1. Paradoxical Role of Muscle During Controlled Movements
D. Gordon E. Robertson, Ph.D.
Yves D. Fortin, M.Sc.
Jean-Marie J. Wilson, M.Sc.
Dan T. Curry, M.Sc.
School of Human Kinetics,
University of Ottawa, Ottawa, CANADA
Supported by the University of Ottawa Rector’s Fund and Sport Canada Applied Research Fund

2. Introduction
Anatomically muscle function is determined by examining how a muscle acts at a single joint during unloaded movements. For a biarticular muscle one joint is fixed while its function is determined at the other joint. The process is then repeated by reversing the fixation.

3. Introduction Cont’d
Unfortunately, this method cannot consider how muscles function when several joints are moving simultaneously and how function is influenced by external loads and constraints.
One scheme classifies muscle as either agonist, antagonist or stabilizer.

4. Paradoxical Role of Muscle
Molbech suggested that biarticular muscle can act paradoxically during “controlled” or constrained movements. That is, when a limb is constrained at one or both ends, a flexor can produce extension or an extensor can produce flexion.
This differs from Lombard’s paradox which considers doubly biarticular muscles at hip and knee during extension. Extension is explained by differential moment arms at knee and hip.

5. Molbech’s Model
A and K are hinges A K X Y
AX = AY

6. Molbech’s Model AX = AY = AK + KH Y H K X A A and K are hinges
H is constrained to Y-axis H A K X Y
AX = AY = AK + KH

7. Molbech’s Model AX = AY = AK + KH Y H O K X A A and K are hinges
H is constrained to Y-axis
O is a muscle origin O H A K X Y
AX = AY = AK + KH

8. Molbech’s Model AP = AK + KH Y P H O K X A A and K are hinges
H is constrained to Y-axis
O is a muscle origin
extend line AK to P on arc of length AK+KH (limb length) O H A K X Y
AP = AK + KH P

9. Molbech’s Model AP = AK + KH Y P H O K L X A A and K are hinges
H is constrained to Y-axis
O is a muscle origin
extend line AK to P on arc of length AK+KH (limb length)
draw ray L from P through O
AP = AK + KH Y P H O K L X A

10. Molbech’s Model AP = AK + KH Y P H O K G L X A A and K are hinges
H is constrained to Y-axis
O is a muscle origin
extend line AK to P on arc of length AK+KH (limb length)
draw ray L from P through O
if ray OG is above L muscle causes extension O H A K L G X Y
AP = AK + KH P

11. Molbech’s Model AP = AK + KH Y P H O K G L X A A and K are hinges
H is constrained to Y-axis
O is a muscle origin
extend line AK to P on arc of length AK+KH (limb length)
draw ray L from P through O
if ray OG is below L muscle causes flexion O H A K L G X Y
AP = AK + KH P

12. Molbech’s Model AP = AK + KH Y P H O K G L X A A and K are hinges
H is constrained to Y-axis
O is a muscle origin
extend line AK to P on arc of length AK+KH (limb length)
draw ray L from P through O
if ray OG is along L muscle acts isometrically
AP = AK + KH Y P H O K G L X A

13. Paradoxical Muscle Activity?
what factors characterize paradoxical activity?
how are muscle contributions determined?
no means of directly measuring multiple muscle forces, simultaneously and in vivo

14. Classification of Muscle Functions
Muscle functions as: agonist, antagonist, stabilizer, extraneous or paradoxical.
Function is characterized by:
its anatomical function (flexor, extensor, . . .)
the joint’s kinematics (flexion, extension, . . .)
the joint’s kinetics (moment of force)
its activity level (EMG above a threshold)
its kinematics (lengthening or concentric, shortening or eccentric, static or isometric)

15. Rectus Femoris during Hip Flexion in Soccer Kicking
hip flexor and knee extensor
hip flexing
hip flexor moment
active contraction
concentric, shortening
Classified:
concentric agonist

16. Gluteals during Hip Extension in Hurdling
hip extensor
hip extending
extensor moment
active contraction
concentric, shortening
Classified:
concentric agonist

17. Erector Spinae while Stooped at Waist
trunk extensor
trunk statically extended
extensor moment
active co/contraction
isometric
Classified:
isometric agonist

18. Assumptions
It was assumed that, in skilled athletes, muscles were only recruited if they were beneficial to the movement.
Extraneous or inappropriate contractions and cocontractions were assumed to be nonexistent.

