at a given joint to a) move limb only
& b) move max possible 'external' weight (including 'internal' limb weight).
Because we need weight & distance (centre of mass) values to calculate torque, it is useful to combine the previous weight & distance values into one table for quick reference;
Area/Weight/Height Breakdown
Head/Neck: 4.97kg/23.46cm
Torso: 33.81kg/51cm
Upper Arm: 2.31kg/29.24cm
Lower Arm: 1.33kg/26.69cm
Hand: 0.42kg/17.68cm
Upper Leg: 7.35kg/39.44cm
Lower Leg: 3.15kg/41.99cm
Foot: 1.05kg/7.14cm
We can now use this table in each of our exercise examples to guide our torque requirements for both the simple movement of our limbs only, and furthermore the torque required to perform an average 1 Rep Max movement.
Note: the foot distance is actually as viewed on a standing person, ie, from the floor/heel to the ankle. However as the average foot length, ie, achillea heel to toe tip is approx 26cm, and because of its lightweight it is perhaps fine/negligible using the 7.14cm measurement.
********************************
We have so far discussed the torque required at the Joint (DOF) itself, however the human body employs Mechanical Advantage in its use of muscle force, insofar the muscle fibres insertion point is usually some distance from the joint (DOF) itself, subsequently requiring less force than if it were inserted/acting on the joint directly.
See illustration below,..
This leads up to consider what if we did not follow the traditional biped design philosophy of inserting the actuators in a direct-drive (directly on DOF) configuration? Instead attaching the actuator at a distance away from the joint, we gain the mechanical advantage the human body employs via the insertion of muscle a number of centimetres away from the DOF, subsequently requiring less force to affect movement.
When thinking about how much torque is required about a joint to articulate a limb, we must consider the weight, length & centre of mass of the attached limb, & also the weight, centre of gravity & sum of inertial forces acting above/supported by the joint.
Because the force required to move a mass linearly increases the further away from the fulcrum it gets, it is beneficial at this stage to overview our average persons weight & height/length illustration again.
Recapping on our regional weights based on an average 70kg person;
Weight Breakdown of 70kg Person
Head/Neck: 4.97kg (7.1%)
Torso: 33.81kg (48.3%)
Upper Arm (each): 2.31kg (3.3%)
Lower Arm (each): 1.33kg (1.9%)
Hand (each): 0.42kg (0.6kg)
Upper Leg (each): 7.35kg (10.5%)
Lower Leg (each): 3.15kg (4.5%)
Foot (each): 1.05kg (1.5%)
If we also assume the person is 170cm in height;
Height Breakdown of 1.7M Person
Head/Neck: 23.46cm (13.8%)
Torso: 51cm (30%)
Upper Arm (each): 29.24cm (17.2%)
Lower Arm (each): 26.69cm (15.7%)
Hand (each): 17.68cm (10.4%)
Upper Leg (each):39.44cm (23.2%)
Lower Leg (each): 41.99cm (24.7%)
Foot (each): 7.14cm (4.2%)
From here, we able to get an better understanding of the torque acting at a given joint to a) move limb only
& b) move max possible 'external' weight (including 'internal' limb weight).
Because we need weight & distance (centre of mass) values to calculate torque, it is useful to combine the previous weight & distance values into one table for quick reference;
Area/Weight/Height Breakdown
Head/Neck: 4.97kg/23.46cm
Torso: 33.81kg/51cm
Upper Arm: 2.31kg/29.24cm
Lower Arm: 1.33kg/26.69cm
Hand: 0.42kg/17.68cm
Upper Leg: 7.35kg/39.44cm
Lower Leg: 3.15kg/41.99cm
Foot: 1.05kg/7.14cm
We can now use this table in each of our exercise examples to guide our torque requirements for both the simple movement of our limbs only, and furthermore the torque required to perform an average 1 Rep Max movement.
Note: the foot distance is actually as viewed on a standing person, ie, from the floor/heel to the ankle. However as the average foot length, ie, achillea heel to toe tip is approx 26cm, and because of its lightweight it is perhaps fine/negligible using the 7.14cm measurement.
********************************
We have so far discussed the torque required at the Joint (DOF) itself, however the human body employs Mechanical Advantage in its use of muscle force, insofar the muscle fibres insertion point is usually some distance from the joint (DOF) itself, subsequently requiring less force than if it were inserted/acting on the joint directly.
See illustration below,..
This leads up to consider what if we did not follow the traditional biped design philosophy of inserting the actuators in a direct-drive (directly on DOF) configuration? Instead attaching the actuator at a distance away from the joint, we gain the mechanical advantage the human body employs via the insertion of muscle a number of centimetres away from the DOF, subsequently requiring less force to affect movement.