European Journal of Applied Physiology

, Volume 100, Issue 5, pp 553–561

Strength, power output and symmetry of leg muscles: effect of age and history of falling

  • Mark C. Perry
  • Serena F. Carville
  • I. Christopher H. Smith
  • Olga M. Rutherford
  • Di J. Newham
Original Article

DOI: 10.1007/s00421-006-0247-0

Cite this article as:
Perry, M.C., Carville, S.F., Smith, I.C.H. et al. Eur J Appl Physiol (2007) 100: 553. doi:10.1007/s00421-006-0247-0

Abstract

Risk factors for medically unexplained falls may include reduced muscle power, strength and asymmetry in the lower limbs. Conflicting reports exist about strength and there is little information about power and symmetry. Forty-four healthy young people (29.3 ± 0.6 years), 44 older non-fallers (75.9 ± 0.6 years), and 34 older fallers (76.4 ± 0.8 years) were studied. Isometric, concentric and eccentric strength of the knee and ankle muscles and leg extension power were measured bilaterally. The younger group was stronger in all muscles and types of contraction than both older groups (P < 0.02–0.0001). Strength differences between the older groups occasionally reached significance in individual muscles and types of contraction but overall the fallers had 85% of the strength and 79% of the power of the non-fallers (P < 0.001). Young subjects generated more power than both older groups (P < 0.0001) and the fallers generated less than the non-fallers (P = 0.03). Strength symmetry showed an inconsistent age effect in some muscles and some contraction types. This was similar overall in the two older groups. Both older groups had greater asymmetry in power than the young (P < 0.02–0.004). Power asymmetry tended to be greater in the fallers than the non-fallers but this did not reach significance. These data do not support the suggestion that asymmetry of strength and power are associated with either increasing age or fall history. Power output showed clear differences between age groups and fall status and appears to be the most relevant measurement of fall risk and highlights the cumulative effects on function of small changes in strength in individual muscle groups.

Keywords

Muscle Ageing Falls Strength Power Symmetry 

Introduction

The ageing process is associated with decreasing muscle strength (reviewed by Macaluso and De Vito 2004) and an increasing risk of falling (Kenny et al. 2001). Any functional activity can occur only when the muscles are capable of generating the force/power critical for that activity. The closer an individual’s strength is to that critical value, the more difficult it is to perform and control the activity.

Many falls in older people occur for medically unexplained reasons and a better understanding of the underlying mechanisms could improve falls prevention. Falls have been associated with low isometric and concentric leg muscle strength by some workers (Gehlsen and Whaley 1990; Lord et al. 1992, 1999; MacRae et al. 1992; De Rekeneire et al. 2003; Takazawa et al. 2003; Robinson et al. 2004) but not others (Schwendner et al. 1997; Daubney and Culham 1999; Skelton et al. 2002; Melzer et al. 2004).

The association between eccentric strength and falling in older people has received little attention in the literature. This is surprising in view of the fact that many older people report particular difficulty in controlling activities with a strong eccentric component such as sitting down or descending stairs (Lark et al. 2003) which may contribute to falls (Startzell et al. 2000). This seems paradoxical in the light of reports that eccentric force generation seems to be relatively well preserved in older people (Lindle et al. 1997; Christou and Carlton 2002; Delbaere and Bourgois 2003) and animals (Phillips et al. 1991) although this is not a universal finding (Poulin et al. 1992; Hortobagyi et al. 1995). Skelton et al. (2002) found no association between falls and eccentric strength in community-dwelling older people but we have found no other work on this relationship and therefore this warrants further investigation.

The generation of power is of key importance for normal movement and its control. While both its components; force (Frontera et al. 2000) and speed (D’Antona et al. 2003) decrease with age, studies directly measuring power output in older fallers are few. Skelton et al. (2002) found that fallers were less powerful than non-fallers, but only when power was normalised for body mass. Levy et al. (1994) noted that the power output in the uninjured limb of elderly people with fall-related hip fractures was 70% less than that of a group of age matched non-fallers. However these subjects may not be able to generate maximal power levels due to pain, reduced stabilisation or weakness following immobilisation.

