Civil Engineering Reference
In-Depth Information
(a)
(b)
0.8
400
0.6
IMP
300
0.4
200
MABP
0.2
100
20
60
100
40
80
20
40
60
80
100
Force (%MVC)
Force (%MVC)
FIGURE 14.4 (a) Shows mean arterial blood pressure (MABP) and intramuscular pressure (IMP) with increasing
contraction force. (b) Shows corresponding blood flow.
of mmHg varies widely between muscles and depends, among other things on the anatomy of the muscle
itself as well as its surroundings. A bulky muscle attains higher pressures than a thin muscle, and a muscle
with bony surroundings or tight fascia shows relatively large increases because of the low compliance of
these surroundings. At high contraction forces, the intramuscular pressure may attain values far above
blood pressure (Figure 14.4) and obviously cause muscle blood flow to be occluded in areas where intra-
muscular pressure exceeds blood pressure, the highest pressures normally occurring deep in the muscles.
However, even at low-level contractions, the complex microcirculatory regulation may become impeded.
First of all, at low blood flow velocities it is not the mean blood pressure but the diastolic pressure that is
decisive for maintenance of blood flow.
34
Further, with prolonged contractions, the muscle water content
will increase
35
and correspondingly, the thickness of the muscle has been shown to increase.
36
Such a state
of edematic tissue with increased volume will per se increase tissue pressure in a deliminated closed
muscle compartment with low compliance. At contraction levels in the order of 5 to 10% MVC, intra-
muscular pressures of 40 to 60 mmHg or more have been reported in muscles such as the m. supraspi-
natus in the shoulder.
37,38
Of note is that intramuscular pressure may increase to higher levels when
dynamic contractions are performed at a force corresponding to the static contractions.
39
However, it
may rather be the duration of increased intramuscular pressure than the absolute level that may be dele-
terious for the muscle and therefore static and not dynamic muscle contractions constituting a risk
factor. Thus, causal relationships between prolonged moderately increased tissue pressure and patho-
genic changes have been studied extensively in relation to compartment syndromes.
40
Pressures above
30 mmHg maintained for 8 h have been shown to induce necrotic changes in the muscle even if no
active contraction was performed and energy demand therefore was minimal.
41
One possible mechanism
is that although initially blood flow is sufficient during low-level contractions, this may not be the case
when the contraction is maintained for prolonged periods. Conditions with low flow and low perfusion
pressure may provoke granulocyte plugging in the capillaries, which effects microcirculation and may
also facilitate formation of free radicals, which have a highly toxic effect.
42,43
Not only muscle tissue
but also the peripheral nerves are sensitive to prolonged or repetitive elevated mechanical pressure. Dys-
function of the sensory perception indicating entrapment of n. medianus and n. ulnaris has been found
among computer users with pain in the hand
elbow regions.
44
Metabolism Adequate muscle blood flow is essential for muscle function because force development
relies on the conversion of chemically bound energy to mechanical energy, a process also called
energy turnover or metabolism. Some chemically bound energy or substrate is located in the muscle
tissue (especially glycogen), but these may become depleted during prolonged activities. Therefore,
wrist and forearm
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