L'anabolisme est maximum avec un entraînement à 75 % du maxi et 1h30 à 3 heures après l'effort
J Physiol 587.1 (2009) pp 211–217 211
Age-related differences in the dose–response relationship
of muscle protein synthesis to resistance exercise in young
and old men
Vinod Kumar1, Anna Selby1, Debbie Rankin1, Rekha Patel1, Philip Atherton1,Wulf Hildebrandt1,
JohnWilliams2, Kenneth Smith1, Olivier Seynnes3, Natalie Hiscock4 and Michael J. Rennie1
1University of Nottingham, School of Graduate Entry Medicine and Health, Derby DE22 3DT, UK
2Anaesthetic Department, Derby Hospitals NHS Foundation Trust, Derby DE22 3DT, UK
3IRM, Manchester Metropolitan University, Manchester M1 5GD, UK
4Unilever Discover R & D, Colworth Science Park, Sharnbrook MK44 1LQ, UK
We investigated how myofibrillar protein synthesis (MPS) and muscle anabolic signalling were
affectedby resistance exercise at20–90%of1repetitionmaximum(1 RM)intwogroups(25 each)
of post-absorptive, healthy, young (24±6 years) and old (70±5 years) men with identical body
mass indices (24±2 kgm−2).We hypothesized that, in response to exercise, anabolic signalling
molecule phosphorylation and MPS would be modified in a dose-dependant fashion, but to
a lesser extent in older men. Vastus lateralis muscle was sampled before, immediately after,
and 1, 2 and 4 h post-exercise. MPS was measured by incorporation of [1,2-13C] leucine (gas
chromatography–combustion–mass spectrometry using plasma [1,2-13C]α-ketoisocaparoate as
surrogate precursor); the phosphorylation of p70 ribosomal S6 kinase (p70s6K) and eukaryotic
initiation factor 4E binding protein 1 (4EBP1) was measured using Western analysis with
anti-phosphoantibodies. In each group, there was a sigmoidal dose–response relationship
between MPS at 1–2 h post-exercise and exercise intensity, which was blunted (P<0.05) in the
older men. At all intensities, MPS fell in both groups to near-basal values by 2–4 h post-exercise.
The phosphorylation of p70s6K and 4EBP1 at 60–90% 1 RM was blunted in older men. At
1 h post-exercise at 60–90% 1 RM, p70s6K phosphorylation predicted the rate of MPS at 1–2 h
post-exercise in the young but not in the old. The results suggest that in the post-absorptive
state: (i)MPS is dose dependant on intensity rising to a plateau at 60–90% 1 RM; (ii) older men
show anabolic resistance of signalling and MPS to resistance exercise.
(Received 7 October 2008; accepted after revision 10 November 2008; first published online 10 November 2008)
CorrespondingauthorM.J.Rennie:University ofNottingham, Division of Clinical Physiology, School ofGraduate Entry
Medicine andHealth, CityHospital, Uttoxeter Road, Derby, DE22 3DT, UK. Email: michael.rennie@nottingham.ac.uk
Exercise is known to stimulate the rate of post-exercise
myofibrillar protein synthesis (MPS) in healthy young
people, the extent of which probably depends upon
several factors including nutritional state, mode, intensity
and duration of exercise. (Chesley et al. 1992; Biolo
et al. 1995; Phillips et al. 1997; Miller et al. 2005;
Dreyer et al. 2006; Drummond et al. 2008; Wilkinson
et al. 2008). However, little or no information is
available in respect of the dose–response regarding exercise
intensity. Muscles of older people are also capable of
increases in MPS after resistance exercise, and feeding
increases this (Sheffield-Moore et al. 2005; Drummond
et al. 2008) but again dose–response information is
lacking.
Normal ageing is associatedwith a loss of skeletalmuscle
mass at 0.5–2% per annum, causing sarcopenia with an
incidence rate of 13–24% in those aged 50–70 years and up
to50%for those in their eighties(Baumgartner et al. 1998).
However, maintaining physical activity appears to preserve
muscle mass (Raguso et al. 2006) and even non-agenarians
can benefit fromresistance training (Fiatarone et al. 1990).
Nevertheless, the dose–response relationship between
increases in muscle synthetic rates and exercise intensity
for older people is unknown. Thus, a comparison of the
effects of exercise between young and old across a full
spectrum of exercise intensity on MPS and associated
changes in signalling activity is unavailable. Here we aim
to fill this gap.