Unilateral measurements of strength have been made in most studies of older people. This also applies to studies of power output, although some have measured power generation from contractions of both legs simultaneously. Age effects on symmetry of strength or power have not been investigated. Symmetry of strength and power on falls risk has been researched by only one group (Skelton et al. 2002) who observed greater asymmetry of leg extension power in elderly community-dwelling fallers than in age-matched non-fallers, although strength symmetry was similar.

No studies were found that directly compared asymmetry of strength or power in young and older people. Such a comparison is important to assess whether any greater asymmetry in fallers is due to an accelerated ageing effect or some other cause.

We measured the symmetry of the strength (isometric, concentric and eccentric) of the knee and ankle flexors and extensors and also power output from leg extension to investigate; (a) the effects of age in healthy young and older people and (b) the association with falling in two groups of older people—those with and without history of falls.

Methods

Subjects

We studied 44 healthy younger subjects aged 18–40 years [29.3 ± 0.6 years (mean ± SE), range 19–41 years], 44 healthy older people (> 70 years, 75.9 ± 0.6 years, range 70–86 years) with no history of falls (non-fallers) and 34 otherwise healthy older people (> 70 years, 76.4 ± 0.8, range 70–87 years) who had a history of medically unexplained falls (non-fallers). Young and non-fallers were recruited from the population of King’s College London students and staff and the population of southeast England through advertisements in local newspapers. The fallers were also recruited from local falls clinics. All subjects were living independently in the community.

Fallers were defined as those having had at least one unexplained fall over the past 12 months (Whipple et al. 1987; Gehlsen and Whaley 1990; Daubney and Culham 1999; Lord et al. 1999).

Exclusion criteria for all subjects were any known cardiovascular disorders likely to be exacerbated by maximal muscle contractions, neurological disorders, musculoskeletal pathology in the lower limbs or spine that might affect performance in the test procedures and dementia.

All volunteers completed a screening health questionnaire before participation and gave written informed consent. Approval for the study was obtained from the local ethics committee.

Strength measurements

These were made bilaterally using an isokinetic dynamometer (Kin Com, Chattanooga Inc., USA). Isokinetic dynamometers have been shown to be reliable for both knee (Ly and Handelsman 2002) and ankle (Morris-Chatta et al. 1994) flexion/extension measurement in older people. The order of testing for flexion/extension and side was randomised. The order of contraction types was isometric, concentric and eccentric for reasons of convenience.

Isometric measurements of the knee muscles were made with the subjects strapped to the dynamometer chair around the waist and shoulders. The axis centre of knee flexion was aligned by eye with the dynamometer lever arm axis centre and the lever arm connected by a cuff to the ankle just above the lateral malleolus. The weight of the lower leg was measured using the inbuilt dynamometer facility for automatic gravity correction. After several practice attempts the maximal voluntary contraction (MVC) force was measured at 90° of knee flexion for both muscle groups.

For measurements from the ankle muscles the subjects were supine and the knees fully extended and supported by a pillow. The axis centre of ankle flexion was aligned by eye with the lever arm axis of rotation and the foot was strapped into the ankle plantarflexion/dorsiflexion apparatus. After several practice attempts MVCs at 20° of plantarflexion and dorsiflexion from the neutral position were obtained. These forces were not corrected for gravity, as any effect was negligible.

In all cases the contractions lasted for approximately 3 sec, with a minimum rest period of 5 sec between each. Two repetitions at each angle were made for each muscle group.

Isokinetic measurements were made in the same positions for each joint. Measurements were made at 50 and 150° s−1 in random order. The testing ranges were 25–80° knee flexion and 0–20° plantarflexion. A rest period of at least 5 sec was allowed between each trial. A minimum of three trials was performed until values decreased, with a minimum rest period of 5 sec between each. The first trials were regarded as practice attempts.

Leg extension power output

The Nottingham Power Rig [University of Nottingham Mechanical Engineering Unit, UK] was used to determine power output during single leg extension (Bassey and Short 1990) with the order of testing randomised.

Subjects sat in the semi-recumbent chair, adjusted so that the knee angle was 10° from full extension when the footplate pedal was at its end position. With the footplate retracted, the subjects extended their hip and knee as powerfully as possible, forcing the footplate forwards, until it reached the end position. A minimum of six trials was obtained on each leg and measurement was curtailed when two successive measurements were below the highest. The highest measurement on each leg was recorded.

