The influence of age dependent changes in muscle components, muscle stiffness and neuro muscular control on the performance during a point-to-point reaction time test

During ageing significant alterations occur in the skeletal muscle. A well
known phenomenon is the age-related muscle atrophy and strength loss, defined
as sarcopenia (1). The process starts at the age of 30-40 years and progresses
insidiously at an average rate of 1% loss of strength per year (2). Above the
age of 70 years the loss rate increases up to more than 3% per year (3). Due to
sarcopenia, activities of daily living in elderly persons necessitate efforts
close to the maximal strength and loss of independency can occur (4). A
parallel change to sarcopenia in the musculo-skeletal system is the loss of
flexibility with advancing age. Although less systematically documented,
cross-sectional studies demonstrate an age-related reduction in range of motion
at different locations (see reference (5) for review). These losses of mobility
occur also in the absence of joint pathology (such as osteoarthritis) and seems
to be related to the biological ageing process and occurs also in the absence
of joint pathology (5). The resistance of joints to movement is mainly due to
the soft tissues surrounding them: mainly the skeletal muscles and tendons
(during the movement and at the end of the movement), the ligaments (especially
at the end of the movement), and to a much lesser extend the skin (5, 6). Based
on the underlying mechanisms, a close relationship seems to exist between
sarcopenia and soft-tissue stiffness in ageing.

At the level of the muscle itself, sarcopenia is characterized by loss of
muscle fibers and atrophy of the remaining muscle cells (7). This phenomenon is
strongly related to increased levels of inflammatory markers, especially
interleukin (IL)-6 and tumor necrosis factor (TNF)-alpha, in the blood
circulation (8, 9) which are important signaling molecules involved in
proteolytic processes. Particularly TNF-alpha, IL-1beta and IL-6 are known to
have cytotoxic and proteolytic properties (10, 11). The exact mechanisms by
which inflammatory cytokines promote myofibrillar proteolysis are not yet
completely understood. Recent insights revealed an up-regulation of the
ubiquitin-proteasome pathway and calcium-activated pathway of calpains induced
by TNF-alpha, and probably also other cytokines (IL-1 or IL-6), and thus
inducing muscle protein breakdown (12-15).

Muscle weakness due to sarcopenia is more important than can be explained by
atrophy alone. The atrophy of type-2 (fast twitch) muscle fibers seems to be
more important than the loss of type-1 (slow twitch) fibers, which might
explain the important loss of muscle strength and explosivity due to sarcopenia
(7). In fact, the absolute loss of explosivity (muscle power, the capacity to
generate a high force in a short time) with ageing is even more important than
the loss of maximal strength (16), indicating that contraction-speed is also
impaired due to sarcopenia. An established illustration of slower movement
speed with ageing is the longer reaction-time in the aged. In fact,
reaction-time (RT) can be divided in decision-time (DT, time necessary to
process a trigger or stimulus) and movement-time (MT, time necessary to execute
a motor task, like pushing on a button or pedal) (17). On a simple
point-to-point RT-test (e.g. moving the finger from one point to another
following a visual stimulus) the increase in MT represents 70% of the total
increase in RT in healthy elderly persons compared to young controls (17).

Interestingly, when normalized to cell size, the contractile strength and
velocity of isolated muscle fibers are not significantly affected by ageing
(18). Supplementary loss of muscle contractile properties might be due to
age-related alterations in the connective tissues surrounding the muscle fibers
(endomysium, perimysium and epimysium). Less described in the context of
sarcopenia is the age-related augmentation of the proportion of non-contractile
tissue in the muscle (19). Besides proliferation of intra-muscular fat-tissue
(19, 20), the formation of both enzymatic and non-enzymatic cross-links between
collagen molecules leads to profound changes in composition of the
muscle-tendon complex as well as its mechanical properties (21). Non-enzymatic
alterations of the extracellular matrix, among which accumulation of Advanced
Glycation End products (AGE, mediated by condensation of a reducing sugar with
an amino group) (22) are related to ageing and can lead to permanent
cross-links. Several AGE's species have been described, from which Pentosidine
and N*-carboximethyl-lysine (CML) are the best chemically characterized (23).
The cross-linking processes (e.g. by Pentosidene) are responsible for an
increasing proportion of insoluble extracellular matrix and thickening of the
tissues, as well as increasing mechanical stiffness and loss of elasticity (21,
22, 24). Also the concentration of circulating AGE in serum increases with
ageing (25), which correlates with AGE in the tissues and the related collagen
alterations (26, 27). Several in-vitro studies have demonstrated increased
stiffness of various tissues (among which tendons) with advancing age in
animals and humans (21). Contrastingly, in-vivo human studies report either
reduced tendon stiffness (28) or increased musculo-tendineous stiffness in
elderly persons (29). Nevertheless, in general it is assumed that age-related
changes in stiffness of the musculo-tendineous complex are related to muscle
weakness and reduced explosivity.