C 2009 The Authors. Journal compilation C 2009 The Physiological Society DOI: 10.1113/jphysiol.2008.164483
212 V. Kumar and others J Physiol 587.1
Onthe basis of a limited pilot study at exercise intensities
of 60–90% (Bowtell et al. 2003), we hypothesized that
the dose–response relationship between exercise intensity
and increases in MPS would be hyperbolic with linear
increases at intensities up to 60% of 1 RM and no further
increase above this value, when all motor units would
probably be activated; on the basis of that work, we also
hypothesized that there would be a latent period of
∼1 h before any rise in MPS occurred. We have
previously shown (Cuthbertson et al. 2005) that in older
men (∼70 years) there is a decreased sensitivity and
capacity of increases of MPS (and associated anabolic
signalling) across a wide range of essential amino acid
availability, a phenomenon we named anabolic resistance.
We therefore hypothesized that decreased sensitivity
and capacity to increase myofibrillar protein synthesis
would occur in older men in response to resistance
exercise.
Methods
Ethics
The study was approved by the University of Nottingham
Ethics Committee and complied with the Declaration of
Helsinki. Written informed consent was obtained from
subjects after explaining the procedures and risks.
Study design
Twenty-five young (24±6 years) and 25 older
(70±5 years) men were recruited for the study.
They were recreationally active, physically independent
and healthy overall, with no sign of insulin resistance
(fasting blood glucose, 4.5±1.0 versus 5.1±0.9mM,
in young and old). Body mass indices were identical
in the two groups (23±4 versus 24±2 kgm2) as were
lean body masses (64±17 versus 57±14 kg) and right
(11.9±2.7 versus 10.7±4 kg) and left (10.5±2.5 versus
9.3±2.3 kg) lean leg masses (all young versus older). The
only major difference was the 1 repetition maximum
(1 RM) weight lifted by unilateral leg extension which was
significantly reduced in the older men (75±14 kg versus
41±11 kg, P<0.05).
Participant screening by an experienced doctor included
a clinical history, a physical examination, an electrocardiogram
and routine blood tests. Subjects were
excluded if they had a history of cardiac, pulmonary,
liver, kidney, vascular or autoimmune disease, clotting
disorders, uncontrolled hypertension, diabetes, thyroid
disorders, obesity, anaemia, cancer, alcohol abuse, visually
obvious muscle wasting, corticosteroid use or joint
pain that restricted movement. Older subjects with
mild controlled hypertension (<140/90 mmHg without
medication) were not excluded but they did not take
medication on the study day. At least 1 week before the
study each subject’s dominant leg maximal strength was
measured on a leg extension machine (ISO leg extension,
Leisure Lines (GB) Ltd) and the subjects were familiarized
with the study protocols. Body composition wasmeasured
using dual-energy X-ray absorptiometry (DEXA; GE
Lunar Prodigy II,GEHealthcare) using themanufacturer’s
software.
The participants were studied after an overnight fast
after normal daily activity. On the morning of the study
(∼09.00 h), they had an 18-g cannulae inserted into the
antecubital vein of each armfor tracer infusion and venous
blood sampling. Blood samples and muscle biopsies were
taken according to the protocol (Fig. 1). Muscle biopsies
were taken fromthe vastus lateralis using the conchotome
biopsy technique under sterile conditions with subcutaneous
1% lignocaine as local anaesthetic. Muscle
tissuewaswashed in ice-coldsaline, blotteddry and frozen
in liquid nitrogen, and stored at −80◦C until analysis. A
primed, continuous (0.7mg kg−1, 1mg kg−1 h−1) infusion
of [1,2-13C2] leucine (99 Atoms%, Cambridge Isotopes,
Cambridge, MA, USA) was started immediately after
the first biopsy and maintained for ∼7 h. After 2.5 h
of infusion, the subjects exercised with their dominant
legs at intensities, randomly assigned, from 20% to 90%
of 1 RM, with five subjects per group per intensity.
The seated subjects performed unilateral leg extensions
and flexions (1–2 s each) with 2min rest between sets.
The schedule of contractions was designed to equalize,
as closely as possible, the volume of exercise, i.e. the
force×time-under-tension product (often described as
‘work’). Thus, at an exercise intensity of 20% of 1 RM,
the subjects completed 3 sets×27 repetitions (reps); at
40%, 3 sets×14 reps; at 60%, 3 sets×9 reps; at 75%, 3
sets×8 reps and those at 90%, 6 sets×3 reps. Total work
output (i.e.% 1 RM×number of repetitions×number of
sets) was 1620–1800 units at different exercise intensities,
and total time-under-tension was obtained bymultiplying
by 4 s.