Statistical analysis

Asymmetry was calculated as the percentage difference between sides. Data were analysed with a multiple ANOVA and post hoc unpaired t Tests where appropriate. Alpha was 0.05 in all cases.

Results

Baseline group characteristics

The younger group were taller than both older groups (P < 0.001), but age-corrected height, which corrects for the vertebral shortening occurring with age and thus remains in proportion to limb length (Sorkin et al. 1999), and other measurements were similar (Table 1).
Table 1

Characteristics of the subjects

 

Age (years)

Sex (M/F)

Body mass (kg)

Height (m)

Age-corrected height (m)

Young

29.3 ± 0.61

18/26

69.4 ± 1.61

1.74 ± 0.01

1.72 ± 0.01

Older non-fallers

75.9 ± 0.61

15/29

70.4 ± 1.60

1.68 ± 0.01

1.73 ± 0.01

Older fallers

76.4 ± 0.78

4/30

70.7 ± 1.97

1.64 ± 0.01

1.69 ± 0.01

The three groups differed in sex ratios and absolute height (P < 0.01), but not in weight or age-corrected height

Absolute strength

The young subjects were consistently and substantially stronger than both older groups (Table 2). There were inconsistent differences between the two older groups. In none of the quadriceps contractions were there any significant differences between them. The fallers were weaker than the non-fallers during isometric contractions of the hamstrings (P = 0.02), dorsiflexors (P = 0.002) and plantarflexors (P = 0.004). This was also found for the faster eccentric contractions in the hamstrings (P = 0.04) and the plantar flexors for the slower (P = 0.03) and faster (P = 0.003) contractions. However the fallers were weaker than the non-fallers for all contractions except the faster concentric ones in the dorsiflexors and overall the fallers had a mean of 85.4% of the strength of the non-fallers (P < 0.001).
Table 2

Voluntary isometric strength of the four muscle groups (N) during isometric and dynamic contractions of the stronger leg

 

