Conscious Drive to Stiffen the Leg Spring

Motor Strategies for an Internal Challenge

Mervyn Travers

, James Debenham

, William Gibson

, Amity Campbell

and Garry Allison

School of Physiotherapy and Exercise Science, Curtin University, GPO box 1987, WA 6845, Bentley, Australia

University of Notre Dame, School of Physiotherapy, 19 Mouat Street, Fremantle, WA 6959, Australia

Keywords: Feed-forward, Stretch-shortening Cycle, Stiffness, Internal Challenge.

Abstract: This study investigated the kinematic and muscle activity profiles at the ankle under two hopping conditions

that consciously altered leg stiffness. Nine healthy volunteers performed multiple trials of bilateral hopping

on a custom built sleigh under two conditions – preferred (PC) and short contact (SC). Leg stiffness, peak

EMG, time to peak EMG and co-activation ratios for the medial gastrocnemius (MG), soleus (Sol) and

tibialis anterior (TibAnt) muscles were compared across conditions. SC hopping resulted in increased leg

stiffness. Importantly, Sol onset shifted from 86ms post-contact during PC to 14ms post-contact for SC.

Similarly, MG onset was 41ms post-contact during PC and 22ms pre-contact for SC. Significantly earlier

onsets of Sol and MG represent a shift into the feed-forward window which was not reflected by TibAnt.

Comparisons revealed no significant differences in co-activation ratios (p>0.05) suggesting that increased

leg stiffness during SC hopping was not a result of increased co-activation. Instead a dynamic strategy

pairing pre-activation with an increased rate of activity of the agonist muscles to develop force in time for

contact with the surface was observed. We suggest that the optimal strategy to consciously drive increased

leg stiffness occurs via a feedforward response.

1 OBJECTIVES

It is well-established that simultaneous contraction

of primary agonist and antagonist muscle groups

(i.e. muscle co-activation) will increase the stiffness

of a joint (Blickhan, 1989). This process is

considered protective, for example when landing

(Santello, 2005); (Yeadon et al., 2010). Furthermore,

such co-activation has been observed at the knee in

athletes following anterior cruciate ligament repair

(Bryant et al., 2009) and in the lumbar region in

clinical pain cohorts (Hodges et al., 2009); (Morris

et al., In Press: 2013); (Moseley et al., 2004); (van

Dieën et al., 2003). The common experimental

paradigm for investigating stiffness modulation

utilises external challenges including hopping and

running on surfaces of varying rigidity (Ferris et al.,

1999); (Ferris and Farley, 1997); (Moritz et al.,

2004), running on uneven surfaces (Müller and

Blickhan, 2010) and even reduced cutaneous

feedback during hopping (Fiolkowski et al., 2005).

The existing literature does not consider the

potential differences in motor strategies responsible

for stiffness modulation in response to internal

challenges. It cannot be assumed that humans would

utilise the same motor strategies to adapt to both

internal and external challenges. In fact, a previous

study has used conscious effort (an internal

challenge) to increase stiffness during hopping by

reducing ground contact time (Hobara et al., 2007).

Interestingly, they observed increased stiffness

without muscle co-activation (Hobara et al., 2007)

suggesting that the conscious drive to produce a

“stiffer” performance may have its own unique

motor strategies. Yet, these specific motor strategies

remain unstudied despite being relevant to

optimising running performance (Hobara et al.,

2010). The specific motor patterns responsible for

stiffness modulation under internal challenges are

relevant to performance, injury and rehabilitation.

The ankle joint is the major determinant of lower

limb stiffness during low load tasks (Farley and

Morgenroth, 1999); (Moritz et al., 2004). This study

examined the muscle activity profile changes at the

ankle associated with consciously driven increase in

leg stiffness during repeated submaximal hopping.

Travers M., Debenham J., Gibson W., Campbell A. and Allison G..

Conscious Drive to Stiffen the Leg Spring - Motor Strategies for an Internal Challenge.

 2013 SCITEPRESS (Science and Technology Publications, Lda.)

2 METHODS

This study utilised a within-subject experimental

design. Nine healthy participants performed multiple

hopping trials on a Custom Built Sleigh Apparatus

(Figure 1). The sleigh incorporated an instrumented

(AMTI forceplate 1kHz sampling) landing platform

allowing the establishment of event markers.

Figure 1: Double leg hopping on the Custom Built Sleigh

Apparatus inclined at 20degrees from horizontal.

Each Trial involved 10 continuous bilateral hops

on the sleigh apparatus (median 6 used for analysis).