Myofibrillar protein synthesis and muscle anabolic
signalling
Myofibrillar protein was isolated as previously described
(Wilkinson et al. 2008). The fractional synthesis rate
(FSR) of myofibrillar protein was determined from
the incorporation of [1,2-13C2] leucine between muscle
biopsies using the labelling of venous plasma α-KIC
as a surrogate for true precursor labelling (Watt et al.
1991) as described in our very recent work (Greenhaff
et al. 2008). The methods for determination of the extent
of phosphorylation of p70s6K1 on Thr389, 4EBP1 on
Thr37/46 and the elongation factor eEF2 on Thr56, were
C 2009 The Authors. Journal compilation C 2009 The Physiological Society
J Physiol 587.1 Age-related effects of exercise on muscle anabolism 213
as described in recent work (Smith et al. 2008) using
antibodies from Cell Signalling Technology Inc. (Beverly,
MA, USA).
Statistical analysis
All data are reported as means±S.E.M. Between and
within-group differences were tested by two-way ANOVA
with a Bonferroni post hoc procedure to identify pair-wise
differences. Significance was accepted as P<0.05. Where
appropriate, correlations were tested by assessing the
existence of a linear fit between variables. All analyses
were made using GraphPad Prism version 5.0 (Graph Pad
Software, La Jolla, CA, USA).
Results
The only distinguishing features between the groups were
age and a51%smaller 1RMleg extension force in the older
men. Thus, the absolute total force–time integral was less
in older men even though the relative work done was the
same.
The basal rates of myofibrillar protein synthesis
were identical in the two groups of subjects
(0.039±0.002 versus 0.043±0.003% h−1, young versus
older, respectively). In both groups there was a doserelated
effect of resistance exercise on myofibrillar protein
Figure 1. Protocol for themeasurement of the relationships of myofibrillar protein synthesis andmuscle
anabolic signal molecule phosphorylation
Each subject was studied over the period shown in the fasted state with 5 young and 5 older subjects carrying out
exercise with their dominant leg randomly assigned to 20–90% 1 RM.
synthesis at 1–2 h post-exercise which was sigmoidal (Fig.
2); thus, there were small increases after exercise at 20
and 40% of 1 RM but a bigger rise to the values at 60,
75 and 90% of 1 RM, that were effectively at a maximal
plateau (i.e.with no significant differences between them).
There was a significant difference between the overall
responses of MPS to exercise in the young and older subjects,
with the area under the curves (rate ofmuscle protein
synthesis×time, i.e.% of total protein synthesized) being
30±6% higher (P<0.04) in the younger men.
No values of synthetic rates are presented for the exercise
period as the exercise period was variable, and in most
cases too short to obtain reliable results. The shapes of the
time courses of the changes in protein synthesis thereafter
were similar in both groups of subjects, with a lag over the
first hour after exercise, a rise whose extent depended on
intensity and the group studied, to a peak at 1–2 h and a
fall thereafter to near basal values.
The values at 60–90% in each group were not different
from each other, and so we averaged the data at these
intensities (Fig. 3).
The values in the young and older men were only
different at 1–2 h. Inspection of the data for the extent of
protein phosphorylation of p70s6K and 4EBP1 revealed a
much greater degree of variation around mean values than
that observed for myofibrillar protein synthesis and it was
therefore difficult to discern more than a broad positive
C 2009 The Authors. Journal compilation C 2009 The Physiological Society
214 V. Kumar and others J Physiol 587.1
Figure 2. Dose–response relationship of myofibrillar protein
synthesis (FSR, fractional synthetic rate,% h−1) measured at
1–2 h post-exercise for 5 young men and 5 older men at each
intensity
The responses of the young men overall were greater than those of
the older men (P < 0.04). The responses between 60 and 90% of
1 RM in young and old were indistinguishable from each other but
those in the young were together significantly higher than in the older
men (P < 0.01) for 15 subjects in each group
relationship between the size of the changes and exercise
intensity (data not shown).
Nevertheless, as for myofibrillar protein synthesis,
there were no significant differences between the
phosphorylation responses of p70s6K and 4EBP1 at 60,
75 and 90% of 1 RM for young and old subjects, and
combining these data produced more coherent images of
the time courses and differences between the two groups
of subjects (Fig. 4).
When the extent of phosphorylation of p70s6K at 1 h
was related to the extent of MPS at 1–2 h there was a
positive correlation for the young but not the old (Fig. 5).