19. Classification of Muscle Function as Paradoxical
To determine paradoxical activity the following must occur:
joint motion must be opposite to muscle’s anatomical function
net moment of force at the joint must be opposite to anatomical function
muscle must be active
muscle must be shortening

20. Methods - Joint Kinematics
The joint motion must be opposite to the accepted anatomical role of the muscle.
For example, for the biarticular hip flexor, rectus femoris, to act paradoxically at the hip then the hip must be extending.

21. Methods - Joint Kinetics
The moment of force at a joint must be opposite to the usual anatomical role of the muscle.
For example, the moment of force at the knee must be extensor for a knee flexor, such as, a hamstring or gastrocnemius muscle, to be considered paradoxical.

22. Methods - Muscle Activity (EMG)
The muscle must be actively contracting.
criterion for significant muscle activity was: muscle EMG level was >25% EMGMVC (i.e., during a maximal voluntary contraction, MVC)
EMG level of less than 25% EMGMVC was considered too low for the muscle to contribute significantly to the movement

23. Methods - Muscle Kinematics
The muscle must be contracting concentrically, that is, it must be shortening.
An isometric or an eccentric contraction of the muscle would imply a stabilizing or antagonist function.

24. Purpose - Squat Lift
The purpose of this study was to determine the functional role of eight leg muscles, four of which are biarticular, during knee extension of loaded and unloaded squat lifting.

25. Squat Lift six experienced male weight-lifters
cinefilm at 50 frames/second
Kistler force platform
80% maximum lift and no load conditions
EMGs from muscles: rectus femoris, gluteus maximus, biceps femoris, semitendinosus, vastus lateralis, gastrocnemius, tibialis anterior and soleus
muscle lengths from Frigo’s and Pedotti’s model with modifications by Hubley

26. Conclusions - Squat Lift
biarticular muscles acted as stabilizers of knee
no paradoxical activity was performed
prime movers were the monoarticular muscles
knee muscles were responsible for starting the descent and preventing hyperextension at the end of the lift

27. Purpose - Ergometer Rowing
The purpose of this study was to determine the functional role of six leg muscles, three of which are biarticular, during drive phase of ergometer rowing.

28. Experimental Set-up Strain gauge force transducer Gjessing ergometer
Flywheel
Workload
adjustment
Foot
stretcher
Biological
amplifier
Kistler force
platform
Charge
amplifier
Bridge
amplifier
A/D
converter
Cinecamera
Microcomputer

29. Methods
four female and four male elite rowers performed on the ergometer while kinematic information was recorded on cinefilm at 50 frames/second
workload 25 N (2.6 kp) for females, N (3.1 kp) for males
ten cycles at 27 strokes/minute
EMGs from MVCs recorded separately

30. Methods - Forces
forces measured by Kistler force platform angled at 10 degrees
sampled at 200 Hz
rotated to Newtonian frame of reference
handle forces measured by in-line force transducer

31. Methods - Joint Kinetics
inverse dynamics to compute the net moments of force produced at the ankle, knee and hip joints
powers produced by moments were computed as product of moment and joint angular velocity (M . w)

32. Methods - Electromyography
muscles investigated were the monoarticular: vastus lateralis, soleus, gluteus maximus and the biarticular: biceps femoris, rectus femoris and gastrocnemius
sampled simultaneously at 200 Hz for ten rowing cycles
CMRR >100 dB, Hz filter, 2000 gain
linear envelope detector (6 Hz cutoff)
ensemble averaged across entire stroke

33. Methods - Muscle Length
instantaneous length of each muscle was computed from the digitized image with to adaptation of the muscle length model of Frigo and Pedotti (1978)
model based on marker kinematics and subject height

34. Results
To provide higher accuracy, the drive phase was divided in three equal parts; each criterion delineated by the classification scheme was verified for each muscle during each third of the drive.