Young

Non-fallers

Fallers

Fallers:non-fallers

Quadriceps

 Isometric

505.7 ± 38.4

319.5 ± 14.5

282.9 ± 25.7

0.885

 Concentric 50° s−1

467.6 ± 30.0

263.6 ± 14.7

234.6 ± 28.6

0.890

 Concentric 150° s−1

346.7 ± 23.1

198.1 ± 9.3

195.4 ± 18.2

0.986

 Eccentric 50° s−1

618.7 ± 37.7

423.0 ± 19.6

390.4 ± 34.0

0.923

 Eccentric 150° s−1

582.3 ± 35.6

409.9 ± 16.2

374.4 ± 28.0

0.913

Hamstrings

 Isometric

191.6 ± 11.4

112.9 ± 4.8

94.5 ± 7.8*

0.837

 Concentric 50° s−1

220.1 ± 14.5

105.2 ± 7.6

94.5 ± 14.1

0.901

 Concentric 150° s−1

211.4 ± 10.8

170.8 ± 6.9

114.4 ± 12.9

0.670

 Eccentric 50° s−1

270.9 ± 17.7

174.7 ± 9.3

150.1 ± 12.7

0.859

 Eccentric 150° s−1

255.7 ± 14.1

188.0 ± 6.9

166.7 ± 10.1*

0.887

Dorsiflexors

 Isometric

231.7 ± 13.9

180.7 ± 9.0

136.1 ± 10.6**

0.735

 Concentric 50° s−1

128.8 ± 8.5

78.9 ± 4.4

76.5 ± 8.3

0.970

 Concentric 150° s−1

89.7 ± 5.7

61.0 ± 2.0

65.8 ± 3.4

0.079

 Eccentric 50° s−1

297.6 ± 15.8

236.7 ± 13.6

200.7 ± 20.1

0.848

 Eccentric 150° s−1

280.4 ± 17.5

185.7 ± 10.5

175.1 ± 19.9

0.943

Plantarflexors

 Isometric

467.3 ± 25.5

340.5 ± 21.1

254.7 ± 21.4**

0.748

 Concentric 50° s−1

406.6 ± 29.1

222.6 ± 16.5

175.2 ± 19.7*

0.787

 Concentric 150° s−1

296.9 ± 22.9

145.6 ± 9.7

124.0 ± 12.1

0.857

 Eccentric 50° s−1

802.9 ± 47.0

548.2 ± 30.3

417.6 ± 34.9**

0.762

 Eccentric 150° s−1

694.6 ± 47.7

417.9 ± 24.0

361.6 ± 30.4

0.865

In all cases both the older groups were significantly weaker than the young subjects (P = 0.003–< 0.0001) and for clarity these differences are not shown. There were relatively few cases of the older fallers being weaker than the non-fallers (*P < 0.02, **P < 0.004). However the fallers showed an overall greater weakness than the non-fallers as shown by the ratios of their strength (P < 0.001)

Absolute power

Greater power was generated by the young subjects (269.5 ± 22.6 W) than both the non-fallers (150.7 ± 9.6 W) and fallers (120.3 ± 13.1 W, P < 0.0001 in both cases, Fig. 1a). The fallers generated less power than the non-fallers (P = 0.03).
Fig. 1

Absolute power and asymmetry of power in the three subject groups. There were significant differences in absolute power between each of the groups. The young subjects showed less asymmetry than both older groups who had similar values. *P < 0.02, **P < 0.004

Asymmetry of strength

During isometric contractions there was a consistent trend for the young subjects to be least and the older fallers to be most asymmetrical in all muscle groups (Fig. 2). In the hamstrings there were no significant differences between the three groups. In all three other muscle groups the young and older non-fallers were statistically similar, although the fallers were more asymmetrical than the older fallers (P < 0.02 for the hamstrings and P < 0.004 for the dorsiflexors and plantarflexors). The only difference between the two older groups was found in the dorsiflexors with the fallers showing greater asymmetry (P < 0.02).
Fig. 2

Asymmetry during isometric contractions of four muscle groups in the young (open bars), older non-fallers (vertical stripes) and fallers (horizontal stripes). Values in the hamstrings (hams) were similar in all three groups. In the quadriceps (quads), dorsiflexors (DFs) and plantarflexors (PFs) the older fallers were more asymmetrical than the young subjects. The only difference between the two older groups was in the dorsiflexors, where the fallers were more asymmetrical. *P < 0.02, **P < 0.004

During dynamic contractions there was no clear pattern between subject groups in the four muscle groups (Fig. 3).
Fig. 3

Asymmetry during dynamic contractions of the quadriceps (a), hamstrings (b), dorsiflexors (c) and plantarflexors (d) in the young (open bars), older non-fallers (vertical stripes) and fallers (horizontal stripes) at two angular velocities. Least differences between groups were seen in the dorsiflexors and most in the hamstrings. Only the slower eccentric contractions of the ankle muscles discriminated between groups, although the direction of difference was different in the dorsiflexors and plantarflexors. *P < 0.02, **P < 0.004

In the quadriceps and hamstrings (Fig. 3a, b, respectively) concentric contractions at both angular velocities asymmetry was significantly less in the young subjects (P < 0.02), with the two older groups being similar. Significant differences during eccentric contractions were found only in the hamstrings at the lower angular velocity (P < 0.004), but again the two older groups were similar.

In the ankle dorsiflexors (Fig. 3c) the only significant differences were found during eccentric contractions at the lower angular velocity where, rather surprisingly, the older fallers were less asymmetrical than their peers who had not fallen (P < 0.02).

In the plantarflexors the only significant differences compared with the young subjects were that the older fallers were more asymmetrical (Fig. 3d) at the lower angular velocities for both concentric (P < 0.02) and eccentric (P < 0.004) contractions. The only difference between the two older groups was during eccentric contractions at the lower angular velocity where the fallers were more asymmetrical (P < 0.02).

Asymmetry of power

While the young subjects showed least asymmetry and the older fallers the most, the only significant differences were between the young and both older groups (Fig. 1b). Twenty-two of the non-fallers and nine of the fallers had asymmetry values > 10%. Compared with the young subjects the non-fallers (P < 0.02) and fallers (P < 0.004) were more asymmetrical. The fallers were more asymmetrical than the non-fallers in power but this was not statistically different because of their increased variability in power.

