1. Force produced during eccentric length changes in active mouse soleus and EDL muscles using the
workloop technique
Miyano CA, Felix I, Hessel AL, Dennison BB, Baker EM, Nishikawa KC
Northern Arizona University Center for Bioengineering Innovation
Introduction and Background Objectives Materials and Methods Cont. Results Cont.
The winding filament hypothesis proposes that the protein
N2A titin (red, below) binds to actin (blue) upon Ca2+ influx in
skeletal muscle, and PEVK titin (green) winds on thin
filaments during force development because the cross-
bridges (purple) not only translate but also rotate the thin
filaments.
The workloop technique
• Workloops are used in muscle physiology to test the
mechanical work and power output of muscle contractions
via in vitro muscle testing
• Force and length changes are combined to create a loop,
whose area constitutes the work produced
Results
Acknowledgements
• Muscles move through eccentric (lengthening) and
concentric (shortening) contractions
• Muscle activation during the shortening phase creates
positive work
• Work is done on the environment by the muscle during
shortening
• Application of stimulation during the lengthening phase
creates negative work
• Work is done on the muscle by the environment
Our objectives with the workloop technique
• To characterize eccentric length changes in mouse soleus and
(extensor digitorum longus) EDL muscles
• To compare eccentric length changes in wild-type
heterozygous mice and muscular dystrophy with myositis
(mdm) mice
• Mdm mice carry a deletion in the N2A region of the titin
gene
• Deletion stops titin from binding during activation, possibly
decreasing residual force enhancement during eccentric
contraction
Questions
• Under optimal conditions, what is the work output of mouse
soleus and EDL muscles in healthy wild-type mice?
• Under optimal conditions, what is the work output of mouse
soleus and EDL muscles in mdm mice?
• Will the workloop profile be different between fast twitch and
slow twitch muscles?
• 30 day old mice
• Soleus and EDL muscles were extracted
• Muscles were attached to an isotonic force lever servomotor
(below)
• Muscles remained in oxygenated Krebs solution while
attached to the force lever or while waiting to be used
Mice genotypes
• Wild-type (+,+)
• Heterozygous type (+,-)
• mdm mutant type (-,-)
Workloop trials
1. Optimal muscle length (Lo) was determined
2. Workloops were run using similar parameters for EDL and
soleus muscles
3. Sinusoidal length changes using a constant strain of + or -
5% of Lo while being phasically stimulated produced the
workloops
4. After 5 active trials with 4 full cycles each, passive trials were
conducted to check the integrity of the muscle
5. 5 passive trials were obtained to compare to active trials
6. Muscles were then removed and weighed to normalize forces
to body size
7. Changes in force production and muscle displacement were
controlled, monitored, and analyzed using in-house software
(LabView)
Parameters
EDL
Duration: 1250ms (5 oscillatory cycles of 250ms each)
Amplitude: ±5% Lo
Frequency 1 and 2: 4hz
Pulse rate: 130ms
Train Duration: 40ms
Train offset: 22.5%
Soleus
Duration: 1250ms (5 oscillatory cycle of 250ms each)
Amplitude: ±5% Lo
Frequency 1 and 2: 4hz
Pulse rate: 200ms
Train duration: 40ms
Train offset: 22.5%
EDL workloops
Soleus workloops
Thanks to the Nishikawa labs for helpful feedback. Our funding
sources include NSF IOS-1025806. NSF IOS 0742483 and
NSF IIP 1237878. We would also like to thank the NAU Bio
Annex for animal care.
Materials and Methods
Stationary End
Lever
Parallel Electrodes
Comparison of negative work between Wild-Type
(healthy) EDL and soleus
Comparison of negative work between genotypes of EDL
and soleus
Conclusions and Future Work
• Total mechanical work absorbed and the rate of force
development were attenuated in mdm mice compared to wild-
type mice
• The workloop profile for wild-type mice was substantially
different between fast and slow twitch muscle in a way that
maximized muscle performance and function
• The results suggest that the deletion in the N2A region of titin
leads to a decrease in negative work during repetitive
movements, however it does not change muscle
activation/deactivation mechanisms.
Future work
• Continued data collection
• Further studies will use more realistic workloop parameters
that focus on physiologically relevant length changes and
stimulation frequencies