Effects of Prestretching Velocities in Leg Extensors on
Mechanical Efficiency and Myoelectrical Activities.
Kazuo Funato* and Mitsumasa Miyashita*** Department of Sports Sciences, College of Arts and Sciences, University of Tokyo** Laboratory for Exercise Physiology and Biomechanics.
Faculty of Education, University of Tokyo

Abstract
Six adult males were studied to examine the utilization of the stored elastic energy. The repetitive knee
bending exercises were performed on a force platform (Kistler, Swiss) at various constant frequencies
ranging from 20 to 80 cycles/min in two different amptitudes of knee angle (60' and 90'). The external mechanical work (W) was calculated from the ground reaction force recorded by force platform.
The oxygen requirement (E) for three minutes exercise was determined by the Douglas bag methods.
EMGs of leg extensor muscles (vastus medialis and rectus femoris) were recorded by bipolar surface
electrodes and the mean amplitude of EMG (mEMG) was obtained. One cycle of knee bending wasdivided into the two work phases (knee extension phase: positive and flexion phase: negative) according to the electrogoniogmm. Positive linear correlations were observed between W, E and mEMG in all

the subjects. Mechanical net efficiency increased significantly with frequency (from 9.55% lo 34.9%).
MEMG during the negative work phase increased with frequency, while it remained almost the same in
the positive work phase. The results suggest that I ) the higher the prestretching velocity of leg extensor
muscles, the more elastic energy stored in the prestretching muscles and 2) the potentiation of myoelectrical activities is increased through prestretching.
Key words: Mechanical efficiency, Knee bending exercise, Muscle elasticity, Myoelectricalactivity
Introduction
A more efficient dynamic muscle contraction can be performed when it follows immediately after
active prestretching of that muscle. This advantage might result from the combined effects of muscle
elasticity and myoelectrical potentiation.
Cavagna et al. (1964) reported that mechanical efficiency in running was higher than the conventionally reported value (about 257o) obtained from isolated muscle preparations. In kalgaroo hopping,on the other hand, mechanical efficiency increased with hopping speed (Cavagna et al. 1977). These
results suggest that the elastic energy stored in the muscle during prestretching (eccentric contraction)
was released during shortening (concentric contraction), so that less energy would be needed to expend

during the subsequent concentric contraction. The effect of prestretching on muscular performance has

been confirmed from mechanical measurements obtained in vertical jumping (e.g. Asmusen andBonde-Petersen 1974b; Bosco and Komi 1979;Bosco et al. l98l).

l0

It is unclear, however, whether or not the increase of prestretching speed will improve mechanicalefficiency and myoelectrical activity. In this study examining the utilization of stored elastic energy
in man, mechanical efficiency and myoelectrical activity were measured by varying the prestretching
speeds of the agonist leg extensor muscles.

Methods
Subjects. Six healthy male students served as subject (age 24.O! 1.5 years, height l7l.7i2.9cm
and weight 65.8 t 5.9 kg).
Experimental Procedures. Each subject performed ten repetitive knee bending exercises on a force

platform (Kistler, Switzerland) with varying frequencies and knee angles of movement. Frequency wascontrolled by an auditory metronome at 20, 35, 50, 65 and 80 cycles/min. A constant range of knee
angle was set on the oscilloscope which was put in front of the subjects so that they could visuallycontrol the angle of their knee at either 60" or 90'. Each subject performed five bouts one day withdiffering frequency at one knee angle and five bouts at the other angle on the next day. During exercise, the subject kept his hand at his waist, his body in an upright position and placed his heels on theforce platform to limit front-back and right-left movements and to make the quadriceps act as theagonist muscle group.
Oxygen requirement. Each exercise was performed for three minutes. Using the Douglas bag methods,

oxygen requirement was determined during last 5 minutes of a 2o-minute rest period, 3 minutes of
exercise and 25 minutes of recovery. Expired air volumes were measured using a dry gas meter. The

02 and CO2 gas fractions were determined with a gas analyzer (1H06, SANEI Inc., Tokyo) which had
been periodically calibrated using the micro-Scholander technique. To obtain mean metabolic power,
net oxygen requirement per second (as expressed by the caloric equivalent) was calculated.

Electromyography. EMGS were continuously recorded from the rectus femoris and vastus medialis

F..qu.^cy20

80cyclelminｄ５ ¨．
４goniogrom

EMG+',.,

r On.9--.1-Opos.
iEx0

rnEMC : Neg. w.p. = iEMcneg. / Dneg.
Pos. w.p. = iEMGpos. /Dpos.

I cycle = (iEMGpos. + iEMGneg.)