Discussion

These data suggest that an increased asymmetry of strength in some leg muscles of older fallers compared with people of a similar age who have not fallen. However this was not a consistent finding for all muscle groups or types of contraction. While the effect of age on muscle strength (sarcopenia) were clearly demonstrated, the majority of strength measurements from individual muscle groups did not show differences between older fallers and non-fallers. However for all muscles combined the fallers were consistently weaker than the non-fallers. Measurements of power, involving a number of muscle groups, were lower in the fallers than the non-fallers and as this was the most functional of the tests performed it may be the most informative in terms of an understanding of a mechanism underlying medically unexplained falls.

Skelton et al. (2002) also reported that fallers were weaker and had greater asymmetry than non-fallers, the latter perhaps due to previous injuries. Ageing is associated with a decline in both strength (Frontera et al. 2000) and contractile speed (D’Antona et al. 2003) and so it is predictable that age related declines in power will be greater than those in strength and speed alone as found here and also by Skelton et al. (1994). For effective and safe control of the body movement, including the prevention of falling after a trip or postural disturbance, strength per se is not the only issue as the speed at which it can be produced is of prime importance, i.e. power output. Strength can clearly be increased in older people by training while is it unclear, and certainly less likely, that contractile speed can be. There are some reports of contractile speed increasing after training (Fisher et al. 1991; Skelton et al. 1995; Delmonico et al. 2005) although it is not clear that this was simply a reflection of greater strength and also the increases were not found during all functional tests. Therefore the maintenance of adequate strength is particularly important in older people.

This is the first study to directly explore asymmetry of strength and power in healthy young and older subjects. Leg extension power was more asymmetrical in the older subjects, but dynamic contractions of the individual muscle groups revealed relatively few differences and these were not consistent for type of contraction or muscle group. Differences in strength symmetry between the fallers and non-fallers were few but only occurred in the ankle muscles. This is an interesting finding but of doubtful relevance as it was not found in all types of contraction and largely agrees with the findings of Skelton et al. (2002) who observed no differences in strength asymmetry between these groups.

The age related decline in strength and power confirms previous work (e.g. Skelton et al. 1994; Hortobagyi et al. 1995; Lindle et al. 1997; Roos et al. 1999; Izquierdo et al. 1999; Trappe et al. 2003; Runge et al. 2004; Petrella et al. 2005). Porter et al. (1997) reported that eccentric dorsiflexion strength did not differ between young and older people while there are reports that eccentric strength is relatively preserved in older people and animals (Poulin et al. 1992; Hortobagyi et al. 1995; Phillips et al. 1991) Other work has shown that even though eccentric lower limb strength may decline with age, it does so at a slower rate than concentric strength (Porter et al. 1995; Lindle et al. 1997). We found a suggestion that eccentric strength was relatively preserved in the dorsiflexors during the slower eccentric contractions only but the difference was small (128, 131 and 148% of isometric for the young, non-fallers and fallers, respectively). In the other muscle groups studied there was no suggestion of a preservation of eccentric strength.

There was a clear trend for strength to be lower in the fallers than non-fallers but this reached statistical significance only in some of the measurements made. The sometimes relatively small differences in individual muscles between the older subjects became very significant when all the data were combined. This highlights the cumulative effect on function of relatively small force decrements in the component muscle actions necessary for functional movement. It also demonstrates the value of measurements such as leg extension power where a number of muscle groups contribute. The fallers generated 85% of the force of the non-fallers, but only 79% of the power. We report here the absolute measurements of force and power while some workers, e.g. Skelton et al. (2002) have reported values normalised for body mass. As can be seen in Table 1 the body mass of the two older groups were virtually identical and similar results were obtained with both absolute and normalised values.

Some studies have reported isometric strength differences in the hamstrings and/or plantarflexors (MacRae et al. 1992; Robinson et al. 2004) but others have not (Daubney and Culham 1999; Skelton et al. 2002; Melzer et al. 2004). Similarly strength differences have been found in the quadriceps by some studies (MacRae et al. 1992; Lord et al. 1992, 1999; Takazawa et al. 2003) but not others (Daubney and Culham 1999; Skelton et al. 2002; Melzer et al. 2004; Robinson et al. 2004). The different findings may partly relate to the different methodologies, age groups and classifications of fallers used. On the other hand, most studies have shown that dorsiflexor weakness is greater in fallers (MacRae et al. 1992; Daubney and Culham 1999; Roma et al. 2001, Takazawa et al. 2003; Robinson et al. 2004).