For eEF2, phosphorylation showed a statistically
non-significant fall of about 20% immediately after
exercise then a rebound to about 120% of basal values by
Figure 3. Time course of the averaged responses to exercise at
60–90% 1 RM of myofibrillar protein synthesis (FSR, fractional
synthetic rate,% h−1) at 60–90% 1 RM for 15 subjects in each
group of young and older subjects
∗P < 0.05. Note that protein synthesis is measured over 2.5 h in the
basal pre-exercise state and then over the periods shown post-exercise.
1 h after exercise with no significant differences between
the two age groups (data not shown).
Discussion
The present results provide new information concerning
the responses ofmyofibrillar protein synthesis andmuscle
anabolic signalling in young and older men to exercise
at a range of intensities, all in the post-absorptive state.
In particular, rather than the hyperbolic relationship we
postulated, they show a sigmoidal dose–response
relationship of myofibrillar protein synthesis to exercise
intensity, with little increase from 20 to 40% 1 RM, then
a bigger rise at 60% of 1 RM, with no significant further
increase up to 90% 1 RM. Older men showed a smaller
response than the young subjects.
We previously showed in a pilot study that isometric
exercise at 60, 75 and 90% of 1 RM increased myofibrillar
protein synthesis at 90–150 min post-exercise to the same
extent (Bowtell et al. 2003); the present results confirmthat
above 60% of 1 RM of isotonic exercise, the stimulatory
effect is maximized. We ensured that the force×time
integral (i.e. total external work) was equalized so far
Figure 4. Time courses of the responses of phosphorylation of
p70s6K and 4EBP1 (arbitrary units as percentage basal for each
subject) averaged for intensities of 60–90% 1 RM
n = 15 in each group. ∗P < 0.05.
C 2009 The Authors. Journal compilation C 2009 The Physiological Society
J Physiol 587.1 Age-related effects of exercise on muscle anabolism 215
as possible so the results suggest that the total energy
expenditure was constant and not a factor affecting the
responses.
Might a change in muscle fibre type composition
explain the age-related differences?
It is known that as contraction intensity increases an
increasing proportion of type 2 fibres are recruited.
Although in the basal or amino acid-stimulated conditions
there is little difference in rates of protein synthesis
in human muscle of markedly different fibre type
compositions (Mittendorfer et al. 2005), it is possible that
at high contraction intensities, type 2 fibres would show
a greater response than type 1 fibres. Indeed, in type 2
fibres, phosphorylation of sarcoplasmic p70S6K1 occurs
to a greater extent (∼25–30% more) than in type 1 fibres
after resistance exercise at 75% of 1 RM (Koopman et al.
2006). This difference was suggested to be due to greater
recruitment of type 2 fibres than type 1 fibres, which
is feasible. Cross-sectional studies comparing individuals
aged 60–70 years to those in their twenties (Larsson, 1983)
suggested a slight increase in the proportion of type 1
fibreswith age.However, other later cross-sectional studies
did not confirm this (see Porter et al. 1995 for review).
Furthermore, when fibre type proportions of muscles
from the same individuals at 65 years and then at 75 years
were assessed, there was a decrease from ∼40–60% of
type 1 content (Frontera et al. 2000). Thus, evidence for a
selective loss of type 2 fibres with age is poor and provides
no explanation for our results.
A loss of totalmuscle mass would not explain the results
given that relative intensity was equalized between groups;
in any case we found little difference in the values of
muscle mass in the young and the older men (although
we may have overestimated this in the older subjects due
to the inability of dual-energy X-ray absorptiometry to
detect interfasicular fat and oedema in muscle). However,
there was a marked difference in strength between the
groups which is most commonly explained as being
due to differences in tendon properties and efficiency of
excitation–contraction coupling in older subjects (Narici
&Maganaris, 2006). Nevertheless, these variations should
not affect muscle protein synthesis per se.
Despite the existence of a blunted response of the
exercise-stimulated rate of myofibrillar protein synthesis
in the elderly, there were no differences in the shape
of its post-exercise time course, or that of anabolic
signalling, between the two groups. Therefore, there was
no indication of any lag in the responses of the older
subjects, as has been reported in a comparison of the
post-exercise changes in young and old subjects fed
after exercise (Drummond et al. 2008). However, it is
noteworthy that we observed a fall in myofibrillar protein
synthesis between 2 and 4 h after the peak at 1–2 h, which
webelieve hasnot been reportedbefore in a fullpaper.The
reason for this fall is puzzling but our exercise stimulus
(presumably volume rather than intensity) might not
have been sufficient to cause a long-lasting effect; after
exercise of a greater volume (8 sets of 8 repetitions at 80%
1 RM) in the post-absorptive state by young subjects, the
stimulatory effects lasted for up to 24 h (Phillips et al.