35. Moment Powers of Male Rowers
1500
RMBS
Hip
1000
RMDH
RMKH
RMNT
RMSD
500
Concentric
-500
Eccentric
Power (watts)
1000
Knee
500
Concentric
-500
Eccentric
Ankle
500
Concentric
Eccentric
-500 10 20 30 40 50 60 70 80 90
100
Time (% drive)

36. Moment Powers of Female Rowers10 20 30 40 50 60 70 80 90
100
-500
500
1000
1500
Power (watts)
RFEK
RFLM
RFLN
RFMM
Concentric
Eccentric
Hip
Knee
Ankle
Time (% drive)

37. Results - Knee
angular velocity curve portrays relatively slow joint motion (below 4 rad/s)
knee moment was extensor for 2/4 females and all males
weak moment for females (>100 N.m) but strong for males (>150 N.m)
moment produced positive work for all males during middle and late drive and for two females during middle drive

38. Results - Knee Kinetics
Figure is from a male subject and depicts the knee’s angular velocity, net joint moment of force and the power produced by the moment of force.

39. Results - EMG
EMG curves show that most muscles contracted in-phase with each other
most were significantly active (EMGmax >25% EMGMVC) during middle and late third of drive
most were below threshold during first third of drive (i.e., end of recovery)

40. Results - Muscle Shortening
muscle velocities were time normalized to the drive duration
positive velocity indicated shortening or concentric contraction (same convention as muscle physiologists)
negative velocity indicated lengthening or eccentric contraction

41. Discussion - Classification
biceps femoris acted paradoxically for knee extension in one female and two males
gastrocnemius acted paradoxically for knee extension in one female and two males
rectus femoris acted paradoxically for hip extension in three of four males and three of four females during late drive

42. Discussion - Powers
female rowers produced majority of work by hip extensors; knee and ankle extensors produced insignificant amounts of work or negative work
male rowers had contributions from ankle, knee and hip extensors; mainly from hip

43. Conclusions - Rowing
Paradoxical activity was detected infrequently during knee extension by the biarticular biceps femoris and gastrocnemius.
More intriguing was the detection of paradoxical activity from the action of rectus femoris during hip extension.

44. References
Carlsoo, S Acta Morphologica Neerlando-Scandinavica, 6:
Elftman, H American Journal of Physiology, 125:
Frigo, C & Pedotti, A Biomechanics IV,
Lombard, WP American Physical Education Review, 8:
Molbech, S Acta Morphologica Neerlando-Scandinavica, 6:
Rasch, PJ & Burke, RK Kinesiology and Applied Anatomy, 6th edition

45. Questions?
Answers?
Thank you.

46. Pectoralis major during horizontal flexion in tennis forehand stroke
horizontal flexor
horizontal flexing
active contraction
shortening
Classified:
concentric agonist

47. Anterior deltoids during arm stand in gymnastics
shoulder flexor
shoulder statically extended
extensor moment
active cocontraction
isometric
Classified:
stabilizer

48. Hamstrings during knee extension in running
knee flexor
knee extending
flexor moment
active contraction
eccentric, lengthening
Classified:
eccentric agonist

49. Neck extensors during keyboarding
neck statically flexed
extensor moment
active contraction
isometric
Classified:
isometric agonist

50. Rectus abdominis during trunk flexion in high jumping
trunk flexor
trunk flexing
flexor moment
active contraction
concentric, shortening
Classified:
concentric agonist

51. Vastus lateralis during knee flexion in soccer kicking
knee extensor
knee flexing
extensor moment
active contraction
eccentric, lengthening
Classified:
eccentric agonist