Neg. w.p. : Negative work phasepos. w.p. : positive work phase/ (Dpos. + Dneg.)
Fig. l. Examples of recordings of tie knee goniogam (upper), EMG from vastus mediatis (middle)
and its integrated EMC (lower) during knee bending at a frequency of 20 cycles/min (left)

and 80 cycles/min (right). Determination of iEMG and calculation of mEMG are also ptesented.

l1during exercise. Bipolar zurface electrodes made of a silver plate l0 mm in diameter were attached
4 cm apart on the belly of muscle. EMG signals obtained during the last 30 s€conds ofeach 3-minuteexercise period were amplified, rectified and integrated (iEMG). As shown in Fig. l, the iEMG during
one cycle of knee bending was divided into two phases: during knee flexion or the negative work phase(iEMGneg) and during knee extension or the positive work phase (iEMGpos). These phases respectively
correspond to eccentric and concentric contractions of the quadriceps. Mean amplitudes of the EMG(mEMGneg and mEMGpos) were calculated from dividing iEMGneg and iEMGpos by the duration ofmuscle activity during negative (Dneg) and positive (Dpos) work phases, respectively.

Mechanicsl work. Potential external energy was determined by the double electrical integrationof the vertical acceleration which was recorded on the force platform. Mean mechanical power was
calculated as external energy divided by duration of positive work phase. Net mechanical efficiency
was determined from the ratio of mean mechanical power to mean metabolic power.

Results

Oxygen requirement increased with the frequency of contraction (Fig. 2). The requirement at 90o
was higher than that at 60" in each subject. On the other hand, oxygen requirement per cycle of knee
bending had a tendency to decrease with frequency.

Although the potential external energy per cycle of knee bending was essentially the same at thefive frequencies because each subject adjusted his movement to a desired knee angle, mean mechanicalpower increased with frequency. Fig. 3 shows a significant linear correlation between mean mechanicalpower and mean metabolic power. As seen in this figure, each plot was scattered across many isoefficiency levels.
Mechanical efficiency increased linearly with increasing frequency (Fig. 4) but mechanical efficiencies at 60" of the nee joint amplitude were not significantly different from those at the 90".

Figure 5 shows comparison of iEMG and mEMG changes relative to the frequency of movement

for all subjects. IEMGs and mEMGs obtained with each subject at the frequency of 20 c/min was
assumed to be 100%. IEMGneg and iEMGpos showed aLmost the same values at the frequency of
20 c/min. At higher frequencies (e.g..80cim), however, iEMGpos decreased to 6-t07o in spite ofan unchanged iEMGneg similar to that obtained at 20 clm. As a result. the reduction in iEMG during
one cycle that occurred with increasing frequency was mainly due to the decrease in iEMGpos. On theother hand, mEMGneg increased with frequency, i.e., the value obtained at the frequency of 80 c/min
was 3.7 -4.4 times gleater than that at a frequency of 20 c/min. MEMGpos showed less change, andconsequently mEMG during one cycle increased with frequency.

A significant positive linear correlations were observed between metabolic power and mEMG ofthe quadriceps (Fig. 6). The correlation coeflicient for all subjects ranged from 0.78 to 0.99.

0 60・● 90・＾●￨ｏ ζ ︶ ヽｏ 夕 ｏ ︒

Ｅ Ｅ ︶ 一メ ●● ＾０ヽσ●ヽＮ０￨￨￨￨・ :̀´ 9

一ｏ ｏ 一● ０も イ‐●′０● Σ＾ ● 一ｆ ヽ一 ︶ ︒ ︒一０．

0

200

400

6000001000

MetoOo c ,owOr(wotts)
Fし 3 Relationship betwcen metaboLc,ower andmcchanical power Dotted llnes denote
isoffrlciency lCvels?o?55065

60°

3090°

Fr.q!.nc,(.y.r./:Y=173+092X,F=066,pく o o01:Y=327+142X,I=087,pく 0001

mih )Fig. 2. Changes in oxygen requLement per min ineach subject (lower) and oxygen require-ment per cycle of knee bending (upper,means 1 sD).
＾ゞ ︶ ヽｏ●●一 一一Ш

￨￨￨

2035
Fr.ou..cY

50

6560

( cYcle / hih )4. Relalionship between mechanical efficiency
and frequency of movemelli.

Y = 11.23 + 0.22X, r = 0.78, p < 0.001

mEMG
(■ )

400／／//′/′
づ200／°だり′―へ'00

tiり(60・ )

︱IEMC
(%)

,00

Trble l. Correlation coefficients between mEMG alxl
metabolic power for a.ll lhe subjects.

Sujb. M. rcctus femods M. vastus medialis

50 65 80

¨2o 3535

HHKSKlKF
MWNY

50 65 80

092*ホ *

095拿 拿*097ホ 摯*093拿 。摯
097摯 摯拿
093摯・I*

080**

078**

096**摯
099**摯
091*拿 ネ
098ホ ホ攣*ネ 拿

Pく 0001
̀3 0001く Pく 001

Fr.qucncy ( cyclc / min )Fi8. 5. Relative comparison of iEMG oower) andmEMG (upper) with frequency of movernent.
Each value indicates mean of all the subjects.

mEMG (o.u.)

Subi : K F.

M.eclus t.nol'.

M voslus hed'ol's;T-:;l:.'t..:.ol. .

Y・ 390・ 003Xr■400

Y・ 111・

6000000

200

400

l,lalobolic powar ( woit )Fig, 6, Relationship between inEMG and metabolic
power for rcctus femoris 0eft) and vastus
medialis (right).

003X

r.o,7098000