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PLOUTZ-SNYDER ET AL.
lar fat stores would increase the overall mean T2 and the T2 of
individual pixels. In fact, a recent study has assumed this and
created algorithms that attempt to identify “fat-free CSA”;
however, the specific threshold criteria were not defined (28).
We cannot exclude the possibility that the higher resting mean
T2 is due to factors other than fat, especially because 24 train-
ing sessions did not alter resting T2 of the whole muscle and,
in fact, increased the resting T2 of the uniform 250-pixel area.
Because we do not know the exact biochemical mechanisms
that regulate T2, it is possible that resting muscle of older
subjects inherently has a higher T2. Regardless of the rest-
ing values, the exercise response is similar in young and
older subjects (Figures 4–7).
Figure 8 and Table 1 show the distribution of the T2 histo-
grams for the young, older untrained, and older trained subjects.
Even though a visibly uniform 250-pixel area was chosen
for analysis, there were significant differences in the variance
of the T2 distributions. Most interesting is that the least
amount of variance among groups occurred at the highest
exercise intensity (see Table 1). When considering the post-
exercise T2 distributions, training generally resulted in less
variance in the T2 distribution (significantly less for the
50% and 75% loads). There are some possible theoretical
explanations for this observation. If training induced an in-
crease in motor unit synchronization, then the T2 distribution
might be less variable. The rationale for this explanation is
that if more motor units are firing at the same time, then
more muscle fibers will be contributing to the elevated pixel
value and there would be fewer “resting” fibers. This could
contribute to a less variable T2 distribution. A second possi-
bility is that perhaps the training made the whole muscle
more homogenous in terms of fiber type and/or metabolic
profile. It has been shown that drastic differences in fiber
type, such as those that occur in different rat hind-limb mus-
cles, can influence the magnitude of T2 response (29). The-
oretically then, training-induced fiber type changes, such as
the rapid reduction of IIb fibers often observed following
resistance training (30), could result in a less variable T2
distribution. Clearly, the MRI spatial resolution typically
used in exercise studies is not able to differentiate individ-
ual muscle fibers or motor units. However, if the physiolog-
ical changes (neural and/or metabolic) were of significant
magnitude and spread over a large area of the muscle,
changes in the T2 distribution might be observed. This is
certainly an area that requires additional research.
that allows for investigation of deep and superficial mus-
cles, including very small muscles, and is highly repeatable
and reliable.
Acknowledgments
Address correspondence to L.L. Ploutz-Snyder, Exercise Science, Room
201, Women’s Building, Syracuse University, Syracuse, NY 13244. E-mail:
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There have been no gender effects previously observed
related to T2 response to exercise in younger subjects, so it
is unlikely that gender effects would be observed in older
subjects. Future studies should document this.
In conclusion, mfMRI is appropriate for use with older
healthy subjects. The T2 response is linearly related to load.
The resting T2 values of older subjects are higher than their
younger counterparts, but the T2 increase is comparable. It
is critical that investigators consider the initial resting T2
before conducting exercise studies, especially with subjects
of varying age. Clearly, mfMRI has enormous potential for
use in the older population. The technique offers several ad-
vantages over other current methods of assessing muscle in-
volvement in exercise. It is noninvasive, does not involve
ionizing radiation, offers unparalleled anatomical resolution