Disturbed neuromuscular control is a supplementary phenomenon that can
contribute to age-related loss of muscle strength and explosivity. Increased
antagonist co-activation has been described for maximal voluntary isometric
contractions in ageing (30-32). Co-activation of the antagonist muscle will
reduce the net force that an individual can exert during contractions of the
agonist muscle. Possibly, antagonist co-contraction increases in order to
ameliorate the joint stability in elderly persons. To date, no conclusive data
are available regarding age-related changes in antagonist co-contraction during
other types of movements like functional (rapid, open -or closed-chain)
movements (33) with or without external resistance.

In summary, there are strong indicators that an important link exists between
loss of flexibility and muscle weakness. Several mechanisms can influence both
flexibility and muscle performance, such as age-related changes at the level of
the soft tissue and changes in neuromuscular control (antagonist co-contraction
and voluntary activation). Their relative contribution to general flexibility
and muscle performance in elderly persons remains unclear. By consequence, it
is difficult to identify the most appropriate intervention strategy in order to
counter age-related stiffness and accompanying muscle weakness.

To investigate the relative contribution of age-related changes in muscle
composition, tissue stiffness and antagonist co-activation on the performance
during a point-to-point reaction time test.

Cross-sectional observational study including healthy young (age 18-30 years)
and 60 elderly (age >=70 years) persons.
The study consists of 4 components:
Component 1:
Movement speed and antagonist co-activation is evaluated during a simple
point-to-point reaction time (RT) task using a specific device (as described by
Gorus et al., 2006), allowing the calculation of decision time (DT) and
movement time (MT). Modifications to the apparatus are applied in the
laboratory allowing an isolated elbow movements during the RT-test. Based on
simultaneous registration of the RT-task, sEMG of the M Biceps/Triceps Brachii
and 3D accelerometry (on the forearm and one the upper arm) the movement and
antagonist co-activation pattern is evaluated. Additionally, electromechanical
properties of the upper-arm muscles are evaluated during voluntary and
electrically evoced contractions. Data from RT-apparatuur, accelerometers en
sEMG will be collected simultanously via a universal amplifier (MPAQ,
Maastricht Instruments, The Netherlands).
primary outcome measures: RT (DT & MT), co-contractionratio (sEMG activity
ratio M Biceps/Triceps Brachii) and movement pattern (timing activity M. Biceps
& Triceps Brachii, acceleration-deceleration) during RT-test.
Component-2. Elongation of the musculo-tendineous complex at the height of the
proximal tendon of the caput breve of the M. Bicpes Brachii (at cracoid
process) and the distal tendon of the caput longum of the M. Triceps Brachii
(insertion at the olecranon) during meximal isometric contraction will be
measured using real time Ultra Sound. (Methods for elongation of achilles and
patella tendon described by Magnusson et al., 2008)). This new method will be
validated using MRI. Simultaneous a method will be explored to register and
evaluate the shortening of the muscle fibers. Recent insights point out the
opposite age related changes in mechanical behaviour of muscle colagen as
increased stiffness (Ochala et al., 2004, Gosselin et al., 1998) and tendon
collagen (decreased stiffness, Magnusson et al., 2008). Therefor it is
important to test the internal resistance in collagen during muscle
contractions of the M. Biceps & Triceps Brachii. A decreased capacity of the
muscle fibers to shorten during contraction will be the parameter to reflect
muscle stiffness Primary outcome: Elasticity muscle fibers and tendon of the
M. Biceps & Triceps Brachii.
Component-3. Using MRI the proportions of intramuscular contractile and
non-contractile tissues are evaluated in tissues of the upperarm muscles as
described by Kent-Braun et al., 2000 using spectroscopy-technology. Bandwidth
of signal intensity similar to intra-muscular fat and contractile collagen will
be tested for optimal signal intensity. The optimal bandwidth will be tested
using a human and animal tissues in vitro. These tissus will be provided by the
experimental anatomy department of the Free University Brussel
Component-4. In all subjecst a serumsample will be taken to test for
inflammatory markers (IL-6, TNF-alpha, Heat Shock Proteins; by means of ELISA
as described by Bautmans et al., 2008 and cross-links (enzymatic and
non-enzymatic, by means of ELISA and/or HPLC).

Differences in RT (DT and MT) between the younger and older age group will be
related to other the primary outcome measures. Finally a regression model will
be calculated to test the proportional contribution of parameters significant
influencing the RT (neuromuscular control [C-1], muscle stiffness [C-2] and
muscle tisuue components [C-3]). C-4 will tested as explanatory model in
relation to other primary outcome measures to gain insight on possible
underlying mechanisms.

The following measurements take place:
5-10 minutes answering of questionnaires - minimal burden - no risk
15 minutes reaction time test - small physical burden - no risk
20 minutes MRI - no burden - no risk according to strict exclusion criteria
5 minutes real time ultra sound - no burden - no risk
5 minutes bloodsample- small burden - no risk

Totally, the research will take one hour of a subject's time. The burden will
be minimal without a risk.