SAQMG2

Epidermal grooves are the creases visible on the surface of skin that form geometric patterns on a microscopic level. Epidermal ridges are the ridges apparent at only some sites and are due to the organised arrangement of the dermal papillae, giving rise to the characteristic fingerprints.

The primary determining factors of the mechanical properties of skin are the fibrous networks of collagen and elastin fibres and the interstitial fluid of the dermis. For the keratinous skin of the soles of the feet and palms of the hand, the thick, keratinous outer layer of the epidermis is also important.

The problem of tearing of a specimen being tensile tested in vitro may be minimised by ensuring that the specimen is clamped evenly so that stress concentrations are not produced and by using adhesive.

There are three phases in a typical tensile stress-strain curve produced in vitro.

Residual strain is the offset of the loading curve to the unloading curve. It gradually reduces to around zero over about six cycles.

Artefacts produced by the initial handling of a skin specimen may be reduced by preconditioning the specimen by subjecting it to several loading cycles until the residual strain is effectively reduced to zero.

If the body position during a tensile test in vivo is not standardised the initial skin tension will vary, so that the duration of the phase I of the stress-strain curve will vary according to the amount of pretension.

Langer lines are evidence of the anisotropic nature and the pretension of skin.

The extensibility of skin varies from site to site according to the functional requirements of the skin.

Three parameters that may be determined from an indentation test are: depth of indentation, initial recovery, and the recovery after a fixed period.

The two parameters may be controlled during an indentation test are applied load and depth of indentation.

The environment of the specimen must be controlled during creep and stress relaxation tests performed in vitro, to prevent the specimen drying out.

The time that an object can be held with a temperature above 43ºC is inversely related to its temperature.

The improvement of the signal could be caused by the reduction in the resistance of the stratum corneum as the subject started to sweat and may indicate that the skin was not adequately prepared. This could be eliminated by adequately preparing the site or by using needle electrodes. The artefact in phase with the subject's walking cycle is probably due to the changes in skin resistance as the subject moves, stretching the skin under the electrodes, and changing the position of the skin relative to the underlying tissues. This could be reduced by placing the electrodes where the skin movement is minimal and by using needle electrodes.

Bony prominences, such as the sacrum and trochanter, are particularly vulnerable to pressure sores.

By using the pressure-time tolerance curve in Figure 14, state whether or not the following pressure applications are safe or unsafe: (i) pressure of 10 kPa for 3 hours - safe (it is below the curve) (ii) pressure of 10 kPa for 8 hours - unsafe (it is above the curve) (iii) pressure of 60 kPa for 1 hour - unsafe (it is above the curve).

A non-blanchable erythemia response indicates that there is severe damage to the skin with leakage of blood into the skin tissue.

The procedures used in Hagisawa's study mimicked the technique used clinically to detect tissue distress.

The skin thickness generally decreases by 20% with age.

The age-related changes observed in indentation test parameters are: slower to reach total indentation, slower to recovery from indentation, and reduction in total indentation.

Two main assumptions usually adopted for a link segment model are that the body segments are rigid and the joints are frictionless.

In inverse dynamics the motion is measured and the resultant force iscalculated.

The three types of forces that act on a body segment are external, internal and inertial.

It is justifiable to treat a link segment model as quasi-static only when theaccelerations are small.

The results of this SAQ are dependent on your own dimensions. As an exampleI will use the following dimensions, height = 193 cm, measured upper limblength = 75 cm, and measured lower limb length = 100 cm.Length of upper limb is calculated as follows using the ratios given in Figure 9:LUPPER = (0.186 + 0.146 + 0.108) H = 0.440 × 193 = 85 cmLength of lower limb is calculated as follows:LLOWER = (0.186 + 0.146 + 0.108) H = 0.530 × 193 = 102 cmIf the calculated limb lengths are compared to measured limb lengths it can be seen that the calculated lower limb lengths are reasonable estimates. However, the calculated upper limb length is 10 cm out. This illustrates the diversity of human dimensions and why standardised data sets should only be used when actual measurements are not available.

To calculate the mass of the thigh, mTHIGH, the ratio reported in Table 1 is used:mTHIGH = 0.100mBODY = 0.100 × 80 = 8.0 kgTo calculate the position of the centre of mass of the thigh relative to itsproximal end, IPROXIMAL, the ratio, 0.433, reported in Table 2 is used:IPROXIMAL = 0.433ITHIGH = 0.433 × 390 = 169 mmTo calculate the position of the centre of mass of the thigh relative to itsproximal end, IDISTAL, the ratio, 0.567, reported in Table 2 is used: IDISTAL = 0.567ITHIGH = 0.567 × 390 = 221 mmThe mass of the thigh is 8.0 kg, and its centre of mass is located 169 mm distal to the greater trochanter and 221 mm proximal to the femoral condyles.Note that the centre of mass is closer to the proximal end than to the distal end. This is mainly due to the shape of the thigh which narrows towards its distal end so that more mass is distributed towards the proximal end. The position of the centre of mass is also dependent on the densities of the various tissues, such as bone, muscle and fat, and how they are distributed.

Calculating the mass of the lower leg and foot complex, m, using the ratio,0.061, in Table 1:m = 0.061 × 75 = 4.575 kgCalculating the radius of gyration about the centre of mass, kCM, using the ratio 0.416, given in Table 2:kCM = 0.416L = 0.416 × 450 × 10-3 = 0.1872 = 0.19 mCalculating the moment of inertia about the centre of mass, ICM:ICM = mAkCM = 4.575 × 0.1872 × 0.1872 = 0.1603 = 0.16 kg m2Calculating the radius of gyration, kP, using the ratio 0.735, and moment ofinertia, IP, about the proximal end:kP = 0.735L = 0.735 × 450 × 10-3 = 0.33075 = 0.33 mIP = mkP = 4.575 × 0.33075 × 0.33075 = 0.50048 = 0.50 kg m2Calculating the radius of gyration, kD, using the ratio 0.572, and moment ofinertia, ID, about the distal end:kD = 0.572 × L = 0.572 × 450 × 10-3 = 0.2574 = 0.26 mID = mkD = 4.575 × 0.2574 × 0.2574 = 0.3031 = 0.30 kg m2The radius of gyration about the proximal end is larger than the radius ofgyration about the distal end because more mass is distributed further from the proximal end than the distal end.

The insertion point of the muscle which connects the two segments does not change. However, the line of action of the muscle line changes as the two segments move relative to one another.

If the sign of the inter-segment force is positive then it means that it is acting to push the segments together.

The joint force is generally much larger than the inter-segment force because it includes muscle forces.

The inter-segment force is the resultant of all the forces crossing the jointbetween two segments.The joint force is the force acting between two segments at the joint.The muscle force is the force produced by the muscles crossing the joint.The bone-on-bone force is the force acting between the bones that form a joint.The ligament force is the force carried by ligaments, joint capsules and muscles that are not contracting.

The net external joint moment is equal and opposite to the net internal jointmoment.

If the mass was doubled then the quadriceps moment would need to bedoubled to maintain static equilibrium.

The interacting filaments in the sarcomeres are called the actin and myosinfilaments or the thin and thick myofilaments.

The maximum active muscle tension is produced when the muscle lengthis approximately equal to the resting length of the muscle.

In a concentric contraction the muscle is shortening and in an eccentriccontraction the muscle is lengthening.

For a certain level of activation, a muscle will produce a greater forceduring an eccentric contraction than during a concentric contraction.

Thus, as the pennation angle increases the physiological cross-sectional area decreases according to the cosine function.

Electromyography is the study of the electrical signal associated with thecontractions of muscles.