Acute effects of contract–relax (CR) stretch versus a modified CR technique
Summary (4 min read)
Introduction
- Contract-relax (CR) stretching increases range of motion (ROM) substantively, however its use in athletic environments is limited as the contractions performed in a highly stretched position require partner assistance, are often painful, and may induce muscle damage, also known as Purpose.
- Regarding autogenic inhibition, a neuromuscular inhibition was thought to occur as the loading of the tendon during the contraction phase of CR activated/stimulated type Ib muscle afferent output from the golgi tendon organs, stimulating inhibitory spinal synapses and hyperpolarizing the dendritic ends of spinal α-motoneurons of the stretched muscle.
- Furthermore, a recent study reported concomitant increases in ROM and reductions in tendon stiffness following isometric contractions performed in the anatomical position (Kay et al. 2015), with the acute increase in ROM being similar to that observed following static stretching.
- Modification of the CR stretching technique to perform the muscle contraction phase with the muscle ‘off stretch’ may provide a similar stimulus whilst reducing injury risk.
Subjects
- Fourteen recreationally active participants (8 women, 6 men; age = 26.1 ± (SD) 9.6 yr, height = 1.7 ± 0.1 m, and mass = 75.6 ± 13.3 kg) with no recent history of lower limb musculoskeletal injury or neurological deficit volunteered for the study after completing a pre-test medical questionnaire and giving written and informed consent.
- The subjects were asked to avoid any flexibility training, intense exercise and stimulant use for 48 h prior to testing.
- All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional research committee, and the study was completed in accordance with the Declaration of Helsinki.
Overview
- The subjects were familiarized with the testing protocol one week prior to data collection and then visited the laboratory on two further occasions under experimental conditions, with trials counterbalanced and separated by one week.
- During the experimental trials, the subjects performed a 5-min warm-up on a Monark cycle at 60 rpm with a 1-kg resistance load.
- The ankle was then placed in the dynamometer footplate in the anatomical position (0°) with the lateral malleolus aligned to the centre of rotation of the dynamometer.
- EMG amplitude was constantly monitored during the passive and active trials to quantify muscle activity (described later).
- The probe was positioned with the proximal end towards the origin of the medial head and the distal end towards the insertion of the Achilles tendon.
Active and passive trials
- During the active trial the subjects were instructed to perform a 5-s ramped maximal isometric plantar flexor contraction to determine maximal isometric strength, EMG activity, and tendon stiffness (described later).
- Two minutes after completing the passive ROM trials the subjects performed either the CR or SRC stretching intervention.
- Furthermore, this ensured that moment data were reflective of the passive properties of the MTC.
- Upon contraction cessation, the ankle was then immediately rotated again at 0.087 rad·s-1 until reaching the point of discomfort with the protocol repeated three further times giving a total duration of 60 s (i.e. 4 ˟ 10-s stretches and 4 ˟ 5-s contractions).
- Two minutes later the subjects repeated the passive and active trials (see Fig. 1).
Plantar flexor moment and ROM
- Maximal isometric plantar flexor moment was recorded pre- and post-intervention during the active trial to determine the influence of CR and SRC stretching on isometric strength.
- Peak isometric plantar flexor moment was also recorded during the four contractions performed during the CR and SRC interventions to determine the average peak loading during each intervention.
- The passive rotation enabled ROM, peak passive moment (stretch tolerance), and the slope of the passive moment curve (indicative of MTC stiffness) to be recorded.
- Joint moment and dorsiflexion angle data were directed from the dynamometer to a high level transducer (model HLT100C, Biopac, Goleta, CA) before analogue-to-digital conversion at a 2000-Hz sampling rate (model MP150 Data Acquisition, Biopac).
- The data were then directed to a personal computer running AcqKnowledge software (v4.1, Biopac) and filtered with a zero lag, 6-Hz Butterworth low-pass filter.
Electromyographic (EMG) activity
- EMG signals collected during maximal volitional contractions as well as during muscle stretches were then processed using a 20- to 500-Hz band-pass filter and converted to root-meansquared EMG with a moving symmetrical 250-ms averaging window.
- The TTL pulse simultaneously placed a marker on the AcqKnowledge (v4.1, Biopac) software while ending the capture of motion analysis and ultrasound data.
