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LAMOUREUX ET AL.
different between the two groups: The old group produced
significantly higher iEMG (p ꢀ .01) at each 100-millisec-
ond epoch from the start of muscle activation up to 500 mil-
liseconds compared with the older group (Figure 2).
ity of the older adults was further evident for the absolute
torque–time curve where they took significantly more time
(almost twice) at each interval to move from a torque of 50
Nm to 100, 150, 200, and 250 Nm (Figure 1). The relative
torque–time curve shows that the older adults also took
longer to reach a percentage of their peak torque, although
the times were not significantly different, probably due to
the large variances found in the groups.
DISCUSSION
The major findings of the present study of old and older
community-dwelling adults showed that further declines
take place in the maximal strength capacity and rapid rates
of torque development in the seventh decade of life. This is
the first study to show that these physiological degenera-
tions were primarily related to a reduction of the lean tissue
mass of the thigh musculature and also to a worsening in the
ability to rapidly increase neural activation. The age-related
degeneration in strength and explosive force characteristics
of the neuromuscular system provides further evidence that
community-dwelling older adults are more likely to experi-
ence difficulties in ADL and are more vulnerable to tripping
and falling (7,12).
Electromyographic Activity
Maximal voluntary neural activation may decrease with
age especially with regards to the rapid activation of the
knee extensors during an isometric MVC (8). As shown in
Figure 2, the older group showed significantly less (p ꢀ
.001) iEMG activity at every 100-millisecond epoch of the
iEMG–time curve. Differences in iEMG between the two
groups ranged from 219% for the first interval (start of con-
traction to 100 milliseconds) to 379% for the last interval
(401 to 500 milliseconds). A similar pattern has been re-
ported between middle-aged and older subjects (12), sug-
gesting that the ability for rapid recruitment of motor units
may decline further with aging. It could be conjectured that
a worsening in the rapid rate of torque development, per-
haps due to a reduced capacity to activate the muscles
quickly, might expose community-dwelling older adults to a
greater risk of tripping and falling. This hypothesis appears
to have some support: It has been shown that the time avail-
able to make initial responses to postural disturbances is
short—usually within 300 milliseconds of the disturbance
(21). Furthermore, simulations of tripping responses have
shown that the available rates of torque development rather
than maximal strength production are often critical to a suc-
cessful restoration of balance in such circumstances (9).
Muscle Strength
The findings of the present study confirm the phenome-
non of an accelerated age-related decline in muscle strength
after the seventh decade of life. We found a 33% decline in
MVC after the seventh decade of life, similar to a 30% de-
crease found in a group of 78- to 81-year-old individuals
compared with a group 10 years younger (14).
Reduction in dynamic strength after 70 years of age was
also evident from the 1-RM values of several key muscle
groups of the lower limb. The differences in 1-RM ranged
from 42% for hip flexion to 65% for plantar flexion (Table
2
). These results have several implications. First, they indi-
cate that the isometric MVC of the knee extensors, although
being a reliable and general measure of leg strength, may
mask individual muscle groups that are perhaps undergoing
varying rates of age-related muscle weakness. Second,
functional independence for community-dwelling elderly
people involves dynamic activities such as chair rising, stair
climbing, and carrying groceries, and a 1-RM method is
perhaps a more functionally specific strength test for older
adults. Our 1-RM results, however, show that dynamic
strength is severely compromised beyond the seventh de-
cade of life, which potentially could threaten the functional
capacity of older community-dwelling adults. Third, mobil-
ity is an imperative requirement for independent living, and
the contribution of the hip abductors, hip extensors, knee
extensors, and plantar flexor muscle groups is critical to the
smooth forward translation of the center of mass during lo-
comotion. Interestingly, these four muscle groups displayed
the highest magnitudes of difference in strength between the
groups, suggesting that mobility may be severely compro-
mised in community-dwelling older adults.
Muscle Mass
Muscle mass usually starts to decrease at 25 years of age,
and by the fifth decade of life approximately 10% of the
muscle CSA is lost. Thereafter, the atrophy accelerates, and
at age 80 a decline of up to 50% of the muscle area has been
reported (4,5). Our study reported a reduction of 15% in
BFLT mass of the thigh in the older group compared with
the younger sample, confirming that aging is accompanied
by muscle atrophy. Our result is comparable to a 25% de-
crease in total-body cell mass (calculated from total body
potassium), which has been reported between similar
groups (14). In addition, in the present study significant cor-
relations found between BFLT mass and isometric MVC
and the summed 1-RM values of the six thigh muscle
groups were relatively strong (r ꢂ .851 and r ꢂ .811, re-
spectively; p ꢀ .001). This finding supports the concept that
age-related muscle weakness is paralleled by a reduction in
muscle tissue.
However, the correlation coefficient between BFLT mass
and the summed 1-RM values was lower and nonsignificant
for the older group (r ꢂ .468, p ꢇ .05) compared with the
old group (r ꢂ .828, p ꢀ .001). This finding suggests the
possibility of mechanisms other than muscle atrophy under-
pinning age-related muscle weakness for the older group.
Several qualitative factors have been proposed and include
a reduction in specific muscle tension (3), a degeneration of
Force Production Characteristics
Several studies have shown that aging is characterized by
a worsening of the rapid force generation capacity of the
neuromuscular system (4,7,12,16). The 47% decline in
MaxRTD reported in this study shows that a further deterio-
ration in the rapid torque development takes place after the
seventh decade of life. The decrease in the explosive capac-