Studies of concentric strength in fallers and non-fallers have reported equally variable results. No differences were found by some groups in the hamstrings (Studenski et al. 1991; Skelton et al. 2002), plantarflexors (Skelton et al. 2002) and quadriceps (Schwender et al. 1997; Skelton et al. 2002). In contrast others have demonstrated deficits in fallers in the strength of the quadriceps (Studenski et al. 1991; de Rekeneire et al. 2003), plantarflexors (Studenski et al. 1991) and dorsiflexors (Studenski et al. 1991; Skelton et al. 2002). Our finding of similar eccentric strength in older fallers and non-fallers is in agreement with the only other similar study (Skelton et al. 2002).

It is possible that strength ad power measurements might be compromised by a failure of voluntary activation due to an unwillingness or inability to produce a movement that was painful. This was not directly tested, but if it were the case then greatest voluntary activation might occur during isometric contractions where no joint movement takes place and least during eccentric contractions which tend to be the most uncomfortable. The indirect evidence is that a failure of voluntary activation did not affect our results. One exclusion criteria for our subjects was the presence of joint pain during maximal voluntary contractions. The data do not suggest most symmetry during isometric contractions and least after eccentric in any of the four muscle groups tested.

Part of the reason for the conflicting results of these various studies may reflect different populations of fallers being studied. Different classifications have been used for identifying fallers. Studenski et al. (1991) and Skelton et al. (2002) used the criteria of having had at least two or three falls in the previous year and therefore were more frequent fallers than the people studied by most other groups. The fallers studied here had fewer reported falls and were relatively active, both physically and mentally. It may well be the case that more significant and functional differences would emerge between older fallers and non-fallers if a more frail population of fallers, or one with a history of more falls, were studied. Our population was relatively unusual in that it included men and it is uncertain whether this impacted on the results.

Cross-sectional studies do not permit causal relationships between falls and strength or power to be examined. However, the falls reported by these subjects were unlikely to have been sufficiently severe to affect muscle or joint function. Previous falls may reduce muscle performance through inactivity due to anxiety about further falls (McKee et al. 1999; Alexander 2001; Scaf-Klomp et al. 2003) but this does not seem to be the case for our subjects.

Biomechanical studies have shown that weakness does not necessarily lead to poorer recovery from tripping (Wojcik et al. 2001; Pavol et al. 2002), but strength and power generation are clearly important for maintaining of safe gait parameters (e.g. De Vita and Hortobgayi 2000; McGibbon et al. 2001; Lamoureux et al. 2003; Takazawa et al. 2003). Power may be more important than strength for recovering from trips as the limiting factor may be velocity of movement (Grabiner et al. 1993; Thelen et al. 1996).

In summary, these results do not suggest that increased asymmetry of leg muscle strength or power generation in the leg extensors are associated with ageing or the incidence of falls in the relatively active group of people studied here. Strength measurements from individual muscle groups underestimate differences between older fallers and non-fallers and measurements obtained from a number of muscle groups are more sensitive. The generation of power was lower in the older subjects and lowest in the fallers and therefore such more functional measurements appear to be more informative. This also highlights the importance of coordinated neural control for safe and effective movement.

Acknowledgments

We are grateful to the European Commission Better Ageing Project (QLRT 2001 00323) and The Charitable Foundation of Guy’s and St Thomas’ for financial support.

Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  • Mark C. Perry
    • 1
    • 2
  • Serena F. Carville
    • 1
    • 3
  • I. Christopher H. Smith
    • 1
  • Olga M. Rutherford
    • 1
  • Di J. Newham
    • 1
  1. 1.Division of Applied Biomedical Research, School of Biomedical and Health SciencesKing’s College LondonLondonUK
  2. 2.School of PhysiotherapyCurtin University of TechnologyPerthAustralia
  3. 3.Academic Department of RheumatologyKing’s College London School of MedicineLondonUK

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