Modulation of neural excitability during and after eccentric muscle action
1 2, B.W. HOFFMAN2, T.J. CARROLL2, A.G. CRESSWELL2 (1 Technische Universität München, Munich, Germany; 2 University of Queensland, Brisbane, Australia)
D. HAHN1,
1 Introduction & Aims
2 Methods – part two
3 Results – part two
For some voluntary eccentric contractions (Westing et al. 1991), the absence of force
enhancement (FE) beyond a corresponding isometric maximum suggests some form
of neural inhibition. However, others have shown that it is possible to achieve 1.1 to
1.5 times greater torques during maximum voluntary eccentric contractions (Lee &
Herzog 2002). Eccentric contractions can also result in a persistent increase in
torque after the muscle has been actively stretched, called residual force
enhancement (RFE; e. g. Hahn et al. 2010). While RFE is associated with an active
and passive mechanical component (Herzog et al. 2010, Leonard et al. 2010), it is
still unclear whether changes in neural activation influence RFE. However, some
evidence suggests that during submaximal voluntary contractions where torque is
kept
electromyography
following
k t constant,
t t surface
f
l t
h (EMG) is
i less
l
f ll i eccentric
t i compared
d
to purely isometric contractions (Oskouei & Herzog 2005), despite discharge rates of
single motor units being unaltered (Altenburg et al. 2008).
Stimulation techniques and muscle responses
Muscle responses to stimulation
810
- All stimulations (single pulses) superimposed on maximal voluntary contractions.
- M-waves and V-waves of SOL after electrical nerve stimulation.
- CMEPs of SOL elicited by electrical stimulation of the cervicomedullary junction.
- MEPs of SOL after transcranial magnetic stimulation of the motor cortex.
Fig. 4: Muscle responses to different
superimposed stimulations during
stretch (light grey) and after stretch
(dark grey) normalised to the isometric
reference
contractions.
V-waves,
MEPs and CMEPs are further
normalised to the sizes (areas) of their
corresponding M-waves The red line
indicates normalised size 1.
- Normalisation of all muscle responses to corresponding M-waves and to isometric
reference contractions.
- No difference in muscle responses (M-wave, V-wave, MEP, CMEP) during the
iso @ 10° and stretch contractions and the iso @ 20° and after stretch
contractions (p > .05).
Based on these observations, the aim of this study was to investigate the neural and
mechanical properties of eccentric and post-eccentric voluntary muscle actions,
during the times where FE and RFE are observed.
- Bigger V-waves after than during stretch (1.19 ± .26 vs. 0.96 ± .23; p ≤.05).
- Bigger MEPs after than during stretch (1.14 ± .24 vs. 0.98 ± .16; p ≤ .05).
2 Methods – part one
- No difference in CMEP responses (p > .05)
Measurement of torque & muscle activity
Fig. 2: Example data of M-waves and V-waves during iso @ 10° and stretch (left) as well as MEPs during
iso @ 20° and after stretch (right). Size of motor responses was determined by areas under the curves
4 Discussion
- Background activity (BGA) of soleus (SOL), medial gastrocnemius (MG) and tibialis
anterior (TA) were obtained prior to stimulations by surface EMG.
3 Results – part one
In contrast to previous findings, our results show the torque produced during an
eccentric contraction is not limited to that which is produced isometrically at a
- Isometric contractions @ 10
10° and 20
20° dorsiflexion,
dorsiflexion isometric-eccentric-isometric
isometric eccentric isometric stretch
contractions from 0°-20° dorsiflexion @ 30°s-1.
Torque & Background Activity
- Ankle plantar flexion joint torque was measured using a BIODEX® dynamometer (n=10).
FE
stim1
RFE
stim2
- FE of 17±8% compared to iso @ 10° and RFE of 9±8% compared to iso @ 20°
(for both, p ≤ .001).
- No difference between FE (p = .617) nor RFE (p = .399) in comparison between
stimulation techniques.
- Similar BGA for SOL, MG and TA during iso @ 10° and iso @ 20° compared to the
isometric-eccentric-isometric contractions during and after stretch (p > .05).
comparable muscle length. Cortical and spinal responsiveness was also similar to
that in isometric conditions, suggesting that the major voluntary motor pathways are
not subject to substantial inhibition during stretch. The failure to produce higher
torques in other studies may have been due to an inability of subjects to fully activate
their muscles when performing maximal voluntary eccentric contractions. However, it
still remains unclear why voluntary eccentric torques are still less than those
predicted, or produced, when muscles are electrically stimulated.
p
byy larger
g V-waves and
Our new results after stretch revealed RFE accompanied
MEPs after compared to during stretch. The increase in MEP size despite stable
CMEPs suggests an increase in cortical excitability, and the increased V-wave
amplitude is consistent with greater motoneuronal output or increased stretch reflex
excitability. Although this illustrates that neuromotor pathways are altered during the
expression of RFE, it is unclear whether these changes underlie the enhanced
torque. Thus, both central and passive mechanisms remain as candidate
mechanisms underpinning RFE.
Fig. 1: Example data of isometric reference contractions (black lines) at 10° and 20° dorsiflexion and an
isometric-eccentric-isometric 20° stretch contraction (red lines). Top traces show torque, bottom traces the
corresponding position of the motor driven dynamometer. The dotted vertical lines indicate the times of the
superimposed stimulations during and after stretch.
References
Fig. 3: Joint torque for isometric contractions at 10° and 20° (light grey) and stretch contractions during and
after stretch (dark grey, left and right, respectively). Red columns show FE during stretch (left) and RFE
after stretch (right).
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Hahn et al. (2010). J Biomech 43, 1503-08.
Herzog et al. (2010). Adv Exp Med Biol 682, 141-61.
Lee & Herzog (2002). J Physiol 545, 321-30.
Contact Information: [email protected]
Leonard et al. (2010). Am J Physiol Cell Physiol 299(6),
C1398-401.
Oskouei & Herzog (2005). J Appl Physiol 98, 2087-95.
Westing et al. (1991). Eur J Appl Occup Phys 62, 104-08.
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