- Tendon length was calculated as the distance between reflective markers A and B (using motion analysis), plus the distance from the actual MTJ position to the distal edge of the image (using ultrasound) in a method identical to that previously reported (Kay et al. 2015).
- Tendon stiffness was calculated as the change in plantar flexor moment from 50-90%MVC divided by the change in tendon length (Nm·mm-1).
Statistical analysis
- All data were analyzed using SPSS statistical software (v.20; LEAD Technologies Inc., USA) and are reported as means and 95% confidence intervals (CI).
- Normal distribution was assessed for pre- and post-group data using Kolmogorov-Smirnov and Shapiro-Wilk tests; no significant difference (P > 0.05) was detected in any variable indicating that all data sets were normally distributed.
- Normal distribution was also examined for change score data in all variables using KolmogorovSmirnov and Shapiro-Wilk tests; a significant difference (P < 0.05) was detected for changes in peak passive moment in CR and SRC conditions and for ROM in the CR condition; no significant difference (P > 0.05) was detected in any other variable.
- Spearman’s rank correlation coefficients (rs) were computed to quantify the linear relationship between the change in ROM and changes in peak passive moment (stretch tolerance) and the slope of the passive joint moment curve (MTC stiffness), muscle stiffness and tendon stiffness in each condition.
- Statistical significance for all tests was accepted at P < 0.05.
Reliability
- Test-retest reliability was determined for peak isometric moment, peak passive moment, ROM, the slope of the passive moment curve (MTC stiffness) and muscle and tendon stiffness in the pre-test data in both conditions.
- Sample size Effect sizes (Cohen’s D) were calculated from mean changes in variables (ROM, muscle and tendon stiffness, and peak passive moment) from previous studies employing similar methods (Kay & Blazevich 2009b; Kay et al.
- To ensure an adequate sample size was recruited for the study, power analyses were conducted using the following parameters (power = 0.80, alpha = 0.05, effect size = 1.0, attrition = 20%).
RESULTS
- Significant correlations were observed between the changes in ROM and peak passive moment (stretch tolerance) in CR (rs = 0.63; P < 0.05) and SRC conditions (rs = 0.71; P < 0.05) indicating that changes in ROM were associated with changes in stretch tolerance after both interventions.
- No difference in the reduction in MTC stiffness was found between conditions (P > 0.05), indicating a similar response after each condition.
- These data are indicative that neuromuscular force generating capacity and reflexive muscle activity were neither inhibited nor potentiated after either condition.
DISCUSSION
- Contract-relax (CR) stretching has been commonly cited as the optimal stretching mode for achieving acute increases in ROM (Funk et al. 2003; Hindle et al. 2012), although the underlying mechanisms responsible for the efficacy of CR stretching to increase ROM remain to be established.
- Given that the muscle length adopted during the muscle contraction phase of CR stretching does not appear to influence the subsequent acute gain in ROM, nor the changes in mechanical or neuromuscular responses, the SRC technique may be useful for safely improving ROM when compared to the standard CR technique.
- Reductions in tendon stiffness have been reported following maximal isometric contractions performed without stretch (Kay & Blazevich 2009b; Kubo et al. 2001), with concomitant increases in ROM being reported that are equivalent to the gains observed after static stretching (Kay et al. 2015).
- These data are similar to those reported in previous acute CR studies where EMG magnitude was unchanged at full ROM (Kay et al.
CONCLUSIONS
- In summary, a significant increase in ROM with reductions in both muscle and tendon stiffness and a concomitant increase in stretch tolerance were demonstrated after both CR and SRC stretching.
- Furthermore, the changes in ROM were significantly correlated with changes in stretch tolerance but not changes in muscle, tendon, or whole MTC stiffness.
- The present study is the first to examine the effect of performing the contraction phase of CR stretching with the muscle ‘off stretch’.
- As no differences in the changes in any measure were evident between conditions, it is likely that similar mechanisms were responsible for the increases in ROM in CR and SRC conditions, regardless of the muscle length at which the contractions were performed.
- These practical improvements may improve the capacity of individuals, coaches and clinicians to facilitate the use of this stretching mode as part of a complete injury prevention strategy in healthy and in at-risk populations in both athletic and clinical settings.
