Download Short Report on the Muscle Length-Tension

Sandra Eames
11/13/2013
A&P 341 Wed. 6pm
Short report
Relationship between Length and Tension in Skeletal Muscle
In this experiment we were trying to determine what relationship between length and
tension in skeletal muscles. As muscles contract, they get shorter because of how the myosin pulls
on the actin. So if the muscle is already shortened that would have an impact on the potential
amount of tension that the muscle could produce. When Calcium binds to the troponin in a thin
filament, it allows the filament to turn and present the binding sites on the actin to the myosin.
The myosin then attaches and pulls on the thin filament, and then detaches and repeats the
process to contract the muscle (Marieb 291). If the sarcomeres are already shortened, there is not
as much room for the myosin to pull the thin filament. If the sarcomeres are drastically
lengthened, there is less contact area between the myosin and thin filament (Staff 65). This could
cause a difference in the tension that the muscle is able to produce. The objective of this
experiment is to determine the relationship between length of muscle and their ability to create
isometric tension.
In this experiment, we used wrist flexion and extension and grip strength to test the
relationship between length and tension of muscles, specifically the finger flexor muscles in this
case. We hooked 3 electrodes to a subjects forearm, one just proximal to the wrist, one in the
middle of the forearm, and one just distal and medial to the elbow to measure EMG (in volts.) We
had the subject hold a hand dynamometer to measure the force of their grip (in Newtons.) We
then tested this by having the subject fully flex their wrist and squeeze the dynamometer as hard
as they could 3 times for 2 seconds each time. We then repeated the procedure with different
lengths of the wrist to see if length had a difference on the amount of tension generated. We
repeated this with wrist angles of 120, 180, and 225 degrees. All of the squeezing force and EMG
was recorded into a software program that enabled us to take averages of grip strength and rEMG
for each wrist angle to determine how much tension was created. We then found the average of
the plateaus of the force for each squeeze, and recorded the average force and rEMG for each
squeezing angle.
The results are just about what we expected. The 90 degree angle yielded the least amount
of force, and the neutral angle yielded the most amount of force. In general, the more neutral the
wrist position, the more grip strength the subject had (fig 1). The sharpest angle, 90⁰, had by far
the weakest force. The partial flexion did not yield very high force. The extended wrist griped a
little harder than the partial flexion, but by far the neutral wrist was able to grip harder than any
of the other positions.
180
160
Mean Force (N)
140
120
100
80
60
40
20
0
Fully Flexed 90⁰ Partial Flex 135⁰
Neutral 180⁰
Extended 225⁰
Wrist Position
Figure 1: Mean Force of Grip Strengths for Varying Wrist Angles
In this experiment, the more neutral the wrist was, the more grip strength the hand had,
and the fully flexed wrist did not have high grip force. These results were to be expected. When the
wrist is fully flexed, it is very hard for one to close their hand, let alone squeeze with force. It is
natural, when squeezing hard, for the wrist to be neutral. It seems to yield the highest force. These
results can be explained quite easily. When the wrist is flexed, the myosin is already in a
contracted sort of state on the thin filament, there is not much room for it to pull on the thin
filament more. This muscle crowding explains why the grip strength for a flexed wrist is lower
than neutral. In an extended wrist, the muscles are stretched already. This means that less of the
myosin is able to come into contact with the thin filament, so there would naturally be less force
because fewer of the myosin heads are able to contact and pull on the thin filament (Staff 65). A
neutral wrist leaves the muscles with the resting length, so there is many myosin heads able to
pull on the thin filament, and there is enough space for the muscle to contract. That explains why
a neutral wrist is able to grip with the most force.
Citations
Marieb E, Hoehn K. Human Anaromy and Physiology, 9th edition. Pearson Education, Inc;2013.
Staff. Z341 Fall 2013 Laboratory Exercises in Human Anatomy and Physiology. Corvallis. OSU print;
2013.