1997). The effects of volume of work and total time under
tension remain to be investigated. Alternatively the fall
might have been due to the lack of amino acid availability
as occurs after feeding when the increase in MPS can be
sustained for at least 24 h (Cuthbertson et al. 2006).
The precise molecular mechanisms by which resistance
exercise stimulates myofibrillar protein synthesis remain
to be determined but it is highly likely that enhanced
phosphorylation of mammalian target of rapamycin
(mTOR) and its downstream effectors, 4E binding
protein 1 (4EBP1) and p70 ribosomal S6 kinase (p70s6K)
are involved. (Deldicque et al. 2005; Cuthbertson et al.
2005;Terzis et al. 2008; Spiering et al. 2008;Wilkinson et al.
2008). Accordingly, we observed quantitatively similar
increases in phosphorylation of both p70s6K and 4BP1,
which were maximal for exercise at 60–90% 1 RM at
Figure 5. Relationship between myofibrillar synthetic rate and
extent of phosphorylation of p70s6K averaged for responses at
60–90% in young subjects (above) and older subjects (below)
There was a significant relationship (P = 0.049) between degree of
phosphorylation (arbitrary units and protein synthetic rate (FSR,% h−1)
only in the young. Note: some points overlaid.
C 2009 The Authors. Journal compilation C 2009 The Physiological Society
216 V. Kumar and others J Physiol 587.1
1 h post-exercise, i.e. just before the period of maximal
increase in myofibrillar protein synthesis, and which
were blunted in the older participants. Furthermore, we
found for the first time in human muscle a positive
correlation between extent of phosphorylation of p70s6K
and MPS, albeit only in the young subjects. The extent of
p70s6K phosphorylation reportedly predicts the extent of
accretion of muscle in rats (Baar & Esser, 1999) and in
weight lifters (Terzis et al. 2008), but no direct correlation
between p70s6K phosphorylation and increases inmuscle
synthesis have been reported and certainly not in human
muscle. This strengthens the support for a major role for
p70s6K in stimulatingMPS after exercise, and short-term
changes in both predict the longer term changes. The lack
of such a correlation in the older subjects is consonant
with a blunted response of MPS to exercise, and reports
that muscle hypertrophy after resistance exercise training
is less in older men (Kosek et al. 2006). Nevertheless, the
fact that both myofibrillar protein synthesis and p70S6
phosphorylation showed identical changes at 60–90%
of 1 RM suggests that muscle adaptation may occur
with exercise at less than the high intensities commonly
assumed to be solely efficacious (Spiering et al. 2008).
The changes in 4EBP1 phosphorylation were more
complex than those for p70s6K, in particular showing
a marked fall in the biopsy taken immediately after
exercise, which presumably mostly reflected the state of
the molecule during exercise, as thereafter the change
was reversed. This fall in 4EBP1 phosphorylation has
been observed before (Dreyer et al. 2006; Koopman et al.
2006) and is likely to be associated with a fall in human
muscle protein synthesis during exercise (Dreyer et al.
2006; Fujita et al. 2008). Although, like that of p70s6K,
4EBP1 phosphorylation showed a peak at 1 h post-exercise
(althoughwithmuchgreater variability),andwithblunted
responses in the older subjects, no significant correlations
with myofibrillar protein synthesis could be observed in
either group.
We were unable to detect any effects of age or
exercise intensity on the extent of phosphorylation of the
elongation factor eEF2, which suggests that modulation of
elongation at least by phosphorylation of eEF2 at Thr56
in the post-exercise period is of little biological relevance.
In summary, we have shown that acute bouts
of resistance exercise at different intensities stimulate
myofibrillar protein synthesis and anabolic signalling in
a dose-dependent manner in both young and old men
in the post-absorptive state. The stimulatory effect of
exercise peaks at 1–2 h post-exercise is suppressed, but not
delayed in older men. Although the extent of p70s6 kinase
phosphorylation predicts the stimulation of myofibrillar
protein synthesis in young men, older men appear not
to match changes in anabolic signalling and myofibrillar
protein synthesis, possibly explaining the deficiency in the
muscle protein anabolic response.
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Acknowledgements
We thank BBSRC (BB/X510697/1 and BB/C516779/1), the EC
EXEGENESIS program Unilever plc and the University of
Nottingham for support. Margaret Baker and Amanda Gates
provided expert clinical assistance for which we are grateful.
C 2009 The Authors. Journal compilation C 2009 The Physiological Society