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Citations
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Cites methods from "Acute effects of contract–relax (CR..."
...This latter method is as effective as more mainstream approaches while reducing the risk of injury (Kay et al., 2016)....
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References
677 citations
"Acute effects of contract–relax (CR..." refers background in this paper
...…to stretch during rotation (indicative of tissue stiffness) are important functional parameters that may affect muscle strain injury risk (Witvrouw et al. 2003), influence the capacity to perform activities of daily living (Mulholland and Wyss 2001), and are compromised with aging…...
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...Both the maximal joint range of motion (ROM) and resistance to stretch during rotation (indicative of tissue stiffness) are important functional parameters that may affect muscle strain injury risk (Witvrouw et al. 2003), influence the capacity to perform activities of daily living (Mulholland and Wyss 2001), and are compromised with aging (Bassey et al....
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535 citations
"Acute effects of contract–relax (CR..." refers background in this paper
...…to several mechanisms, including reductions in tissue stiffness (Kay et al. 2015; Morse et al. 2008), altered peripheral (afferent) output (Avela et al. 1999, 2004), and dampened pain, pressure or stretch perception increasing stretch tolerance (i.e., the capacity to tolerate increased…...
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510 citations
"Acute effects of contract–relax (CR..." refers background or methods in this paper
...The subjects were then seated in the chair of an isokinetic dynamometer (Biodex System 3 Pro, IPRS, Suffolk, UK) with the hip flexed to 55° and knee fully extended (0°) to ensure all plantar flexor muscles could be strongly activated and were at an appropriate length to contribute strongly to the total passive and active joint moments (Cresswell et al. 1995; Kawakami et al. 1998)....
[...]
...…3 Pro, IPRS, Suffolk, UK) with the hip flexed to 55° and knee fully extended (0°) to ensure all plantar flexor muscles could be strongly activated and were at an appropriate length to contribute strongly to the total passive and active joint moments (Cresswell et al. 1995; Kawakami et al. 1998)....
[...]
395 citations
"Acute effects of contract–relax (CR..." refers background or methods in this paper
...…peripheral (afferent) output (Avela et al. 1999, 2004), and dampened pain, pressure or stretch perception increasing stretch tolerance (i.e., the capacity to tolerate increased loading prior to terminating the stretch; Magnusson et al. 1996; Mitchell et al. 2007; Weppler and Magnusson 2010)....
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...Effect sizes (Cohen’s D) were calculated from mean changes in variables (ROM, muscle and tendon stiffness, and peak passive moment) from previous studies employing similar methods (Kay and Blazevich 2009b; Kay et al. 2015; Kubo et al. 2002; Magnusson et al. 1996)....
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...1999, 2004), and dampened pain, pressure or stretch perception increasing stretch tolerance (i.e., the capacity to tolerate increased loading prior to terminating the stretch; Magnusson et al. 1996; Mitchell et al. 2007; Weppler and Magnusson 2010)....
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375 citations
"Acute effects of contract–relax (CR..." refers background or methods or result in this paper
...Importantly, both CR and SRC stretching techniques cause an acute reduction in muscle and tendon stiffness, and this broader adaptive response may be an important adaptation that underpins the superior efficacy of CR stretching to acutely increase ROM compared with static stretching, which only influences muscle properties (Kay and Blazevich 2009a; Morse et al. 2008)....
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...…improve ROM with these improvements thought to be attributable to several mechanisms, including reductions in tissue stiffness (Kay et al. 2015; Morse et al. 2008), altered peripheral (afferent) output (Avela et al. 1999, 2004), and dampened pain, pressure or stretch perception increasing…...
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...Consistent with previous findings (Kay and Blazevich 2009a; Morse et al. 2008), the ultrasonography data in the present study revealed a reduced GM muscle stiffness Fig....
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...2008), studies employing ultrasonography techniques have found muscle stiffness to be reduced after an acute bout of static stretching, whereas tendon stiffness remained unaltered (Kay and Blazevich 2009a; Morse et al. 2008)....
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...The relative stiffness, and consequently the deformation, of muscle and tendon are distinct during low-velocity passive joint rotations towards maximal ROM (Blazevich et al. 2014; Morse et al. 2008), however the energy transfer through the tendon and muscle are identical as these tissues are arranged in series....
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