Research Review By Kevin Neeld©

Date Posted:

May 2010

Study Title:

Stretching versus strength training in lengthened position in subjects with tight hamstring muscles: A randomized controlled trial


Aquino CF, Fonseca ST, Goncalves GGP et al.

Author's Affiliations:

Fundacao Educacional de Divinopolis, Minas Gerais, Brazil; Graduate Program in Rehabilitation Sciences, Universidade Federal de Minas Gerais, Brazil, Nucleo de Integracao das Ciencias do Esporte, Brazil; Center for the Ecological Studies of Perception and Action, University of Connecticut, USA; Centro Universitario de Belo Horizonte, UFMG, Brazil.

Publication Information:

Manual Therapy, 2010, 15, 26-31.

Background Information:

Joint range of motion (ROM) is inherently dependent upon muscle (and fascial) length. Stretching has generally been recommended as a means of improving muscle length, and therefore joint ROM. However, improvements in muscle length following stretching protocols are likely do to a combination of an increase in stretch tolerance (a neurological effect) and a change in the viscoelastic behavior of the muscle (a mechanical effect) (1). This brings into question the long-term efficacy of stretching interventions.

A change in muscle length will likely have an effect on the length-tension curve of the respective muscle, including a possible change in the location of peak torque (2,3). Strength training is known to increase the number of muscle fibers in parallel (hypertrophy). There is some evidence to suggest that strength training in a lengthened position may increase the number of sarcomeres in series, therefore increasing the muscle length. Theoretically, these strength training-related changes would remain for a longer duration than stretching-related changes because the musculotendinous complex is actually undergoing structural changes.

The purpose of this paper was to compare the effects of a stretching or strengthening in a lengthened position intervention on changes in hamstrings flexibility, peak torque angle, and stretch tolerance.

Pertinent Results:

  • The strength training group experienced a significantly greater change of the hamstrings peak torque in the direction of knee extension compared to the stretching (p=0.0004), and control groups (p=0.001). There were no differences between the stretching and control group (p=0.577).
  • There were no significant differences between the strengthening (-4.00 ± 6.46), stretching (-3.53 ± 6.62), and control (-1.31 ± 5.33) groups in flexibility changes. Negative values represent a change in flexibility toward increased knee extension (e.g. improvement in hamstring extensibility).
  • During passive knee extension, the strengthening (p=0.001) and stretching (p=0.001) groups experienced a significantly greater increase in stretch tolerance compared to the control group, but the strengthening and stretching groups did not differ (p=0.245).
  • The maximal passive resistance torque increased significantly more in the strengthening (p=0.001) and stretching (p=0.001) groups compared to controls, but the strengthening and stretching groups did not differ.

Clinical Application & Conclusions:

Theoretically, training in a lengthened position should add sarcomeres in series to the muscle, hence increasing its length (4). This study did not find a significant change in flexibility for any of the groups, which is probably due to assessment validity issues (discussed below).

Despite this, this study provides evidence of a shifting in the length-tension relationship of the knee flexors following training in a lengthened position. This is further evidence of the body’s incredible ability to adapt to the demands we place on it. If the angle of peak torque moves toward a lengthened position following training in a lengthened position, it’s likely that the exact opposite would also be true (peak torque moves toward a shortened position following training in a shortened position).

This could have substantial implications for clients who appear to have “short” muscles in some areas and overly “long” muscles in others. By training the longer muscles in a shortened position and shorter muscles in lengthened position, balance can be restored across joints. A few examples would be to train the neck flexors, thoracic extensors, single-joint hip extensors, and ankle dorsiflexors in the shortened position, as these muscles are commonly long due to our sitting positions and footwear.

Study Methods:

Forty five undergraduate students with tight hamstrings (? 30° knee extension deficit with the hip flexed to 90°) were recruited, and randomly divided into one of three groups (strengthening, stretching, or control). Hamstring flexibility was identified as the knee ROM when a specific torque was applied to stretch the muscle.

This torque was attained by having the examiner move the subject’s knee, while set up in an isokinetic dynamometer with the hip joint in 110° flexion, through extension and identifying the torque at which firm resistance to stretch was first identified. This same torque was used during post-testing to ensure the same torque was used to stretch the hamstrings in both tests.

Stretch tolerance was defined as the as the angle at which the subject reported discomfort, and the maximal passive resistance torque measured at this same angle. Peak torque angle was assessed both passively and actively (concentric contraction) through a ROM of 0-90° of knee flexion. Seven trials of each were performed, after which the highest and lowest values were discarded; the remaining values used for analysis.

The strengthening protocol involved both concentric and eccentric contractions with the knee moving through an ROM that included only the final 30° of extension. Three sets of 12 repetitions were performed at 60% of the participant’s one repetition maximum, with a two-minute rest between sets. The load was increased 10% when the participants could perform at least one rep above the established number on two consecutive days.

The stretching group performed four sets of 30 second holds. With one foot on the ground and the “stretch leg” propped on an adjustable bench, participants were instructed to lean the trunk forward. Caution was taken to minimize accessory movement. Both the strengthening and stretching protocols were performed on both sides, three times per week for eight weeks.

A one-way ANOVA was used to assess differences amongst groups in these variables: flexibility change, angle of discomfort change, maximal passive resistance torque change, and peak torque angle change. Significance was set at ? = 0.05; however, after Bonferroni corrects based on the number of contrasts performed, ? was modified to 0.017.

Study Strengths / Weaknesses:

This is one of the first studies to compared ROM and torque changes associated with a strengthening versus stretching protocol. There were a few notable weaknesses that need to be pointed out. First, using the same torque value to assess terminal knee extension ROM presupposes that muscular hypertrophy did not result from the training program.

If muscle hypertrophy occurs, it is likely that passive stiffness increases within the muscle. This simply means that the muscle would be more resistant to passive movement, and it would take more absolute force to achieve the same range of motion. Since the strengthening group did improve their ROM (albeit, non-significantly), it’s possible that the magnitude of this change was masked by hypertrophic changes.

The high variability in the flexibility values suggests that the author’s choice of assessment strategies may have not been the best. The authors note that this variability could be due to seasonal changes in room temperature in their testing facility. Lastly, the group sizes were relatively small (n=15 per group) and were comprised mainly of females (n=13 per group) and young individuals (mean age = 21.33). As a result, it would be inappropriate to blindly accept these results as concrete across both genders and all ages.

Additional References:

  1. Taylor DC, Dalton JD, Seaber AV, & Garret WE. Viscoelastic properties of muscle-tendon units: the biomechanical effects of stretching. American Journal of Sports Medicine 1990; 18:300-309.
  2. Williams PE, & Goldspink G. Changes in sarcomere length and physiological properties in immobilized muscle. Journal of Anatomy 1978; 127:459-68.
  3. Brocket CL, Morgan DL, & Proske U. Human hamstring muscles adapt to eccentric exercise by changing optimum length. Medicine and Science in Sports and Exercise 2001; 33:783-790.
  4. Lynn R, & Morgan DL. Decline running produces more sarcomeres in rat vastus intermedius muscle fibers than does incline running. Journal of Applied Physiology 1994; 77:1439-1444.