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Research Papers

Local Dynamic Stability of Spine Muscle Activation and Stiffness Patterns During Repetitive Lifting

[+] Author and Article Information
Ryan B. Graham

School of Physical and Health Education,
Nipissing University,
100 College Drive, Box 5002,
North Bay, ON P1B 8L7, Canada
e-mail: ryang@nipissingu.ca

Stephen H. M. Brown

Department of Human Health
and Nutritional Sciences,
University of Guelph,
50 Stone Road East,
Guelph, ON N1G 2W1, Canada
e-mail: shmbrown@uoguelph.ca

1Corresponding author.

Manuscript received February 6, 2014; final manuscript received September 30, 2014; accepted manuscript posted October 16, 2014; published online October 30, 2014. Assoc. Editor: Kenneth Fischer.

J Biomech Eng 136(12), 121006 (Oct 30, 2014) (9 pages) Paper No: BIO-14-1070; doi: 10.1115/1.4028818 History: Received February 06, 2014; Revised September 30, 2014; Accepted October 16, 2014

To facilitate stable trunk kinematics, humans must generate appropriate motor patterns to effectively control muscle force and stiffness and respond to biomechanical perturbations and/or neuromuscular control errors. Thus, it is important to understand physiological variables such as muscle force and stiffness, and how these relate to the downstream production of stable spine and trunk movements. This study was designed to assess the local dynamic stability of spine muscle activation and rotational stiffness patterns using Lyapunov analyses, and relationships to the local dynamic stability of resulting spine kinematics, during repetitive lifting and lowering at varying combinations of lifting load and rate. With an increase in the load lifted at a constant rate there was a trend for decreased local dynamic stability of spine muscle activations and the muscular contributions to spine rotational stiffness; although the only significant change was for the full state space muscle activation stability (p < 0.05). With an increase in lifting rate with a constant load there was a significant decrease in the local dynamic stability of spine muscle activations and the muscular contributions to spine rotational stiffness (p ≤ 0.001 for all measures). These novel findings suggest that the stability of motor inputs and the muscular contributions to spine rotational stiffness can be altered by external task demands (load and lifting rate), and therefore are important variables to consider when assessing the stability of the resulting kinematics.

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Figures

Grahic Jump Location
Fig. 1

(a) Experimental setup used for repetitive lifting and lowering between SH (1) and knee height (2). (b) Each of the 30 lift/lowers consisted of moving from a target at position (1) to a target at position (2) and back, to the beat of a metronome.

Grahic Jump Location
Fig. 2

Exemplar EMG linear envelopes, 3D angular and 2D positional data from one participant during each of the five repetitive lifting scenarios. EMG: TES, LES, MF, LD, IO, EO, RA. Angles: F/E, LB = lateral bending, RT = axial rotation. Positions: SH, HA, A/P, I/S = inferior/superior.

Grahic Jump Location
Fig. 3

The process of state space reconstruction and local dynamic stability analysis using the spine rotational stiffness data. (a) Original 3D spine rotational stiffness data, as well as the Euclidean norm stiffness at each point in time. (b) The reconstructed dynamics in state space using a reconstruction dimension of six and a time delay of 16 samples (0.5 s). (c) Expanded view of a local region on the attractor (outlined in b), displaying the diverging Euclidean distance (dj) of nearest neighbors after an infinitesimally small perturbation. (d) Average logarithmic rate of divergence for all nearest neighbor pairs over 0.5 cycles.

Grahic Jump Location
Fig. 4

ANOVA and post hoc results between the three load conditions (0%, 5%, and 10% maximum back strength at 12 lifts per minute). (Top) Current study results for the local dynamic stability of muscular contributions to spine rotational stiffness (stiffness—λmax), the local dynamic stability of the full state space trunk muscle activity (full trunk EMG—λmax), and the local dynamic stability of the low back muscle activity (low back EMG—λmax). (Bottom) Results from Ref. [11] for the local dynamic stability of spine kinematics (kinematics—λmax), the mean muscular contributions to spine rotational stiffness (mean stiffness) (note: maximum stiffness showed the same findings), and the minimum muscular contributions to spine rotational stiffness (minimum stiffness). † indicates a significant main effect of load on the corresponding dependent variable (p < 0.05). ---- red lines indicate a significant post-hoc pairwise comparison between the different load conditions (p < 0.05). Error bars represent standard deviations.

Grahic Jump Location
Fig. 5

ANOVA and post hoc results between the three rate conditions (6/min, 12/min, and 18/min at 5% maximum back strength load). (Top) Current study results for the local dynamic stability of muscular contributions to spine rotational stiffness (stiffness—λmax), the local dynamic stability of the full state space trunk muscle activity (full trunk EMG—λmax), and the local dynamic stability of the low back muscle activity (low back EMG—λmax). (Bottom) Results from Ref. [11] for the local dynamic stability of spine kinematics (kinematics—λmax), the mean muscular contributions to spine rotational stiffness (mean stiffness) (note: maximum stiffness showed the same findings), and the minimum muscular contributions to spine rotational stiffness (minimum stiffness). † indicates a significant main effect of rate on the corresponding dependent variable (p < 0.05). ---- red lines indicate a significant post-hoc pairwise comparison between the different rate conditions (p < 0.05). Error bars represent standard deviations.

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