IntroductionMaximal force is decreased when the

homologous contralateral limb is activated

Recognized for a century (Mosso, 1892)Bilateral deficit (BD, Ohtsuki 1983)

occurs with maximum voluntary bilateral activation of contralateral limb muscles

BD = bilateral force < sum of unilateral

Literature Review Ohtsuki, grip strength finger max unilat (1981) Isometric vs. dynamic contractions with males

Coyle, DKE no change w/velocity, (1981) Vandervoort, DLP increase in BD w/vel, (1984) Secher, ILP no change with angle, (1988) Schantz, IKE facilitation , (1989)

EMG decline vs. no-decline Vandervoort, decline with BD increase (1984) Schantz, no decline with BD (1989)

Literature Review Cont... Fast vs slow twitch muscle fibers Vandervoort, DLP, (FT) (1984) Grabiner, IKE, (FT) rate of torque (1993) Secher, ILP, (ST) pharmacological (1978) Brown, DKE, (ST) BD & vel (1994)

-20

-15

-10

-5

0

5

BD%

60 120 180 240 360

Velocity

ExtFlex

Literature Review Cont... Facilitation = bilateral force > sum of

unilateral Homologous facilitation maximally (Brown,

1994) Nonhomologous facilitation sub-maximally

w/speech and finger amplitude (Kelso, Tuller and Harris, 1983)

Limited resources or inability to activate?

Maximum Bilateral Contractions Are Modified By Neurally Mediated Interlimb Effects

J.D. Howard and R.M. Enoka, J. Appl. Physiol. 70(1): 306-316, 1991.

Purpose Is BD due to neural mechanisms?Does EMG decline during BD? Is BD exhibited with nonhomologous

muscles?Does the effect of e-stim on

contralateral limb differ between subjects with different degrees of BD?

Subjects22 males (19 to 39 yrs of age)2 experiments (18 in exp. 1 & 12 in exp.

2)Exp. 1 (3 groups of 6 )

weightlifters (WL) (1 year bilateral) cyclists (CY) (1 year competition) untrained (UN) (no training)

Exp. 2 (all subjects naive)

MethodsR and L knee extension (1100) and L

elbow flexion (900) max forceSupine position with either arm/leg or

leg/leg isometric contractions

ApparatusE-stim - four 3x6 electrodes over R

quadsEMG - bipolar electrodes over belly of

VL and BF of R legEMG - over biceps and triceps of L arm

ProtocolAll trials - 3s max force with 30s rest

(1:10)Exp. 1 - unil and bilat max L & R knee

ext and max R elbow flex Exp. 2

- max R knee - max L knee w/ R leg e-stim - max L knee w/o R leg

Data AnalysisBilateral Index (BI) =

[100 x (bilateral)] - 100

RU+LULeg/leg & arm/leg BI for force (BIf) and

EMG (BIe)EMG filtered and rectifiedForce from single max repetition

EMG Analysis (typical)

Force

EMG

Filter

L only R onlyBilateral

Exp. 1 ResultsLeg/leg task exhibited

WL BIf = +6.2+4.7% ^ / BIe= +13.7+12.0% ^ CY BIf = -6.6+7.1% * / BIe= -11.5+4.9% * ^ UN BIf = -9.5+6.8% * ^ / BIe= +1.2+22.9%

0

200

400

600

800

1000

1200

1400

N

WL CY UN

FORCE

LL BiRR Bi

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

N

WL CY UN

EMG

LL BiRR Bi

Exp. 1 Results Cont...•Arm/leg task exhibited

WL BIf = -6.9+8.0% / BIe= -0.3+4.1%CY BIf = -1.4+15.1% / BIe= -11.7+11.3%UN BIf = -4.8+7.5% / BIe= -7.1+16.3%

0

200

400

600

800

1000

1200

1400

N

WL CY UN

FORCE

ArmBiLegBi

Exp. 2 Results

-9.7-4.2

42

6.2 5.8

25.5

54.6

16.2

-10

0

10

20

30

40

50

60

N

Deficit Facilitation

BI forceBI emgR ELL FL

• 2 groups (n=6) either deficit or facilitation•L leg MVC (w or w/o) R leg e-stim

• L leg EMG was equal w or w/o R leg e-stim

Discussion BD is reliable for untrained subjects BD is not always present in trained subjects Some subjects exhibit facilitation Interlimb interactions exist on a continuum

BD BFNull

Purpose Is BD due to neural mechanisms?Does EMG decline during BD? Is BD exhibited with nonhomologous

muscles?Does the effect of e-stim on

contralateral limb differ between subjects with different degrees of BD?

Does EMG decline w/Bie?

• leg dataEMG & force = parallel

• r - EMG & force is variable

• EMG on only one muscle of quads

Nonhomologous Muscles BD? Inability to activate a large muscle

mass?Division of attention between arm/leg?BD not associated with nonhomologous

muscles

ElectromyostimulationE-stim of contralateral limb causes facilitation of MVCSubjects were unable to voluntarily

exert maximal forceNeural integration from contralateral

feedback causes facilitationAmount of facilitation mediated by BD

or BF group

ConclusionsBD is a local neural phenomenon that is

influenced by afferent feedbackBD exhibition depends on neural

integration between peripheral and central sources

Related InvestigationsBD is greatest in dominant limb

(Ohtsuki, 1983)BD is greater in paired proximal than

paired distal limbs (Asanuma, 1989)Specificity of BF to the limbs practiced

(Thorstensson, 1979)Multiple degrees of freedom act as a co-

ordinative structure (Kelso, 1979)

Homotopic inhibition of mirror image in motor cortex at high levels of activation, (Asanuma, 1962)

Inhibition:1 Mirror image2 Within hemis3 Non-homol in diff hemis (model by Archontides, 1992)

Related Investigations

Future Research Include females

Perform dynamic movements

Change velocities

Monitor EMG from whole muscle group

Pair different limbs