Participants minimised any associated knee flexion

(no external fixation was used) so that performance

was primarily driven by the ankles. Three trials were

performed under two different conditions – preferred

ground contact time (PC), and with as short a

contact time as possible (SC).

Surface EMG for the medial gastrocnemius

(MG), soleus (SOL), and the tibialis anterior

(TibAnt) muscles was collected using an AMT-8

(Bortec Biomedical Ltd.) system. The EMG signal

was full wave rectified and onsets detected using the

integrated protocol (Allison, 2003). Trial linear

envelopes were created using a fourth-order zero-

lag, Butterworth low-pass filter (10 Hz) and

temporally synchronised to foot contact. Ensemble

average LE were determined for a 760ms window

(280ms pre-contact to 480ms post-contact). The

feedforward window was defined at 33ms post-

contact (Voigt et al., 1998). EMG signals were

integrated in 20ms epochs (IEMG) for the 760ms

window. The median peak of 10 PC hops was used

as a 1.0 arbitrary unit for amplitude normalisation

(Allison et al., 1993).

Leg Stiffness (K) was calculated using the

formula below (Dalleau et al., 2004):

K = (M x П(t

+ t

)

----------------

((t

+ t

/ П) – (t

/ 4)))

Figure 2: Formula for estimating leg stiffness (K); M =

body mass; tf= flight time; tc = ground contact time.

Co-activation was defined as the ratio of the

agonist (MG and Sol) and antagonist (TibAnt)

muscle activity and co-activation ratios were

labelled MG/TibAnt and Sol/TibAnt.

Paired samples t-tests were used to compare

differences in K, co-activation ratios and onset times

between conditions. A linear mixed model was

utilised to identify any significant difference in onset

times for each muscle grouped for condition and

side. It was further used to investigate any

interaction between condition, side and muscle with

onset time as the dependant variable.

3 RESULTS

The participants demonstrated a significant increase

in K during SC hopping (Table 1).

Table 1: Leg Stiffness (kN.m-1) by Condition.

Condition Stiffness (SD) p

PC 9.20 (2.58)

SC 14.16 (3.09)* p <.001

The peak EMG amplitude was not significantly

different for any muscle between PC and SC.

Further, there was no significant interaction between

side and hopping condition (F =.182, p = .671) nor

main effects for side (F =.284, p = .596) or condition

(F = .690, p = .409).

Sides were pooled for EMG onsets and time to

peak EMG as there was no significant interaction

between side and muscle or condition (p > 0.05).

The Sol onset time was 86ms (95% CI 58ms to

114ms) post-contact for the PC condition and 14ms

(95% CI -7ms to 36ms) post-contact for the SC

condition (F = 58.145, p <.001). The MG onset time

was 41ms (95% CI 25ms to 57ms) post-contact for

the PC condition and 22ms (95% CI 35ms to 9ms)

post-contact for the SC condition (F = 56.137, p

<.001). The TibAnt onset was not altered

significantly between conditions (p = .062).

Peak Sol activity occurred at 200ms (95% CI

184ms to 216ms) post-contact for PC and was

significantly earlier (p <.001) for SC occurring at

114ms post-contact (95% CI 101ms to 125ms).

Similarly, peak MG activity occurred at 195ms post-

contact (95% CI 179ms to 211ms) and was

significantly earlier (p <.001) for SC occurring at

102ms post-contact (95% CI 91ms to 114ms). The

time to peak activity for TibAnt was not

significantly different between conditions (p < .05).

Finally, comparisons revealed no significant

differences in MG/TibAnt and Sol/TibAnt co-

activation ratios between PC and SC (p>0.05).

4 DISCUSSION

Both MG and Sol demonstrated earlier onsets during

SC hopping. This represented a change from

potential feedback latency to a clear feedforward

response with onsets occurring within the defined

33 (+/- 7)ms window (Voigt et al., 1998).

Importantly, this was not matched by TibAnt.

Specifically, our findings demonstrate that in the

presence of a controlled environment and self-

regulation of the pending challenge and

consequences (i.e. the choice of hopping contact

time on a stable sleigh) individuals may choose a

feedforward strategy instead of the established co-

activation strategy. We observed a dynamic strategy

of pre-activation with an increased rate of activity of

the agonist muscle to develop force in time for

contact with the surface.

5 CONCLUSIONS

This study investigated the neural control of

consciously driven increase in joint stiffness during

submaximal hopping. We observed a stiffer hopping

performance driven by a feedforward strategy

confirming our hypothesis that internal challenges to

performance have their own unique motor strategies.

ACKNOWLEDGEMENTS

This research was partly supported by a grant from

the Neurotrauma Research Program – WA.

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